INST

~ r'ubli- cations

OF

t/) NBS SPECIAL PUBLICATION 400-72

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Semiconductor Measurement Technology:

NBS/RADC Workshop

Moisture Measurement Technology

for Hermetic Semiconductor Devices, II

4J0-72

1932
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Semiconductor Measurement Technology: \LIBEAEY

NBS/RADC Workshop

^'^^"^

Moisture Measurement Technology uSi

for Hermetic Semiconductor Devices, II '^'> 9<x>-73r

Proceedings of the NBS/RADC Workshop

held at the National Bureau of Standards,

Gaithersburg, MD

November 5-7, 1980

Elaine C. Cohen and Stanley Ruthberg, Editors

Semiconductor Materials and Processes Division

Center for Electronic and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

This activity was sponsored by:

Rome Air Development Center RBRE

Griffiss AFB, NY 13441

and

National Bureau of Standards

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1 982
Library of Congress Catalog Card Number: 82-600503

National Bureau of Standards Special Publication 400-72

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-72, 302 pages (Apr. 1982) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1982
Table of Contents

Page

Preface vl

Abstract 1

1. Introduction 1

2. Session I Mass Spectrometry Measurement of Moisture 3

2.1 The Paradox of Moisture Measurement - A Modern Tetralogy

John C. Pemicka and Bvuoe A, Raby (Pemiaka Cor*povation) 3 2.2 Three Volume Calibration Valve - Calibration and Operation

Procedure for Carrousel Mass Spectrometer System

Kenneth L. Perkins (Rockwell International Corporation) . 8

2.3 Gaseous Compositions of Hermetic Package Cavity Amblents

Robert K. Lowry (Harris Semioonduotor) 15

2.4 Correlation Between Mass Spectrometer and Aluminum Oxide

Sensor Measurements of Moisture In Hermetic Packages

Rebeaoa J. Gale (Bell Laboratories) 19

2.5 Moisture Standards for Mass Spectrometers

Benjamin A. Moore (Rome Air Development Center) 32

2.6 Method 1018.2 Certification Results

Benjamin A. Moore (Rome Air Development Center) 39

3. Session II Moisture Sensors 49

3.1 A Procedure for Preparing Hermetic Packages with Known

Moisture Levels

Malcolm L. White and Robert E. Sarmons

(Bell Laboratories) 49

3.2 A Surface Conductivity Moisture Monitor for Hermetic IC

Packages

Robert K. Lowry (Harris Semiconductor) 64

3.3 Some Observations on the Response of Dew Point Detection Chips

Malcolm L. White and Albert F. Walcheski (Bell Laboratories) 76

3.4 Cross Correlation Experiments on Different Types of

Sensors

Michael G. Kovaa (University of South Florida) 79

3.5 Moisture Sensors, Mass Spectrometry, and MIL Standards

John Hale (Texas Instruments, Inc.) and Victor Fong

(Panametrics, Inc.) 90

3.6 Dew Point Moisture Measurements

Bert A. Unger and Peter R. Bossard (Bell Laboratories) . 98

3.7 Water Vapor Measurements in Integrated Circuit Packages Using

an Infrared Diode Laser

J. A. Mucha and Peter R. Bossard (Bell Laboratories) . . 105

ill
4. Session III Process Control 110

4.1 A Recent Evaluation of AI2O3 Moisture Sensors in Metal Hybrid

Packages

Robert F. Maeko (Hughes Aircraft Company) 110

4.2 Moisture Monitoring and Control During Assembly of LSI

Circuits via In-Situ Moisture Sensors

Saeed E. Siddiqui (Digital Equipment Corporation) .... 113

4.3 Moisture Failures in Hybrids

E, Wes Poate (General Dynamics) 117

4.4 Test Method 1018.2 - A Progress Report

Robert W. Thomas (Rome Air Development Center) 126

4.5 Hybrid Stress Testing in Wet and Dry Packages

Michael Richtarsic (Martin-Marietta Corporation) .... 128

5. Session IV Moisture physics 129

5.1 Conceptual Model of Aluminum Corrosion of an Integrated

Circuit

Allen R. Bailey (AMP Inc.) 129

5.2 Conductivities and Electrolytic Properties of Adsorbed Layers

of Water

George B. Cvijanovich (AMP Inc.) 149

5.3 Microenvironments and Accelerated Testing

Michael G. Kovac (University of South Florida) 165

5.4 Moisture Failure Mechanism

George H. Ebel (Singer Company) 175

5.5 Experiences in Microcircuit Moisture Problems

James R. Duffy (Hughes Aircraft Company) 178

5.6 Thermodynamic and Kinetic Considerations of Moisture Sorption

Phenomena

J. Gordon Davy (Westinghouse Defense and Electronic

Systems Center) 184

5.7 The Detection of Cracks In Ceramic Packages by Vapor Condensation

Aaron Der Mardevosian (Raytheon Corporation) 201

5.8 Thresholds for Corrosion

Aaron Der Marderosian (Raytheon Corporation) 212

6. Session V Packaging 213

6.1 Moisture Content of Solder Glasses

Rama K. Shukla, J. SinghDeo, Nirmal K. Sharma, and

Richard Blish (Intel Corporation) 213

6.2 Dry Sealing Glasses - A Summary of Research

Robert K. Lowry (Harris Semiconductor) 220

6.3 what's Wrong with Cerdips?

Robert {/. Thomas (Rome Air Development Center) 234

6.4 Moisture Impermeable Polymers

Philipp wh Schuessler (IBM Corporation) 239

6.5 Plastic Packaging

Dimitry Grabbe (AMP Inc.) 246

iv
7. Session VI Hermeticity, Fact or Fiction 247

7.1 A Method of Assessing The Surface Conductivity o. Plastic

Encapsulated Integrated Circuits

Robert P. Mevrett (British Teleaom Research

Laboratories) 247

7.2 Internal Moisture Measurement of IBM Integrated Circuit

Memory Package

Henry C, Baron, F. Richard Moser, and John Susko (IBM

Corporation) , 258

7.3 A Method of Leak Detection and Location for Conformally

Coated Packages

George E, Ebel and Richard A, Be Cvistofava (Singer

Company) 271

7.4 Improved RTV Silicone for IC Encapsulant

(Thing P. \fong and Donald E» Maurer (Western Electric

Company) . . , 275

7.5 Leak Testing Electronic Components

Paul R. Forant (Varian) 281

8. Workshop Participants . . . • 2B9

8.1 List of Speakers ............ . 289

8.2 List of Attendees 291

V
Preface

A second workshop on Moisture Measurement Technology for Hermetic Semiconductor Devices was conducted as part of the Semiconductor Technology Program in

the former Electron Devices Division of the National Bureau of Standards

(NBS); it was another in a series dedicated to the furtherance of the measurement technology needed by the semiconductor device industry in its

attempt to provide to its customers products that are based on the most advanced technology, yet have high reliability and the affordable costs which

result from high yields. It was held at the Gai thersburg, Maryland, facility

of the National Bureau of Standards on November 5 to 7, 1980, under the co- sponsorship of the National Bureau of Standards and the Rome Air Development

Center (RADC). Representatives from industrial, governmental, and academic

organizations concerned with device manufacture, analysis, and instrument

design participated.

The workshop provided a forum for reporting the progress that has been made

since the previous workshop of March 22 to 23, 1978, in the measurement of

moisture in hermetic semiconductor devices, in the correlation between measurements made by different procedures, in test environments, and in-process

control. Thirty-six formal talks as well as numerous question and answer

sessions were included. The 99 attendees represented a broad spectrum of the

semiconductor and related communities concerned with moisture measurement and

control.

The workshop was hosted by S. Ruthberg of NBS; R. W. Thomas of RADC was

chairman; E. C. Cohen of NBS coordinated workshop arrangements; E. J. Walters

of NBS provided editorial assistance and supervised processing; and J, S.

Halapatz of NBS provided typing and rework.

It is hoped that this report will convey the spirit of the workshop to those

who could not attend.

Disclaimers

The views and conclusions expressed are those of the authors and do not nec- essarily represent the official policies of the Department of Defense, Department of Commerce, or the United States Government.

Certain commercial equipment, instruments, or materials are identified in

this report in order to adequately specify the experimental procedure. In no

case does such identification imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.

Papers in this volume, except those by National Bureau of Standards authors,

have not been edited or altered by the National Bureau of Standards. Non-NBS

authors are solely responsible for the content and quality of their submissions.

vi
Semiaonduotov Measurement Teohnology:

NBS/RADC Workshop

Moisture Measurement Technology for Hermetic Semiconductor Devices, II

- A Workshop Report -

Chairman: Robert W. Thomas

Editors: Elaine C. Cohen and Stanley Ruthberg

Coordinator: Elaine C. Cohen

The workshop, one of a series concerned with measurement problems

in integrated circuit processing and assembly, served as a forum to

examine the progress that has been made in the measurement and

control of moisture in hermetically packaged semiconductor devices.

While moisture-induced failure modes and mechanisms have been ex- tensively documented, the lack of accurate and reliable measurement

of the moisture content itself has been a major obstacle to meaningful efforts to limit and control this pervasive contaminant.

Manuscripts are provided of 36 presentations which detail the progress that has been made in mass spectrometer measurements and

calibration of internal package moisture, in increased assurance

with moisture sensors, in testing, and in package control.

Key words: analysis of moisture content; hermetically packaged

semiconductor devices; mass spectrometer measurement; moisture;

moisture generators; moisture sensors; quality control; reliability

of semiconductor devices; semiconductor devices.

1 . Introduction

A workshop on Moisture Measurement Technology for Hermetic Semiconductor

Devices was held at the National Bureau of Standards in Gaithersburg, Maryland, on November 5 to 7, 1980, cosponsored by the National Bureau of Standards and Rome Air Development Center. It was another in a series concerned

with measurement problems related to integrated circuit processing, assembly,

and reliability. This was a second workshop and a third meeting in a series

at NBS concerned with the moisture measurement problem. The first workshop

on moisture measurement was sponsored by ARPA/NBS on March 22 to 23, 1978

(see NBS Special Publication 400-69, Semiaonduotov Measurement Teohnology:

ARPA/NBS Workshop V, Moisture Measurement Technology for Hermetic Semiconductor Devices, May 1981). The second meeting was specifically concerned with

the use of the mass spectrometer for the measurement of interior moisture

levels in hermetic packages, was limited to those actively engaged in the

process, and was sponsored by NBS on June 6 to 7, 1979.

The accurate determination of the moisture content in hermetically packaged

semiconductor devices continues to be a difficult problem, but progress on

its measurement is being made. At the time of the first workshop, inter laboratory measurement precision was poor, although it appeared that single-laboratory precision of mass spectrometer measurements was adequate. It also

appeared that the use of moisture sensors was becoming practical, but correlation to the mass spectrometer was low. It was clear from the first work1
shop that much more had to be done to characterize mass spectrometers and

moisture sensors in terms of all the variables, conditions, and applications

that could affect the measurement. By the time of the ad hoc invitational

meeting specifically on the mass spectrometer, measurements obtained on moisture prefilled test packages still evidenced erratic results. Whenever the

reliability community preferred that a 20 percent interlaboratory precision

be demonstrated before Test Method 1018 of MIL-STD-883 (Internal Water Vapor

Content) became effective, the results obtained on the test packages sometimes gave this precision (at 5000 ppm^ water) and sometimes were very much

worse. It was clear from the data presented at this meeting that test packages were very difficult to produce with any consistency and that an independent method was required to evaluate mass spectrometer performance. This was offered in the form of a three-volume artifact through which a gas flow of

known moisture content could be passed from an absolute generator and additionally measured with a calibrated dew-point meter. The three volumes were typical common packages in use. The contained moist gas is then introduced

to the spectrometer from the selected volume.

Considerable detail and significant progress were documented at this second

Moisture Measurement Workshop. The use of the three-volume artifact has

verified that suitable ( < 20 percent) interlaboratory precision can be obtained with the mass spectrometer and that the desired precision is available

with the mass spectrometer on metal test packages of approximately 0.01 cm^

volume at the 5000 parts -per-million-by-volume moisture content level. Cross

correlations between the mass spectrometer and surface sensors are improving , but much more in the way of definitive characterization is required. Other

methods of measuring interior moisture were presented , such as by derivative

infrared absorption spectroscopy. Process control for moisture is improving.

However, precision on Cerdip-type packages has yet to be obtained and efforts

on moisture concentrations < 5000 ppm^ must be continued.

The workshop comprised 36 presentations in 6 sessions with topics ranging

from mass spectrometry measurement to hermeticity. Each session included a question and answer period. Further discussion continued at informal evening

receptions
2. SESSION I MASS SPECTROMETRY MEASUREMENT OF MOISTURE

2.1 The Paradox of Moisture Measurement

A Modern Tetralogy

John C. Pemicka

Bruce A. Raby

Pernicka Corporation

450 E. Middlefield Road

Mountain View, CA 94043

(415) 969-0220

Abstract: This tetralogy is an exposition of four subjects which influence

the mass spectrometric determination of water in semiconductor packages.

These concerns are: calibration procedures; the automation of instrumentation and data processing; the perturbation of gas flow during sample

transfer; and the detrimental effects of oxygen's chemical reactivity.

The discussions also lead to a consideration of several paradoxes which

entangle moisture measurement.

Key Words: Algorithms; calibration; chemical reactions; gas flow; gas

transfer; mass spectrometer; moisture measurement; oxygen; software;

sorption; water.

1. INTRODUCTION

The mass spectrometer is one of the most effective tools being used to measure

moisture in semiconductor packages (1) . Several problem areas have been identified

by the authors and others which can unknowingly lead to a distortion of the final

measurement result (2) . Several experiments with nitrogen/argon/water and helium/

water samples have been conducted using abnormal instrument parameters in order to

emphasize effects which require careful consideration. The problem areas have been

divided into the following four basic subgroups: A. general calibration techniques,

B. software and data processing techniques, C. effects of sorption, etc., on gas

analysis, and D. I wonder where the oxygen went?

2. EXTENDED SUMMARY

A. General Calibration Techniques

There are two general methods of calibrating a mass spectrometer for quantitative

analysis (3). The best method is to measure directly the parameters of interest;

the other is to estimate them from instrumental transfer functions. Methane serves

as the example to illustrate the two methods.

The direct method measures both the sensitivity coefficient and the actual spectrum

of the substance of the mass spectrometer which will be used for analysis. Sensitivity is the intensity of the chosen analytical indicator peak divided by the

partial pressure of that gas in the inlet. For methane at an iglet pressure of

1.85 Torr and choosing m/g = 15 with an intensity of 3.16 x 10 A, the sensitivity

coefficient is 1.71 x 10 A/T. Measurement of methane's spectrum shows that

m/e = 2 has an intensity of 2.03 x 10~ A; therefore, its interference coefficient

is 0.0643 with respect to m/e = 15 as indicator. This contribution to mass 2 by

methane is one of the corrections which must be applied to the quantitation of

hydrogen.

3
Estimation of sensitivity involves a computation based on four parameters:

relative ionization efficiency, transmission rolloff, relative multiplier gain,

and fragmentation pattern. Thus, for methane, the correction factor for m/e = 15 is 1/(0.9143 X 1.0 x 1.2 x 0.3867) = 2.3570. The alternative to actual

measurement of methane's spectrum from a given instrument is to estimate it

from tables of standard reference spectra (4) . Again, one needs to compute

necessary interference coefficients.

Calibration of mass spectrometers for moisture Is presently an Indirect process

which Involves the use of a package simulator and moisture generator system (5)

The most accurate calibration for a given system can be accomplished using

"standard" packages (6) . The paradox is that standard packages are difficult

to manufacture and are being used to test the indirect calibration method. In

addition, uncontrolled package lots are continually used by individuals to

study calibration accuracy between various instruments.

Experiments have shown that some indirect calibration procedures require molecular

flow during the sample transfer (7). This would imply that optimization for

sample size should be a consideration for system design. Since the cavity size

range of packages extends from 0.001 sec to over 10.0 sec, optimization would be

at best a compromise. The paradox in this case is that we should minimize the

surface area of the sample volume in order to reduce surface interactions with

moisture (8). The result Is gas transfer In a nonmolecular flow regime. The

following paragraphs describe how to handle this situation.

B. Software and Data Processing Techniques

The necessity for complex calibration corrections requires a simple, automated

method of operation for practical applications such as implementation of MilStd 883B, Method 1018.2, Procedure 1.

A computer operating with a fully prompted BASIC program controls the mass spectrometer, acquires the data, and processes those data. The acquisition phase of

the program obtains background data, sample data, and a safeguard scan of the

entire mass spectrum to search for unexpected materials or compounds.

The raw data are corrected for instrunent background, sensitivity, and Interfering

spectral peaks. The computer^ generates a final analytical report and, in addition,

plots both the gas transfer curves for all the gases of interest and a complete

mass spectral scan over the instrument's scan range.

The system operator can transfer all the above Information to an archive file.

The raw data are retained in the event reconstltutlon of the plots and report are required

Since the computer has ready access to data which are catalogued by customer name

and date, it is fairly simple to provide production trends as well as performance

profiles with a minimum amount of effort. The data can be presented in tabular

or graphical format, can be averaged over any time interval, and standard deviations

may be computed for selected groups of packages.

The gas transfer curves for water and other gases provide invaluable information

about a given product. Yet, in many cases, these curves are either ignored or non-existent depending on the technique for data collection. This paradox centers

on the prevalent attitude "all I want to know is if the sample passed or failed

4
the test", but in order to make the determination, gas transfer curves must be

considered and the following paragraphs illustrate their importance.

C. Effects of Sorption, etc., on Gas Analysis

Accurate quantitation of package atmosphere constituents requires careful integration of the corresponding transfer curves. It is easy to integrate those

cases which transfer in molecular flow and have no complicating perturbations.

One can readily determine the exponential coefficient, end-point, and integral

of these ideal curves. From the differential equation of molecular flow, SP + = 0, we derive (S = pumping speed or conductance, V = chamber volume, P = pressure)

:

-St_

P = P e V The pressure (or signal) at any time

t = ^ In (P /P ) Time to the end point

e.p. S ^o e.p.'^ ^

t

e.p.

P V e p

Pdt = o (1 - e Y^"^') Integral of the substance

Nonideal curves result from nonmolecular flow regimes, pressure oscillations,

saturation effects, and sorption phenomena. Transition and viscous flow occur

for those combinations of dimensions and pressures which favor gas phase collisions rather than gas-wall collisions. Oscillations may be produced in cases of

sudden transients (e.g., opening a valve to produce a step change in pressure)

or when the source of gas is pressure dependent (permeation through a membrane,

sorption-desorption, and viscous flow)

.

Saturation effects occur when source pressure is high enough to saturate the

ion-extraction voltage or to cause ion-molecule reactions in the ionizer. A

high ion current impinging on the electron multiplier can saturate the multiplier's

ability to produce secondary electrons and transport them to the anode.

Sorption phenomena are complex and include the chemistries of sorbent and sorbate,

the transfer of energy during a collision on a surface (accommodation coefficient),

and all the ramifications of the sticking coefficient and sojourn. Sticking

coefficients vary from 10"^^ to 1, but the ones of interest here range from

0.01 to 0.9. All of the above perturbations are, in fact, seen in the course of

analyzing samples and standardizing.

0.10 sec gas samples of nitrogen/argon/water and helium/water mixtures were

introduced into a mass spectrometer inlet to reproduce a typical test configuration

for a semiconductor package. The inlet was operated in both a preconditioned

and nonpreconditioned mode. The resulting gas transfer data were plotted on log-linear paper, with signal intensity on the log axis and time, or the linear axis.

The resulting straight lines could be misinterpreted as molecular, ideal gas

flow. Careful examination of the relative slopes provides a strong indication

that the gases are transferring from the inlet to the mass analyzer in transition

flow. In addition, the structure of the water transfer curve is a function of the

initial equilibrium state of the inlet as well as the relative competition between

water and the host gas to stick to the surfaces of the inlet.

5
The structure of the moisture transfer curve is dominated by the transient response

of the inlet and the initial equilibrium state of the inlet. One should be

cautioned that assumptions which imply that the structure of moisture transfer

curves can be interpreted as contributions of ambient and desorbed moisture from the

package under test are erroneous, and calculations based on the above assumption

can result in errors of a factor of two or larger for certain package types.

Therefore, it is recommended that integration be carried to the end point by

an ensemble or trapezoidal algorithm. Another choice is to integrate by one

of the above techniques until the transfer curve becomes well behaved, then

compute the end point, and complete the integration theoretically. For cases in

which moisture is high and the transfer is never well behaved, integration can be terminated when the running integral exceeds the 5000 ppm criterion.

End point determination of a typical moisture transfer curve provided an integral

correction coefficient of 3.5 when compared with integrals of 98 percent of the

host gas. This value compares favorably with the overall correction factor of

3.32 measured using Method 1018.1, Procedure 1, calibration techniques.

D. I Wonder Where the Oxygen Went?

The problem of moisture determination in semiconductor packages is further complicated by the presence of oxygen in the host gas. Oxygen can react to produce both

volatile and nonvolatile compounds, and these reactions interfere with the

quantitative determination of oxygen as well as water, hydrogen, carbon monoxide,

and carbon dioxide.

Oxygen reacts with hydrogen and hydrocarbons to form water; it reacts with carbon,

carbides, and hydrocarbons to form CO and CO^. It reacts, also, with metals to

form oxides. The result of all these side reactions is a depletion of oxygen,

hydrogen, and hydrocarbon; conversely, there is an enhancement of carbon oxides.

One particular study showed that 20 percent of the oxygen went into carbon

oxides, 20 percent to water, 40 percent to metal oxides, and 20 percent

remained as oxygen.

Some remedies are to reduce filament temperature, use oarbide-free metals, reduce

system background (residual) gases, and coat reactive surfaces (e.g., gold plate

copper gaskets)

Calibration with pure oxygen and oxygen/water mixtures can be misleading, since

it might take up to 50 hours to reach equilibrium in the average mass spectrometer.

The most conservative approach to calibration with oxygen is to use the same

relative amount in the same matrix as the unknown sample.

3. ACKNOWLEDGMENTS

All facilities and experiments were provided for and conducted by the Pernicka

Corporation with many helpful suggestions from R. W. Thomas and B. A. Moore of

RADC, and R. J. Gale of Bell Telephone Laboratories.

6
4 . REFERENCES

1. Thomas, R. W. , and Meyer, D. E., Moistvire in SC Pacikages, ^eiMdlState

Technology, Vol. 17, No. 9, September 1974.

2. Pernicka, J. C, Moisture Measurement by Mass Spectrometer, Semiconductor

Measurement Technology: ARPA/NBS Workshop V, Moisture Measurement Technology for Hermetic Semiconductor Devices, NBS Spec. Publ . 400-69 (March

1978), p. 42.

3. Dawson, P. H. (editor), Quadrupole Mass Spectrometry and Its Applications,

Elsevier Scientific Publishing Company, New Yorl?., 1976.

4. Stenhagen, E. , Abrahamsson, S., and McLafferty, F. W. (editors). Atlas

of Mass Spectral Data, Interscience Publishers, New York, 1959.

5. Perkins, K. , Three Volume Calibration Valve - Calibration and Operation

Procedure for Carrousel Mass Spectrometer Systems, this report.

6. Moore, B. A., and Der Marderosian, A., Moisture Standards for Mass Spectrometers, this report.

7. Merrett, R. P., A Dynamic Method of Calibrating a Mass Spectrometer Used

for Measuring the Water Content of Semiconductor Encapsulations, Semiconductor Measurement Technology: ARPA/NBS Workshop V, Moisture Measurement

Technology for Hermetic Semiconductor Devices, NBS Spec. Publ. 400-69

(March 1978), pp. 33-37.

8. Roth, A., Vacuum Technology, North-Holland Publishing Company, New York,

1976.

7
2.2 Three Volume Calibration Valve - Calibration and Operation

Procedure for the Carrousel Mass Spectrometer System

Dr. Kevniof:h L. Perkins

Rockwel] Tt) Lernat lot-, jl I Corporation

3370 rliraloma iVvence

Mail Stafioii D?03

Anaheim, Calif orni'?. 92803

(714) 632-2329

j^bstract: Pvesults cbtair.sd using th^i 'chree vclurja caiibraLion

valve (TVCV) v/ith a carrousel quacirupole i?.ass spectrcraeter system

show that it is ar i ndl ? p o n s a b le • ad d i. t i r/ a .

' Use oi the TVCV revealed

two ImportanL facts. One v.'a;-. tb.at; consiiilent syKteiii conditions must

be established prior to each analysis to obtain reproducible watervapor -.neasuiements . The proceaure found suitable for th.is purpose

was to precondition the systen: by analyzing a room air saraplc and

then evacuating the sample cLiamber to 2 x 10-S Torr, The other

was thai; the water-vapor senBj.tivicy factor is not constant j but

varies with the sample volune and the water-vapor concentratioa.

iliis lac-uas Lhat to obtain accurace ineasurements , the systenj itbst

be calibrated over a range of water-vapor concentrations for che

volume of the package being analyzed. Since Rockwell's primary

concern is hybrids, the system was calibrated using the largest

volume of the TVCV. Samples containing 5000 PPM^ were analyzed

first, and the system sensitivity factor was adjusted to give a corresponding measured value. Samples containing &pproxim,ately

1000, 3000, and 9000 FFMy v;ere then analyzed and correction factors

calculated. These results were plotted on semi-log paper in the

fcr.-n most convenient to the operator, i.e,, Measured Water-Vapor

Content versus Correction factor.

Key Words', Mass spectrometer; mass spectrometer calibration^, xraas spectrometer calibration factor; mass spectrometer sensitivity

factor; moisture analysis; moisture measurement: three volume

calibration valve; three volume calibrator; water-vapor measurement.

1. INTRODUCTION

Accurate measurement of the water-vapor content ct the internal atm.ospheres

of hemeticaily sealed packages has proven to bo a much mere difficult task

than originally assumed^ The problem lies in properly calibrating the mass spectrometer system., and in establishing consistent base-line system conditions

prior to each measurement. A company funded study to solve this problem was started this past year at Rockwell International Corporation. This paper

reviews the v7ork perforn^ed to date, presents tb.e results obtained, and describe

the v/crk that rem.ains to be done to achieve the desired goal of being able

to measure accurately the water-vapor content of the internal atmospheres of

hybrid packages of any free internal volume and water-vapor concentration.

8
2. THE KEY TO THE PFOiVLEM - THE THREE VOI,U>iE CAT.IBPATION VALVE

A three vo.lurae calibration valve (TVCV) and a general purpose Innnidifier v;ere obtained from Fernicka Corporation and incorporated in the Rockvell carrouseltype quadrupole mass spectrometer moisture/gas analysis system. The TVCV

contains three separate chambers with volr-mos of approximately 0.01, 0.1, and

0.8 cc. The general purpose humidifer is ?. two pressvire type and v.'hen connect

to a source of dry nitrogen provides nitrogen of fixed inoisture content- determined by the source pressure. This setup provides the capability to generate

samples of three different volumes (simulating three different si;';e packac>es)

containing any desired concentration of v/ater-vapor do^m to approximately 350

P?M^.. The water-vapor concentration can be calculated from the source pressur

but an optical dew point hygrometer was installed downstream from the TVCV to

obtain more accurate and convenient measurements.

2.1 Initial Results Obtained Using The TVCV And Determination Of Base-Line

System Conditions

During the first several weeks after installing the TVCV, many analyses were

made using primarily the 0.1- and 0.8-cc volumes at known water-vapor concen- trations around 1000, 5000, and 10,000 P?M^,. A review of the results obtained

sb.owed the following:

1. The carrousel-type mass spectrom.eter moisture/gas analysis S3^stem

must be preconditioned prior to each analysis in order to obtain

accurate, reproducible results

o

2. The water -vapor sensitivity (or calibration) factors determined at

5000 PPM^ are different for the 0.1- and 0.8-cc volumes.

3. The water-vapor calibration factor determined at 5000 PPM for the

0.8-cc volume gives measured values that are too low at 1^00 PPM

and too high at 10,000 PPM^. ^

The necessity for preconditioning the system prior to each analysis became

obvious after the first few days of using the TVCV. During this time, samples

V7ere analyzed using the three different volumes, different concentrations of

water vapor, and different sample chamber base pressures. In general, measure

cents for samples of the same volume and water-vapor content were not repeatable. Review of the data showed that the measurements obtained varied with

the sample chamber base pressijro (the pressure to which the sample chamber

is evacuated prior to introducing the sample into it) and with the watervapor content of the sample just previously analyzed. For example, typical

results obtained for the first 0.8-cc volume sample analyzed in the morning

after the chamber had been pumped down overnight at 100°C to a pressure of

approximately 10~8 Torr arc given in Table 1 for samples containing approximately 1000, 5000, and 10,000 PPH-^i, and those for a sequence of analyses on samples containing approximately 5U00 PPM^^ are given in Table 2. The results

given in Table 1 show that tiie sample chamber is moisture starved after the

overnight pump-down at lOO'^C and must be preconditioned witti moisture in some way prior to analyzing samples. The results given in Table 2 show that

successive analyses of 5000 samples eventually provide this preconditioning. The last entries in Table 2 also show that the measured watervapor content depends on the sample chamber base pressure.

9
As a result of these observations, the decision was made to preconditiori the

system by anletting and analyzing a room air sample. After considerable

additional experimentation with the sample chamber b^^se pressure, the optimum

value was determined to be 2 x 10~8 Torr. Also, as a result of this additional

experience, it was decided to inlet and analyze a room air sample prior to

each analysis to assure that consistent base-line system conditions are established

2,2 Use Of The TVCV To Calibrate The Carrousel-Type Hoisture/Gas Analysis

System

The primary interest at Rockwell is hybrids with internal free volumes ranging

from approximately 0.5 to 2.5 cc. Based on the second result stated in the

previous section that the water-vapor sensitivity (or calibration) factors

determined at 5000 PPM^ are different for the 0.1 and 0.8-cc volumes, use of

the 0.8-cc volume of the TVCV is probably not completely adequate to calibrate

the system over the entire range of interest. Additional work to investigate

and correct this situation is briefly outlined in the last section of this

paper. Fortunately, the calibration factor required for the 0.8-cc volume was smaller than that required for the 0.1-cc volume, so the calibration factor

determined using the 0.8-cc volume will probably give corrected measured values

of water-vapor content that are slightly larger than the actual values.

Using the preconditioning procedure established in the previous section to assure

consistent base-line system conditions prior to each analysis (i.e., analysing

a room air sample and evacuating the sample chamber to 2 x 10~8 Torr prior to

each analysis), 0.8-cc volume samples containing approximately 5000 PPM^ watervapor were analyzed and the system water-vapor sensitivity or calibration

factor was adjusted to give a corresponding measured value. Then, 0.8-cc

volume samples containing approximately 1900, 5000, and 9000 PPHy watervapor were analyzed to determine the correction factors (i.e., the factors

by which the measured values must be multiplied to obtain the known values)

required for these water-vapor concentration levels. The results are given

in Table 3. As shown, the previously determined water-vapor calibration

factor is correct at approximately 5000 PPM (i,e., the correction factor

is 1). However, at approxiEiately 1000 and 9000 PP>L,, the required correction

factors are 1.95 and 0.81, respectively. These results were plotted on semilog paper as shown in Figure 1 in the form most convenient to the operator

(i.e., Measured Water-Vapor Content versus Water-Vapor Content Correction

Factor) to give approximate correction factors for all measured values of

water-vapor content in this range.

3. WOPJC STILL TO BE PERFORMED

As stated in the previous section, the present TVCV is probably not completely

adequate to calibrate the Rockv^ell moisture/gas analysis system over the entire

range of interest. Further work will be performed to determine the quantitative

dependence of the water-vapor calibration factor on sample volume and watervapor content for sample volumes corresponding to the free. inteEtral' voliimes

of commonly used production hybrids (i.e., approximately 0.5 to 2.5 cc) . To

atrfcom.plish this, the large chamber (0.8-cc volume)_ qf^ Jthe^present TVCV will

be modified to obtain several volumes covering this range, and s^.ples con- taining known water-v^por concentrations ranging from approximately lOGO to

10
10,000 P?M^, will be analysed for (^acli of these volur.GS. The relalilonship

betvjaen the measured r::\d Icik-uTi va vapor concentrations will be calculated

and the data will be plotted and as simplified as possible to provide a family

of curves or a nomograph covering the entire region of interest.

4. CONCLUSIOinS

The TVCV and larger voJ.ume variations of it are indispensable additions to

the carrousel-type q^uadru^ole mass spectrometer moisture/gas analysis system

to provide the capability to measure accurately the water-vapor content of

the internal atmospheres of packages of any free internal volume and watervapor concentration. Tvo requirements must be met to obtain accurate watervapor measurements using this system. One is that consistent system conditions

must be established prior to performing an analysis. This requirement is met

by preconditioning the system by analyzing a room air sample and evacuating

the sample chamber to 2 x 10-8 Torr prior to each analysis, Tlie other is

that the system must be calibrated over a range of water-vapor concentrations

for the voluDe of the package being analyzed. The TVCV provides the capability

to meet this requirement for packages of free internal volumes ranging from

0.01 to 0.8 cc. Larger volume variations of the TVCV are required to meet

this requirement for packages of larger free internal volumes.

5, ACKNOl-JLEDGMENTS

The author would like to thank Dr. J. J. Licari and Mr. M. W. McMurran for

their support, consultation, and encouragement during the performance of this

and all other work in the area of moisture measurement.

371-525 0-82-2

11
12
Table 1. Typic.?.! Results For First Sample.

After Overnlsht Purap-Down at 100°C

Sample Chamber

Base Pressure

(Torr)

Known

Water-Yapcr Content

(PPM^)

Measured

Water-Vapor Content

(PPM^)

0.8 X

10~^ 1000 Zero

0.8 X 10"^ 5200 892

0.85 X 10"^ 9400 5584

Table 2. Typical Results For A Sequence Of Runs

After Overnight: Pump-Down At 100°C

Sample Chamber

Base Pressure

(Torr)

Kno\ra

Water-Vapor Content

Measured

Water-Vapor Content

(PPMy)

0.8 X 10~^ 5250 908

1.0 X 10~^ 5100 2067

1.2 X lO"^ 5400 2950

2.0 X 10~^ 5000 3990

2.5 X 10"^ 4950 4937

3.5 X 10"^ 4950 5197

4.0 X 10~^ 4880 6141

13
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14
2.3 Gaseous Compositions of Hermetic Package Cavity Ambients

Robert K. Lowry

Harris Semiconductor, Products Division

Melbourne, FL 32901

(305) 724-7566

Abstract: Mass spectrometric measurement of internal water vapor

content also yields data on other volatiles in the package cavity.

Identities and concentrations of all the gases present can supply

useful information for packaging technology improvements to

produce cleaner and drier parts. This paper describes some of

the relationships found between levels of moisture and associated

levels of N2, O2, H2, CO2, and Ar in several different package

styles.

Key Words: Gas analysis; gases in hermetic packages; hermetic

IC packages; internal water vapor; mass spectroscopy; moisture

measurement

.

1. INTRODUCTION

An important failure mechanism of integrated circuit chips contained in

hermetically sealed enclosures is electrogalvanic corrosion of metallization

caused by excessive moisture within the sealed cavity [1,2]. Mass spectrometric methods have been specifically developed to measure internal water

vapor contents of a wide variety of microelectronic packages [3,4]. An

advantage of mass spectroscopy, as opposed to in-situ means of moisture

sensing, is the detection of all volatile species present within the package

cavity. Knowing complete package ambient compositions assists development

of improved packaging technology, and can provide information on seal

integrity as well as chemical events within the cavity during or subsequent

to seal formation.

This paper reviews a body of mass spectrometric analysis data obtained since

July 1979 on Cerdip, braze seal, and TO can package styles. Many of the

samples came from package engineering studies of internal water vapor content

in which purposely wet parts were produced. Hence, the compositions reported

here are not entirely representative of a routinely assembled, product. All

analyses were carried out according to Procedure 1 of the MIL-STD-1018. 2 test

method. Some interesting trends involving levels of water vapor and the

other volatile species in the various packages are discussed.

2. DISCUSSION

Cerdip

Ceramic dual- in-line packages are composed of an alumina base and cap. The

IC chip is attached to the base, and the lid is sealed to the base using a solder glass to enclose the chip in a hermetic cavity. Sealing is done in a

15
temperature profiled furnace. Seal ambient is either natural or synthetic air.

In decreasing order of concentration, the gaseous components within the Cerdip

cavity are typically CO2, Ar, and H2O.

Carbon dioxide within Cerdips is derived from organic binders and vehicles used

in glazing the alumina piece parts. Variability in burn-out of these organics

prior to and during seal causes substantial variation in the C02 content of

individual Cerdip packages. Concentrations usually range from 2,000 to 20,000

ppmv. A few specimens may contain significantly more or less than these amounts.

When analytical results for 146 Cerdip parts are arranged in order of increasing

moisture content, an associated plot of corresponding CO2 content is completely

random. There is no relationship between level of water and level of CO2.

The analytical data for the same 146 parts were arranged in order of increasing

Ar content. The Cerdips seem to comprise three distinct groups. One group

contains little or no Ar, representing packages sealed in synthetic air. A

second group contains more than 7,000 ppmv Ar, representing packages sealed in

natural air (which contains 9,300 ppmv Ar) . A third group contains 200 to 600

ppmv Ar. These parts were sealed in synthetic air, but for some reason contain

finite amounts of Ar which were not originally present in the seal ambient.

This Ar is presumably contributed by air entrapped in the sealing glass. However,

these parts could also have been fine leakers into which a small amount of ambient

air has diffused.

An associated plot of H2O contents with increasing Ar shows a weak dependence.

Parts sealed in synthetic air tend to be slightly drier than those sealed in

predried natural air. When the associated CO2 data are placed on this plot,

the random nature remains. There is no relationship between CO2 contents and

H2O contents or the type of air used for sealing. Parts sealed in synthetic

air, which itself contains no CO2, generally have as much CO2 as parts sealed

in natural air.

Another trend within Cerdips relates package H2O level to the ratio of the

principal constituents N2:02. Parts sealed in natural air have an N2:02 ratio

of about 4:1. Those sealed in synthetic air may have ratios ranging from 4:1

to as much as 15:1, presumably due to inhomogeneities in mixing of the component

gases in the seal furnace. A plot of H2O content of the 146 part sample group

shows that the higher N2:02 ratios produce substantially drier packages. For

example, of 20 specimens whose N2:02 ratio exceeded 8:1, only two contained H2O

greater than 1,000 ppms. At N2:02 ratios of 8:1 or less, about half the specimens

contained H2O greater than 1,000 ppmv. The N2:02 ratio of 8:1 seems to be a critical value, as that is where a precipitous decline in average package moisture

content occurs as the ratio increases. The reason for this observation may be

due to one or both of two factors. Synthetic air may simply be inherently drier

than dried natural air. However, an N2 enriched cavity ambient may induce a physico-chemical condition within the cavity which by some means lessens the

amount of available water vapor. It must be emphasized that sealing Cerdips in

a highly N2 enriched ambient does not guarantee dryness. Eleven of the 146

specimens had N2:02 ratios greater than 30:1 (i.e., little or no detectable O2)

Seven of these had H2O greater than 1,000 ppmv. Sealing with some O2 present

is necessary to achieve optimum ambient and is also necessary for proper formation

of the solder glass seal.

16
Braze Seal

These packages consist of a ceramic body with a metal cavity for the chip onto

which a metal-plated lid is brazed at low temperature to form a seal. The parts

studied here were brazed in an N2 atmosphere. In decreasing order of concentration, the gaseous components inside their cavities are typically N2 , H2O,

and CO2.

These particular packages were gold plated and were baked in H2 prior to seal.

Consequently, the finished packages contained H2 as a minor component. When

analytical results for 56 braze seal specimens are arranged in order of

increasing moisture content, an associated plot of the specimens' corresponding

H2 content shows a well defined sympathetic trend between the two species.

Hydrogen content ranges from one to 10 volume percent as H2O content ranges

from 300 to 5,000 ppmv. As H2 level rises, H2O level also rises. It thus

appears that greater availability of H2 increases H2O content within the cavity.

This may be due to internal chemical reactions where H2 reactant yields H2O as

a product. Excessive H2 atoms may occupy sorption sites on the gold surfaces

within the cavity where H2O might otherwise have been tied up. Out-diffusion

of H2 incorporated within the gold plating may be accompanied by desorption of

H2O as well.

Careful calibration of the mass spectrometer each time these packages were

tested was carried out with known gas mixtures to insure that H2 observations

were not measurement artifacts.

Carbon dioxide contents of these parts typically range from 100 to 2,000 ppmv.

When the CO2 data are placed on the plot of increasing H2O for the 56 specimens,

the result is completely random. As in Cerdips , there is no relationship

between amount of CO2 and amount of H2O in braze seal packages.

TO

These are the "can" style packages having the chip mounted on a gold-plated

header and encapsulated inside a cylindrical lid welded to the header. The TO

package lids in this study had nickel-plated interior surfaces. Weld sealing

was done in an N2 atmosphere near room temperature. In decreasing order of

concentration, gaseous components within the TO cavity were typically N2 , H2O,

CO2, and H2.

When analytical results for 36 TO specimens are arranged in order of increasing

H2O content, an associated plot of corresponding CO2 content shows a definite

sympathetic trend between the two species. As H2O increases from 100 to 8,000

ppmv, CO2 also increases from 500 to 6,000 ppmv. This is unlike the results

for Cerdip and braze seal parts in which no relationship existed between H2O

and CO2 contents.

Hydrogen content of TO cans ranges from less than 10 to 500 ppmv. When the H2

data are placed on the plot of increasing H2O for the 36 specimens, the result

is completely random, with no sympathetic trend evident.

17
3. CONCLUSIONS

Mass spectroscopy yields useful information about all gaseous components within

a hermetic package, in addition to its H2O content. Interior surfaces of the

package cavities, whether alumina, glass, or metal, and the texture and con- dition of these surfaces, obviously play a significant role in overall hermetic

package chemistry. Major trends identified include the independent nature of

CO2 and H^O content in Cerdips and braze seal, while these species show a dependency trend in TO. H2 and H^o levels are definitely related in braze

seal, but are independent of each other in TO. Both braze seal and TO contain

significant quantities of CO2, considering that their sealing is carried out

in N2 . H2O content of Cerdips may well be reduced by enriching the N2 content

of the sealing ambient. These and other trends in hermetic package ambient

chemistry are easily identified by mass spectroscopy, but emphasize the need

for reproducible analytical technique and further efforts in calibration for

measurement accuracy.

REFERENCES

1. Kolesar, S. C. , Principles of Corrosion, J^^ Electrochem. Soc123

,

155-167 (1976).

2. Koelmans, H. , Metallization Corrosion in Silicon Devices by

Moisture- Induced Electrolysis, J. Electrochem. Soc. 123

,

168-171 (1976).

3. Meyer, D. E., and Thomas, R. W. , Moisture in Semiconductor Packages,

Solid State Technology 17 (9), 56-59 (September 1974).

4. Thomas, R. W., Microcircult Package Gas Analysis Techniques, 14th Annual

Proceedings , Reliability Physics 1976 , Las Vegas, Nevada, April 20-22, 1976,

pp. 283-294.

18
2.4 Correlation Between Mass Spectrometer and Aluminum Oxide

Sensor Measurements of Moisture in Hermetic Packages

Rebecca J. Gale

Bell Laboratories

555 Union Boulevard

Allentown, PA 18103

(215) 439-6768

Abstract: Correlation has been obtained between mass

spectrometer measurements and aluminum oxide sensor

(Mini-Mod-A, Panametrics, Inc., Waltham, MA 02154)

measurements of the water vapor content of hermetic

packages. Side-brazed, multilayer ceramic packages

were seam sealed in a controlled humidity glove box at

moisture levels of 11,000, 6100, 5100, and 2600 ppmV in

nitrogen. Aluminum oxide sensors which were calibrated

using an optical dew point hygrometer were sealed

inside some of the packages at each moisture level,

providing monitors for each lot. The preparation of

the packages and calibration of the sensors is

described in a separate paper by M. L. White and R. E.

Sammons. The calibration of the mass spectrometer was

accomplished using two independent methods to determine

the sensitivity factor for water vapor: 1) the use of

packages containing 6100 ppmV of water vapor in nitrogen (as measured by the aluminum oxide moisture sensors) as standards; and 2) the use of bursts of room

air with a dew point hygrometer determination of the

actual moisture content of the air. Using either calibration technique, the mass spectrometer measurements

were found to be within + 25% of the aluminum oxide

sensor measurements at each of the four moisture levels.

Key Words: Humidity? mass spectrometry; moisture sensors; packaging; reliability; standard packages.

1. INTRODUCTION

The measurement of the quantity of moisture inside hermetically

sealed packages is necessary for the development of packaging

techniques which will provide a sufficiently dry environment to

assure reliable device operation. Thomas [1] has given a review

of various failure modes of devices in hermetic packages which

have been attributed to high moisture content.

19
The magnitude of allowable water vapor content has been a subject

for debate for many years, primarily due to the lack of an established method for water vapor determination. The upper limit

currently accepted for hermetically packaged parts for use in the

Bell System is 5000 ppmV (parts per million by volume or 0.50% by

volume). The acceptable level specified in MIL-STD-883B Method

5005.4 is 5000 ppmV + 20% for high reliability applications.

Several different approaches have been taken to develop a method

for measuring water vapor sealed inside hermetic packages. An

important consideration is that the technique must be calibrated

using known quantities of moisture before meaningful results can

be obtained. A basic division in the methods currently being

developed can be made between nondestructive and destructive

testing .

The techniques which can be considered nondestructive* involve

the use of a moisture sensitive device characteristic or a moisture sensor which has been placed in the package with the device

to measure the water vapor content in the package after sealing

[2,3,4,5]. Since such measurements do not break the hermetic

seal, if the device is operational and the measurement indicates

that the water vapor content is within the specified limits, the

device can be used. However, the in-situ monitors must be placed

in the package to be tested before it is sealed, and therefore,

unless a sensor is mounted in every package, the sampled package

will not be truly representative of the manufactured product.

For more detailed discussion of moisture sensors, consult the

references given above.

The technique which is the subject of this paper falls into the

category of destructive testing: mass spectrometr ic analysis of

the gas that flows out of a package into a vacuum after the lid

has been punctured. With this approach, a package chosen at random from a manufacturing lot can be tested, thus providing an

analysis for moisture as well as for other gases which should be

representative of the current manufacturing process. If the test

results show that the moisture content of the package is within

the specification, it is obvious that the tested device cannot be

used because the package is no longer hermetic. Nevertheless,

the technique can be used in conjunction with a valid sampling

procedure to obtain the necessary information in a statistical

sense.

Thomas [6] has described the method of mass spectrometr ic

analysis of the gases in hermetic packages in detail. This has

been the primary method for the moisture analysis of semiconductor device packages for several years [1,6,7,8]. The major

* White and Sammons [4] have found that each aluminum oxide

sensor must be individually calibrated after all the

processing is completed. This requires delidding the package

and consequently becomes a destructive technique.

20
problem with this technique has been the lack of correlation of

results obtained by different analytical laboratories due to the

various calibration procedures employed by each [9,10],

This paper describes a mass spectrometer system, the calibration

procedure employed and the correlation which has been established

using calibration packages and moisture sensors [4], The calibration packages were side-brazed, multilayer ceramic packages

with metal lids which were hermetically seam sealed in a glove

box at various humidity levels. At each level, aluminum oxide

moisture sensor chips which were calibrated using an optical dew

point hygrometer were sealed inside some of the packages. The

preparation and calibration of these packages is described in a separate paper by M. L. White and R. E. Sammons [4].

2. PROCEDURE

2.1. Apparatus

The apparatus used in this work is a Batch Inlet Hermeticity

Tester and Moisture Analysis System coininercially available from

Pernicka Corporation (Mountain View, CA) . A schematic diagram of

the system is given in figure 1. The system, constructed from

304 stainless steel, consists of two chambers, a sample chamber

where up to 12 packages can be mounted, and a detector chamber

containing the mass spectrometer, which are separated by a pneumatic valve and a variable conductance orifice. Each chamber is

pumped by a separate turbomolecular pump. As depicted in figure

1, an oven encloses the two chambers. The temperature inside the

oven is maintained at 100 C to minimize adsorption of water on

the walls of the chambers. At this temperature, the base pressure in the sample chamber is £ 1 x 10~° Torr and that in the

detector is <_ 1 x 10"^ Torr.

The samples are mounted on a carousel such that each package can

be rotated into position under the puncturing mechanism, which is

a micrometer-driven needle. A cross section of the sample

chamber and the puncturing device is shown in figure 2.

The detector is a Uthe Technology International (Sunnyvale, CA)

Model lOOC quadrupole mass spectrometer which is interfaced to a Tektronix (Beaverton, OR) Model Tek 31 calculator. The scan

drive of the mass spectrometer can be set to any mass number

between 1 and 300 AMU (Atomic Mass Units) by sending a 0.0000 to

9.9995 Volt signal from the calculator to the mass spectrometer

via the interface. The output of the mass spectrometer, an analog voltage of 0 to 10 Volts representing the ion current, is

digitized to a 4 digit number by an A/D converter in the interface and is then printed by the calculator. The mass peak maxima

are determined by monitoring the output signal as the scan drive

is incremented in 0.1 AMU steps. Eight mass peaks are scanned

21
sequentially under program control, and the measured mass peak

values are printed a specified number of times with a repetition

every eleven seconds.

2.2. Calibration of the Mass Spectrometer

Preliminary results indicated that the response of the system to

various gases, particularly oxygen and water, was dependent upon

the recent history of the system. In order to establish a reproducible background or baseline of the residual gases, the system

was conditioned using room air each morning before calibration

and testing.

This procedure consists of flowing room air through the sample

chamber (PVl in figure 1 closed) into the mass spectrometer at ~1

X 10~5 Torr for 30 minutes. The purpose of this procedure is to

allow the stainless steel walls of the chambers to reach equilibrium with oxygen and water vapor such that gettering of oxygen

or water is minimized when testing packages. The system is then

pumped to the base pressure before continuing. This conditioning

procedure is subject to variations due to the changes in the

moisture levels in the air from day to day and from season to

season. However, we can assume that the walls of the chamber are

saturated since for a relative humidity of 40% at 70 F, which

corresponds to 10,000 ppmV (parts per million by volume or 1% by

volume) ,the conditioning would expose the walls of the detector

chamber to "100 monolayers of water vapor and several hundred

monolayers of oxygen.

2.2.1. Standard Gas Mixture

When the system reached the base pressure after conditioning with

room air, the mass spectrometer was calibrated for helium,

methane, nitrogen, oxygen, argon, and carbon dioxide using a mixture prepared and analyzed by Airco (Riverton, NJ) as shown in

table 1. This was achieved by allowing the calibration gas to

flow through the sample chamber (PVl in figure 1 closed) into the

mass spectrometer chamber. The pressure in the mass spectrometer

chamber was allowed to stabilize in the range of 1-2x10"^ Torr

with a corresponding pressure in the sample chamber of 0.2 Torr

which approximates the maximum pressure rise when a calibration

package is punctured. The mass peaks of interest were scanned

over in 0.1 AMU steps to determine the positions of the maxima.

The positions of the peak maxima did not vary by more than +0.1

AMU from day to day.

The leak valve (MVl in figure 1) was then closed, and the system

was pumped to the base pressure again. The calibration gas continued to purge the manifold as monitored by the flow meter.

When the base pressure was attained, the calibration gas was

analyzed in a manner analogous to package testing. The turbomolecular pump was valved off from the sample chamber and the

22
valve between the sample chamber and the mass spectrometer was

opened. Ten mass scans were made for the residual levels of

hydrogen, helium, methane, water, nitrogen, oxygen, argon, and

carbon dioxide. The valve between the chambers was then closed

and the calibration gas was leaked into the sample chamber to

approximately the pressure measured by the capacitance manometer

when a calibration package was punctured (~0.2 Torr) . The valve

to the mass spectrometer was again opened and the eight mass

peaks were again scanned repetitively until >^95% of the gas was

pumped away. To correct for the background gases, the average of

the ten background scans was multiplied by the total number of

scans and subtracted from the total signal measured for each

peak. If this corrected signal was less than twice the standard

deviation of the ten background scans, it was taken to be zero.

The corrected signals for the eight mass peaks were then summed

and the percentage of each computed.

At least three such calibration runs were made and the results

were averaged for each mass peak. The sensitivity factor for

each of the gases in the standard gas mixture was then determined

by comparing the average value with the analysis supplied by

Airco shown in table 1, That is, the signal for each mass peak

was multiplied by a factor before the corrected signals were

summed in order to obtain agreement with the Airco analysis. The

factors for six of the seven gases remained constant to within

+20% over the period during which these experiments were performed. The factor for oxygen was found to vary depending on the

recent history of the system. Since hydrogen and water were not

included in the mixture, sensitivity factors for these gases

could not be obtained using this gas mixture.

2,2,2, Calibration Packages

Since we wanted to measure the moisture levels inside hermetic

packages, the first approach to calibration of the system for

water vapor was to use side-brazed, multilayer ceramic packages

with metal lids which had been seam sealed in a controlled

environment at various known moisture levels. The amount of

water vapor in the packages after sealing was monitored at room

temperature by aluminum oxide moisture sensors (Mini-Mod-A,

Panametrics, Inc., Waltham, MA). These sensors, which were

sealed inside some of the packages in each lot, were calibrated

using a controlled-humidity glove box and a General Eastern

(Watertown, MA) Model 1200 AP optical dew-point hygrometer. For

the lot of calibration packages which were used as standards, the

average moisture level determined by the aluminum oxide sensors

was 6100 ppmV (0.61%) with a range of +25%. A more detailed discussion of the package preparation and sensor calibration is

given by White and Sammons [4].

The test procedure when using the calibration packages as standards was as follows: packages which had been sealed at various

moisture levels [4] were loaded into the system. After pumping

23
the system overnight at 100 C, the base pressure of '^^l x 10"^

Torr in the sample chamber and <1 x 10~^ Torr in the detector

chamber was attained. The system was then conditioned with room

air and the calibration procedure described in section 2.2.1.

above was performed.

Before testing packages and between analyses, the system was

again pumped to the base pressure. The measurement for each

package was made by first closing PVl (see figure 1) in order to

isolate the sample chamber and then by opening PV2 to connect the

sample ciiamber and mass spectrometer. Ten scans of the background were made. PV2 was closed and the package was then punctured. The pressure rise in the sample chamber was measured

using the capacitance manometer D2. PV2 was again opened and the

eight mass peaks of interest were scanned until >^95% of the gas

was pumped away.

The data were then corrected for the background gases, and the

relative amounts of each gas were calculated as described in section 2.2.1.

The packages which were sealed at 6100 ppmV (0.61%) were used as

standards. One or more of these packages were tested during each

series of measurements in order to determine the sensitivity factor for water.

2.2.3. Room Air and Optical Dew Point Hygrometer

In order to provide an independent calibration procedure for

water vapor, a General Eastern (Watertown, MA) Model 1200 AP optical dew-point hygrometer was added to the system. Its calibration was compared with the same model dew point hygrometer calibrated by R. Sammons and M. White [4] by flowing nitrogen containing various amounts of water vapor through the two hygrometers connected in series. The agreement was found to be within

20% as shown in table 2. The discrepancy at the lowest values is

due to the fact that these values represent the lower limit of

detection of the hygrometers, which is determined by the ambient

temperature (the two hygrometers had different cooling jackets).

Room air was used to achieve an independent calibration of the

mass spectrometer for moisture using a compressor to provide the

flow rate necessary to obtain an accurate reading from the dew

point hygrometer (See figure 1). These experiments were performed as described above, but the calibration for moisture was

made using samples of room air and the dew-point hygrometer,

rather than the 6100 ppmV packages.

2.3. Results

Table 3 presents the results obtained using the packages sealed

at 6100 ppmV to calibrate the mass spectrometer for water vapor.

The samples tested were calibration packages sealed at 11,000,

24
5,100 and 2,600 ppmV.

The results for the analysis of the moisture content of the calibration packages using room air to calibrate the mass spectrometer for water vapor are shown in table 4.

2.4. Discussion

As shown in tables 3 and 4, the agreement between the mass spectrometer measurements and the sensor chip measurements of moisture in the calibration packages (mean values) is within +25%,

which is the range of the moisture levels measured by the aluminum oxide sensors in the lot of calibration packages used as

standards containing on the average 6100 ppmV of water vapor.

The independent calibration using room air confirms that the

technique of White and Sammons [4] produces packages containing

known levels of moisture as shown in table 4. This packaging

facility can be used to seal devices hermetically in packages at

known moisture levels which can then be subjected to accelerated

stress. The results of siich experiments should establish the

upper limit for moisture in hermetic packages for each particular

device.

In order to calibrate the system at lower moisture levels and for

packages of different internal volumes, a gas humidifier and a three volume inlet system (burst mode calibrator) have been added

to the system, as shown in figure 3.

Gas containing various levels of moisture is generated by

saturating the gas with moisture at room temperature and elevated

pressure. The gas is then expanded to atmospheric pressure.

The inlet system is designed such that the three volumes are constantly being purged by the humidified gas which then flows into

the dew point hygrometer allowing accurate measurement of the

moisture level and assurance that equilibrium with the walls of

the inlet system has been reached. The three volumes are 0.01

cm3, 0.1 cm3, and 0.8 cm^. Additional volume can be added to the

largest volume by means of a Swageloc connector.

Although metal lid packages are furnace sealed in pure nitrogen,

mass spectr ometr ic analysis of the package contents has shown

high levels of hydrogen (>1%). A further complication is that

some ceramic packages are glass sealed in oxygen containing

environments. Recent experiments with the humidity generator,

the burst mode calibrator and various gas mixtures have shown

that the presence of hydrogen or oxygen at levels >^ 1% can affect

the apparent sensitivity for water vapor relative to that for

water vapor in pure nitrogen. To measure accurately the moisture

content in these packages, it has been found that the mass spectrometer must be calibrated using mixtures of composition similar

to package contents.

25
3. CONCLUSIONS

A technique for calibrating a mass spectrometer system for

measuring the moisture content in hermetic packages seam sealed

in pure nitrogen has been developed, ^4odif ications have been

made to provide calibration for different package volumes and for

the entire range of moisture levels. Techniques for measuring

the moisture content of hermetic packages containing >^1% hydrogen

or oxygen are currently being developed.

REFERENCES

[1] Thomas, R. W. , Moisture, Myths and Microcircuits , IEEE

Transactions on Parts, Hybrids and Packaging PHP -12 , 167-

171 (1976).

[21 Kovac, M. G. , Chleck, D. , and Goodman, P., A New Moisture

Sensor for In-Situ Monitoring of Sealed Packages, Solid

State Technology 2_1, 35-39, 53 (1978).

[3] Sosniak, J., and Unger, B. A., Moisture Determination in

Hermetic IC Packages by the Dew Point Method Using AC Capac-

-^^^ itance and Conductance Measurement, lEDM Technical Digest,

• Washington, D.C. , 1978.

[4] White, M. L. , and Sammons, R. E., A Procedure for Preparing

Hermetic Packages with Known Moisture Levels, this report,

[5] Lowry, R. K. , et al «

, Characteristics of a Surface Conductivity Moisture Monitor for Hermetic Integrated Circuit

Packages, Proceedings of the 17th Annual International

Reliability Physics Symposium, 97-102 (1979).

[6] Thomas, R. W. , Microcircuit Package Gas Analysis, Proceedings of the 14th Annual International Reliability Physics

Symposium, 283-294 (1976).

[7] Thomas, R. W. , and Meyer, D, E. , Moisture in SC Packages,

Solid State Technology 3/7, 56-59 (1974).

[8] Fisher, H. D. , Analysis of Volatile Contaminants in Microcircuits, Solid State Technology _21_, 68-70,82 (1978).

[9] Thomas, R. W. , Moisture Measurements and Reliability - An

Overview, Semiconductor Measurement Technology : Reliability

Technology for Cardiac Pacemakers III, NBS Spec. Publ.

400-50 (March 1978), pp. 43-46.

26
[10] Perkins, K. L. , Moisture Measurement Studies, Semiconductor Measurement Technology : Reliability Technology for

Cardiac Pacemakers III, NBS Spec, Publ . 400-50 (June 1979)

pp. 60-65.

27

371-525 0-82-3
PV7 PV8 PV9 PVIO PVU PV12 PV13

FLOW METER OEVK POINT HYGROMETER /'-^ MV2 VALVE PV5 ^ PV4

Figure 1 . Schematic diagram of the mass spectrometer system.

100 Vs TURBOMOLECULAR PUMP

Figure 2. Cross section of the sample chamber.

28
PV7 PV8 PV9 PVIO PVll PV12 PV13

GAS

SUPPLY LEAK 1MV5 VALVE MV4 MV3

OVEN

HUMIDIFIER

MASS

SPECTROMETER

.J

1?.°)[-<^PV1B

Figure 3. Schematic diagram of the mass spectrometer system

with modifications.

29
Table 1

Analysis of Standard Gas Mixture (Airco)

Component

He 1 i um

Methane

Ox yg en

Argon

Carbon Dioxide

Ni trogen

Concentration (ppmV)

1047

1037

976

1130

5180

Balance

Table 2

Comparison of Two Optical Dew-Point

Hygrometers in Series on an H2O/N2 Gas Stream

White and Sammons Author

DPH (^C) ppmV H2O DPH ("O ppmV H

-40.7 77 -43.7 52

-40. 6 78 -43.4 55

-40.2 81 -43. 3 55

-10.9 2123 -10.8 2150

-8.2 2742 -8.7 2 619

-3.8 4218 -3.8 4218

-3.5 4320 -3.5 4320

-3.3 4398 -2.8 4671

-2.8 4671 -3.0 4574

30
Table 3

Using Calibration Packages to

Calibrate the Mass Spectrometer at 6100 ppmV

(% = ppmV X 10^)

Moisture

in

(%)

Le\^el

Package

Moisture Measured Number of

By Mass Spectrometer Packages

( % ) Analyzed

Mean Range

1. 10

0.61

0. 51

0. 26

1.27

0. 62

0. 44

0. 24

1.25-1.29

0. 57-0. 68

0. 36-0. 48

0. 20-0. 27

3

12

4

6

Sensor chip measurement in sample(s) at each level

Table 4

Using Room Air and an Optical Dew-Point Hygrometer

to Calibrate the Mass Spectrometer for Moisture

(% = ppmV X 10^)

Moisture Le\^el

in Package

(%)

Moisture Measured Number of

By Mass Spectrometer Packages

(%) Analyzed

Mean Range

1.10

0. 61

0. 51

0. 26

1.20

0. 64

0. 46

0.19

0. 92-1. 45

0. 49-0. 82

0. 40-0. 52

0. 12-0. 22

Sensor chip measurement in sample(s) at each level

31
2.5 Moisture Standards for Mass Spectrometers

Benjamin A. Moore

Rome Air Development Center

RBRE

Griffiss AFB, NY 13441

(315) 330-4055

Abstract: The results of prior attempts at producing and distributing

(via analytical round robins) standards are discussed with respect to

sealing techniques, package materials, and screening procedures. The

culmination of learning experiences associated with the manufacture of

an acceptable standard is the TO-18 candidate that was used in the

second phase of the program to certify analytical facilities as capable of performing accurate moisture analysis in accordance with

Method 1018 (Internal Water-Vapor Content) of MIL-STD-883B. The

results show correlation among test laboratories analyzing the TO-18

and strongly suggest these samples are good candidates for static

moisture standards.

Key words: Certification; mass spectrometry; Method 1018;

quantitative analysis; standards; water vapor.

1. INTRODUCTION

The objectives of this experiment were: (1) to compare calibration done

externally during the Method 1018 certification process [ 1] to actual

microelectronic package moisture analysis by mass spectrometric methods, (2) to

provide a data base for determination of the suitability of the candidate samples

for use as moisture standards for future recertif ication purposes (Method 1018

calls for biennial examination of certified laboratories) , and (3) to provide

comparison of certified laboratories for additional confirmation of analytical

capabilities.

The effects of package materials, construction, and different sealing techniques

present many problems in producing an acceptable static (a sample whose exact

value cannot be independently determined at the time of assay) standard. Early

attempts to produce a viable standard were not entirely successful. While an initial analytical round robin was encouraging (fig. 1) , a second met disaster

due to the inadvertent inclusion of foreign material (fig. 2) . Based upon this

anomalous behavior and other unanticipated technical problems encountered during

the development of the TO-18 standard, the Joint Electron Device Council (JEDEC)

Subcommittee, Task-14, which was involved in implementing Test Method 1018,

Internal Water-Vapor Content, of MIL-STD-883B, could not accept this standard as

an effective means of cross-correlating moisture determinations. An alternate

method, employing identical calibration equipment and a National Bureau of

Standards (NBS) traceable referee, was proposed by RADC and accepted by this

subcommittee and mass spectrometer analytical facilities. However, development

of the TO-18 standard continued and samples, manufactured to the state of the

art, were provided as a secondary phase of the Method 1018 certification program.

32
2 . PROCEDURE

Using the same calibration and analysis procedures as employed in the Method 1018

certification process, each laboratory analyzed several of the latest TO-18 type

metal can candidate standards manufactured by Raytheon and provided by RADC.

Additionally, samples provided by Bell Labs were included. Analyses of those were optional. The results of this experiment are summarized in tables 1-4.

Table 1 compares the results of moisture analysis of the TO-18 samples among the

four commercial analytical facilities and RADC. The results indicate excellent

correlation among all the laboratories. A few analyses were rejected as being

outside confidence limits. Since these were spread over several laboratories, it

is likely that the anomalies are in the samples rather than the analyses. The

standard deviations indicate an analytical uncertainty of less than +10 percent

of the mean value.

The results of analyses of the Bell Lab's samples present some questions. As

detailed in table 2, Lab B data are much lower than data from other participating laboratories. In an attempt to resolve this incongruity. Lab B con- ducted a bakeout time versus moisture concentration experiment. The results

are shown in table 3 and seem to indicate the depletion of moisture with

increasing bakeout. While detailed discussion of these samples will be found

in the paper by M. L. White and R. E. Sammons of Bell Laboratories [2] it is

interesting to note that the standard deviations for these analyses are again

approximately ±10 percent of the mean values,

A comparison of Method 1018 certification data with the TO-18 analyses is made

in table 4. It should be noted that the 0.01 cm^ burst data were generated by

slightly variable moisture concentrations (as determined by an NBS traceable

dewpoint hygrometer) encountered during the field certification process. Comparison of analyses of nominally generated 5000 ppm gas samples with standards

packaged to contain a similar concentration is encouraging. There was approximately the same uncertainty in the field and transfer standard measurements.

One of the most important aspects of this experiment was the independent

determination by three of the analytical laboratories that their systems reacted

differently to moisture in the presence of nitrogen than in the presence of

oxygen (room air) . This determination was the result of comparison of Method

1018 certification data (using H^O-N^ mixtures) , the TO-18 samples (containing room air constituents) , general purpose humidifier (GPH) generated mixtures, and room air samples. This effect is explored in a paper by J. C. Pernicka and B. A.

Raby [3]

.

3. CONCLUSIONS

The following conclusions are made with respect to the transfer standard experiment:

(1) Correlation among the test laboratories has been established by the TO-18

samples.

33
(2) Calibration and analysis techniques certified during the Method 1018 program have been proven valid for in- situ moisture measurements of the TO-18

candidate standard.

(3) The T0-18s are good candidates for Method 1018 recer tif ication standards.

Still to be determined is the long-term moisture stability of the standard.

(4) At the time of this writing, further study of the Bell Lab's package is

required before acceptance of analytical data. The report from Bell Laboratories

on these samples may have resolved this problem [ 2]

REFERENCES

1. Moore, B. A., Method 1018 Certification Results, this report.

2. White, M. L. , and Sammons, R. E., A Procedure for Preparing Hermetic

Packages with Known Moisture Levels, this report.

3. Pernicka, J. C, and Raby, B. A., Mass Spectrometr ic Analyses of

Semiconductor Package Atmospheres, this report.

34
RADC MOISTURE STANDARDS

MIL

SPEC

± 2Q%

r ^

LAB D

1 1 —

1 1

1 1 X Is X S xx' '

(Sd 6b X \ 1 'Of 'O' ^ 1 1

1 I

! 1

lmB C

i 1 '

'

1

1

I 1

1 ® ® 1 ® X X

1 1

1 1

1 I

LAB B

1 1

1 I 1® X® XX 1 ® ®X X

' (5?) X X

1 1

1 1

LAB A

1 1

1 1

1 ^ 1

' X® ® X><3® ' X

1 ®XX 1

1 1

1 1

RADC

j

1 ® ® ^X X

' X X® XX X 1 ® ®

1 ®x '

1 ® 1

1 1 1 \ 1 \ 1

\

\— !

1

1000 ZDOO 30DO 4000 5000 6000 7000 8000 9000 10000

MOISTURE CONTENT IN PPM

Figure 1. Results - Round Robin I.

35
36
METHOD 1018 MOISTURE ANALYSIS CERTIFICATION

STANDARD SAMPLE RESULTS

Table 1. TO-18 Standard,

Laboratory Trials Dates Mean, PPm^ Std Dev, ppm^

RADC 13^^^ 4 5280 436

C

8^^^ 2 5160 403

D 7 1 4767 156

B 5 1 5398 443

A 4^^^ 1 5335 282

Table 2. Bell Lab's Standard.

Laboratory Trials Dates ^''"^ Mean, ppm^ Std Dev, ppm^

RADC 9 3 4070 424

C 10 1 4000 390

D 5 1 3222 305

B 5 1 1250^"^^ 100

A») ...

NOTES

(1) Number of different days

(2) One analysis rejected as

(3) Two analyses rejected as

(4) See Lab B bakeout experii

(5) Chose not to participate

on which determinations were made,

outside confidence limits,

outside confidence limits,

lent data; bake time 20 hrs.

in optional standard analyses.

37
Table 3. Influence of Bake Temperature on Bell Lab's

Moisture Standard (by Lab B)

Bake Time, hr Moisture Concentration ppm,

A4

O

8 1960

10 2160

12 1680

20 1250

Table 4. Comparison of Method 1018 Certification Moisture

Analyses and Similar Volume Standard.

Mean, ppm^ Std Dev, ppm^

3 3 Laboratory 0.01 cm Burst TO-18 Std 0.01 cm Burst TO-18 Std

C 4920 5160 113 403

D 4556 4767 704 156

B 5202 5398 380 443

A 5411 5335 303 382

38
2.6 Method 1018.2 Certification Results

Benjamin A. Moore

Rome Air Development Center

RBRE

Griffiss AFB, NY 13441

(315) 330-4055

Abstract: The presence of moisture within a microelectronic package

can have a detrimental effect upon the reliability of the enclosed

device. In order to screen these reliability hazards from military

electronic, avionic, and armament systems, a military test method,

1018.2 (Internal Water-Vapor Content) was generated that detailed ac- ceptable methods of water-vapor analysis. The most prevalent method

was mass spectrometr ic quantitative analysis of integrated circuit

package ambients. However, problems arose in developing a suitable

"standard" for assurance of accurate moisture analysis in order to

obtain correlation among various analytical facilities. The described

program, a result of Government, electronic industry, and analytical

laboratory efforts, resulted in the acceptance of several laboratories

as certified to perform Method 1018.2 analysis. Details of the effort,

as well as results, are presented and lead to the conclusions that (1)

practical criteria were developed and met, (2) common calibration

equipment aided the correlation between laboratories, and (3) dynamic

and static standards gave correlative results.

Key Words: Calibration; certification; mass spectrometry; Method

1018.2; quantitative analysis; water vapor.

1 . INTRODUCTION

The purpose of this work was to demonstrate the capability of the analytical

laboratories to perform accurate, reproducible mass spectrometr ic analysis of a known concentration of moisture (nominally 5000 ppm^) to within + 20 percent over

a volume range of 0.01 cm to 0.8 cm . The program certified laboratory calibration equipment and procedures.

The presence of moisture within a microelectronic package can have a detrimental

effect on the reliability of the enclosed device. Data, both generated and

collected at the Rome Air Development Center (RADC), specifically demonstrated

that the presence of moisture in an integrated circuit (IC) package, either by

itself or in concert with other contaminants, aided and abetted by surface

defects on the enclosed device, resulted in unreliable behavior ranging from out

of tolerance electrical parameter drift to catastrophic failure. The potential

inclusion of such reliability hazards in military electronic, avionic, and

armament systems is unacceptable. Therefore, a screening procedure to minimize

the presence of such devices in military systems was established. This procedure

is Method 1018.2 (Internal Water-Vapor Content) of MIL-STD-883B.

39
However , in order to earn acceptance as a viable test method by the Joint

Electron Device Engineering Council (JEDEC) , which coordinates the generation

and acceptance of device and package specifications between industry and the

Government, it was necessary to demonstrate that the analytical capability

existed for accurately determining moisture in IC packages. Task Group-14

was established to provide guidance and oversee resolution of technical problems associated with the implementation of Method 1018.2

Early attempts to produce a static moisture standard, e.g., TO-18 type metal cans containing "known" (via assay) amounts of moisture, resulted in several problems

which could not be resolved in an economical, timely manner. Preliminary work at

RADC involving calibration by the simulation of actual package puncturing suggested a viable alternative to static standards. The technique involved the

trapping of a volume of gas (approximately equal to that of an IC package) of

known moisture concentration between two valves and then immediately bursting

this gas into an analysis system.

A Task-14 meeting was held at the National Bureau of Standards (NBS) in early

June 1979 at which these efforts were reviewed. This dynamic approach to

moisture calibration was accepted provided that an absolute reference was available that could be used to confirm the moisture concentration of calibration

gases. An optical dewpoint hygrometer, already on order by RADC, was chosen as this reference with the requirement that it be calibrated at NBS and transported

to each analytical facility for primary determination of moisture concentration

of calibration gases. Concurrent with this meeting, a Moisture Measurement by

Mass Spectrometer users' program was also held at NBS. This meeting was attended

by representatives from commercial gas analysis facilities, IC manufacturers,

and RADC. At ^his time,^ a commercially available method of bursting known

volumes (0.01 cm , 0.1 cm , and 0.8 cm ) of gas, the Three Volume Calibration

Valve (TVCV) manufactured by Pernicka Corporation, was introduced and accepted

by the participants as standard calibration equipment. Also, NBS suggested a relatively low-cost, facile method of generating known concentrations of

moisture. Pernicka agreed to manufacture this piece of equipment, known as a General Purpose Humidifier (GPH), which was also accepted as part of the

standard moisture calibration equipment. Utilization of this equipment allowed

direct comparison of each laboratory's calibration techniques and analytical

procedures.

The establishment of a 5000 ppm (+ 20 percent) certification point for three

volumes was the result of: (1) initial empirical judgement as to an upper bound

which would not allow liquid condensation on a chip surface during temperature

cycling (frostpoint rather than dewpoint) , (2) the Task-14 committee's evaluation of this criteria and the expected precision of mass spectrometer determinations using the dewpoint hygrometer, TVCV, and GPH, and (3) an approximation of

the range of IC package volumes currently encountered. Contaminant matrix

effects and moisture threshold values for various failure mechanisms are yet to

be determined and, therefore, preclude the establishment of experimentally

demonstrated moisture limits for various device types and manufacturing technologies.

40
2 . PROCEDURE

I

The net result of interplay among the JEDEC subcommittee, mass spectrometer

users, and the Government was a technical certification procedure that would

assure the availability of accurate analytical facilties. The salient aspects of

this program were (1) the equipment, (2) the experimental plan, and (3) the

results.

2.1 Equipment

As previously mentioned, the basic equipment employed during the certification

process included an optical dewpoint hygrometer, a TVCV, and a GPH. The optical

dewpoint hygrometer is shown in figure 1. It is a General Eastern Model 1200 APS

thermoelectrically-cooled, optically detected, condensation hygrometer. The

sensor body, shown on the left with optional mirror microscope (used to determine

the state of condensate on the mirror - dew or frost) is of special construction

employing a stainless steel skin. The mirror is illuminated with a lightemitting diode (LED) . Condensation is indicated by reduced reflected light

reaching a photo detector. As gas flows through the body, the mirror is cooled

until condensation is detected. Chiller current is then reduced until only a thin layer of condensate remains. The thickness of this layer is maximized for

the type of condensate present (auto-reflectance) . Other operational controls

include an automatic programmable mirror cleaning sequence. This instrument was calibrated at NBS using the two-pressure humidity generator, Mark 2. Results of

the calibration are given in table 1. Of course, field accuracies will not

approach these figures, nor can NBS make any statement concerning field generated

data. However, traceability to NBS and confidence in the instrument was established.

Figure 2 is a picture of the TVCV connected to a GPH. The GPH consists of two

stainless steel cylinders, one filled with a water saturated medium, the other

acting as a pressure ballast. By varying the pressure in the saturator cylinder,

one can generate concentrations of moisture that can be calculated from basic

properties. Expanding the gas into room atmosphere, while knowing the temperature of the saturator, permits determination of moisture concentration for an

ideal gas mixture via the relationship below where V is the vapor pressure of

water at saturator temperature and P^ represents totaf^ pressure in the saturator

%H-0 = (V X 100)/P . 2 ' p " g

Derivation of this relationship is given in Appendix A. If necessary, corrections for non ideal gas behavior can be made. However, at the pressures and

temperatures encountered in generating 5000 ppm moisture, deviations from

ideality are negligible. Nitrogen gas was used during the certification process

in order to: (1) provide a relatively inert mixing medium for moisture, and (2)

approximate the sealing gas contained in a majority of IC packages. Considering

oxygen enhancement of moisture sensitivities encountered at some laboratories

and reported during this workshop [1], this was judicious choice. Nitrogen,

saturated with moisture at saturator pressure and temperature, is expanded to room pressure and flows through the TVCV. The valve is constructed (see the

cross-section given in fig. 3) such that the calibration gas constantly purges

41
each of the burst volumes and exits to atmosphere. One side of each toggle valve

tandem isolates the analysis system from the calibration gas flow. When the

other side of the tandem is closed, the trapped volume represents a calibration

burst which is accomplished by quickly lifting and then releasing the isolation

toggle valve. Since the valves are 0-ring sealed, minimum movement of the valve

produces best results in terms of reproducibility and hermeticity.

Unfortunately, the seal limits the temperature to which the valve may be heated

while maintaining proper operation. Operation at room temperature is

recommended by Pernicka. Each facility involved in the certification program

purchased the GPH-TVCV calibration pair and employed them in establishing

certifiable calibration techniques.

2.2 Experimental Plan

The basic procedure involved in certifying candidate laboratories included: (1)

installation of a Government GPH-TVCV pair on the analysis equipment, (2) generation of nominal 5000 ppm moisture in nitrogen using the GPH, (3) determination

of the calibration gas ^ewpoint using the NBS traceable hygrometer, and (4)

production of a burst of the calibration gas at known volume using the TVCV with

subsequent mass spectrometr ic quantitative analysis. In order to be certified

for a package volume range, a laboratory had to pass at two adjacent volumes of

the TVCV. As required in the test method, deviations of greater than 10 percent

in system moisture sensitivity factors at two neighboring volumes require interpolation of sensitivity factors for intermediate samples. As previously

mentioned, the pass/fail requirement of 5000 ppm {+ 20 percent) was established,

in part, by the JEDEC subcommittee's evaluation of anticipated measurement

precision. Additionally, each laboratory was required to demonstrate three

successful determinations at each volume. If one failure occurred in the first

three tests, two additional successful analyses were required. This is the same requirement as is found in MIL-STD-883B, Method 5005.5, Qualification and

Quality Conformance Procedures, Group D (package related tests for all classes)

Therefore, each analytical facility was certified to the same requirement as will

exist for IC packages. Data generated at the inauguration of the certification

program indicated no appreciable differences among the Government and various

analytical laboratories with respect to the GPH-TVCV calibration equipment.

Subsequently, the requirement for installation and use of the Government provided GPH-TVCV pair was waived.

As an additional confirmation of the certification program, static TO-18

standards were distributed to each laboratory and were analyzed sometime after

the certification was accomplished. The purpose of this experiment was to

establish a data base for possible future recertification standards as well as to provide direct comparison between laboratories.

2.3 Results

Laboratory certification results to date are summarized in table 2. Each tested

facility was successful in meeting the stated certification criteria. Figure 4 depicts the overall spread in analytical accuracy for each laboratory.

The results of the TO-18 transfer standard experiment are given in table 3. As

reported earlier during this workshop [ 1] , the presence of oxygen in these

42
standards confirmed that certain mass spectrometers exhibited enhanced sensitivity to moisture in the presence of oxygen. This required that the affected

analytical facilities determine the relationship between sample gas oxygen con- centration and moisture sensitivity. As indicated in table 3, this correction

led to correlative results.

3. CONCLUSIONS

The results of the certification program indicate, within the scope of experimental conditions, that (1) and accept/reject criteria are realistic and can be

met, (2) all laboratories employing the GPH-TVCV calibration system were able to

establish accurate, reproducible calibration procedures, (3) laboratories using

a dynamic GPH-TVCV calibration system correlate when analyzing a static (TO-18)

standard, and (4) multiple, acceptable sources for accurate moisture analysis in

support of Method 1018.2 are now available.

REFERENCES

1. Moore, B. A., "Moisture Standards for Mass Spectrometers," this report.
i
^ ^ ^

^^^^

3 VOLUME CALIBRATION

VALVE

Figure 3. Cross-section of three-volume-calibration valve.

45
Table 1. Calibration of a dewpoint hygrometer.

Model 1200 APS, S/N 92474, PRT 13929,

NBS Identification No. H-3484.

(a) (b)

Time

(Min.)

Dewpoint

(°C)

Readout

(°C)

Hygrometer, H-3484

Resistance

(ohm) t68(°C)

Remarks

31 24.67 24.6 109.752 + .003 24.73

41 0.27 0.3 100.155 + .009 0.36

25 -15.34 -15.4 94.011 + .007 -15.14

45 -18.40 -18.5 92.699 + .048 -18.44 Chiller -10° + 5°C

35 -35.54 -35.5 85.904 + .072 -35.48 Chiller -10° + 5°C

60 -45.20 -45.2 82.114 + .065 -44.94 Chiller -25° + 5°C

465 -58.91 77.137 + .088 -57.31 Chiller -25° + 5°C

65(^) -58.91 -58.0 77.121 + .115 -57.35 Chiller -25° + 5°C

(a) The readings were at steady state for the listed period of time. The readings

were averaged over this period.

(b) Dewpoint of sample air generated by the NBS two-pressure humidity generator.

(c) The average reading over the last 65 minutes of the 465-minutes interval.

Table 2. Method 1018.2 certification results

pass/fail ratio.

3 Sample Volume, cm 0.01 0.1 0.8

Lab

A 3/0 3/0 4/1

B 3/0 4/1 4/1

C 3/0 3/0 3/0

D 4/1 3/0 3/0

E 3/0 4/1 3/0

46
Table 3. Comparison of Method 1018 Certification Moisture

Analyses and Similar Volume Standard.

Mean, PPm^ Std Dev, PPn»^

3 3

Laboratory 0.01 cm Burst TO-18 Std 0.01 cm Burst TO-18 Std

C 4920 5160 113 403

D 4556 4767 704 156

B 5202 5398 380 443

A 5411 5335 303 382

47
APPENDIX A

CALCULATION OF MOISTURE CONCENTRATION USING THE GPH

(IDEAL GAS ASSUMED)

Pj^ (atmosphere)

%RH = — ; 7 X 1 00 ( 1 ) Note that P, = P, + gauge reading

P2 (generator) 2 1^^ ^

(%RH) (Vp)

atm ' temperature, T

P' %H20 = (2) Vp = vapor pressure of water at GPH

G

(%H20) (P ) from (2) %RH = / | ^

(3)

Equating (1) and (3)

"

P (%HpO) (P ] atm ^ atm-^ lOQx—- = (4)

G P

V X 100

Simplifying (4) %H20 = (5)

G

A. Measure GPH temperature (room temperature if at equilibriiam with

surroundings )

,

B. Measure Pq which is gauge pressure plus atm pressure,

C. Look in table for vapor pressure of water vapor at GPH temperature,

D. Calculate %H20 generated, and

E. Conversion of %H20 expressed as ppm^ to dew point.

48
3. SESSION II MOISTURE SENSORS

3.1 A Procedure for Preparing Hermetic Packages with Known

Moisture Levels

Malcolm L. White

Robert E. Sammons

Bell Laboratories

555 Union Boulevard

Allentown, PA 18103

(215) 439-7457

Abstract: Side-brazed, multi-layer ceramic packages containing known

moisture levels have been prepared by hermetic sealing in a humidity con- trolled glove box. This control is achieved by saturating nitrogen with

water vapor at slightly higher than room temperature and combining it in

known ratios with dry nitrogen as the gas supply for the glove box.

Metal lids are gold-tin alloy sealed on ceramic packages using a parallel

seam sealer in the glove box. The concentration of water in the package

after sealing is determined with commercial porous aluminum oxide sensor

chips. These chips are individually calibrated subsequent to all measurements on the sealed package to account for calibration shifts occurring during assembly heat cycles.

Continuous monitoring shows an increase in moisture level in the package

immediately after sealing, followed by a gradual decrease over a 200-hour

period at room temperature to a stable value within 20 percent of the

controlled humidity at the time of sealing. Using this technique, it is

possible to assemble packages at any desired humidity level, with or without in-situ moisture sensors.

Key Words: Dew point; hermetic packages; mass spectrometer; seam sealing; sensor chips; standards; water vapor.

1 . INTRODUCTION

There has been a great deal of interest in recent years in the moisture content of hermetically sealed packages because of the importance of moisture

levels on the reliability of integrated circuits sealed in these packages [1].

There are basically three techniques used for making these moisture measurements: 1j mass spectrographic analysis by puncturing the lid and sampling the

gas in the cavity [l]; 2) determining a dew point by cooling the package and

detecting the condensation of water by measuring a leakage between an i-nterdiggtated metallized pattern on a chip mounted in the cavity [2]; or a similar

technique, but using a capacitance pattern that is laser scribed in gold

metallizing in the bottom of the package [3]; and 3) measuring the change in

impedance of a porous alumina dielectric in a thin film capacitor structure on

a chip as a function of moisture [4].

There has been lack of agreement, not just between the various techniques for

measuring moisture, but even within a single technique. A mass spectrometer

"round robin" run between five laboratories in 1978, for example, showed large

variations between the laboratories, and lack of agreement with the moisture

49
level at which packages were filled [5].

The experience with the Western Electric production line at Allentown, Pennsylvania has been that samples of hermetic packages, all sealed at the same time in

belt furnaces or parallel gap seam sealers and sent for mass spectrometric analysis to the same analytical lab at different times, or sent to two different

laboratories, have shown widely variable results [6].

One way to establish some confidence in the analytical results would be to

have available packages containing known amounts of humidity. This memorandum

will describe the equipment and a procedure that was developed to prepare such

packages. This involved the use of the alumina dielectric chips for monitoring the moisture level in the packages; a number of interesting effects in the

use of this chip were found, and these will be described and discussed.

2. PROCEDURE

2 . 1 Apparatus

Figure 1 shows a sketch of the equipment used for preparing standard packages.

A glove box containing a seam sealer was modified to provide a nitrogen

ambient of known hiimidity. The level of moisture in the glove box was set by

metering incoming dry and wet nitrogen, and was monitored with a dew point

hygrometer. Humidity levels from 10 ppm to 10,000 ppm could be attained with

this system. ^

The package assembly is done with a Model 95 Parallel Seam Sealing System.* A

gold-plated Kovar lid is solder sealed onto a side-brazed ceramic body by

passing the package (with a lid clamped on it and a solder pre-form under the

lid) between two tapered wheel electrodes which pass a high current between

the edge of the lid and the electrodes. This current flow locally melts the

solder (Au/Sn alloy) which then solidifies to fonn a hermetic seal of the lid

to the package.

The sealing equipment is inside a glove box, with the gas supply modified to

provide for controlled, constant humidity. This was done by diverting some of

the incoming dry nitrogen, saturating it with water and then recombining it

with the dry gas in variable proportions, as shown in the bottom part of fig- ure 1 . Both wet and dry branches are put through flow meters to monitor the

relative amounts of each. The wet side then goes through two water saturators. The first is heated to about 50 C to counteract the cooling effect from

evaporation of water into the dry nitrogen. The second saturator along with a water trap is kept in a 30°C constant temperature air bath, so that the

nitrogen is considered to be saturated at that temperature. The wet gas is

then combined with the dry gas within the air bath, and the combined flow put

into the glove box. At the exit of the glove box is an optical dew point

* Solid State Equipment Corporation, Fort Washington, Pa. 19034.

+ Obtained from boiling liquid nitrogen; water content of less than 20 ppm.

50
hygrometer* to monitor moisture levels in the glove box. A leg of the gas

system goes from the wet nitrogen side directly to the hygrometer, so that the

dew point of the saturated nitrogen can be checked at any time. (The part of

the line outside of the air bath is heated to prevent condensation.) For

measuring dew points below the lower limit of the General Eastern optical dew

point hygrometer (-40 C) , a Panametrics Model 700 hygrometer with a lower

limit of -80 C dew point was used inside the glove box.

In using the system, the dry and wet nitrogen flows are adjusted to attain the

proportion of the two gases that will give the desired final concentration of

water vapor, assuming complete saturation of wet nitrogen at 30°C (41,800 ppm ).

The temperature of the air bath is controlled with ±0.5°C, so the saturation ^ values vary from 40,700 to 43,000 ppm which is a 5 percent variation or a 0.5 C variation in dew point. The total gas flow (wet plus dry) into the

glove box varies from 42 CFH (ft. /hr.) at the higher moisture levels to 75

CFH at lower levels. The gas flow is kept at a particular value for at least

an hour to attain a constant humidity in the box. After this time the dew

point hygrometer agrees within 1.5 C dew point of the values calculated from

the gas flow. The reading of the dew point hygrometers is taken as the humidity level in the box.

Because the accurate measurement of humidity was essential to this whole

investigation, the General Eastern dew point hygrometer was calibrated as follows. A cold finger was fabricated from a 3/8" diameter copper rod, polished

on one end and inserted through a rubber stopper with a polished end flush

with the top of the stopper. This assembly was then placed into a Dewar

flask containing crushed ice and water and put into the glove box. The moisture level in the box was then raised slowly and the reading on the hygrometer

noted when moisture condensed on the polished copper surface. This procedTjire was repeated with the flask filled with a frozen slush of perchlorethylene to

give a nominal temperature of -22 C.

The temperature of the polished copper surface was measured with a surface

temperature probe , ^ which in turn was calibrated with ice (0°C) or frozen perchlorethylene (-22°C). Table 1 shows the data used for determining the temperature of the polished copper surface and the hygrometer readings at the

point of condensation. The probe temperature on the solid phase is subtracted

from the melting point value to get a correction for the accuracy of the

probe. This probe correction is then applied to the reading on the copper

surface to determine its actual temperature. The last column is the hygrometer reading at which condensation was first seen on the copper surface. The

range of this reading is within 0.5 C of the corrected copper probe temperature.

* Model 1200Ar, General Eastern Instruments, Watertown, MA 02172.

+ Prepared by pouring liquid nitrogen into perchlorethylene until a slush

forms.

§ Model 392 digital heat-prober thermometer with a #123 fast surface probe,

William Vahl Corp., Los Angeles, CA 90066.

51
Two techniques were used to determine the temperature that the package attains

during the sealing procedure on the parallel seam welder. Initially, a thermocouple fastened to the inside bottom of a package was monitored during a sealing cycle. This showed a maximum temperature of about 75°C on the second

pass.* The second technique used temperature recording indicators in the

package, noting the color change after sealing. These indicated that a temperature range of 110-120 C was attained. From these observations an assumption was made that the gas in the package reaches a temperature of about

100°C. Hence, the pressure in the package at room temperature is approximately

0.8 atmospheres, based on a calculation using the ideal gas law. This figure

is then used for determining the concentration in parts per million (by

volume) at 0.8 atmosphere from the measured dew point in the package.

2.2 Use Of Sensor Chips

In order to have confidence in the level of humidity in a sealed package, it is

necessary to have a sensor sealed in the package that can detect moisture. A

commercially available chip that ^ is reportably capable of doing this is sold

under the trade name MINI-MOD-A. This 60 x 60 mil chip consists of a top

layer porous gold electrode, a porous alumina layer, and a bottom aluminum

electrode. The admittance of this capacitor structure changes with the water

vapor pressure in the surrounding ambient [4]. These chips can be die bonded

into a package and then wire bonded so that the admittance can be measured

after the package is hermetically sealed. It is necessary to calibrate the

chips by measuring the admittance as a function of humidity. This was done in

the same glove box as used for the seam sealing.

Figure 2 shows the calibration curves obtained for two different die bond preform materials (and temperatures) and for no die bonding at all. The values

on the ordinate are proportional to the admittance. Where no die bonding was

done, the chip is held only by the wires used for electrical connection

between the chip and the package. Two different chips were calibrated with

each preform; as will be discussed later, they do not have consistent calibration curves, so there is a range of values for each group. It is apparent

from the plot that as the temperature of the die bonding increases, the curves

become flatter, resulting in less moisture sensitivity of the chip. Because

of the difficulty in wire bonding chips that are not bonded at all to the substrate, and because of the tendency for wire breaking and shorting when only

wire bonding is used, it was necessary to die bond the chip to the package.

It was found that soft solder (60% Sn, 40% Pb) , with no flux, could be used at a temperature of 200°C which is low enough to minimize the effect on the calibration curve, but high enough not to melt during subsequent wire bonding and

baking treatments. Organic adhesives were intentionally avoided because of

possible water adsorption/desorption effects.

* The package makes two passes through the welder, sealing two opposite

sides on each pass.

+ "Temp-Plate" decals, William Wahl Corp., Los Angeles, OA 90066.

§ Panametrics Inc., Waltham, MA 02154.

52
After a die-bonding procedure had been established, a number of sensor chips were

die and wire bonded into side-brazed ceramic packages.* The packages were then

cleaned in boiling Freon TMS"*" for ten minutes, followed by a 2-hour bake at

150°C in nitrogen. This is a standard cleaning and baking procedure used for

all the packaging in this study. A lid with an 80 Au/20 Sn alloy sealing ring

(M.P. = 280°C) was then sealed on with the seam sealer, as described earlier.

The lid was then mechanically removed from the package and the chip calibrated

in the glove box at several humidity levels. Figure 3 is a plot of the curves

obtained on five chips from each of two lots of sensors; the chips within each

lot were calibrated at the same time. It is apparent that there is a considerable spread of values, even within a lot. In lot 72 four of the five chips

have very similar curves, but the fifth chip is significantly different. In

lot 120 there is a more even distribution of curves. An important conclusion

from this study is that every sensor chip has to be individually calibrated to

be assured of accurate results.

As pointed out earlier, another important factor in the use of the sensor

chips is the effect of heating on the response of the chip. Figure 4 shows

the effect of some processes involving heat treatments on chip calibration.

Chips were calibrated after 1) bonding, cleaning, and baking, 2) seam sealing

and delidding, and 3) a furnace sealing operation in which the package is put

through a belt furnace. This type of furnace has several temperature zones

that raise the package temperature to a maximum of 350°C and then back to room

temperature. The total time in the furnace is about nine minutes; of this,

about three minutes is above 300°C. Figure 4 shows that there is a slight

change in the calibration curve after the seam sealing operation; there is a more significant change after the furnace sealing, because of the higher temperature exposure. This is a plot for only one chip; as shown in figure 3

there can be significant variability between chips - some show considerably

more of a decreased response after furnace sealing than the chip shown in fig- ure 4. In using these chips, therefore, it is important to calibrate them

after all heat treatments and measurements on the sealed package.

As discussed in a previous section, the final pressure in the package at room

temperature after seam sealing is about 0.8 atmospheres. Literature from the

manufacturer^ states that the chip response is a function of water vapor pressure or dew point, rather than volume concentration (ppm ). To confirm this,

the following experiment was done. A calibrated sensor chip was put into a stoppered glass jar which could be pumped down with a hand pump, inside the

glove box. The humidity level in the box was set to a particular level and

sensor readings made as the pressure was lowered. Table 2 shows the change in

response as the pressure was lowered at two moisture levels. As the pressure

at the chip was lowered, the admittance reading of the sensor decreased. The

dew point, taken from the calibration curve, was converted to ppm at each

pressure, using a published conversion graph [7], This ppm value was within

experimental error of the value in the box at all pressures. Since the volume

concentration is independent of pressure, this confirms the response of the

* Kyocera International, Inc., Bridgewater, NJ 08807'

+ E. I. DuPont de Nemours & Co . , Wilmington, DE 19898.

§ Panametrics, Inc., Waltham, MA 02154.

53
sensor chip to dew point.

2.3 Preparation of Standard Packages

After establishing confidence in the sensor chips by individually calibrating

them after all heat treatments, the chips could now be used for measuring the

humidity inside a sealed package, as the next step in preparing standard humidity packages. In preparing such standards, it is important that the surfaces

within the package are equilibrated with the moisture in the vapor phase in

the cavity, so that there is no change in humidity level with time.

Initially, sensor chips were die bonded with Sn/Pb solder into ceramic packages, and the packages, along with the lids with preassembled sealing preforms, were baked at 150°C for 16 hours in dry nitrogen. The lids were then

sealed on the packages on the seam sealer with the glove box at a stabilized

humidity. The sealed packages were leak tested to check the hermeticity .*

They were then stored at room temperature while being monitored by the sensor

chip. Figure 5 shows a plot of the dew point in the package as a function of

storage time for two packages sealed at each of three levels of humidity. The

dew point was determined from individual calibration curves of the sensor

chips done after the packages had stabilized and the experiment was complete.

The humidity in the sealed package drops steadily from the original value in

the glove box during sealing. After 150 hours of stabilization, the humidity

in the package appeared to level out. The packages were then heated at 100°C

for 5 hours and allowed to stabilize at room temperature again. An increase

of moisture of several degrees of dew point was observed at all levels. The

moisture then dropped over the next 50-100 hours to where it had been heating

before.

The initial drop in moisture is presumably due to adsorption on the interior

walls of the package. The 100 C bake (to simulate that done during mass spectrometer analysis) desorbed some of this moisture, which then readsorbed as the package sat again at room temperature. The final level of moisture in the

package is significantly below the value in the glove box during the sealing;

the amount of drop is not a constant dew point difference at the different

levels or a fixed percentage change, so it would be difficult to be sure of

the final level of moisture without a sensor chip in the package.

If the adsorption in the package after sealing could be reduced or eliminated,

there would be a better chance of maintaining the moisture at the same level

as that in the box during the sealing. Some work at other laboratories [s]

has shown that when a gold-plated Kovar lid is baked extensively, it can act

as a getter for moisture. This could explain the adsorption effect shown in

figure 5-

To evaluate this effect, some standards were made in the same way as before,

but the lids and packages were not baked. Figure 6 shows the stabilization

curves for these packages, again using the sensor chip calibration curve determined after the packages had stabilized. There is now an immediate increase

in the moisture level on sealing followed by a decrease over 150 hours to a

* Radioisotope Fine Leak Test, Military Standard 883B, Method 1014.3, Test

Condition B (5xl0~^ atm cm-^/sec. maximum leak rate).

54
constant level within 700-1000 ppm of the dew point in the box at the time of

sealing. The increase in moisture on sealing is presumably due to desorption

from the walls of the cavity caused by the heating during the sealing operation. This moisture then readsorbs gradually onto the same surface.

In order to check the reproducibility of the stabilized value in relation to

the box value during sealing, five packages were sealed (with sensor chips)

with the box at 5000 ppm (-2.5 C dew point); they were allowed to stabilize

at room temperature and ^he moisture in the sealed package determined from the

sensor chip response, using the calibration curve obtained after the chip had

stabilized. Table 3 shows the results. The stabilized volume concentration

showed a range from 5400 ppm^ to 7000 ppm , or an average of 1100 ppm higher

than the concentration in the box during sealing. This agrees within experimental error with the results shown in figure 6. The average value for the

nominal 5000 ppm standards, therefore, is 6100 ppm with a standard deviation

of 700 ppm_^. Thus, reliable standards can be made without in-situ sensors.

There was initially some concern that there would be difficulty in getting a good flow of the lid sealing alloy in high humidity environments on the parallel gap sealer so that the packages would not be hermetic. This was not the

case, for leak test yields of greater than 90% have been achieved.

2.4 Analysis of Packages by Mass Spectrometer

A number of packages were sealed at known moisture levels as standards for

analysis by two commercial laboratories by mass spectrometer. Some of the

packages contained sensor chips, so that the stabilized moisture level in the

package could be confirmed. The results are shown in table 4. The stabilized

values shown by the sensor chip were just outside of the low end of the range

expected for standards (table 3). Both laboratories show significant variability in their analyses, with Lab A showing somewhat high average results and

Lab B somewhat low. Overall, however, the values are in the right range,

although not within the +_20^ hoped for.

Analyses were also made on another set of standards using a mass spectrometer*

at Bell Laboratories, Allentown, Pennsylvania. The details of this system have

been previously described [9]. The results are shown in table 5. The agreement

between the sensor chip values and measured values is very good at all levels.

2.5 Analysis of Packages by Dew Point Technique

Additional analyses of standard packages were made using commercial dew point

chips that were die and wire bonded into the package before sealing at a known humidity. It was found that in order to obtain a response from the chip

it was necessary to do some special cleaning. The details of this procedure

have been described elsewhere [lO]. Two techniques were used for cooling the

chip, a spot cooling done at Bell Laboratories, Allentown, Pennsylvania and a total package cooling done at Harris Semiconductor [10]. In both techniques

the peak response of the leakage current was taken as the dew point.

* Batch Inlet System from Pernicka Corp., Sunnyvale, CA.

+ HI 0-5501 -6 Moisture Sensor Chip, Harris Semiconductor, Melbourne, FL 32901

55
Table 6 shows the results. The agreement between the two methods is excellent; the absolute values, however, are lower than the 5800 ppm expected from

these standards. This may result from the use of the peak value of leakage

current as an indication of the dew point; as discussed earlier, there are some who feel that the initial increase in current is a better measure of the

dew point [3]. If this were to be used, these packages would show a higher dew

point, which would bring the measured values more into agreement with the

actual value.

2.6 Effect of Heat Treatment on Packages

It has been reported that when ceramic packages are heated to 1 06°C and

higher, moisture desorbs from the walls and can increase the moisture content

of a sealed package [l ]. Two experiments were run to examine this effect.

The first consisted of doing a mass spectrometer analysis .at two different

temperatures. Table 7 shows the results obtained when some standard packages

were analyzed first at 100 C and then at 40 and 30 C by Rome Air Development

Center. It is apparent that the water content at 100°C is 500 ppm to 600

ppm_^ higher than that at room temperature.

The second experiment is the one shown in figure 5, where a 100°C/5-hour bake was done on some packages that had stabilized at each of three levels. Table

8 tabulates the data from figure 5 and shows that there is an increase in

moisture level in the package of 300 ppm to 800 ppm , depending on the stabilized value. This value agrees well with that found from the previous

experiment (500 ppm and 600 ppm ). The moisture level in these packages was then monitored over the next 100 hours; the last column of table 8 shows that

the moisture level returned, in that time period, to within 300 ppm of the

original stabilized value.

These data indicate that the 100°C heating done before or during the mass

spectrometer analysis will increase the moisture in the package level by only

about 500 ppm_^.

3. CONCLUSIONS

Hermetically sealed ceramic packages with metal lids containing known amounts

of water vapor can be prepared using standard sealing techniques in a glove

box with controlled humidity. If the package and lid are given the proper

pretreatment (cleaning, but no extensive baking), the final level of moisture

in the package is within 1000 ppm of the level in the glove box during the

sealing. These packages can be used as standards for analytical determinations of water vapor, or for preparing samples at different humidities for

reliability studies.

Packages prepared in this way at 4000 and 5000 ppm^ were analyzed by mass spectrometer by two commercial laboratories and found to give average results

within 1500 ppm of the known values, although there was a considerable spread

in values repo r^ed by each laboratory. Analyses by an in- house mass spectrometer at four humidity levels gave average values within 10^ of the known value

at levels above 5000 ppm and within 30^ at the 2600 ppm level. Analysis at

the 5000 ppm level with dew point detection chips in the package shows

56
average values within 23% of the known value.

It has been found that the MINI-MOD-A sensor chips are reliable indicators of

moisture levels in packages if they are individually calibrated after all

heat treatments.

Heat treating the packages at 100°C causes an increase in the moisture level

of a sealed package of about 500 ppm^. This suggests that the packages are usable as standards for mass spectrometer analyses which are usually run at

100°C.

ACKiroWLEDGHENTS

The authors are indebted to R. J. Gale for the Bell Laboratories mass spectrometer analysis; to R. W. Thomas and B. A. Moore at Rome Air Development

Center for mass spectrometer analysis as well as several helpful discussions

to A. F. Walcheski for dew point determinations at Bell Laboratories; and to

R. K. Lowry for dew point determinations at Harris Semiconductor.

57
REFERENCES

1. Thomas, R. W. , Moisture, Myths and Microcircuits , IEEE Trans, on Parts,

Hybrids, and Packaging, PHP-12 , 167-71 (September 1976).

2. Lowry, R. K. et al . , Characteristics of a Surface Conductivity Moisture

Monitor for Hermetic Integrated Circuit Packages, Proceedings of the

17th Annual International Reliability Physics Sjrmposium, San Francisco,

CA (1979).

3. Sosniak, J., and linger, B. A., Moisture Determination in Hermetic Packages by the Dew Point Method Using AC~ Capacitance and Conductance Measurement, lEDM Technical Digest, Washington, D.C. (1978).

4. Kovac, M. G., et al. , A New Moisture Sensor for In-situ Monitoring of

Sealed Packages, Proceedings of the 15th Annual International Reliability

Physics Symposium, Las Vegas, NV (1977).

5. Perkins, K. L. , Comparison of Mass Spectrometric Moisture Measurements by

Different Laboratories, Semiconductor Measurement Technology: ARPA/NBS

Workshop V. Moisture Measurement Technology for Hermetic Semiconductor

Devices, NBS Spec. Pub. 400-69 (March 1978), pp. 58-68.

6. Striny, K. M. , unpublished data.

7. General Instruments Corp., Selecting Humidity Sensors for Industrial Processes Handbook, Watertown, MA (1978)

8. Thomas, R. W. , private communication.

9. Gale, R. J., Correlation Between Mass Spectrometer and Aluminum Oxide

Sensor Measurements of Moisture in Hermetic Packages, this report.

10. White, M. L., and Walcheski, A. F. , Some Observations on the Response of

Dew Point Detection Chips, this report.

58
DRY N2

FLOWMETERS —

WATER

SATURATORS

EXPERIMENTAL SETUP Figure 1. Apparatus

CONSTANT TEMPERATURE

AIR BATH

(30°C)

9 4

t.o

DIE BOND

.9 PREFORM TEMP

.8 0, • NONE 25° C

. Au/Sn 350°C

.7 Au/6a 550°C

.6 NO PREFORM

Au/Ga (550°C)

-40 -30 -20 -10 DEW POINT (t)

+10

10 100 1000 5000 10000 PPMy AT 1 ATM Figure 2. Effect of Die Bond

Procedure on Sensor Chip

Calibration

59

371-525 0-82-5
LOT 72 LOT 120

1.0 CHIP No 104 IrfHlr INO 1 O 1 .9 CHIP No 105 A CHIP No 138

.8 X- CHIP No 106 0 CHIP No 139

CHIP No 107 V CHIP No 140

.7 * CHIP No 108 Q CHIP No 141

-30 -20 DEW POINT (°C)

100 1000 PPMu AT 1 ATM

5000 10000

Figure 3. Variability of Sensor

Chip Calibrations

1.5

1.0

.15-60

10

AFTER BONDING AND CLEANING AFTER SEAM SEAL

AND DELIDDING AFTER FURNACE SEAL AND DELIDDING

-50 -40 -30 -20 DEW POINT, "C

-10 +10

1000 PPMy AT 1 ATM

5000 10000 Figure 4. Effect of Processing

on Sensor Chip Calibrations

(X103)

Figure 5. Stabilization After 150°C/16 Hour Figure 6. Stabilization With No Baking

Bake on Lids and Packages of Lids and Packages

60
Table 1 - Calibration of Dew Point Hygrometer

Solid Probe Read. Probe Probe Read. Actual Hygrometer Read.

Phase on Solid Corr'n on Cu Cu Temp. at 1st Condensation

Ice (0°C) 1.5°C -1.5°C 2.3°C 0.8°C 0.6° to -0.9°C

Perchloro- -21.1°C -0.9°C -17.1°C -18.0°C -18.2° to -17.4°C

ethylene (-22 C)

Table 2 - Effect of Pressure on Sensor Chip Response

Dew Point

ppm in

Box

Press, at

Chip, atm

Sensor

Reading

From Calibration Curve, C

ppra at

Chip Press

2800 1 .0 0.285 -9 2800

0.74 0.270 -13 2700

0.47 0.250 -18 2800

0.21 0.225 -26 2700

6500 1 .0 0.350 +1 6500

0.74 0.325 -2 6900

0.47 0.300 -6.5 7400

0.21 0.250 -18 6200

Table 3 - Reproducibility of Stabilized Values Sealed at 5000 ppm^^ Glove Box Level

Package

No.

Hours to

Stabilize

Stabilized ppm

Stabilized D.P. in Sealed Package

0

From Sensor Chip, C Corrected to 0.8 atm

Change From

Sealing PPj5^

137 143 -2.0 6400 +1 400

138 122 -4.0 5400 +400

139 122 -2.5 6100 +1 100

140 122 -4.0 5400 +400

141 122 -1.0 7000 +2000

Average 6100 +1 100

61
Table 4 - Analysis of Packages by Commercial Laboratories

Glove Box Sensor Chip Lab A Lab B

Level, ppm^^ Value, PPro^ n Range Ave. n Range Ave.

4000 4300 3 4100-7100 5800 2 2900-3300 3100

5000 5300 7 4200-8800 5600 5 3300-6800 5000

Table 5 - Analysis of Packages by Bell Labs - Allentown

Mass Spectrometer

Sensor Chip Results, ppm

Value, ppm Mean ^nge n

11,000 12,000 9200-14,500 7

6100 6400 4900-8200 4

5100 4600 4000-5200 5

2600 1900 1200-2200 3

Table 6 - Analysis of 5800 ppm^^ Packages By Dew Point Chips

Cooling No. ppm at 0.8 atm

Procedure Samples Range Ave.

Spot 5 3100-5400 4300

Total 5 4200-5400 4600

62
Table 7 - Analysis of 6100 PP^^ Packages By RADC Mass Spectrometer

Temp. During No. ppm

V Analysis Packages Range Ave.

100°C 3 7500-8100 7800

40°C 1 - 7200

30°C 1 - 7300

Table 8 - Effect of 100°C/5-Hour Bake on Moisture

Level in Packages (Average of two samples at each level)

Stabilized ppm^ After

lOOOC/5 Hrs.

ppm

TIncrease

ppm^ 100 Hrs

After Bake

7300 8100 800 7000

2800 3200 400 2800

1000 1300 300 1200

63
3.2 A Surface Conductivity Moisture Monitor for Hermetic IC Packages

Robert K. Lowry

Harris Semiconductor, Products Division

Melbourne, FL 32901

(305) 724-7566

Abstract: An in-situ surface conductivity sensor for measuring water

content of hermetic package cavity ambients is described. The sensor

is a 50 X 95 mil chip whose surface consists of an interdigitated

pattern of aluminum stripes on silicon dioxide. The chip is mounted

and wire bonded as a test vehicle into the package configuration

whose moisture content is to be determined. The hermetically sealed

specimen package is cooled in a temperature bath with 50 V dc applied

to the sensor. As moisture condenses onto the sensor surface, the

leakage current of the metal pattern rises. The temperature value

of the leakage current peak represents complete condensation of all

available water vapor, and this is nomographically converted to ppmv

water content.

Sensor performance is evaluated via correlation experiments with mass spectroscopy and volume-effect sensors. Use of the sensor to estimate

levels of metal ions within the package cavity is also described.

A revised sensor design, now undergoing testing, is described. The

revised sensor incorporates an on-board diode structure permitting

accurate measurement of actual sensor surface temperature as the

specimen package is cooled.

Key Words: Gas analysis; hermetic IC packages; in-situ moisture monitor; internal water vapor; moisture measurement; surface conductivity

moisture monitor.

1. INTRODUCTION

Moisture within hermetic packages may cause premature device failure due to

electrogalvanic corrosion of chip metallization [l-3] . Knowledge of package

moisture contents thus becomes a critical parameter both for operating reliability as well as process technology improvements and quality control [A-b]

Mass spectroscopy has been the definitive method for measuring package moisture

[7-8]. It is relatively expensive, either for acquiring the instrumentation or for per-sample charges by service laboratories. It is destructive to the

sample package. If not executed carefully, it can also be destructive to the

encapsulated device precluding further failure analysis. Results are not

obtainable at the time that the need for them arises.

An in-situ monitor, in which a sensing device is mounted directly in a saiupie

package or incorporated into the design of a circuit, offers rapid availability

of moisture data. Analysis cost per sample is inexpensive, only about 10 percent

of the cost of a mass spectrometer analysis. This means that statistically

64
significant numbers of packages can be analyzed on a more frequent basis.

The in-situ monitor thus enables more exacting process development experimentation and process control measurements, and speeds up vendor and assembly

lot qualifications. Moreover, the in-cavity sensor gives a real-time dynamic

analysis of moisture within a package. The determination can be made repeatedly

so that moisture conditions can be monitored as a function of part storage or operating lifetime.

Several types of in-situ sensors for integrated circuit packages have been

developed. One type operates via changes in resistance of a cobalt oxide

structure with relative humidity [9] . Another type utilizes a porous Si02

structure whose resistance changes with relative humidity of the package [lO] . Sensors composed of a porous AI2O3 structure which trap diffused water molecules

causing an impedance change as a function of humidity have become a common means

of detecting moisture in sealed packages [ll-isj . A correlation study of AI2O3

sensor performance with mass spectroscopic measurements has been made [IAJ . Yet another type of in-situ sensor has recently been reported for IC package

applications. It is composed of a film of ultrafine particles of Sn02 averaging

100 A in diameter, and is extremely sensitive to water vapor in air ambient at room temperature [15]

.

Feasibility exists with most passivated circuits to make a simple determination

of whether or not water condenses onto the chip surface. This enables an alternate type of in-situ vehicle operating via surface adsorption rather than volumeeffect diffusion. Effects of condensed moisture on surface electrical conduction

of oxides and glasses have been studied [l6-173 . A sensor operating on the

principle of changes in surface conductivity as water condenses onto an oxide

surface has been described [is] . A surface conductivity type sensor has also

been employed in a study of hermetic package leak rates £l9j . A rapid dewpoint

technique suitable for rapid package screening has been reported using passivation

of the actual IC chip itself as a detector of condensed moisture [20-21] .

This paper describes a nonglassivated in-situ sensor wherein the water-sensitive

surface is composed of an interdigitated pattern of aluminum metal stripes.

The surface conductivity of the pattern increases as water condenses onto its

surface and peaks at the temperature where condensation ceases. Since negligible amounts of foreign ions are present to cause stray surface leakage

currents, a sensor of this type is a more dependable moisture detector than a glass surface. The quantitative features of this sensor concept are investigated via correlations in which moisture measurements on packages containing

sensors are made by mass spectroscopy. Other features of the sensor including

its use as a detector of ionic impurities are also presented.

2 . EXPERIMENTAL

Description of the Sensor and Measurement Method

Figure 1 shows the layout of a surface conductivity cell. Sensor chips are made via simple wafer fabrication steps during regular product line operations.

Aluminum is vacuum-deposited onto oxidized silicon wafers. A mask/photoresist

step delineates an interdigitated aluminum stripe pattern of 5 X 8 mils. The

aluminum stripes are nominally 10,000 angstroms thick and 0.3 mil wide. Sensor

65
chips, each containing four cells like that shown in figure 1, are then diced

from the wafers and mounted, wire bonded, and sealed into those packages and

by those sealing processes whose moisture properties are to be investigated.

Pin 1 of a sealed package containing a sensor chip is soldered to leads of a

50 V dc power supply. Pin 2 is soldered via a 10 picoamp sensitivity currentvoltage conversion to an X-Y recorder. The specimen is placed into a thermostatted fluorocarbon bath containing FC43 fluid. -The bath is heated to 100°C

to assure desorption of water molecules' from internal cavity walls. With 50 V

applied across the sensor, the bath is then cooled at approximately 10°C/min

while the recorder monitors current output of the aluminum pattern. Normal

quiescent current is 10~^^ amps. When cavity water vapor begins condensing

on the sensor surface, the current rises. When all water has condensed, the

current reaches a maximum value. A typical conductivity plot is shown in

figure 1. From the temperature of the peak current value and the pressure

of the hermetic cavity (reasonably estimated from the law of Gay-Lussac:

P « T, V constant), the nomograph in Figure 2 yields package moisture content

in parts per million by volume (ppmv)

Correlations With Mass Spectroscopy: Cerdip Packages

A variety of Cerdip packages containing sensors was processed using different

types of solder sealing glass under various sealing conditions. Moisture

content of each specimen package was determined with the enclosed sensor, and

then subsequently determined by mass spectroscopy using MIL-STD-883B, Method

1018, Procedure 1.

Figure 3 is a 1:1 correlation plot of sensor and mass spectrometer results.

The shaded area indicates a +20 percent range applied to the 1:1 correlation

line. This error bar was not extended to the low ppmv portion of the graph

as good agreement is not expected there. Many of the samples measured by the sensor contained less than 700 ppmv water. During these experiments, the

cooling profile was often halted there since strict correlations for specimens

containing only a few hundred ppmv water were not sought. Data points with

arrows signify samples with moisture contents less than the value indicated

by the method whose axis is parallel to the arrow.

These observations are made about data in figure 3: of 18 samples in the

1,000 to 10,000 ppmv range, eight (44 percent) fall within +20 percent of 1:1

agreement with mass spectroscopy. The remaining 10 read higher by sensor. For these quantities of water vapor, the sensor may be a more efficient net

collector of water molecules than a mass spectrometer. In the mass spectrometer system, molecules must travel relatively large distances to detection

once they leave (or if they leave) the opened package. Many opportunities

occur for chemisorption en route to reduce the apparent water content. Within

a sealed cavity, however, vapor molecules need travel only a very short

distance to the condensation/detection surface, permitting an in-situ device

to detect practically all available water molecules.

Of 42 samples measuring less than 400 ppmv by at least one of the methods,

12 (29 percent) fall within +20 percent of 1:1 agreement.

Only four samples indicated greater than 10,000 ppmv by both measuring methods.

Mass spectroscopy gave the higher reading for each of these. For large amounts

66
of water, mass spectroscopy may provide the more accurate reading since quantities of condensed water may saturate the sensor surface and reduce its

ability to detect additional condensed amounts. For such very wet packages,

of course, the need for quantitative accuracy is slight.

Of 36 samples measuring less than 700 ppmv by at least one method, 17 appear

to read significantly higher by mass spectrometer than by sensor. This is

the same tendency as for parts containing greater than 10,000 ppmv but is the

opposite of the tendency for parts between 1,000 and 10,000 ppmv. At lesser

water contents, it is possible that the sensor could be a less efficient

collector of water molecules than a mass spectrometer. Microdroplets of

water may be condensing between metal stripes without bridging them so that

the sensor yields a reading lower than that determined by mass spectroscopy.

On the other hand, the behavior of less than 1,000 ppmv of water vapor

released into a mass spectrometer for analysis can be unpredictable. A

contrast in mass spectrometer performance is evident for 18 of the 20

packages which contained less than or equal to 700 ppmv water according to

the sensor. These 18 samples were prepared simultaneously in one sealing

operation using vitreous-glazed Cerdip parts in an effort to achieve maximum

and reproducible dryness throughout the entire group. Nine of these 18

were analyzed by mass spectrometer A and the other nine by mass spectrometer

B. The nine analyzed by A ranged from less than 100 to 950 ppmv water content,

all in reasonably good agreement with the dryness condition indicated by the

included sensor. Of the nine analyzed by B, four ranged from less than 100

to 680 ppmv, in good agreement with other results. The other five analyzed

by B, however, ranged from 1,500 to 2,300 ppmv, grouped substantially above

the other 13 samples. Similar poorly agreeing results from mass spectrometer

B are evident for packages indicated by the sensor to contain less than or equal to 400 ppmv and less than or equal to 100 ppmv water. It is unclear

whether this is a measuring or calibration discrepancy which occurs from

time to time in mass spectrometer B. Of the 36 packages indicated by the sensor to contain less than or equal to 700 ppmv water, 19 (53 percent) were

so verified by the mass spectrometers.

Table 1 summarizes the data of figure 3 in terms of disagreement between the

two measuring methods in verifying package dryness relative to specific

moisture levels in the 1,000 to 5,000 ppmv range. For example, if 1,000 ppmv

were a desired maximum moisture content, 49 of the 58 specimens are undisputedly

above or below that figure. Nine of the specimens would be disputed relative

to 1,000 ppmv. Eight of these measure greater than 1,000 ppmv by mass spectrometer but less than 1,000 ppmv by sensor, while one sample measured

greater than 1,000 ppmv by sensor but less than 1,000 ppmv by mass spectrometer.

It appears that the frequency of "disputes" drops as actual water content

increases. There are, however, more samples near 1,000 ppmv than 5,000 ppmv.

It does appear that the higher the water level to be certified, the greater

the likelihood that the sensor would yield a failing result. Note that two

disputed samples near 2,000 ppmv fall within the +20 percent error bar.

Correlations With Mass Spectroscopy: Metal Packages

Initial use of the sensor in metal packages such as T099 cans gave unsatisfactory results. All cans tested would show no detectable water, though

subsequent mass spectroscopy showed that some cans contained significant

67
amounts of water vapor. Even sensors in cans sealed in wet air showed no detectable moisture.

It was found that during cooldown in the FC-43 temperature bath, the metal walls

of the cans cooled considerably faster than the sensor chip. Preferential

adsorption of water molecules onto the metal walls caused no liquid water to

condense onto the sensor. This effect was eliminated by coating external

metal surfaces of the cans with RTV sealant compound number 106. This acted

as an insulator to the slow cooling rate of metal surfaces to rates less than

that of the chip within the package.

Initial measurements on samples of three—way correlations in table 2 yielded

no detectable water. The cans, subsequently coated with RTV sealant and then

remeasured, showed easily detectable cavity moisture. This made possible the

three-way correlation experiment discussed below and provided a routine method

for analyzing metal packages.

Three-Way Correlation.

Use of external insulation on metal cans enabled an experiment in which moisture

was measured on the same specimen package by three different means. A series

of T099 cans was prepared in which each can contained a commercially available

AI2O3 volume effect sensor chip mounted next to a surface conductivity sensor

chip. Water contents of the cans were determined according to each of the sensors and then subsequently by mass spectroscopy. Table 2 summarizes

results. The two in-situ sensors give good agreement in both dry and relatively

wet packages. Mass spectroscopy, however, gave wetter readings than the sensors

for all samples.

Detection of Ionic Contamination Using Surface Conductivity

The surface conductivity sensor also provides information about cleanliness of

the sealed package cavity. Amplitude of the current peak (see figure 1) is

independent of the quantity of water in the package, but will vary according to

concentrations of dissolved ions.

The curve families in figure 4 shows the typical conductivity peaks obtained

for vitreous and nonvitreous glass—sealed Cerdip packages. Peak amplitudes are typically 10~^ to 10"'' amps for Cerdips sealed with vitreous glasses (V^ and V2)

and 10~^ to 10"^ amps for Cerdips sealed with devitrifying glasses (D^ and D2).

These magnitudes shown in figure 4 are obtained reproducibly for the four

distinct glasses indicated, regardless of the actual quantity of water within

the package.

Relationship of peak amplitudes to concentrations of ions in water condensed on

the sensor was verified by the following experiment. Solutions containing

varying amounts of lead or sodium ion (common in solder sealing glasses) were

prepared by serial dilution of 1.0 M solutions of their nitrate salts. Dilution

was with deionized water containing added nitrate ion such that each working

solution was constant at 6 X 10"^ M nitrate concentration. These solutions were

applied in drop form to a sensor mounted in a delidded package. The sensor was thoroughly rinsed with deionized water between applications of each solution.

Conductivity measured by the sensor for each cation concentration was determined

68
using 5 volts dc. Background current for 6 X 10~-^ M nitrate black solution is near 1 microamp. As cation concentration of the applied solutions increases,

conductivity measured by the sensor rises as plotted in figure 5.

The data of figures 4 and 5 show that the interdigitated sensor does provide

an estimate of ionic contents of a hermetic cavity when detectable water vapor

is present. Caution in interpretation is necessary since leachability of

package materials is widely variable. The sensor offers a means of studying

the role of leached ion species in device failure mechanisms.

Effect of Package Temperature on Moisture Desorption

Water content of a Cerdip package containing 1,300 ppmv water was measured

repeatedly using initial temperature values of 25°, 40°, 75°, 100°, and 125°C

at the start of the cooldown cycle. The surface conductivity plots are shown

in figure 6. Regardless of initial temperature used, the maximum conductivity

of the specimen's sensor always corresponded to a water content of 1,300 ppmv.

Even at 125°C, no additional water was desorbed to the cavity ambient. This

suggests that cycling of a hermetic specimen higher than room temperature is

not necessary to assure that all available water within the cavity will

condense onto the sensor. It also is indicative of the test-to-test reproducibility of the sensor chip.

Performance Crossover of Sensors in Wet and Dry Packages

The ability of conductivity sensors to respond to a marked change in package

ambient was established by the experiment summarized in table 3. Two Cerdip

specimens containing sensors were selected, one of which indicated no detectable

water (less than 200 ppmv), and one which contained 2500 ppmv water. A minute

hole was drilled through the lid of each sample exposing the cavities and sensors to ambient. The initially dry package was exposed to live steam for

two minutes. The small hole was then immediately plugged with RTV silicone

which was allowed to set in order to reseal the package. Subsequent to this

treatment, the sensor indicated 120,000 ppmv moisture. The initially wet

package was baked in a dry nitrogen ambient for 1 hour at 250°C. During

the last 15 minutes of the bake, the hole was plugged with clay, after which

the clay-covered hole was also coated with RTV to reseal the package. Subsequent to this treatment, the sensor indicated no detectable water (less than

200 ppmv). Thus, sensors remain sensitive to large increases (e.g., loss of

hermeticity) or decreases (due to desiccants) in cavity moisture.

Effect of Elevated Post Seal Temperatures on Sensor Sensitivity

The possibility exists that sensors' aluminum metallization may self-passivate

due to temperature-induced formation of AI2O3 if oxygen is present during

package seal. Any such passivation would reduce sensitivity to condensed

water.

Five Cerdip parts were assembled using devitrifying solder glass with a seal

profile known to contain oxygen. Initial sensor readings indicated 2,500 to

10,000 ppmv in these parts. The parts were then baked at 150°C for 16 hours,

followed by 250°C for 2 hours. Remeasurement showed that originally indicated

water contents of each package were unchanged.

69
Aluminum surfaces of these sensors were compared to those of unused sensors

by scanning Auger microscopy. Careful Ar"*" depth profiling showed that about

75 A of aluminum oxide was present on the surface regardless of whether the sensor was used many times within a package or was obtained fresh from dice

storage. This thickness of AI2O3 represents native oxide growth, and it

does not increase with sensor usage. It may well offer the desirable effect

of stabilizing surface properties of the sensor during long-range or repeated

moisture measurements.

Correlation With Gold Metallized Sensor

Performance of aluminum as an active sensor component was compared to that

of gold. A number of chips were fabricated geometrically identical to the

aluminized sensor but having gold metallization. The gold sensors were

mounted directly adjacent to aluminum sensors in six ceramic packages.

Table 4 shows the comparative moisture values obtained. Variation between

two sensors in the same package exceed a 20-percent differential in only one of the six samples (number 3). For the other five samples, variation

averages 10 percent.

The agreement is added evidence that passivating oxidation does not occur

to aluminum surfaces to reduce sensitivity of the pattern to changes in

conductivity caused by subsequent water condensation. It also indicates

that for gold and aluminum patterns of identical geometry, there is no preferential attraction of condensing water droplets to either metal surface.

A Modified Surface Conductivity Sensor

The sensor heretofore described has been recently modified. The new design

consists of a concentric rectangular pattern of aluminum lines. The entire

chip is covered by the single pattern instead of the previous quadrant

arrangement. Metal line-and-space dimensions are the same. A diode structure is included on the chip at the edge of the metal pattern. This diode

enables direct measurement of the actual surface temperature of the moisturesensitive entity of the device.

The new device is expected to offer improved sensitivity and measurement

accuracy. Routinely used moisture-^sensitive surface area is quadrupled.

The temperature parameter of the measurement is obtained directly from the

sensing surface within the specimen rather than indirectly from the temperature bath fluid external to the specimen. Capabilities of the new sensor design are presently undergoing testing.

3 . CONCLUSIONS

Many uncertainties are involved in the interactions of water molecules with

surfaces. Much remains to be learned about the dynamics of such interactions

with surfaces ranging from smooth metals to rough and chemically reactive

glasses. What is learned will influence measurement technologies and will

establish levels of chemically available water inherent in particular microelectronic packaging materials and processes. In-situ sensors are necessarily a part of such studies.

70
Quantitative accuracy and precision of both mass spectroscopic and in-situ

sensor techniques are to be further refined. Parts-per-million-by-volume

values have been based upon proved mass spectrometer calibration methods,

but without reference to a primary standard. Reproducibility with small

amounts of water vapor in systems of variable design and materials is yet to

be well defined. The sensor ppmv values are derived from relative humidity

concepts, but again are not referenced to a prirtiary standard. Hence, the

analytical figures of merit cannot be known.

The correlations of figure 3 substantiate both methods of package moisture

measurement. This is especially significant when it is realized that the

two methodologies are entirely different.

The interdigitated aluminum sensor provides an effective means of determining

moisture within a hermetic package. It also indicates approximate levels of

ionic contamination inherent in packaging systems. It offers a useful alternative to mass spectroscopy for measuring hermetic package moisture content

for quality control and packaging development programs.

ACKNOWLEDGMENTS

J. M. Bird provided technical support by assembling many specimen packages for

testing. The technical assistance of L. A. Miller in design and operation of

the moisture sensor test apparatus is gratefully acknowledged.

REFERENCES

1. Kolesar, S. C, Principles of Corrosion, J^. Electrochem . Soc . 123 , 155-167 (1976).

2. Eisenberg, P. H. , Brandewie, G. V., and Meyer, R. A., Effects of Ambient

Gases and Vapors at Low Temperatures On Solid State Devices, 7th New York

Conference on Electronics Reliability (May 1966)

3. Koelmans, H. , Metallization Corrosion in Silicon Devices by Moisture-Induced

Electrolysis, J. Electrochem Soc. 123 , 168-171 (1976).

4. Thomas, R. W. , Moisture, Myths, and Microcircuits , IEEE Transactions on

Parts , Hybrids , and Packaging , PHP- 12 , 167-171 (September 1976).

5. Zierdt, C. H. , JEDEC Committee Correspondence, MIL-STD-883B, Method 1018,

Internal Water Vapor Content (August 1977).

6. Lowry, R. K. , VanLeeuwen, C. J., Kennimer, B. L. , and Miller, L. A.,

A Reliable Dry Ceramic Dual In-Line Package, Proc. IEEE Intl. Rel. Phys.

Symp., San Diego, CA, April 17-19, 1978, pp. 207-212.

7. Meyer, D. E., and Thomas, R. W. , Moisture in Semiconductor Packages,

Solid State Technology 17, 56-59 (September 1974).

71
8. Thomas R. W. , Microclrcult Package Gas Analysis Techniques, Proc.

14th Intl. Rel. Phys. Symp . , Las Vegas, NV, April 20, 1976, pp. 283-294.

9. Frazer, L. E. , and Fraioli, A. V., U.S. Patent 3,943,557,

March 9, 1976.

10. Burkhardt, P. J., and Popaniak, M. R. , U.S. Patent 4,057,823,

November 8, 1977.

11. Kovac, M. G. , Performance Characteristics of AI2O3 Moisture Sensor Inside

Sealed Hybrid Packages, Proc. ISHM Symp., Baltimore, MD, October 24-26,

1977.

12. Kovac, M. G. , Chleck, D. , and Goodman, P. , A New Moisture Sensor for

"In-Situ" Monitoring of Sealed Packages, Proc. 15th Intl. Rel. Phys.

Symp., Las Vegas, NV, April 13, 1977, pp. 85-91.

13. Finn, J. B. , and Fong, V. , Recent Advances in AI2O3 "In-Situ" Moisture

Monitoring Chips for Cerdip Package Applications, Proc. 18th Intl. R.el

Phys. Symp., Las Vegas, NV, April 8-10, 1980, pp. 10-16.

14. Cordasco, V. T. , Control of Microcircuit Contamination, RADC-TR-75-217

Final Technical Report, AF 30602-74-C-0203 (March 1976).

15. Ogawa, H. , Nishikawa, M. , Abe, A., and Hayakawa, S. , A New Type Ultrafine

Particle Gas Sensor, Electrochem . Soc . Extended Abstracts 80 (2),

1367-1369 (October 1980).

16. Comizzoli, R. B. , Bulk and Surface Conduction in CVD Si02 and PSG

Passivation Layers, J^. Electrochem . Soc . : Solid- State Science and

Technology 123 (3), 386 (1976).

17. Kawasaki, K. , and Hackerman, N. , On the Variation of Surface Conduction

Current on Porous Vycor Glass by the Adsorption of Water Vapor, Surface

Science 10, 299-302 (1976).

18. Merrett, R. P. , and Sim, S. P. , Assessment of the Use of Measurement

of Hermetic Semiconductor Encapsulations, Semiconductor Measurement

Technology : ARPA/NBS Workshop V, Moisture Measurement Technology

for Hermetic Semiconductor Devices, NBS Spec. Publ. 400-69 (March

1978), pp. 94-107.

19. Der Marderosian, A., and Gionet, V., Water Vapor Penetration Rate Into

Enclosures With Known Air Leak Rates, IEEE Transactions on Electron

Devices ED-26 (1), 83 (January 1979).

20. Bakker, N. , In-Line Measurement of Moisture in Sealed IC Packages,

Philips Telec. Rev. 37 (1), 11-19 (March 1979).

21. Merrett, R. P., Sim, S. P., and Bryant, J. P. , A Simple Method of

Using the Die of an Integrated Circuit to Measure the Relative Humidity

Inside Its Encapsulation, Proc. IEEE Intl. Rel. Phys. S5Tnp. , Las Vegas, NV,

April 8-10, 1980, pp. 17-25.

72
FIGURES

SURFACE CONDUCTIVITY SENSOR

AMPS

10-7^

10-8 r

10-9 r

10-10-

100 60 40 20 0 -20 -40

TEMPERATURE OC

Figure 1. Layout and typical response

curve of surface conductivity sensor.

IN-SITU SENSOR/MASS

SPECTROMETER CORRELATION

100

HI 1,000 10,000

10000 8000

6000 mo

4000

3000

2000

1000 800

60-

50-

+40- 30-

+20-

10-

0-

9 -50—

sS

-60-

-140

-120 -too

-+80

-+60

•+50 -+40 -30

-+20

-+10 0

--10

--20

--1M

130

—140

150

600—

500— —10 —9 400— —8

300— —

u.O

250— —

E

E 200— 4

150— —

100—

75—

50-

-1000 -900 -800

-700

-600

-500

-400

Figure 2. Nomograph for conversion

to ppmv.

MOISTURE SENSOR, PPMV

Figure 3. 1:1 Correlation plot of sensor and mass spectrometer results.

73
104

103

NANOAMPS

102

101

A

-

/ \ D2

/ \ —

/ ^

//

/ \

1

—1

1

1

11n / \ —

' r\ vi

Pb & Na ION CURRENT PLOTS

300 -F

20 40 60 80 100120140160

ION CONTENT, PPM

Figure 4. Typical current values of

conductivity peak amplitudes for

certain devitrifying (Dj^, and

vitreous (V^, sealing glasses.

Figure 5. Sensor peak amplitudes

in response to known quantities

of dissolved ions.

CURVES RUN AT VARYING

INITIAL TEMPERATURES

1250c

40OC

250c

TEMP, oc 60 40 20 0 -20 -40

Figure 6. Effect of initial

package temperature on quantity

of moisture detected within a package (Cerdip)

74
TABLES

Table 1

DISPUTED MEASUREMENTS RELATIVE

TO SPECIFIED AMOUNTS OF WATER

n = 58

HI M.S. HIC. S. LOC.S. LOM.S. TOTAL

lOOOppmv

2000ppmv

BOOOppmv

8

4

0

1

4

2

9 (16%)

8 (14%)

2 (3%)

Table 2

THREE-WAY CORRELATION

VOLUME- SURFACE

EFFECT CONDUCTIVITY MASS

SENSOR SENSOR SPECTROMETER

SAMPLE (ppmv) (ppmv) (ppmv)

<100

<100

17,500

20,500

20,500

22,000

<700

<400

16,000

22,000

20,000

22.000

890

3,000

30,000

38,000

42,000

40.000

Table 3

Table k

DRY

PACKAGE

WET

PACKAGE

CROSSOVER EXPERIMENT

INITIAL SENSOR PACKAGE

SAMPLE RESPONSE TREATMENT

SENSOR RESPONSE

AFTER TREATMENT

200ppmv

2,500ppmv

CAVITY EXPOSED TO

STEAM

EXPOSED

CAVITY n2

BAKED

120,000ppmv

200ppmv

GOLD SENSOR/ALUMINUM

SENSOR CORRELATION

PACKAGE

12

3

4

5

6

GOLD

2,600ppmv

1,900

2,400

2,400

1,700

1.900

ALUMINUM

2,200ppmv

2,000

1,500

2,200

1,500

2,000

75

371-525 0 - 82
3.3 Some Observations on the Response of Dew Point Detection Chip

by

[Malcolm L. White and Albert F. Walcheski

Bell Laboratories

555 Union Boulevard

All en town, PA 13103

(215) 439-7457

EXTENDED SUMMARY

The response of a commercial dew point sensing chip* has been

found to be very sensitive to the treatment of the chip p;:ior to

sealing it in a hermetic package. Chips without any precleaning

that were put into side brazed ceramic packages containing about

5000 ppm^ water vapor [1] and solder sealed at a low temperature

showed no response while monitoring direct current leakage down

to -35*^^ using either a spot cooling or a total package cooling

technique. When the lid was removed from these packages and

moisture condensed from room air by cooling the chip, or by

breathing on the chip, the condensate was in the form of very

small droplets. With this dropwise condensation, there is not a continuous film of water between the interdig itated electrodes

that are used for sensing leakage. Thus, there is not a significant change in the surface leakage current.

The dropwise condensation of water results from the hydrophobic

condition of the chip surface. This, in turn, is caused by the

presence of organic contaminants on the surface of the silicon

dioxide [2] which is the dielectric between the electrodes.

Removal of this organic material will restore the dielectric surface to its normal hydrophilic condition [2]. The condensation

of water will then be filmwise, resulting in a complete conducting path between the electrodes on the chip.

An oxygen plasma treatment is known to be very effective in

removing organic contaminants [3] from surfaces, so some moisturesensing chips were cleaned with this technique. The condensation

of water on these chips was observed to be much more filmwise, so

that there was a continuous path of water between electrodes.

Packages were then assembled to contain 5000 ppm^ water vapor

with chips that had been cleaned in oxygen plasma. There was now

a very strong electrical response, with both the spot and total

cooling techniques, at about the expected dew point of -5^C, thus

confirming the importance of having the condensate occur on a

* HlO-55001-6 Moisture Sensing Chip, Harris Semiconductor

Products Division, Melbourne, FL 32901.

76
hydrophilic surface for conductivity-type moisture sensors. The

occurrence of organic contamination that leads to the hydrophobic

condition of a chip surface is probably more likely to occur in a seam-sealing or furnace-sealing operation, which is done at a relatively low temperature in a nonoxidizing atmosphere. In

Cerdip sealing, however, a higher temperature is used, which combined with the air atmosphere normally employed will remove

organic contamination [2], so that the chip surface is cleaned

during the sealing operation. Thus, the condensation should be

filmwise and a good response obtained at the dew point. It is

concluded, therefore, that dew-point-sensing chips are most reliable when used in Cerdips. If there is no response of a dewpoint-sensing chip in other types of packages, it should not be

automatically assumed that the package is dry; the lack of

response may be due to a hydrophobic condition on the chip surface.

Key Words: Contamination; dew point; hermetic packages; moisture;

packaging; water vapor.

77
REFERENCES

White, L. , and Sammons, R. E., "A Procedure for Preparing Hermetic Packages with Known Moisture Levels", this

report

White, M. L. , "The Detection and Control of Organic Contaminants on Surfaces", Clean Surfaces , G. Goldfinger, ed.

,

Marcel Dekker, Inc., Nev; York, 19 70 . Bonham, H. B., and Plunkett, P. U. , "Surface Contamination

Removal from Solid State Devices by Dry Chemical Processing", Proc. 4th Ann. Symp. on Contamination Control, Washington, D.C., 1978.

78
3.4 Cross Correlation Experiments on Different

Types of Sensors

Michael G. Kovac

University of South Florida

College of Engineering

Tampa, FL 33620

(813) 974-2581

Abstract: This paper describes experiments conducted to determine

the correlation between the aluminum oxide. sensor and the surface

conductivity sensor. It Is shovm that the correlation exists If

the onset of conduction (not the peak) Is taken as the dew point In

the case of the surface conductivity sensor. The "time response"

of both types of sensors Is described , and Its effect on Interpretation of results Is outlined.

Key Words: Aluminum oxide moisture sensor, moisture sensors, pn junction temperature sensor, surface conductivity sensor, time response of

moisture sensors.

1 . INTRODUCTION

During the past few years several techniques [1,2,3,4] have been used to measure the amount of moisture inside a hermetically sealed microelectronic package. Some of these techniques are destructive and are used on a sampling basis, while others are nondestructive. The most widely employed destructive

technique involves the use of the mass spectrometer to measure the gases es- caping from a microelectronic package punctured in vacuum. While the mass spectrometer is an Instrument that is generally considered to be a "first

principles" machine, its adoption as a primary reference measurement technique

for trace amounts of water vapor in packages has been hampered by many engineering and materials problems.

The most widely employed nond est ructive techniques for moisture detection

are shown in figure 1. The most prevalent sensors/techniques are listed in

the first three boxes: the aluminum oxide sensor; the surface conductivity

sensors; and the interelectrode capacitance-sensing technique. These three are described in more detail below. Under the general category of "other sensors

and techniques" are listed some less common approaches. For example, the

adsorption of infrared radiation by moisture can be utilized in a technique

called "Infrared Derivative Spectroscopy" [5]. Another technique that has

been suggested [6] is the use of a quartz crystal to detect moisture. In

this proposed technique, the quartz crystal frequency is characterized over

the temperature range of -55 to 0°C in a dry environment. The mass loading

caused by condensed water will cause a quartz sealed in a wet package to have

a different frequency versus temperature characteristic. The use of a

79
"charge flow" transistor has also been suggested [7] as a moisture sensor. Basically this device is a "gateless" MOSFET whose "turn-on" time can be characterized as a function of moisture in the surrounding ambient. Package leakage currents as a function of temperature has also been suggested [8] as a detector of hermetically sealed moisture.

Interelectrode Capacitance-Sensing Technique

Two techniques (figure 2) have been developed that make use of the interelectrode (or parasitic) capacitance between metallization lines on an IC die.

The first of these, the Philips Technique [10], measures the capacitance between two lines as a function of temperature. When water condenses on the

surface, an increase in capacitance (typically 1 pf) is observed. In the

Merrett Technique [4], the capacitance of two metallization lines is measured

at two different frequencies while the temperature is maintained constant.

The ratio of these capacitances can be correlated to the moisture content of

the package.

The Al ^O ^ Sensor and the Surface Conductivity Sensor

The aluminum oxide [1] sensor (figure 3) has been used for several years to

determine the moisture content of hermetic packages. It is currently available from Panametrics, Inc. of Waltham, Massachusetts. The interdigitated

surface conductivity sensor [2] ("dew point" sensor) is currently available

from Harris Semiconductor, Melbourne, Florida. Because of its simple structure (a pair of interdigitated aluminum electrodes on oxidized silicon)

,

many IC-manufacturing companies fabricate these in-house. In the paragraphs below the correlation of these two sensors is discussed.

2. EXPERIMENTAL SETUP

The experimental setup for the cross correlation of the aluminum oxide and

the surface conductivity sensor is shown in figure 4A. The two types of sensors were mounted on a "TO" style header along with a digital IC die.

The header was placed in a stainless steel chamber whose environment could be controlled. A cold finger was located beneath the header to

provide spot cooling of the sensors. Known amounts of moisture were generated

using a two-step dilution of saturated water vapor. The dew point generated

by the moisture generator was monitored using a General Eastern, Inc. (Watertown, Massachusetts) optical dew point hygrometer Model 1200 AP. Both input

to the chamber and output from the chamber were monitored. (Note: for clarity two hygrometers are shown; however, only one hygrometer is used in con- junction with valves.) The phase of the condensed water could be observed by

a microscope located above the window.

The Objective

The objective of the experiment was to compare the response of the surface

conductivity sensor with that of the aluminum oxide sensor in a flowing sys80
tern. The question to be resolved was: what point in the current-versus-temperature characteristic curve of the conductivity sensor corresponds to the

"true dew point"? Figure 5 is a typical response of a surface conductivity

sensor operated with 50 Vdc across it. The significant feature to note is

that the peak at -18°C correlated very well with subsequent mass spectrometer

analysis. This fact was very repeatable. Is, then, the "dew point" of the

package -18°C? As is pointed out below, this apparent correlation at -18°C was con incidental. The curve of figure 5 was typical of the result obta3.ned

with a Cerdip package. The temperature on the horizontal axis was that

measured by a thermocouple in intimate contact with the package. The package

was cooled by being immersed in a Fluor inert bath.

Figure 6 shows the response of a surface conductivity sensor (Harris) in the

moisture calibration chamber. The optical dew point hygrometer was reading

at +8°C. The aluminum oxide sensor was also reading +8°C as would be ex- pected since it was calibrated in the chamber. Note that the Harris sensor

begins to show a major increase in conduction at +8°C. In this case, the

temperature was measured using a pn junction on the header. Therefore, it is

the onset of "conduction" in the Harris sensor that corresponds to the actual

dew point*; this is to be contrasted to the peak current which correlated

with the mass spectrometer. It is important to note that the temperature

scale in figure 5 should probably be shifted to the right by 5°C or 10°C

to account for the temperature difference between the surface of the sensor and

the outside of the package. The temperature differential between the chip

surface and the outside of the package (or ambient) is a function of the

thermal impedance path. For metal packages with soldered or eutectlcally

bonded die, this differential could be a few degrees (or even less) , whereas in

a ceramic package or a hybrid package with multiple substrates the differential

could be 15 °C.

PN Junctions as Temperature Sensors

When measuring the surface temperature of a chip using a diode as the sensor,

problems can arise. Refer to figure 7. The diode is usually calibrated in a dry environment, resulting in a calibration curve as shown in the left of the

figure. Usually a constant current (e.g., 10 yA) is used to calibrate it.

Once moisture condenses on the surface, however, there exists the possibility

that a shunt path is established. This would result in an incorrect determination of surface temperature. The most reliable data would, therefore, be

those taken as the package is initially cooled down.

3. PRELIMINARY TESTS ON SEALED PACKAGES

Metal lids were sealed unto the headers (figure 4B) containing the Harris sensor

* Note: The conventional concept of "dew point" when applied to small

cavities having absorbants, limited amounts of water and walls at different

temperatures needs to be clarified [11],

81
and the Panametrlcs aluminum oxide sensor. Varying amounts of moisture had

been sealed into these packages. It was not possible to measure the dew point

of these packages due to the water condensing on the metal lid before on the sensor. It is very difficult to isolate thermally the lids of all metal

packages. Unless the surface of the sensor reaches the dew point before the

other surfaces of the packages, no water will be available for establishing

leakage currents in the sensor. (This is not a problem in the moisture

calibration system described in section 2 above, since there is an "infinite" supply of moisture available.)

Time Response of the Al^O ^ Sensor

The problem of "time response" of the Al^O^ sensor is entirely different from

the "time response" of the surface conductivity sensor described above. Inherently, the aluminum oxide sensor has a time constant of a few seconds at

worst. However, when sealed in a small cavity, the "observed" time response

can be markedly different [9] - even up to days long! Figures 8 and 9 show

the time response of aluminum oxide sensors in hybrid packages that were

either plunged into a 100°C oven (figure 8) or taken out of a 100°C oven (figure 9) . In the case of the "cool down" mode, it was over 4 days before the

devices had returned to their initial condition. Further work in cross-correlation of the surface conductivity sensors and the aluminum oxide sensors

inside sealed packages is in process.

LIMITS OF DETECTION FOR SURFACE CONDUCTIVITY SENSORS

Sensors (Harris type, Merrett Technique, etc.) utilizing the mechanism of

condensed water on their surface require a precise knowledge of the temperature and pressure to convert the readings to ppm^ (via nomograph)

A Cerdip package is sealed in an oven operated at 400 to 500°C. At room temperature its interval pressure is less than one atmosphere as determined by

the gas law:

T^ is the temperature at which sealing took place. T2 is the measured temperature where condensation begins. P]^ is the pressure in the cavity at the

time of seal (one atmosphere), and P2 is the pressure in the cavity at T2

The temperature at which the seal takes place is not precisely known since

it usually takes place in a belt furnace with a temperature ramp. In addition, there is an uncertainty in the measured "dew point".

Figure 10 shows that if there is a + 2°C spread in T2 and a + 25°C spread in

T]^, the calculated ppm^ can range from 3,400 ppm^ to 6,000 ppm^.

For low values of moisture, the surface conductivity sensor relies on the fact

82
that the condensed can exist in a supercooled state. As a practical matter, water will not stay liquid much below -15 or -20°C, although -30°C has

been observed in other situations. Figure 11 shows that if we assume -15 °C

is the supercooled limit, a Cerdip packa.ge (with a P2 pressure of 0.35 atmospheres) will have a minimum detectable water limit of 4,800 ppm^. The value

for a -20°C supercooled assumption is 2,800 ppmy. The conclusion is that the

"dew point" sensor will not be useful if the moisture to be detected is below

these values. Although the current maximum allowable level is set at 6,000

ppm^, it is felt that this will be reduced in the future as reliability

specifications become more stringent.

5 . REMARKS

It has been shown in a calibration chamber that the surface conductivity sensor correlates with the aluminum oxide sensor if the point of the onset of conduction is used as the "dew point". The shape of the current curve after the

conduction process has begun is dependent on (and indicative of) several factors

the rate of cooling; the thermal path seen by the surface conductivity sensor;

the package interval surface condition, etc.

It was also pointed out that the ultimate limit of water that can be detected

by the surface conductivity sensor is determined by the lowest temperature that

will permit supercooled water to exist.

ACKNOWLEDGMENTS

Portions of this work are supported by funds from Rome Air Development Center

under the Post-Doctoral Research Program (F30602-78-C-0120 and contract

F30602-80-C-0168)

.

83
REFERENCES

1. R. W. Thomas, "Moisture, Myths and Microcircuits" , Proceedings 26th

Annual Electronics Components Conference, San Francisco, CA, April

26, 1976.

2. M. G. Kovac, "Performance Characteristics of AI2O3 Moisture Sensor

Inside Sealed Hybrid Packages", ISHM Symposium, Baltimore, MD, October

24-26, 1977.

3. R. K. Lowry, C. J. Van Leeuwen, B. L, Kennimer , and L. A. Miller, "A Re' liable Dry Ceramic Dual In-Line Package", International Reliability

Physics Symposium, San Diego, CA, April 19, 1978.

4. R. P. Merrett, S. P. Sim, and J. P. Bryant, "A Simple Method of. Using

the Die of an Integrated Circuit to Measure the Relative Humidity

Inside Its Encapsulation", Proc. International Reliability Physics Symposium, Las Vegas, NV, April 8-10, 1980, pp. 17-25.

5. P. R. Bossard, "Moisture Measurement Using Infrared Derivative Spectroscopy", this report.

6. R. W. Thomas, private communication.

7. S. D. Senturia, C. M. Sechen, and J. A. Wishnewsky, "The Charge-Flow

Transistor: A New MOS Device", Applied Physics Letters, 3^ (2),

January 1977, pp. 106-108.

8. S. Zatz, "A New Simplified Method to Measure Moisture in Micro Enclosures", Martin Marietta internal report.

9. M. G. Kovac, D, Chleck, and P. Goodman, "A New Moisture Sensor for ' InSitu' Monitoring of Sealed Packages", Proceedings of the 15th Annual

International Reliability Physics Symposium, Las Vegas, NV, April 13,

1977.

10. N. Bakker, "An In-Line Measurement of Moisture in Sealed IC Packages",

Phillips Telecommunications Review, _37, pp. 11-19, 1979.

11. J. Gordon Davy, "Thermodynamic and Kinetic Considerations of Moisture

Sorption Phenomena", this report.

84
TYPES OF IN-SITU MOISTURE SENSORS/TECHIJIQUES

A1203

THIN FILM SENSOR

SURFACE CONDUCTIVITY

SENSOR

INTERELECTRODE CAPACITANCE

SENSING

OTHER SENSORS S TECHNIQUES

• PANAMETRICS • HARRIS

• MOSFET HICROLAB

IN-HOUSE

• PHILIPS TECHNiaUE MERRETT TECHNIQUE

Figure 1

INTERELECTRODE CAPACITANCE SENSING

• INFRARED

• QUARTZ

PACKAGE LEAKAGE

• CHARGE FLOW TRANSISTOR

ABOVE "CONDENSATION" TEMPERATURE

WATER

BELOW "CONDENSATION" TEMPERATURE

THE PHILIPS TECHNIQUE

& THE MERRETT TECHNIQUE

Figure 2

PADS

• DEW POINT SENSOR

( SURFACE EFFECT )

AI2 O3 SENSOR

( VOLUME EFFECT )

SECTION A-A

Figure 3

85
EXPERIMENTAL SET-UP FOR CORRELATION OF MOISTURE SENSORS

Optical Dewpoint Moisture Sensors Hygrometef? and Temperature Diodes Mounted on Header

Optical Dewpoint

Hygrometer*

Figure 4

TYPICAL RESPONSE* OF INTERDIGITATED

SURFACE CONDUCTIVITY SENSOR"

86
RESPONSE OF HARRIS SENSOR IN MOISTURE CALIBRATION CHAMBER

+H0 +20 0 -20 -to

SENSOR' TEMPERATURE, c'

Figure 6

PN JUNCTION AS TEMPERATURE SENSOR |

550 600 650

Forward voltoge, mV

Figure 7

87
SEALED PACKAGE SENSOR READINGS

VS, TIME "HEAT UP" MODE

TIME, MIN.

Figure 8

SEALED PACKAGE SENSOR READINGS

VS. TIME "COOL DOWN" MODE

1) days

Figure 9

88
UNCERTAINTY BANDS CAUSED BY ± 2 C SPREAD IN T2 AND ± 25c

SPREAD IN T-^ (sealing TEMPERATURE)

NOMOGRAPH FOR OEWPOINTS a PPmy ASA FUNCTION OF P

Figure 10

LIMITS OF DETECTION FOR CONDENSATION TYPE SENSORS

ASSUME: SUPERCOOLED HoO LIMIT IS -20 C

PACKAGE press. MINIMUM DETECTAT CONDENSATION ABLE WATER (in POINT TEMPERA- PPMy) TURE (in ATM.)

0.39* 2,800 2,m

1,00 laoo

ASSUME: SUPERCOOLED H2O LIMIT IS -15 C

PACKAGE PRESS. AT CONDENSATION

POINT TEMPERATURE (IN ATM.

)

MINIMUM DETECTABLE WATER ( IN PPMy)

0.38' 0.13"

1,00

4,800

4,000

2,800

PRESSURE CALCULATED ASSUMING GLASS SEAL FUSED AT 400^C. PRESSURE CALCULATED ASSUMING GLASS SEAL FUSED AT 300 C.

Figure 11

89
3.5 Moisture Sensors, Mass Spectrometry, and MIL Standards

John C. Hale Victor Fong

Panametrics, Inc.

221 Crescent St.

Waltham, MA 02154

(617) 899-2719

Texas Instruments Inc.

Military Products Group

P.O. Box 6448 MS/3028

Midland, TX 79701

(915) 685-6708

Abstract

This paper describes a series of experiments, where two types of

"in-situ" moisture sensor chips were studied, installing them in

three ceramic glass-sealed package styles. Both vitreous and nonvltreous sealing glasses were used. The subsequent data were analyzed in terras of mass spectrometry results, and performed in accordance with MIL-STD-883B, Method 1018, Procedure 1. An aluminum

oxide moisture sensor chip was found to offer a straightforward

correlation. Due to difficulties encountered in the procedure of

data acquisition, it was not possible to determine a similar correlation when using a surface conductivity-type moisture sensor chip.

Nonetheless, the surface conductivity chip was found to be useful

in terms of identifying wet packages and dry packages, even though

quantification of moisture levels is difficult if not uncertain.

The reasons for measurement ambiguities of the surface conductivity

sensor as a system are discussed with projections for future work

necessary. Each moisture measurement system is discussed in terms

of its respective leak detection capability. A nonvitreous seal

glass was found to cause relatively high failing moisture levels,

while two vitreous sealing glasses were found to give dry packages.

Key Words: Aluminum oxide sensors; Cerdip; Cerpak; leak detection; mass spectrometry; Method 1018; moisture sensors; surface conductivity sensors.

Moisture sensor chips appear to be an attractive means of determining moisture

levels inside microcircuit packages [1,2,3,4,5]. They offer a means of

internal process control, which is a more time- and cost-effective alternative

to mass spectrometry. In varying degrees of ease, they also offer leak detection capability [5]. Although there are differences in initial chip costs, the

staff of Texas Instruments' Military Products Group is intent upon studying the

overall effectiveness of two commercially available moisture-sensor chips in a comparative way. Since Method 1018 of MIL-STD-883B requires correlation with

mass spectrometry results where the onus is upon the user, it seemed reasonable

to get a first-hand conception of how easily a mass spectrometry correlation

can be accomplished for each of the subject type chip sensor system.

The experimental procedure is shown in the flow diagram of figure 1. The

objective of the procedure is to obtain three sets of mass spectrometry data.

The first would provide a means to measure the moisture content of each package

with an aluminum oxide sensor chip and by mass spectrometry. The second set of

data would have mass spectrometry measurements and surface conductivity chip

measurements of the same packages. The third set of data would have only mass spectrometry measurements, but a dummy chip of equivalent volume, and surface area would be present in each package for that set of data.

1. INTRODUCTION

2. EXPERIMENTAL PROCEDURE

90
Connnon to all three sets of data would be what we refer to as a J-PAK, a 14-pin Cerdip using KC-1 sealing glass; a W-PAK, a 14-pin Cerpak, using a type 7589 nonvitreous sealing glass; and finally, a WA-PAK, another 14-pin

Cerpak of similar internal volume as the latter at 0.01 cm^, but externally

smaller than all three packages using KC-lM sealing glass. The 14-pin

Cerdip J-PAK had an internal volume of 0.03 cm3. Five consecutive date

codes would be involved.

The readings of the aluminum oxide sensor were taken on a commercially

available admittance amplifier, which excites the sensor at a 1-V

peak square wave at 770 Hz. Such readings are performed in a matter of a few seconds. The packages were then analyzed by Oneida Research Services,

Inc. A calibration curve was generated from six of these post-analyzed

packages. No effect of package style was observed in the calibration curve The calibration was performed using a gas flow dilution system similar to

the type used by the National Bureau of Standards, where the sensors are

?-xposed to precisely knora dew points to which values of the commercially

available readout instrument can be read and recorded [6]. The previously

described meter readings, which were in arbitrary units, can then be

converted to dew point or parts per million by volume (PPM^) at any pressure.

Intended only as a further cross check, the dummy units without sensors were also measured by mass spectrometry.

At this writing, the mass spectrometry measurements on the group with

surface conductivity sensors have not as yet been completed due to some re-evaluation of sensor readings which appears necessary. The measurement

of the surface conductivity sensors was performed strictly in conformance

with the manufacturer's instructions and guidance. Basically, each package

has to be heated and cooled in a fluorocarbon bath at a controlled rate

with a thermocouple attached to the package. A 50-V dc bias is applied

to the sensor by means of solder connections. During the cooling program

of the package, a current peak or inflection is sought corresponding to a value of temperature observed on an X-Y recorder. Ignoring set-up time,

it takes at least 15 minutes to accomplish the cooling procedure. The

observed temperature • is neither the chip temperature, nor the psychometric

dew point.

3. SUMMARY OF RESULTS

Results to date are summarized in table 1. The J-PAKs, using KC-1 sealing

glass, and the WA-PAKs, using KC-lM sealing glass, both of vitreous type, are dry. The W-PAKs, using a 7589 nonvitreous glass were found to be weto

There are two sets of "actual" mass spectrometer measurements, which are mean values of a set of measurements performed on the sampling of dummy

packages and an identical sampling performed on packages containing aluminum

oxide sensors. In the light of these mass spectrometer measurements, the

columns identified as "predicted by sensor" may seem redundant. Mean

values of dew points were calculated from each package style measured by

an aluminum oxide sensor. The predicted value by mass spectrometer is

then determined by the previously reported work of Finn and Fong for

comparison of results with "this experiment", in order to project upon

what could be used in the absence of mass spectrometry^ measurements \vere

performed on the same packages [5]. Ranking of each package style and

glass appears contradictory in the case of the WA-PAKs. The contradiction

will be explained later. The "N.D." values shown ly the surface

91

371-525 0-82-7
sensors indicate the packages are dry, but also reflect the difficulty

encountered at quantifying results, which will be explained later. The

19000 PPM-y value predicted by the surface conductivity sensors was determined

by the mean values of the temperature at where the current peaks, using the

manufacturer's recommended method of data analysis,

4. DISCUSSION OF RESULTS BY ALUMINUM OXIDE SENSORS

Figure 2 shows a curve which summarizes the results of all the same packages

measured both by mass spectrometry and by aluminum oxide sensors. Points

on the bottom are all data points measuring one or less PPMv at one atmosphere

by sensor, and are not considered in the weighting of the best fit smooth

curve. The dotted curve is from the data of Finn and Fong previously

reported [5]. Both curves show that in order for any sensor to work measuring

moisture at or below 25°C in the vapor phase, it must be able to resolve

-32°C dew/frost point in order to pass/fail at 5000 PPM mass spectrometer

V

test level. Again, it can be seen that the W-PAKs with their devitreous

7589 sealing glass are wet, compared to the KC-1 and KC-lM glasses used in

the J-PAKs and WA-PAKs, respectively. There are two outliers appearing as crosses (X) for the J-PAKs, The topmost outlier indicates that a failure

in the 6000 PPMv range belongs in the distribution. In similar fashion,

another outlier is also indicating that a mass spectrometer value higher

than the distribution shown in the curve at about 3000 PPMv belongs in

the J-PAK distribution. Inspection of the mass spectrometer data in

table 2 of the dummies shown one J-PAK at 6200 PPMv and another at 3060

PPM . The sensors ranked the WA-PAKs driest of all packages, challenging the

corresponding measurements made on the same packages made by the mass spectrometer. Again, inspection of the mass spectrometer measurements made on table 2 shows that on another occasion, the mass spectrometer would be in better agreement with the sensor's evaluation, partially explaining the contradictory results shown in the summary of table 1. These results are indicative of the

problems one might encounter when measuring moisture values in the neighborhood of 1000 PPM where standards at that level are not yet available, and

where package voYumes are at the 0.01 cm-^ level.

5. DISCUSSION OF RESULTS BY SURFACE CONDUCTIVITY SENSORS

When using the surface conductivity sensor technique on the wettest packages,

the W-PAKs, the observed temperatures where the current peaked ranged from

-HOC to +29°C, where the median fell at about +70°C as shown in figure 3.

According to the manufacturer's methodology, this would correspond to a 19000 PPMv mass spectrometer value. With such measurements and only having

separate sets of mass spectrometer measurements, it was difficult to make

any reasonable judgement as to what value of temperature peak corresponded

to a failure, knowing that the majority are at least statistically failures.

The results with relatively dry packages were still another matter^ An

examination of figure 4 shows no peak down to -40^0. Reversing the cooling

process by heating shows a peak always occurring at or around 0°C. This,

by the way, is a test for open circuit conditions, when no peaks were found.

Thlo observation shows that the sensor requires condensate to form in the

liquid phase to show an inflection or peak. The reverse, dotted curve shows ^ that the melting frost or ice will not yield supercooled water, and that

frost or ice will not provide the conduction necessary. There are still

other considerations. The thermal resistance between the exterior ceramic

92
package bottom and the top active surface of the chip is the highest of

any of the three-dimensional directions of the package o This means that

the chip surface would be the last to cool. Consequently, and especially

in a dry package, all other interior surfaces would become parasitic to

those few monolayers of water necessary for the chip to conduct, assuming

the better situation that the condensate is water. Spot cooling could

be an answer to such a problem, and is presently under investigation with

preliminary results appearing quite promising [7]. Still another consideration that needs to be addressed is the fact that supercooled water is

a nonequilibrium condition, where ice or frost can form at any temperature

below 0°C, with the likelihood escalating very rapidly below -20°C. How

manageable this situation might be could be better answered if more precise

knowledge of the chip temperature were provided by a sensing element such

as a calibrated diode ^ Obviously, many other mechanisms might be better

understood if more precise knowledge of the chip temperature could be

provided.

The surface conductivity sensor, even against the background of present

difficulties, can still be considered useful in determining a wet package

versus a dry package. With further refinement in technique and/or design,

the surface conductivity sensor concept could very well approach the quantification capability of other methods

o

6. SENSORS AND MASS SPECTROMETRY IN LEAK DETECTION

Scarcely visible is the fact that the military would waive the necessity . of a fine and gross leak test if a moisture sensor chip were installed in

a microcircuit to be delivered [8] . All packages in this experiment were

screened for "leakers" beforehand. Yet, the aluminum oxide moisture

sensor found "escapes" ©r units that had escaped detection. The symptoms

are immediately recognizable as a relatively high and unsteady meter

reading on the admittance amplifier previously described. Leakers can be confirmed, if not quantified in relationship to standard techniques,

by simply exposing the package connected to the meter to some ambient of

extreme moisture or absence of moisture, such as the inside of a dry box.

In table 3, the sensors were exposed to an environment of dryness of -7U0C

dew point o In a matter of hours, the meter readings reached the terminal

values shown in the table, which were above the equilibrium value; for a -74°C dew/frost point. Reversing the process and exposing the sensors to

a 1G°C dew point ambient, the meter readings increased to a value approaching

that dew point in a matter of hours. Unit number 3 in table 3 remained

unchanged, since it was not a leaker either by sensor indication or by

standard helium leak test. Unit number 2 is a leaker, although unconventionally defined by the sensor, and disputed by a conventional leak test,

which indicates otherwise. It can be speculated that leak tests which

include pressurization can cause leaks or plug them. Some of the corresponding mass spectrometer data are shown to give some insight into what

parameters measured by mass spectrometry could distinguish leakers from

nonleakers. In this particular case the mass spectrometer did not obviate

leakers. In fact, the presence of Freon was only indicative of unit number

3's process history. The same can be said of the rare gasses data which

also confirm the unit's process history.

There were insufficient data to evaluate the surface conductivity sensor's

capability as a leak detector. It can be assumed, however, that the surface

conductivity sensor can detect leaks by means of moisture intrusion.

93
7 . CONCLUSIONS

Aluminum oxide moisture sensors can be easily correlated in a straightforward

manner to mass spectrometer measurements o The original correlation work of

Finn and Fong was essentially repeated and confirmedo

Surface conductivity sensors are useful in that they can distinguish a wet package from a dry package. The actual levels of moisture so measured

can be difficult to quantify o With further systems and technique refinement,

which was found to be necessary, the surface conductivity sensor as a concept might very well approach the quantification capabilities of other

systems and techniques,, There is reason to believe that when properly

used, aluminum oxide sensor chips are at least as accurate as any present

method for measuring moisture levels inside microcircuit packages.

The results of this experiment indicate that any sensor chip utilized to

measure the vapor phase moisture level inside a microcircuit package at

or below 25°C has to be able to resolve at least a -320C dew/frost point

in order to determine pass/fail results corresponding to a 5000 PPM mass

spectrometer measurement.

In using moisture sensors to satisfy Method 1018, an option in lieu of showing

correlation with mass spectrometry should be offeredo This may take the form

of a guardbanded dew point of, for example, a -40°C dew/frost point for

packages where the data reported are applicable.

A nonvitreous type sealing glass, 7589, was found to be excessively wet

in terms of packages, and as a result of this, it is no longer considered

for use in Tl's Military Products Group. The continued use of KC-1 and

KC-lM, two sealing glasses of determined dryness, is being further evaluated

and considered.

The aluminum oxide moisture sensor was found to be an effective leak detector.

The same could be true of the surface conductivity sensor, but in that case,

such opportunity did not present itself.

94
8 . REFERENCES

1. Kovac, M.G., Chleck, D., and Goodman, P., "A New Moisture Sensor for

"In-Situ" Monitoring of Sealed Packages", Proceedings of the 15th

Annual International Reliability Physics Symposium, Las Vegas, NV,

April 13, 1977.

2. Kovac, M.G., "Performance Characteristics of AI2O3 Moisture Sensors

Inside Sealed Hybrid Packages", ISHM Symposium, Baltimore, MD,

October 24-2 5, 1977.

3. Meyer, D.E., "Miniature Moisture Sensors for In Package Use by the

Microelectronics Industry", Reliability Physics Symposium, Las Vegas,

NV, April 1975.

4. Lowry, R.K. , Miller, L.A., Jonas, A.W., and Bird, J.M., "Characteristics

of a Surface Conductivity Moisture Monitor for Hermetic Integrated

Circuit Packages", 17th Annual Proceedings, International Reliability

Physics Symposium, San Francisco, CA, April 24-26, 1979.

5. Finn, J.B., and Fong, V., "Recent Advances in AI2O3 "In-Situ" Moisture

Monitoring Chips for Cerdip Package Applications", Proceedings of the

18th Annual International Reliability Physics Symposium, Las Vegas, NV,

April 8-10, 1980.

6. Semiconductor Measurement Technology : ARPA/NBS Workshop V, Moisture

Measurement Technology for Hermetic Semiconductor Devices, NBS Spec.

Publ. 400-69 (March 1978).

7. Bakker, N. , "In-Line Measurement of Moisture in IC Packages", Phillips

Telecommunication Rev. Edi. , Volume 37, No. 1, pp. 11-19.

8. MIL-STD-883B, Test Methods and Procedures for Microelectronics,

August 31, 1977, Method 5008.

95
EXPERIMENTAL PROCEDURE

J-PAC

14-PIN CERDIP

KC-1 GLASS

V = 0.03 cm' I

W-PAC

14-PIN CERPAK

7589 GLASS

V - 0.01 cm'

i

WA-PAC

14-PIN CERPAK

KC1 - M GLASS

V = 0.01 cm'

3 PKGS./WK. WTTH SURFACE CONDUCTIVrTY SENSOR CHIP

3 PKGS./WK. WITH ALUMINUM OXIDE SENSOR CHIP

3 PKGS./WK. WITH DUMMY IC OF EQUIV. VOL. CHIP REPEAT 5X TO YIELD 5 CONSECUTIVE DATE CODES

SEAL AT

460°C., AIR SAME FURNACE

MEASURE PKGS.WITH

A UO, SENSORS

R6A

MEASURE PKGS. WITH

SURF. COND. SENSORS —|rga I" 'DUMMY PKGS. RGA - H

Figure 1

103

SENSOR /MASS - SPECTROMETER

CORRELATION

AFTER FINNBlFONO

• WA KC-IM.

I02 io5 10^ io5 MASS SPECTROMETER ANALYSIS AT IO0°C.. PPMv

Figure 2.

SURFACE CONDUCTIVITY

MEASUREMENTS

W-PAC

^ +7 M 19,000 miv

~1 1 ~-I 1 1 ^ 1 r~

100 80 60 40 20 0 -20 -40

TEMP., '»C.

Figure 3.

SURFACE CONDUCTIVITY

MEASUREMENTS

J - PAC AND WA - PAC

N = 14 ^o-c.

I I

* I

I '

/

I I

/ i ^ HEATIM

' \ ^ omm /' \.

100 80 BO 40 20 0 ^^^0 ^

TEMP.,°C.

Figure 4.

96
MEAN MASS SPECTROMETER VALUES IN PPM«

PKG. STYLE

ACTUAL,

PKGS. WITH DUMMIES

ACTUAL

PKGS. WITH

A1,0, SENSORS

PRH)OH) BY A, D, S»SOR PREDICTED BY

SURF. CONDUCTIVITY pa FINN-FONG PER THIS EXP. SENSOR

J 2054 (2) 1402 (1) 950 1020(2) N.D. W 10787(3) 11 200 (3) 11 000 12 300 (3) 19 000(3) WA 931 (1) 1725(2) < 550 (1) N.D.

Table 1

PKG. STYLE PPMv PKG. STYLE PPMv PKG. STYLE PPMv

J(x) 393

987

X = 931

6200/ W» 5207 WA * 1413

3060'' 5362

1766 1990

1636 18700 1268 8555 2140 21654 IT- =512

1978 2761 543 4161 1855 2577/ X = 10787 1693 cr = 7907 1745

1336 S MASS SPECTROMETER ^

MEASUREMENTS, DUMMIES

= 2054

= 1301

Table 2

LEAK DETECTION OF W-PACs

BY VARIOUS METHODS

A1,0, SENSOR METER READINGS HEUUM

LEAK!

RATE AMT-CM-/I

MASS SPECTROMETER PPMv

UNfT

NO. AS

REC'D

IN - 74''C

D.P. ATM.

In + 10*C

D.P. ATM. PdESSURE

TORR. ARBON He FREDN

1 1.1 • 0.354 0.44 41.6x10' 0.18 8570 NO ND

2 0.44* 0.322 0.42 1 xlO* 0.18 9157 NO ND

3 0.22 0.22 0.22 1 xlO* 0.18 8948 ND 3587

'UNSTEADY METER READINGS

tALL UNTTS PREVIOUSLY PASSED

FINE AND GROSS LEAK TESTS.

Table 3

97
3.6 Dew Point Moisture Measurements

by

Bert A. Unger and Peter R. Bossard

Bell Laboratories

600 Mountain Ave.

Murray Hill, NJ 07974

(201 ) 582-2555 or 3189

Abstract: Moisture measurements by the dew point technique have been

reported by several investigators. Recording either the capacitance or leakage between interdigitated lines as a device is cooled results in a curve that rises steeply, peaks out, and falls rapidly as the temperature

is lowered. Data showing the strong dependence of the dew point measurement on cool-down rate and temperature sensor positioning are presented.

Data on the accuracy and sensitivity of the leakage current and capacitance

technique using a laser machined sensor are presented.

Key Words: Capacitance; cooling rate; dew point; leakage current.

' I. INTRODUCTION

Many failures of integrated circuits (IC's) in hermetic packages have been

attributed to corrosion of metallization or device leakage by high levels of

water vapor in the package [1,2]. Attempts to come to grips with this problem

have been hampered by the inability to measure repeatably and accurately the

quantity of moisture inside small hermetic packages. Without such a measure,

reliability factors and allowable package leak rate cannot be accurately

established.

There has been considerable effort devoted to developing techniques for

measuring the quantity of moisture in packages. Three techniques have generally

been used for this purpose: 1) analysis of the package atmosphere by a mass spectrometer [3], 2) measurement of a moisture-sensitive and suitably

calibrated device characteristic [4,5], and 3) determination of the dew point

by observing the effect of vapor condensation on surface leakage or capacitance

[6-9].

This paper reports on some studies of moisture determination by the dew point

technique by both dc leakage and capacitance measurements.

As moisture-laden air is cooled, a saturation temperature is reached where

condensation begins - called "the dew point temperature". Once this dew point

temperature has been determined, the partial pressure of the moisture in the air

and the other properties of the mixture at any temperature can be obtained from

standard psychometric charts and tables. The determination of the dew point

98
relies on a physical process - a phase change (condensation of vapor) , rather

than the calibration of some sensor parameter. This feature of the dew-point

technique is very attractive since it measures the parameter that is most

significant in characterizing the corrosion susceptibility of the silicon chip

enclosed in the package - namely, the temperature at which moisture condenses

on the chip surface. Since the dew point defines the vapor pressure of water,

the quantity of water vapor contained in the enclosure and, therefore, available

for reaction at the chip can be determined. Note that this does not require

knowledge of the total pressure inside the package. For these reasons, our interest and that of others in the industry, concerned with the reliability of

microcircuits enclosed in hermetic packages, has been focused on the dew-point

technique

.

II, DEW-POINT MEASUREMENTS

Dew-point determinations are made by cooling a sensor consisting of

interdigitated lines, as shown in figure 1, while measuring either the

capacitance or leakage current of the sensor. Condensation of moisture on the sensor initiates a sharp increase in leakage current, since the condensate is

a much better conductor than the sensor surface, or a rise in capacitance since

the dielectric constant of the condensate is up to 60 times that of air. With

continued cooling of the sensor, the curve rises to a peak and then drops

rapidly. The drop is associated with the freezing of the condensate on the sensor surface. A typical plot of temperature versus leakage and capacitance

is shown in figure 2. These plots were made in succession with a package

open to room atmosphere. The dew point of the ambient is indicated on the graph

and the good agreement between the measured ambient dew point and the rise in

the capacitance or leakage curves is evident. There has been some industry

reports that associate the dew point with the peak of the curves. The curves

of figure 2 and the visual observation of condensate forming when the curve

starts to rise are confirmation that the start of the increase in leakage or capacitance is associated with a dew point. The peak of the curve is associated

with the quantity of condensate, the sensitivity of the instrumentation, and the

thermal characteristics of the system. The peak may be several tens of degrees

centigrade below the actual dew point. Although we know that the dew point is

associated with the rise in the curve, the precise point to be selected as the

dew point is not well defined. The moisture-measuring technique described by

Bossard and Mucha will be used for this purpose [10]

.

III. MEASUREMENT APPARATUS AND SENSORS

The dew-point measuring system consists of:

1 ) A thermal-electric spot cooler that contacts the package and selectively

cools the package in the area of the sensor. The cooling/package system is

flushed with dry N2 to prevent condensation on the package exterior and the

possibility of the outside condensate influencing the measurement. The dewpoint temperature is either measured on the interior surface or on the

cooling stud and is related to the internal surface temperature as will be

discussed below.

99
2) Power supplies, timers, and temperature instrumentation to provide

programmed package bakeout and cool down.

3) Leakage instrumentation with a measurement capability of 10 "'"^ amperes

and an ac capacitance meter with measurement capability of 10""'"^ farads.

4) A programmed control system that ties the above system together, and

provides automated operation and hard copy plots of the dew point curves.

The dew-point measuring system is programmed to raise first the temperature of

the package to 70° C and then to lower the temperature at a programmed

cooling rate. The initial high-temperature operation is an initialization

procedure for all packages.

Three types of sensors have been used in this program; 1) A sensor on silicon

consisting of photolithographically defined interdigitated lines made by the

Harris Semiconductor Corp. was used primarily for leakage measurements and some capacitance measurements. The long lengths of lines, large spacings (several

mils) between lines, and high parasitic capacitance of this chip made it less

desirable for capacitance measurements than the other sensors. 2) An

interdigitated pattern laser machined from the die attach pad in the bottom of

hermetic packages was used for many tests. Lines and spaces of this pattern

were approximately 0.5 mil. The pattern was stitch bonded to the package bond

pads for measurement. 3) The third type of sensor was the same laser machined

interdigitated pattern as above except it was produced on a special lid. The

lid consists of several cofired layers of ceramic with feed-through lines

printed on some cofired surfaces that provide access to the internal pattern

from the outer surface. The lid permits measurement of moisture in packages

that contain IC chips bonded as if it were a standard production package.- Figure

3 is a photo of an hermetic package with a laser machined sensor and figure 4

is a diagram of a ceramic lid with a similar sensor.

In figure 5 we have plotted the rate of cooling of the sensor as a function of

temperature for two different power inputs to the thermoelectric coolers. Curve

A had the largest initial power input to the coolers and had the largest cooling

rate; curve B had a reduced power input to the coolers and, therefore, had a reduced cooling rate. Both of these cooling rates are reproduced when the input

power levels are reproduced. Figure 6 has the difference in the temperature at

the sensor (Tp) and the top of the cooling stud (Ts) (Ts - Tp) plotted as a function of the sensor temperature. Curve A in figure 6 results from the

package being cooled with the cooling rate shown by curve A in figure 5. At

-30°C there is a 16° difference between the top of the stud and the sensor. For the slow cooling rate the difference is 12°C. This clearly points out the

necessity for measuring the temperature in the package or calibrating it as a function of cooling rate. In all our measurements of dew points, the cooling

rate was measured as a function of temperature. Curve B in figure 5 is typical

of the measured cooling rates in our dew point measurements; using curve B in

figure 6 the temperature of the sensor is always known within 2°C.

Figure 7 has the typical change in capacitance of a laser scribe moisture sensor

as a function of temperature at various frequencies 20, 40, 70 100 KHz in an open package. All these curves would indicate the same dew point within 2°C.

The dew point in the room shown by the vertical line is about -6°C.

100
Conclusions

Data have been presented that demonstrate the strong dependence of the indicated

dew point in an hermetic package on the cool-down rate. Therefore, the temperature at the sensor must be measured or suitably calibrated in order to determine

accurately the initial dew point in the package. Instrumentation is available

that allows routine measurements of dew points down to -30° C in hermetic

packages using either leakage current or capacitance techniques. When

calibrated, the dew-point technique provides a quick, easy method for determining

the humidity level in an hermetic package in a nondestructive manner.

101
REFERENCES

1. A. Ertel and H. Perlsteln, "The Hermeticity Hoax," Proceedings 1978 Annual

Reliability and Maintainability Symposium.

2. A. Shumka and R. R. Piety, "Migrated-Gold Resistive Shorts in Microcircuits,

13th Annual Proceeding^, Reliability Physics, 1975.

3. R. W. Thomas, "Moisture Myths and Microcircuits," 2 6th Electronic Components

Conference Proceedings, 1976.

4. L. E. Miller and I. M. Mackintosh, "Reliability of Silicon Transistors and

Diodes," Conference on Reliability of Semiconductor Devices, Jan. 12 and 13,

1961.

5. M.. G. Kovac, D. Chleck and P. Goodman, "A New Moisture Sensor for In-Situ

Monitoring of Sealed Packages," Solid State Technology, Feb. 1978.

6. S. Zatz, "A New Simplified Method to Measure Moisture in Micro Enclosures,"

24th Electronic Components Conference Proceedings, 1974.

7. R. K. Lowry, et al., "A Reliable Dry Chemical Dual In-Line Package

(CERDIP)," 16th Annual Proceedings, Reliability Physics, 1978.

8. R. P. Merrett and S. P. Sim, "Assessment of the Use of Measurement of

Surface Conductivity as a Means of Determining Moisture Content of Hermetic

Semiconductor Encapsulations," Semiconductor Measurement Technology: ARPA/NBS

VJorkshop V, Moisture Measurement Technology for Hermetic Semiconductor Devices,

NBS Spec. Publ. 400-69 (March 1978), pp. 94-107.

9. J. Sosniak and B. A. Unger, "Moisture Determination in Hermetic Packages By

the Dew Point Method/' International Electron Devices Meeting, 1978.

10. J. Mucha and P. R. Bossard, "Water Vapor Measurements in Integrated

Circuit Packages Using an Infrared Diode Laser," this report.

\

\

102
FIG. 3 INTERDIGITATED SENSOR LASER MACHINED

ON THE BOTTOM OF A HERMETIC PACKAGE.

TEMPERATURE deg .C

FIG. 2 PLOT OF THE NULLED CAPACITANCE VERSUS

TEMPERATURE AND LEAKAGE CURRENT VERSUS

TEMPERATURE.

FIG. 4 INTERDIGITATED SENSOR LASER MACHINED

ON A CERAMIC LID.

103
0 6r Or

-40 20 -10 0 10 TEMPERATURE deg C

-4

Q. -a -

-12 -

-16-40 -30 -20 -10 0 10 20 TEMPERATURE deg C AT PACKAGE SURFACE Tp

FIG. 5 PLOT OF THE COOLING RATE OF THE SENSOR

VERSUS TEMPERATURE FOR TWO DIFFERENT

POWER INPUTS TO THE THERMOELECTRIC

COOLERS.

FIG 6 PLOT OF THE DIFFERENCE IN THE TEMPERATURE AT THE SENSOR ( Tp ) AND THE TOP OF

THE COOLING STUD (Ts) AS A FUNCTION OF

SENSOR TEMPERATURE.

60i-

-20 -10 0 10 TEMPERATURE deg C

FIG. 7 PLOT OF THE NULLED CAPACITANCE OF THE

SENSOR VERSUS TEMPERATURE FOR FOUR

DIFFERENT FREQUENCIES; 20, 40, 70, lOOKHz.

104
3.7 Water Vapor f^easurenents in Integrated Circuit Packages

Using an Infrared Diode Laser

J. A. Mucha and Peter R. Bossard

Bell Laboratories

600 Mountain Avenue

Murray Hill, NJ 07974

(201) 582-3659 x3189

Abstract: The need for reliable and accurate measurements

of water vapor in circuit packages for process control and

reliability assurance is well established. A new technique

for measuring the water content in hermetic packages has

been developed. These measurements have been made on sealed

packages that have been ruptured so that the internal atmosphere is exposed to a test cell. The time-dependent

derivative spectra obtained from the test cell using a tunable infrared laser operating in the 6-ijm region are used to

determine the water vapor content in a manner that is

independent of the cell walls. The experimental technique

and the calibration proced^jre that eliminates the effect of

the test cell walls on the results of the measurements are

presented. Detection limits of 1000 ppn from packages with a volume of 40 yl have been achieved.

Key Words: Derivative spectroscopy; diode laser; humidity;

infrared; microcircuits ; moisture; reliability; water vapor.

1. INTRODUCTION

There has been considerable effort devoted to developing techniques for measuring the moisture content of integrated circuit

(IC) packages. Three techniques have generally been used for

this purpose: (1) mass spectrcxnetric analysis [1], (2) measurement of a suitably calibrated device characteristic [2], and (3)

dew-point determinations based on dc surface leakage or capacitance [3,4]. In this report, we wish to introduce a new method

which utilizes derivative infrared (6-iJm) spectra, obtained using

a tunable lead-salt diode laser, to measure the water-vapor content of hermetic packages.

2. EXPERIMENTAL REQUIREMENTS

As in the mass spectrometric analysis, the infrared method

requires that the package be ruptured and the contents exposed to an external test cell. In such cases, three dynamic processes

become active: (1) expansion of the package contents to a new

equilibrium pressure, (2) adsorption (or desorption) of water

vapor on the test cell walls, and (3) the temporal response of

the detection system itself. Each of these processes has

105
associated with it a characteristic time constant v^ich we will

define as x^, x^, and x^, respectively.

It is clear that in order to make an accurate measurement in such

a dynamic system, one desires x^ £

t^^ « t , so that the measurement is made at the new equilibrium pressure before any significant adsorption has occurred. If ^3 < x^ £ x^^ or x-|^ << T2 < X3,

the measurement is doomed to be systematically low. Of course,

if these are indeed constants, a simple correction factor is all

that is necessary to yield an accurate result, ""^-j^ falls into

this category since it depends primarily on cell geometry, temperature, and sealing pressure of the package, all of which are reasonably reproducible. Similarly, X, is a constant of the

detection system. However, the rate of adsorption (i.e., x^) can vary significantly from measurement to measurement since it

depends critically on the previous exposures of the system sur- faces and the gas phase concentration of adsorbing species in the

test cell. Thus, extensive and complex calibration procedures

are necessary when the condition ''•3 <.'^]^ ^ 2 satisfied.

Because of the dynamic nature of the measurement, a method

capable of elucidating these dynamics was designed with time

resolution consistent with the ideal condition noted above. A

one-dimensional diffusion calculation based on a model consistent

with our experimental apparatus indicates Z 0.01 s (95% of

final equilibrium) . This value is at least two orders of magnitude shorter than any adsorption rate we have observed experimentally, thus satisfying the x^ « x^ requirement. Since x ^ can be

chosen to match t-^, the technique should allow trace amounts of

water vapor in IC packages to be measured accurately and with

confidence, even though the adsorption characteristics of the

test cell may vary. In addition, the ability to monitor accu- rately the time dependence of the water vapor in the test eel],

offers the ability to determine the magnitude of any systematic

errors associated with the interplay of the system dynamics.

3. EXPERIMENTAL TECHNIQUE

Figure 1 shows a diagram of the experimental arrangement used in

this study. The collimated output of a cw PbS-|^_^Se diode laser

is passed through a 16 cm absorption cell, dispersed by a 0.5 m

Czerny^urner monochromator , and focused onto a HgCdTe infrared

detector. The laser frequency is tuned to a rotation-vibrational

transition in the 6.3 ym infrared band of water vapor by adjusting

the dc injection current level. By simultaneously inposing a

small, 1 kHz ac current (i.e., frequency modulating), and using

phase-sensitive detection techniques, the derivative of the

absorption band is detected and displayed on a recorder. Figure

2 shows a typical trace for a partial pressure of water comparable to those expected after the contents of a package are exposed to the test cell. An absorption trace at a higher pressure_|s included for comparison. The narrow spectral bandwidth

(~10 cm ) characteristic of these lasers insures extremely high

selectivity for water vapor [ 5 ] . Furthermore , under the

106
experiinental conditions typically used for analysis, the derivative arrplitude (P-P) varies linearly [5] with the partial pressure

of water vapor which sitrplifies instrument calibration.

-4 The test cell is evacuated to a pressure less than 10 Torr and

sealed. The device-under-test (DOT) is then ruptured mechanically and allowed to expand into the cell v^ere the total sample

pressure (typically, 100-300 m Torr) is measured with a capacitance manometer. The frequency of the laser is pre-set to the

maximum of the derivative signal (A in Figure 2) and the timedependent output of the lock-in amplifier is displayed on a recorder and digitized using high speed electronics. The time

resolution is limited to approximately 3 times the time constant

of the lock-in amplifier.

4. RESULTS AND DISCUSSION

The solid curves in Figure 3 show the time dependence of the peak

derivative amplitude for various detection time constants. These

were obtained v^en a 55 yl simulator volume (an integral part of

the test system) , that was in equilibrium with a continuous flow

of a standard water vapor mixbjre, was suddenly released into the

test cell. It is clear that within 10 s, two-thirds of the

water vapor initially released has been adsorbed by the test cell

walls. The dashed line represents the magnitude of the derivative signal the same mixture is continuously flowing through

the test cell at a pressure that equals the total pressure in the

cell after the simulator volume had been released. This can be

considered the true signal level since equilibration under flow

conditions is rapid. From the initial transient response when

the simulator volume is released, errors of 30% and 20% are observed for the two longer time constants; however, virtually no error is observed using a 0.03-s time constant. This clearly

demonstrates the importance of making the measurement in a time

that is small compared to the time constant associated with

adsorption. The slope of the curve in Figiire 3(C) gives the

decay rate of water vapor in the test cell which allows one to

assess and correct for any systematic error should it be evident.

None have been observed with time constants <_ 0.03 s.

The signal is calibrated in absolute partial pressure of water

vapor (or concentration if preferred) by flowing the standard

mixture through the cell at several measured pressures. The com- position of the mixture can also be determined in a similar manner using known pressures of pure water vapor as a primary

standard. Thus, measurements with this system are standardized

directly, and the accuracy does not depend on comparison with

some other system. It is important to stress that small simulator volumes are employed only to verify measurement consistency

and are not required for calibration.

Measurements made at ambient temperatures in a cell constructed

from Pyrex glass and stainless steel show a detection limit

(S/N=l) of 1000 ppm for packages with internal volumes of 40 yl.

107

371-525 0 - 82
These materials and conditions which exhibit extremely high

adsorption characteristics were chosen deliberately in order to

provide the most stringent test of the time-resolved measurement

technique. Elevated temperatures and wall coatings can be used

to reduce the rate of adsorption, and thereby relax the time con- straints necessary for an accurate measurement. Optimizing the

laser intensity and length-to-volume ratio of the test cell will

enable us to measure hundred parts-per-million levels in an accurate and reliable manner. Work along these lines is currently in

progress and a more detailed account of this method will appear

elsevhere [6]

REFEREiCES

1. R. W. Thomas, "J-ioisture Myths and Microcircuits , " IEEE Trans, on Parts, Hybrids, and Packaging, PHP-12 , 167-171 (1976).

2. L. E. Miller ^nd I. M. Mackintosh, "Reliability of Silicon

Transistors and Diodes , " Conference on Reliability of Semiconductor Devices, January 12, 13, 1961.

3. M. G. Kovac, D. Chleck, and P. Goodman, "A New Moisture Sensor for In-Situ Monitoring of Sealed Packages," Solid State

Tech., 21, 35-39, 53 (1978).

4. B. A. Unger and P. R. Bossard, to be published.

5. J. A. Mucha, to be published.

6. J. A. Mucha and P. R. Bossard, to be submitted to Anal. Chem.

108
CRYOGENIC TEMPERATURE

GRATING SPECTROMETER

FIG 1 HUMIDITY MEASUREMENT SYSTEM FOR IC PACKAGES

LASER FREQUENCY mi HMH ISSOIUTKM AMOVTKM AND DBUVATIVE TRACH KM WATK

VAMt TXAMrrOM AT 1MZ.41 CM -i

FLOWING GAS SIGNAL

A- Is B-0.3S

C- 0.03 s

4 5

TIME (sec)

FIQ. 3 TIME DEPENDENCE OF THE DERIVATIVE SIGNAL OBTAINED FROM A 56^i

SIMUUTOR VOLUME FOR THREE DIFFERENT TIME CONSTANTS (A-C). ARROW

INDICATES TIME WHEN CONTENTS WERE RELEASED INTO TEST CELL.

109
4. SESSION III PROCESS CONTROL

4.1 A Recent Evaluation of Al^O^ Moisture Sensors in Metal Hybrid Packages

Robert F. Macko

Hughes Aircraft Company

11940 Jefferson Blvd.

Culver City, CA 90230

(213) 391-0711 X7823

Abstract

Aluminum-oxide moisture sensors were installed in over 125 hybrid

packages and the interior moisture concentration was measured as a function of assembly and processing procedures. Calibration of the sensors was made before and after processing. The time residual

moisture content could only be determined after extended baking at

temperatures greater than 100°C. The sensors were found to be

effective in the resolution of problems in packaging and in process

variations.

Key Words: Hybrids; moisture measurement; oxide moisture sensors.

Aluminum-oxide moisture sensors were installed in over 125 hybrids which

contained moisture-sensitive microelectronics in a 1.25 x 1.25-inch metal

butterfly package. Each hybrid contained four LM108A operational amplifiers

on a substrate with a potential leakage path which could develop in the

presence of moisture and contamination. A leakage current of a few pico amps

would be sufficient to cause an out-of-spec performance.

Extensive effort was made to reduce the moisture level as well as contamination. The hybrids were cleaned frequently at various stages of the assembly,

cure, trim, and rework process. The moisture sensors proved to be a very

effective process control and failure diagnostic tool. They provided early

confidence in the efforts taken to obtain and maintain a maximum desorbed

moisture content of less than 500 ppm measured shortly after the sealed

hybrid was removed from a 24-h stabilization temperature bake of 125°C. The

average maximum desorbed residual moisture content was less than 300 ppm and

was achieved after production oven vacuum-bake durations of 48 to 65 h at

125°C with minimum production scheduling impact (fig. 1). The extended

vacuum-bake process was generally performed during weekends.

Rework of these hybrids provided a unique opportunity to conduct an evaluation of the sensor's data repeatability, sensitivity, and thermal stability.

Measurements made shortly after seam weld seal established a moisture level

baseline of 20 ppm at 25°C. This baseline was used to evaluate each sensor's

output and performance in the batch processing of 10 to 30 hybrids. Very

favorable moisture sensor performance results were obtained with baseline

data variations of less than 10 percent. The sensors exhibited minimal shift

in performance after exposure to the hybrid processing temperatures of 125°C

to 150°C and temperature cycling from -55°C to 125°C for an average cumulative sum of approximately 300 h. Repeated processing during rework showed

110
little effect on the calibration and the performance of the moisture sensor

(fig. 2).

The cost effectiveness of the moisture sensor was repeatedly demonstrated in

the resolution of problems in package beads and sealing, process variations,

and equipment malfunctions. The capability to measure the moisture content

quickly and simply saved many hours and days and enabled fast and sound manufacturing decisions. The extreme sensitivity of the sensor to small changes

in moisture content flagged potential leakers prior to detection by the

standard leak detection methods.

Evaluations were also made of the epoxy influence on moisture content and the

effectiveness of alternate epoxy applications and cures. The high moisture

retention characteristics of the epoxy used in earlier hybrids were confirmed

by these evaluations.

The most significant conclusion made during this evaluation and a vacuum-bake

experiment was that each hermetic hybrid would exhibit the same baseline

moisture content regardless of the vacuum-bake duration (2 to 96 h). Only

after the sealed hybrid was exposed for 24 to 48 h to a desorbing environment

of more than 100°C and less than 1 50°C could the true residual moisture

content be measured. If the hybrid's temperature exposure never exceeded

25°C, then the baseline conditions at sealing in the drybox would probably be

maintained. However, if the hybrid were to be exposed to or operated in

temperatures exceeding 100°C, then extremely high moisture contents could

develop unless extended vacuum-bake durations were employed (fig. 3).

It is highly recommended that each production hybrid prototype be instrumented with one or preferably two moisture sensors and exposed to various vacuum bake durations to determine the maximum residual moisture content.

Ill
DEW/FR05T rotNT.

fPMy AT 1 ATM Figure 2. M/S calibration curve.

il 1 1 1 1 1 1 1 M 24 0 12 24 36 48 IMC. MOUn

Figure 3. Typical hybrid moisture content profile.

112
4.2 Moisture Monitoring and Control During Assembly of LSI

Circuits via In-Situ Moisture Sensors

Saeed H. Siddiqul

Digital Equipment Corporation

75 Reed Road

Hudson, MA 01749

(617) 568-4573

Abstract: Aluminum oxide based in-situ moisture sensors have

been successfully used for on-going monitoring of moisture in

package sealing environment during the assembly of LSI circuits.

After establishing the initial correlation between the mass spectrometry and in-situ moisture sensor data, the on-going

moisture monitoring via moisture sensors provided a relatively

simple, quick, inexpensive, and convenient means for getting information regarding moisture contents in a package-sealing environment. The details of moisture-monitoring activities at two seal

plants are described and the effectiveness of in-situ moisture

sensors in providing us with information regarding actual and potential problems pertaining to moisture contents in the packagesealing environment are discussed.

Key Words: Aluminum oxide; Cerdip packages; IC assembly; in-situ

moisture sensors; LSI circuits: mass spectrometry; on-going

monitoring activity; package- sealing environment.

1. INTRODUCTION

In-situ moisture sensors have been successfully used for on-going monitoring

and control of moisture in a package-sealing environment during assembly of

LSI circuits. Aluminum oxide moisture sensing chips of recent design [1,2]

capable of providing information regarding internal water vapor content of

cerdip packages have been employed in the work reported in this presentation.

These moisture sensor chips provided us a relatively simple, quick, and

convenient means for monitoring moisture in a package-sealing environment

on a routine basis in a manufacturing environment. Initially the moisture

measurements were made using the in-situ moisture sensors as well as the mass spectrometry technique and a correlation between the moisture sensor

and mass spectrometry data was established. This correlation was used

to establish the limits for allowable moisture contents in the package-sealing

environment • and the in-situ moisture sensors were then used to monitor the

changes in water vapor content of a package-sealing environment on an ongoing basis.

A system to monitor the moisture contents in the package-sealing environment via in-situ moisture sensors at an offshore assembly house was established.

Such a system did indeed provide us useful information on a weekly basis and was a tremendous help in alerting us when this particular IC assembly house

started having problems with their one and only supplier of N2 gas. Our

continuous monitoring of water vapor contents in the sealing environment

indicated that things were not getting any better. Our suspicions were

confirmed when subsequent deliveries of N2 gas far exceeded the maximum

113
allowable moisture contents as measured by other moisture measuring instruments. This situation eventually led to a shutting down of all sealing operations at this particular assembly house until the arrival of an N2 tanker

from the United States.

Encouraged by the success of using in-situ moisture sensors to monitor the

moisture contents in the package-sealing environment at one plant, we now

have instituted similar controls at other IC assembly plants.

2. PROCEDURE

The initial correlation between moisture sensor chip measurements ' and mass spectrometry measurement was established by having a large number of moisture

sensor chips packaged according to standard cerdip assembly procedures in an actual manufacturing environment. All of the moisture sensor readings

were taken using the Panametrics hygrometer model 771MM. Calibration curves were then generated for half of these sensors and the rest of the packages

were subjected to detailed gas analysis by mass spectrometry technique. A

correlation was then established between the moisture readings from the

two techniques. After the initial correlation was established, the on-going

moisture monitoring was initiated by having the moisture sensors packaged

on a weekly basis along with the production units. The moisture sensor readings were converted to equivalent PPMy moisture levels and the results

were used to monitor the quality of a package-sealing environment on a weekly

basis. The results of such a monitoring activity for two assembly houses

are shown in figures 1 and 2.

3. RESULTS AND DISCUSSION

The moisture-monitoring activity for seal plant A shows that for several

weeks after the start of this activity the moisture contents in the packagesealing environment were fairly constant and below the maximum allowable

limit. However, beginning week #4, we started observing a trend towards

higher moisture contents. The product sealed during this week and for

subsequent weeks was put through special screening tests designed to detect

any moisture related problems. Also, the seal plant was notified of this

problem. Because of this, 1) the moisture-monitoring instrument used on the line at this plant was found to be defective and was repaired and recalibrated, and 2) the supplier of N2 gas admitted having problems with the purity

of its gas. Continued monitoring of incoming gas at the seal plant resulted

in the inability of their only supplier to supply them with quality gas.

This led to a total shut down of sealing activity at this particular plant.

The restart was only possible after supply of gas from the United States.

The quality of package sealing gas at seal plant B has been consistently

good during the time period shown in figure 2. However, lately some fluctuations have been observed and the cause (s) of these fluctuations are presently under investigation.

A. CONCLUSION

The in-situ moisture sensors, if used in a carefully planned way, do indeed

provide a valuable means for on-going monitoring of moisture contents in the

package-sealing environment. The results presented here have shown the

114
effectiveness of such a system as a means of routine process control of a package-sealing environment during the assembly of Cerdip packages. The

moisture levels as measured by this technique are basically qualitative

in nature; however, for an on-going process control type of monitoring these sensors are more than adequate to do the job.

REFERENCES

1. Kovac, M.G., Chleck, D. , and Goodman, P., A New Moisture Sensor for

"In-Situ" Monitoring of Sealed Packages, Proaeedings of the ISth Annual

International Reliability Physios Symposium, Las Vegas, NV, April 13-15,

1977.

2. Finn, J.B., and Fong , V., Recent Advances in AI2O3 "In-situ" Moisture

Monitoring Chips for Cerdip Package Applications, Proceedings of the 18th

Annual- International Reliability Physios Symposium, Las Vegas, NV, April

8-10, 1980.

\

115
I 'llltm J I I I I .„_l I I I I I I I

Figure 1. Moisture monitoring activity for seal plant 'A'.

i 300

1200

1 HtO

j

'

S 500

300

100

0 —«—-1 1 1 1

1 14 15 Ifc 1

Figure 2. Moisture monitoring activity for seal plant 'B'

116
4.3 Moisture Failures in Hybrids

E. Wes Poate

General Dynamics

P. 0. Box 2507

Pomona, CA 91766

(714) 620-7511

ABSTRACT

Moisture-related failure modes are prevalent through the industry, accounting for approximately 25 percent of all hybrid rework. However, even today

these failure modes are commonly ignored or misunderstood.

A brief overview of some of the most common sources of moisture found in

hybrids and some of the most common moisture-induced failure modes are presented. A synopsis of experiments performed in a production facility is

included

.

Key Words: Adsorption; corrosion; dew point; failure modes, hybrid

manufacturing; moisture sources.

INTRODUCTION

Many studies have been made in both industry and the government sector in

order to determine a normal industry-wide distribution of hybrid microcircuit failure mechanisms. Results of some of these studies place

contamination-related failure modes in the region of 21.4 percent [1] to

25.0 percent [2], the third most commonly encountered failure mode after

active device rework and lifted wire bonds, Figure 1. These figures, however,

show only the gross, obvious contamination, not the subnanogram quantities

which are not necessarily visible, yet undoubtably account for a significant

portion of the most common failure mode in the industry, active device

failures

.

Results of a survey of moisture sources and moisture-related failure modes

are presented, along with the results of some experiments designed to

determine the quantity of mass outgassed by an epoxy as a function of time,

determine the quantity of moisture sealed into a hybrid package, and show

the damage done to hybrids by saliva.

COMMON MOISTURE SOURCES

Six moisture sources which are found commonly throughout the industry are herein described. They are as follows:

People

Atmosphere

Adsorbed Moisture

Particulate Matter

Epoxies

Insufficient Preseal Baking

Each of these is a factor which must be considered in the fabrication and

assembly of microelectronic parts.

117
In hybrid manufacturing one of our greatest assets is also one of our severest sources of contamination, the people who make the parts. The

human body is composed of many substances which can be extremely deleterious to a hybrid microcircuit . These include acetoacetic acid, fumaric

acid, glutamic acid, as well as various fatty acids, carboxylic acids,

amino acids, and nucleaic acids; salts such as NaCl, KCl, CaCl2, and

MgCl2; and water, which composes on the order of 73 percent of the human

body. Table 1 lists all of the human bioelements.

Another source of moisture is the room atmosphere which can supply water

to the various contamination reactions as they proceed, as well as being

a source for surface adsorbed moisture, condensed moisture, and the water

found in epoxies. In order to control this moisture at a tolerable level,

a good rule of thumb is to maintain the ambient atmosphere to which the

part is exposed during fabrication, assembly, and especially testing at a humidity of less than 43 percent [3]

Adsorbed water which adheres to all exposed surfaces is another problem.

Adsorption is the preferential concentration of a species at the interface between two phases. Adsorbed moisture can be broken down into two

categories, chemically adsorbed (chemisorbed) moisture which involves a chemical interaction with the transfer of electrons between the water and

the surface, and the physically adsorbed moisture which arises from intermolecular forces involving permanent dipole, induced dipole, and quadrupole

interactions. This water is right on the hybrid surface where it can do

the most damage.

Particulate matter is considered for two reasons. First, it may contain

and/or outgas water. Secondly, it presents surface active sites which provide secondary valence forces allowing water to condense at a much higher

temperature than the normal ambient dewpoint. Appendix I. Too much care

cannot be taken in ensuring the removal of all unwanted particulate matter

from the part.

Epoxies are another significant source of moisture as well as other contaminants. Vacuum microbalance data Scotchcast 281 indicate that water and

other contaminants continue outgassing indefinitely and can be fairly well

described by Figure 2 and the following Equation [4]

m^ = ma> (1-e )

where m is the loss of mass of the material at time t, moo is the total mass

lost at t =oo, and x is the time constant with which the species outgasses

from the material. Figure 2. In many hybrid packages there is sufficient

epoxy to saturate the package with water vapor many times over [5].

Insufficient vacuum baking of the package prior to sealing is another

potential problem area [6,7]. Many of the failure modes we observe today

require the presence of water in order to occur. If the water is removed

from the package, the part degradation will either stop immediately or gradually consume any remaining secondary or catalytic contaminant and then come to a halt. Thus, the purpose of baking the parts, prior to sealing in an inert atmosphere, is to remove as much moisture as possible from the

package walls, epoxies, etc. The duration of the baking cycle becomes

118
critical since many hours are required to remove both the adsorbed and

absorbed water from the part and package surfaces and any epoxies that may

be present in order to ensure that the final package environment is con- ducive to failure-free operation.

MOISTURE-RELATED FAILURE MODES

Three of the most commonly encountered failure modes are as follows:

Migration of conductors

Inversion of active devices

Corrosion of components

Each of these can easily account for the failure of a hybrid and requires

or is enhanced by the presence of moisture.

Metallic migration requires a potential difference between two adjacent

conductors, the presence of ionic contaminants, and the presence of water.

With a bias applied to the conductors the ions in water enable the conductor

to migrate from the more negative conductor to the more positive, causing

a short circuit between these conductors. A static potential has been

found to provide sufficient bias to enable migration to occur in gold. The

migrated conductor shorts form a characteristic dendritic pattern and are easily recognizable, Figure 3. Only minute quantities of ionic contaminants

are necessary to cause this failure mode. Gold, for example, requires less

than 10~^ grams/cm^ of KCl with a 12 volt applied bias in order to migrate.

Active device functions can also be affected by moisture. Water in the

presence of sodium will supply a hydrogen atom which will rapidly diffuse

through the passivating layer (s) of the semiconductor and form a charge

plane at the Si-Si02 interface (8), Figure 4. The manner in which it

affects the semiconductor depends on the nature of the device and the quantity of contaminant, however, increased leakage currents, failure of FETs

to enhance or deplete properly, or total lack of function are not uncommon.

Corrosion of conductors and resistors is not uncommon. The manner in

which the corrosion occurs may vary with the other contaminants present,

but the end result is generally fatal for the part. One of the most

commonly observed forms of corrosion is the action of chlorine and water

to oxidize the metal, in which chlorine acts as a catalyst and the reaction

proceeds until either all of the conductor or all of the water is consumed.

Figure 5.

SUMMARY

Keeping the hybrid as dry as possible during fabrication, assembly, and testing cannot be overly emphasized. This helps protect the hybrid from migration, inversion, and corrosion, as well as resistor drift, and various

galvanic reactions. To protect the part during its lifetime, a lengthy

vacuum bake followed by an inert atmosphere hermetic sealing of the package

(with no interim exposure to any humid environment) will help to ensure

that the part has a fighting chance to function as it was designed to without failing due to contamination in the future.

119
ACKNOWLEDGMENTS

Many thanks to Harold Koenigsberg, Bill Smiley, and Jack Spoor for their

support and especially to Bob Thomas for his invaluable technical guidance.

1. A. Koudinaris, A. Kamensky, and I. Pratt, "Active Device Prescreening

for Hybrids," IEEE Transactions on Components, Hybrids and Mfg. Technology; Vol. CHMT-2, No. 2, pp. 247-254, June 1979.

2. H. M. Waldron, III, L. F. Buldhoup, "Hybrid Technology Cost Reduction

Reliability Improvement Study," Boeing Aerospace, II, March 1978.

3. M. Antler, et al., "Effect of Environment on Electrical Contacts: A

Discussion at the 1973 Holm Seminar," IEEE Transactions on Parts, Hybrids

and Packaging; Vol. PHP-11, No. 1, pp. 57-67, March 1975.

4. J. Blanco, E. W. Poate, and R. Viswanathan, Internal research, HAC

IDC 7621-14/27, Sept. 19, 1979.

5. A. Czanderna, R. Vasofsky, and K. Czanderna, "Mass Changes of Adhesives

During Curing, Exposure to Water Vapor, Evacuation, and Outgassing,"

Proc. 1977 Int. Microelectronics Symp., Baltimore, MD, October 24-36,

1977, pp. 197-208.

6. R. Thomas, "Moisture, Myths, and Microcircuits , " IEEE Transactions on Parts, Hybrids, and Packaging; Vol. PHP-12, No. 3, pp. 167-171, Sept.

7. R. Thomas, "Microcircuit Package Gas Analysis," IEEE Reliability Physics

Symposium, 1976, pp. 283-294.

8. S. Hof stein, "Proton and Sodium Transport in Si02 Films," IEEE Transactions

on Electron Devices, Vol. ED-14, No. 11, Nov. 1967, pp. 749-759.

9. C. J. King, "Separation Processes," McGraw-Hill, New York, 1971.

10. A. W. Adamson, "Physical Chemistry of Surfaces," John Wiley & Sons,

New York, 1976.

11. G. G. Hill, Jr., "Introduction to Chemical Engineering Kinetics and

Reactor Design," John Wiley & Sons, New York, 1977.

REFERENCES

1976.

120
Figure 1. Distribution of hybrid microcircuit

failure mechanisms (1)

.

^ 15

BAKE-OUT TIME (HRS)

Figure 2. Mass of Scotchcast 281 outgassed as

a function of time for 75 hours at 150<^C in a vacuum microbalance at 15^ Torr (4)

.

121
Figure 3. Gold dendrites on

a microwave integrated circuit.

Figure 4. A depletion mode

Field Effect Transistor.
Figure 5. Cracked passivation and

semiconductor corrosion.

BIOELEMENTS

0 OXYGEN

c CARBON

H HYDROGEN

N NITROGEN

P PHOSPHORUS

S SULPHUR

Na+ SODIUM

K+ POTASSIUM

Mg+2 MAGNESIUM

Ca+2 CALCIUM

ci- CHLORINE

Mn MANGANESE

Fe IRON

Co COBALT

Cu COPPER

Zn ZINC

1 IODINE

Table 1. Bioelements found in the

human body.

123

371-525 0-82-9
APPENDIX

FORMATION OF A NEW PHASE - NUCLEATION

There appears to be some confusion in the industry with reference to the

term dew point. The dew point refers to the temperature at which a given

moisture quantity in the vapor phase will autonucleate, coalesce upon

itself to form a droplet without the intervention of a foreign substance.

Since the Gibbs free energy for the formation of a cluster incorporates

surface energy if a foreign solid is present, it may provide sites for

condensation requiring less surface energy by partially substituting a solid-liquid interface for a liquid-vapor interface, ignoring effects of

secondary valence forces.

In the formation of the moisture phase, in the absence of foreign surfaces,

the normal sequence of events is as follows: small clusters of molecules

form and grow by aggregation to the point of becoming recognizable droplets which may finally coalesce to yield massive amounts of the new phase.

Observation indicates that this sequence of events does not take place if

the vapor pressure is only slightly over the saturation value. This

impedance of the new phase is associated with the extra surface energy of

small clusters that make their formation difficult (10).

With a suitable solid foreign surface, however, the vapor phase may condense

a vapor pressure less than the saturation value. If the surface energy at

the liquid-solid interface is less than that for the liquid-vapor interface, then nucleation will more readily occur at the solid.

In the absence of surface tensional effects, the free energy of formation

would be given by the free energy to transfer n moles from the vapor phase

to the liquid phase,

AG = nkT (AHv/R)

1T T

A-1

A liquid drop, however, possesses a surface energy equal to the surface

area A^y = Anr^ multiplied by the surface energy Y^y is now

AG = -nk (AH^/R)

1T T

V L + Anr^Yj^^ . A-2

Figure A-1 shows the variation of AG as a function of drop size (10).

If a foreign surface is the site of condensation, then two changes must be

made to equation A-2. First, the liquid phase no longer forms a sphere,

so that 4nr^ will be replaced by Ajjj as the surface area between the

liquid and vapor phase. Secondly, a factor must now be considered for

the area and surface energy of the solid liquid interface which is now

present. The AG for this latest addition is Ag^YsL*

124
AG = -nk (AH^/R) +

LV ^SL^SL A-3

Thus, as can be seen from equation A-3 with temperature and moisture content

remaining constant, the free energy of formation will vary as a function of

the interfacial surface energy of the foreign particle and the ratio of

A^^/Ag^ for a given total surface so that porosity is a factor which must

be taken into account.

Hence, condensation may not necessarily occur at the dew point, but may in

fact occur at a temperature higher than the dew point depending upon whether

the water molecules condense on themselves or a foreign surface.

Y

=

T =

A =

C, =

AH = V

k =

m^ =

nioo =

n =

R

=

r =

T, =

T = V

t =

surface energy

time constant with wich a species outgasses from a material

surface area

Chow's constant

latent heat of vaporization

Boltzmann constant

mass loss at time t

mass loss at »

moles

universal gas constant

radius

liquid temperature (°K) at which the saturation pressure

is equal to the actual pressure

,0 s vapor temperature ( K)

time (sec)

Figure A-1. Variation of AG

with drop size. Ordinate

represents 1013ag, in erg.

at 0°C.

125

0 2 4 6 8 10 12 I' Radius of nucleus, A.
4,4 Test Method 1018.2 - A Progress Report

Robert W. Thomas

Rome Air Development Center

RBRE

Griffiss AFB, NY 13441

(315) 330-4632

Abstract: As a result of a three year program coordinated by the

Joint Electron Device Engineering Council, a decision has been made

by the Defense Electronic Supply Center to implement Test Method

1018.2 of MIL-STD-883 on 2 February 1981 for JAN devices.

Correlation experiments indicate that better than —20%

transferability has been achieved between test laboratories on

sample packages with known moisture content. It is recommended that

the new Test Method should only be used as a pass or fail determinnation at 5000 or 6000 ppmv and for package volumes of .01-. 85 ccm. This recommendation is justified on the basis that correlation has

not been established at other limits. Other moisture measurement

techniques have been or are in process of being developed which

offer alternatives to the mass spectrometry technique.

Key Words: Analytical laboratories, correlation, microcircuits, MILSTD-883, moisture measurement, moisture standards.

The test method for internal water vapor content was first published 31 August

1977. Since then, through a interactive program coordinated by the Joint

Electron Device Engineering Council (JEDEC) , both government and industry have

developed a carefully controlled program of calibration and correlation for

internal moisture.

During the early correlation experiments, it became clear that ordinary CERDIP

packages, sealed under seemingly identical conditions, did not have a sufficiently tight distribution to be used for correlation moisture standards,

especially when trying to achieve +20% accuracy at 5000 ppmv. Even TO-18

packages, if used without pre-inspection for internal glass cracking, gave a distribution from 2000 to 20,000 ppmv moisture when sealed in dry nitrogen.

There was obviously something to learn in this area and so in parallel with the

TO-18 standards, the Core 14 group from the JEDEC JC13.5 Committee recommended

that the group adopt and test a dynamic calibration technique known as the

three volume calibration valve and moisture generator. The tests of this

apparatus proved most encouraging. Generating known amounts of moisture

measured with an NBS calibrated dewpoint hygrometer, procedures were developed

at each of the analytical laboratories which demonstrated calibration

transferability. As a final check on the calibration process, a large number

of TO-18 packages were sealed using special cleaning and preconditioning

operations to produce a very tight distribution of moisture. These samples were sent out to the laboratories for analysis and it was determined that by

using the three volume dynamic calibration technique tljiat the package moisture

correlation between laboratories was better than —15%. Based on these

126
calibration and correlation experiments, it was jointly recommended by the

Core 14 Committee to JC-13 that all was in order to implement MIL-STD-883 Test

Method 1018.2 on a limited basis. This was spelled out explicitly in MIL-STD883 Test Method 1018.2. In summary, it was recommended that Method 1018.2 be

used only on packages covered by the three volume generator (i.e., .01cc-.85cc)

and only at 5000 ppmv for monolithics and 6000 ppmv for hybrids. It was clearly pointed out that correlation was established only at 5000 and 6000 ppmv

and that a specification calling out moisture limits of less than 5000 ppmv

could not reference Mil Std 1018.2 as a test method, and in fact, that lower

limits were not yet accurately measurable or correlatable.

During the Summer and Fall of 1980, the Defense Electronic Supply Center (DESC)

and RADC visited the analytical labs gathering the necessary documentation on equipment and operating procedures for the purpose of granting full

suitability status to the four analytical laboratories. In parallel with these

visits, DESC dated the revised test method on 4 November 1980 and publication

is expected by 30 November 1980 which will allow implementation of the moisture

limits called out in MIL-STD-883, Method 5005 and 5008, and MIL-M-38510 on 2 February 1981.

Current work on the specification is concerned with minimizing the interlab

correlation differences when large absorbing surfaces or materials are present, such as desiccants and organics. This is especially important since

many manufacturers are using thin organic coatings to prevent soft errors

caused by alpha particles in high density memories.

One company is currently using Procedure 3 Method 1018.2 which utilizes a surface conductivity sensor mounted in the CERDIP package to measure the

moisture in the package. During correlation experiments between the mass spectrometer and the sensor, it was shown that all packages with conductivity

peaks below -20 C demonstrated less than 5000 ppmv moisture at 100 C as measured by the mass spectrometer. Therefore, the pass or fail point for this

package type became the minus 20 C conductivity maximum observed using

carefully controlled point cooling under the sensor.

Use of the aluminum oxide sensor in CERDIPS has also been established by

spectrometer correlation experiments. In this case, for a particular

manufacturer and sensor, readings of less than 300 ppmv moisture at 25°C would

guarantee moisture readings of less than 5000 ppmv at 100°C by the mass spectrometer. Rome Air Development Center (RADC) has also recommended that mass spectrometer correlation experiments with sensors from the same production lot shall not be required for similar style packages (i.e., 14, 16,

24 pin CERDIPs) which use the same sealing furnaces and sealing glass. This

is intended to minimize the cost of package qualification where the sensor application has been previously demonstrated.

Future techniques, such as the one to be presented at this workshop by R.

Merrett of the British Telecoms Laboratories, look very promising. In this

procedure, the integrated circuit chip itself is used to establish moisture

content, not only in hermetic packages but plastic encapsulated packages as well.

With combined efforts of the manufacturer, analytical laboratories, and the

customer, the program to eliminate moisture as a cause of failure will surely

succeed.

127
4.5 Hybrid Stress Testing in Wet and Dry Packages

Michael Richtarsic

Martin-Marietta Corporation

Sand Lake Road

Orlando, FL 32855

(305) 352-2685

Paper not submitted.

128
5. SESSION IV MOISTURE PHYSICS

5.1 Conceptual Model of Aluminum Corrosion of An Integrated Circuit

Allen R. Bailey

AMP Inc.

3705 Paxton Street

Harrisburg, PA 17111

(717) 780-6859

A conceptual model of a real Integrated Circuit (IC) surface has

been developed. The model is applied to the corrosion of aluminum

and the localized chemistry of aluminum corrosion. All corrosion

starts out as a localized attack at a defect site, or area of high

stress. Once started, a micro-corrosion cell forms, and the

corrosion rate is independent of surface conductance and applied

voltage.

The model can be used to develop a method for reliable encapsulation of IC devices.

Key Words: Corrosion of an IC; IC surface; localized corrosion;

surface model.

In this paper, we wish to develop a conceptual model of an IC surface that

will enable us to understand the surface chemistry of encapsulation. In our

opinion, protection of the surface interphase is the key to achieving high

reliability. The real world of an IC can be represented by the following

picture. Figure 1 is a picture of a high-speed, bipolar memory chip and

figure 2 is a close-up of the surface of the same chip. Figure 3 is a picture of a triple-track test chip and figure 4 is also a close-up of the surface of the same chip. As can be seen, the macrostructure of an IC surface

can vary widely, with many asparities and surface defects present. To make

matters even worse, we have organic film adsorbed from the environment on top

of an adsorbed water layer of unknown thickness and composition. Figure 5 is

a representation of the surface of a real chip and forms the basis for understanding the IC surface and aluminum corrosion.

We need to develop an atomic model of the surface to provide a framework for

building a corrosion model. We have adopted the representation shown in

figure 6 as an atomic model [2-6]. At the glass or aluminum surface, we have

hydroxyl groups vertically oriented and randomly distributed at a density of

3 to 7 per 100 nm^, and covered with 3 to 1 5 layers of adsorbed water and

organics [1-3]. The top 30 to 50 A of the surface is Si02, whether the substrate is silicon dioxide, or silicon nitride [4-6]. In this analysis, we will assume that the surface is on the atomic or chemical level as is represented in figure 6. Figures 5 and 6 represent the surfaces that we need to

protect.

1. INTRODUCTION

2. STRUCTURE OF THE IC SURFACE

129
The Si02 and ^^2^3 ^'toms form a pucker-hexagon. Only those hydroxyl groups

occurring at an apex can readily react. Water or hydroxyl groups occurring

in cells cannot react because they are inaccessible. Furthermore, water

residing within these cells will not be removed by high temperature and vacuum. W^ter within the cells will not contribute to surface conductivity, but

can provide an ion pathway from the bulk of the glass to the surface. Heating to 250°C will remove the bulk mobile water and some of the loosely bound

water (transition layer), but the structured water, or "ice-like," will not

be removed. The hydroxyl concentration will be reduced to 2 to 3 hydroxyl

groups per 100 nm^ [5,7]. To remove all of the water transition layer would

require even more drastic conditions and recrystallization of the surface

[9].

On the IC surface, we have an electrolyte formed from the adsorbed water and

diffusion of the Na"*", K"*", and Cl~ ions [10-15]. These ions will diffuse into

the adsorbed water and form a solution with a concentration of 10"^ to 10"^

moles/1. This is sufficient to conduct a current and form an electrochemical

corrosion cell. While these surface ions might be removed by deionized water

rinse, thermodynamic equilibriiim would result in more ion diffusion out of

the passivation layer and the re-establishment of electrolyte solution.

Cracks, pits, crevices, and grain boundaries in the passivation layer provide

pathways for ions to migrate to the surface.

If we expose the IC surface to gradually increasing humidity, we will have

water molecules bombarding the surface. Some will attach and migrate to the areas with high affinity for water (most likely the pores, cracks, and defects in the passivation layer). As the water is adsorbed, it will initially

follow a B.E.T., a type of adsorption isotherm, a = °=-m [10] where,

a = surface conductance,

°^ = is B.E.T. coefficient in volume fraction, and

m = volume adsorbed.

As more water builds up, a continuous layer is formed. The continuous layer can be a monolayer, but multilayers will form at the active or high energy

site. At a relative humidity of greater than 1 0 to 30 percent, bulk water is

present as a continuous phase on the surface. Above 70-percent relative

humidity, Si02 has a surface conductance of 0.34, which is the same as con- ductance measured for Si02 immersed in 18 megohm water [10].

3. ALUMINUM CORROSION

This section will consider the mechanism of the induction and propagation of

either pits or intergranular corrosion of aluminum. The concept of breakthrough or pit potential has not been applied to an active IC device, but the

results of corrosion studies on aluminum can be applied [15-18]. Aluminum is

sensitive to the attack of halogens such as F~, Cl~, Br~, I", and other metal

ions such as Cu"^"*", Na and K"*". One percent to four percent of copper and

one to two percent silicon are added to the aluminum to prevent diffusion and

both Cu and Si will lower the breakthrough potential of aluminum [15-18].

Figure 9 is a plot of breakthrough or pitting potential of aluminum versus concentration for Cl~ ions [15] . Note that Cl~ ions lower the breakthrough

potential and also lower the free energy for corrosion.

130
We now have all the basic ingredients for corrosion of the metallization.

The interconnects and bond pads form the electrodes, the applied voltage or metallization provides the electrical field, or potential, and the adsorbed

water forms the electrolyte. Figure 8 is a plot of the free energy versus Cl~ concentration for aluminum [13]. To have a significant effect on the free

energy of corrosion requires ion concentration reduction below 10"'* moles/1.

This is not likely to be achieved in practice. Furthermore, ions can migrate

under an applied electrical field and can create a high localized ion concentration at some point. The important thing to note from figure 8 is that

aluminum prefers to be in the ion state; however, aluminum is covered with

25 to 30 A of aluminum oxide (AI2O2) which inhibits further corrosion of the

aluminum unless damaged or missing.

To prevent corrosion of the aluminum requires that we prevent formation of

the electrolyte and/or the migration of ions. Aluminum is an amphoteric

metal and can react anodically or cathodically. Table 1 lists some of the

possible electrochemical Al reactions and their potentials [19]. While the

electrode potential, or Gibb's Free Energy, can tell one whether a reaction

will occur, thermodynamics says nothing about the rate at which the reaction

will occur. Let's look at some proposed mechanisms for Al corrosion.

4. CATHODIC CORROSION OF ALUMINUM*

[15-19]

Cathodic processes involve neutralization of the positive charge. Figure 9

is a representation of a growing cathodic corrosion pit. The applied potential has caused Na"*" and K"*" ions to migrate to an "active site" or defect on the aluminum surface where the ions have fit themselves into a missing cathodic vacancy. Here the Na"*" or K"*" ions act as a "quasi-catalyst.

1. Na"*"(g) + e~ = Na(g)

2. Na(g) + Al(g)(H20) = Na"'"(g) + Al ( g )

( . . . .OH) + H+ e"

3. 4(0H)- + Al° = Al(OH)- + 3e-^

(s) - surface species

Figure 10 represents the Al reaction site on the surface within the pit. The

Na"*" is reduced at the surface where it can instantly react with adsorbed

water to form an A1....0H complex. Once the second OH reacts, the Al is now

detached from the AI2O3 network and can be hydrated by H2O. The

Al ( OH) ^ ( H2O) 2 species has a negative charge which causes desorption and

Al ( OH) ( H2O) 2 to migrate away from the surface. As the Al(OH)^(H20)2

migrates away from the surface, the pH changes from pH 2.8 to 4-5 and decomposes.

4. Al(OH)^ = Al(OH)34' + OH"

The precipitated AKOH)^ forms a plug that isolates the pit solution from the

bulk solution. As the pit grows, the anode and cathode migrate apart and

water vapor can now condense within the pit. Bulk corrosion can be

considered to start when the anode and cathode are at infinity or at disThe cathode is the electrode where cations lose their charge. The anode is

where anions lose their charge. Aluminum can be solubilized or corroded by

either process.

131
tances equivalent to IC spacing. The bulk corrosion is electron transport

controlled while pit corrosion is diffusion controlled.

5. ANODIC CORROSION OF ALUMINUM*

[22-26,28-30]

Figure 11 represents an anodic corrosion pit.

5. AL(OH)^ + PO^^ = AlPOi^+ + 4 OH"

6. Al + 30H- = AL(0H)3 + 3e7. AL(0H)3 + 0H~ = Al(OH)^

Here Cl~ or S~ ions have migrated to an active site, where they act as a quasi-catalyst for the corrosion process by penetrating the AI2O3 surface,

and increasing the rate of hydrolysis of the Al.

The anodic reaction site is represented by figure 1 2. Cl~ ions have replaced

the OH" groups on the surface, probably by formation of Al-(0-Cl)(g)

complex liberating H"*". When the second Cl~ ion substitutes on the aluminum

atom, the aluminum is detached from the AI2O3 network and can be hydrated.

The additional Cl~ ion reacts with the hydrated aluminum and probably forms

the Al (Cl^^ ) ( H3O) ^ complex that migrates from the surface. A pH gradient

exists at the surface, and as the pH decreases from 14 to 9, in the cell

solution, the Al (Cl^ ) ( H^O) ^ decomposes and forms AKOH)^.

8. Al(CL^) (HgO)^ + 1 H2O = A1(0H)3>I' + 4C1~ + 5H+

The Cl~ ion is regenerated and the hydrogen ion can diffuse to a cathodic

site and form H2+. We now have a growing pit and the Cl~ ion is acting like

a "quasi-catalyst." The reaction rate is diffusion controlled, but the

distances are very short (50 to 100 A); therefore, the rate can be explosive.

Bulk corrosion occurs when the anodic and cathodic sites have migrated apart

and can be considered at infinity. At this point, mass transport takes over

control of the corrosion rate.

The presence of P2O5 the glass can result in formation of phosphoric acid.

9. P2O5 + 3H2O = SHgPOj^

10. HjPO^ = H2P0^ + H"'' 11. H2PO- = HPO^ + H+

12. HPO^ = P0= + H+

13. HPO= = PO^ + Al(PO^) + 3eMixed phosphates can also be formed. The electrons liberated by the phosphate can break down H2O and form 0H~, which immediately decomposes more Al.

14. 2A1 + 6H+ = 2A1+3 = 2A1+3 + 3H2 + Ge"

15. O2 + 2H2O + 4e~ = 4(0H~)

Basically, these reactions can be used to describe the process of corrosion.

Most IC corrosions start out as a localized attack at the defect site or area

of high stress. Once the attacking species such as Cl~ or Na"*" ions have

* See footnote, p. 131.

132
migrated to high corrosion initiation area, they must overcome the polarization, and diffuse to the electrode surface to initiate the corrosion process.

The ions such as CI" compete with O2 and OH" adsorbed at the electrode surface and aid in solubilizing the aluminum.

After the pit is started, micro-corrosion cells form cathodic and anodic

sites within the pit. Solvation occurs at leading edges and precipitation at

the trailing edges. Precipitation drives the reaction to the right. The

corrosion rate in the pit is diffusion controlled and is independent of surface conductance and the applied voltage. The corrosion rate is accelerated

by the Free Energy charge and the precipitation of the aluminum. Eventually

the pit becomes large enough that bulk corrosion takes over. The reaction

rate will then depend on the applied voltage.

The major effect of the applied voltage on corrosion is on the ion migration

and initiation. Once bulk corrosion starts, increasing the bias voltage can

force reactions to occur that would never occur in actual use. Acceleration

tests may not be measuring reliability because the corrosion reactions may

not be the same as those observed in the field failure even though the reaction products are the same. The acceleration factor determined from an 85°C/85-percent RH test or pressure pot test may be used for a reliability

prediction of a particular chip, process, and test procedure, but any change

in the wafer manufacturing or packaging process will affect the acceleration

factors and the changes are not predictable.

Since the same corrosion products can be formed by a number of different

processes, the failure analysis of the corrosion products will not

distinguish which process forms them, and it is possible for an anode or cathode site to form on any + or - electrode. The pit potential is formed by

the corrosion process. The migrating ion, such as Cl~ or Na"^/ controls the

corrosion chemistry. If the migrating ion is a stronger acid than aluminum,

the pit becomes acidic. If the migrating ion is a stronger base than

aluminum, the pit becomes basic.

6. CONCLUSIONS OF CORROSION OF AN INTEGRATED CIRCUIT

[32-43,45]

1. Corrosion of an IC starts initially as a pit corrosion process.

2. The applied potential or bias causes ions to migrate to a defect

site where the pitting process starts. Once the pitting corrosion

is initiated, the pitting process generates its own potential.

3. Aluminum can be corroded anodically or cathodically and these sites

can form on a positive or negative electrode.

4. The key to preventing the corrosion of aluminim is blocking the

hydration of the interphase and preventing ion mobility. The

pitting corrosion process on aluminiam can be prevented by the use of an inhibitor such as a chromate or silicate. Chemical conversion

coatings have been used to protect aluminum in salt water

environments.

133
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1. Eirich, F, R., Factors in Interface Conversion for Polymer Coatings,

Interface Conversion for Polymer Coatings , P." Weiss and C. Dale Cheever,

eds. (Elsevier, New York, 1968).

2. Holland, L. , The Properties of Glass Surfaces (J. Wiley, New York,

1964)

3. Pantelides, S, T. , Some Properties of the Oxides of the Tetrahedral

Semiconductors and Oxide-Semiconductor Interfaces, J. Vac. Sci . Technol. r4 (4), 1965 (1977).

4. Spaepen, F. , A Structural Model for the Interface Between Amorphous and

Crystalline Si or Ge, Acta Metallurgical 26 , 1167 (1978).

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6. Morrison, S. R. , The Chemical Physics of Surfaces (Plenum Press, New

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7. Arkles, B. , Tailoring Surfaces with Silanes, Chemtech 7, 713 (1977).

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Survey of Corrosion Failure Mechnisms in Microelectronic Devices, RCA

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Physics , Las Vegas, Nevada, April 1976, p. 42.

14. Lycondes, N., The Reliability of Plastic Microcircuits in a Moist

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134
16. Galvelle, J. R. , DeMicheli, S. M. , Muller, I. L. , DeWexler, S. B. , and

Alanis, I. L. , Critical Potential for Localized Corrosion of Aluminum

Alloys, NACE, Houston, p. 580, 1974.

17. Kaesche, H. , Pitting Corrosion of Aluminum and Intergranular Corrosion

of Aluminum Alloys, NACE , Houston, p. 516, 1974.

18. Wood, G. C. , Sutton, W. H. , Richardson, J. A., Riley, T. N. K. , and

Malherbe, A. G. , The Mechanism of Pitting of Aluminum and Its Alloys,

NACE , Houston, p. 526, 1974.

19. Lurie, J., Handbook of Analytical Chemistry , p. 268 (Mir Piiblishers,

Moscow, 1975).

20. Shumka, A. , and Piety, R. P. , Migrated-Gold Resistance Shorts in

Microcircui ts , 13th Annual Proc, Reliability Physics , Las Vegas,

Nevada, April 1975, p. 93.

21. Grunthaner, F. J., Griswald, T. W. , and Clendening, P. J., Migrating

Gold Resistance Short: Chemical Aspects of a Failure Mechanism, 1 3th

Annual Proc, Reliability Physics , Las Vegas, Nevada, April 1975, p.

99.

22. Anderson, W. T. , Christou, A., and Sieger, R. J., Ionic Contamination - Humidity Effects on GaAs FETs , 17th Annual Proc, Reliability Physics , San Francisco, California, April 1979, p. 127.

23. Sbar, N. L. , Bias-Humidity Performance on Encapsulated and

Unencapsulated Ti-Pd-Au Thin-Film Conductors in an Environment

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1718 (1979).

26. Uhlig, H. H. , Corrosion and Corrosion Control (J. Wiley, New York,

1971 ) .

27. Olson, D. L. , Patil, H. R. , and Blakely, J. M. , Influence of

Contaminates on the Surfaces Self-Diffusion Coefficient of Gold,

Metallurgica 6, 229 (1972).

28. Whittington, J. K. , Malloy, G. T. , Mastro, A. R. , and Hutchens, R. D.

,

Internal Conformal Coatings for Microcircui ts , IEEE Trans. Components,

Hybr. Mfg. Technol . CHMT-1 , 416 (1978).

29. Christou, A., Griffith, F. R. , and Wilkins, W. , Reliability Testing of

Fluorinated Polymeric Materials (FNP) for Hybrid Encapsulation, 1 6th

135
Annual Proc«, Reliability Physics ^ San Diego, California, April 1978,

p. 194.

30. Christou, A., and Wilkins, W. , Assessment of Silicone Encapsulation

Materials: Screening Techniques, 15th Annual Proc, Reliability

Physics , Las Vegas, Nevada, April 1977, p. 122.

31. Gregoritsch, A. J., Polyimide Passivation Reliability Study, 1 4th Annual

Proc, Reliability Physics , Las Vegas, Nevada, April 1976, p. 28.

32. Leidheiser, H. , ed.. Corrosion Control by Coating (Science Press,

Princeton, 1979).

33. White, M, L. , Encapsulation of Integrated Circuits, Proc. IEEE 57 (9),

1610 (1969).

34. Trudel, M. L., The Adhesion of an RTV Silicone Rubber Encapsulant in

Hybrid Integrated Circuits, Proc. IEEE 59 (10), 1468 (1971).

35. Johannson, 0. K. , Stark, F. O. , Vogel, G. E. , Lacefield, R. M. , Baney,

R. H. , and Flanegan, 0. L. , Wetting, Adsorption and Bonding at Glass

Fiber-Coupling Agent-Resin Interface, ASTM STP 452 , p. 169, 1969.

36. Erickson, P. W. , Glass Fiber Surface Treatment: Theories and Navy

Research, AD 426, U.S. Naval Ordinance Lab., November 1963.

37. Ishida, H. , and Koenig, J. L. , Effects of Hydrolysis and Drying on the

Siloxane Bond of a Silane Coupling Agent Deposited on E-Glass, J. Poly

Sci: Poly. Phys. Ed . 18 , 233 (1980).

38. Silicon Compounds , Petrarch Systems Inc., Levittown, PA, 1979.

39. Morgan, P., ed., Glass Reinforced Plastics , Idiffe and Sons, London

(Philosophical Library, New York, 1954).

40. Zisman, W. A., Influence of Constitution on Adhesion, Handbook of

Adhesives , I. Skiest, ed. , p. 33 (Van Nostrand, New York, 1977).

41. Plueddemann, E. P., and Stark, G. L. , Proc. SPT 28th Conf. Reinforced

Plastics , p. 21-B, 1973.

42. Plueddemann, E. P., Clark, H. A., Nelson, L. E. , and Hoffman, K. R. , New

Silane Coupling Agents for Reinforced Plastic, Modern Plastics , August

1962.

43. Plueddemann, E. P., Interface Phenomena in Polymer Matrix Composite

Material, E. P. Plueddemann, ed. (Academic Press, 1974).

44. Mittal, K. L., Factors Affecting Adhesion of Lithographic Materials,

Solid State Tech . 22 (5), 89, (May 1979).

45. Fowkes, M. , and Mostafa, M. A., Acid-Base Interactions in Polymer

Adsorption, Ind. Eng. Chem. Prod. Res. Dev. 17, 3 (1978).

136
DEC Specimen 5000X

e 2. Magnified surface of chip of figure

137
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Draper specimen 500X

Figure 3. Structure of a triple- track test chip.

Draper specimen 5000X

Figure 4. Magnified surface of the triple-track test chip of figure 3,

138
139
Bulk

,0 ^..Q ^ .^a^.^QWater;

Ice Like y,

@ © © @ ® ®

© © Q © ©

0III0 0 0 0 0 I & (Si)

^

isij (Si;

© © © ©

(17)

SiJ — Silicon Q - Oxygen O -Water

Figure 6. An atomic model of an IC surface for corrosion analysis.

140
141

Figure 9. _Breakthrough or pitting potential of aluminum versus concentration of CI ions.

143
Na"^ + e" = Na

Na + H2O = Na"^ + OH" + H"^ + E"

3 (OH)" + Al° - Al(0H)3 + 3e~

2Al + 6H"^ = 2Al"^^ + 6H2 + 3e"

Figure 10. Cathodic corrosion of aluminum.

144
145
AlCOH)^

-0.30 scE -O.AO E°scE 0.30 s.ce

1. Al + 30H- - Al(0H)3 + 3e"

2. Al(0H)3 + OH' = AlCOH)/^"

3» Al + ^ Cl" = Al(Cl)/^' + 3e"

ii. Al(Cl)£|" + 6H2O = 2Al(0H)3 + eH"^ + 8Cl

5. P2O5 + 3 H2O + 2H3P0ij

6. 2Al + eH"^ = 2Al''^ + 3H2 + 6e~

7. O2 + 2H2O + 4e" = ^1(0H")

Figure 12. Anodic corrosion of aluminum.

146

TABLE 1

ALUMINUM POTENTIAL

Cathodic E°(n)

AlO^" + 2 H^O + 3 e~ = Al + 40H~ -2.35

Al (OH)^" + 3e~ = Al + AOH~ -2.31

Al (Cl^)7 + 3e" = Al + 4C1~ -2.24

4

Al F,~ + 3e~ = Al + 6F" -2.07

D

2h'^ + 2e~ = 0.00

O2 + H^O + 4e~ = 4 (0H~) -2.07

2H2O + 2e~ = 2 (0H~) + H2 -1.77

Anodic

Al = Al^"^ + 3e~ 1.66

2 Cl~ = Cl^ + 2e~ 1.40

2H2O = O2 + 4H"'' + 2e" 1.20

2H2O = U^O^ + 2h"^ + 2e" 1.80

Na = Na"^ + e~ 2.713

K = k"*" + e~ 2.925

148
5.2 Conductivities and Electrolytic Properties of

Adsorbed Layers of Water

George B, Cvijanovich

AMP Incorporated

3705 Paxton Street

Harrisburg, PA 17111

(717) 780-5404

Abstract

A model of the mechanism responsible for failures of integrated

circuit (IC) devices encapsulated or unencapsulated is discussed.

The functioning of this model is based on the fundamental

assumption that electrical conductivity is essentially a nonlocal

process. This means that the motion of an ion is a function of the

field values met not only at the position of that ion, but also in

the immediate neighborhood of it. This fact greatly modifies

electrochemical properties in adsorbed layers and on surfaces

exposed to electrochemical interactions. From the investigations

presented in this paper, it follows that the testing techniques

practiced in the selection of encapsulants, at the present time,

should be modified. In addition, the above analysis of nonlocal

interactions can be applied to other surface phenomena as well as

to other types of bulk nongap conductivities.

Key words: adsorbed water; electrical conductivity; nonlocal process; surface conductivity; surface phenomena.

1 . Introduction

An adsorbed layer of water can form an electrolyte solution on an integrated

circuit (IC) surface either by solvating metal ions or by the presence of

contaminants in the adsorbate. The dissolved ions can carry currents that

may initiate corrosion (or become a source of electrical noise) when a potential is applied or when different metals are present. This type of mechanism

is responsible for the majority of permanent failures of IC devices, encapsulated or unencapsulated [1]. The question arises, therefore, how to prevent

this process and how to recognize the limits at which it will occur.

The usual approach to reliability testing is to expose the device to an 85-

percent RH and 85°C environment, apply a bias potential, and monitor the

change in leakage current between conducting elements of the IC package. The

measurements are then compar'sd with the data obtained at normal operating

conditions in order to determine the acceleration factors or other quality

criteria. There are a number of tables and plots in the literature on corrosion and reliability physics [1,8-10] showing the results of these

measurements.

In the theoretical field, a paper written by Cerofolini and Rovere [2]

describes an analysis of corrosion of microelectronic circuits based on simplifying assumptions centered around the mechanism of electrolytic

conduction. Most of the work in this field, however, was based on the

149
hypothesis that the adsorbed layer of water behaves as a quasi -normal fluid

in contact with a smooth surface, forming a series of monomolecular layers.

However, adsorbed water on the surface of a solid, especially on an ordered

(crystalline) surface, does not behave as an ordinary liquid, at least not at

distances as far from the surface as comparable to molecular dimensions (few

molecular diameters). In addition, the relative topographic roughness of an

IC surface surpasses by far any molecular dimensions of the adsorbed water

film. As experiments have shown, even on a very smooth surface, the

properties of adsorbed water become properties of bulk water gradually after

several molecular layers [3]. In such layers the specific conductivity has

different properties from those of electrolyte-supported ion transport.

For example, in experiments with silica gel in the presence of adsorbed

water, Anderson and Parks [2] have observed conductance due to proton hopping

as the dominant charge transfer with some 0H~ groups detached from the Si-OH

bond as dominant donors. In this model the 0H~ group is assumed to be

chemisorbed on the silicon surface, and the water layer is hydrogen bonded on

the hydroxy1 groups.

Thus, most of the electrolyte -like conduction may occur in the upper mobile

layers of adsorbed water into which solvated ions migrate through immobile

layers or in which contaminants are present.

There are other phenomena due to the transport of ions in these layers. Upon

application of bias potentials, some of the solvated ions can become

irreversibly electroplated out of the solution. This means that under

certain conditions, there may be a hysteresis effect, rather than a local

one, as was observed on aluminum metallization on Si^N^^ with 1 0-V bias

applied [1]. Hence, at some point, temperature cycling may even improve the

corrosion resistance during the test period by plating out some of the ions.

It also means that in the presence of relatively rich water adsorbate, the

corrosion effect will be slow in showing up, if the immobile layer has

adequate structure to slow down or to prevent the diffusion of ions into the

mobile layer. Therefore, we can expect a complex conductivity phenomenon

that may mask the true nature of the corrosion process at the surface of a microelectronic circuit.

In general, it is difficult to separate these effects from each other by

simple measurements of leakage currents alone in conditions of given

temperature and humidity [9]. Nevertheless, we can get a glimpse of what is

really happening if we can analyze the specific conductivity as collective

phenomena in the vicinity of a solid surface.

In the proposed model of a nonlocal collective conduction in an adsorbed

layer of water, ions move surrounded by a cluster of water molecules of nonnegligible size relative to the thickness of that layer. Such collective

motion through an electrolyte can be described by an integral equation of

motion, where the kernel function in the integrand represents the functional

relation of the mobility of ions at some point in the layer to the properties

of that layer in the immediate neighborhood of that point [7]. Thus, several

monomolecular layers of the adsorbate will play a collective simultaneous

role in the corrosion or ion transport role.

150
2. Nonlocal Collective Conduction

Conventionally, in IC reliability tests the overall conductance is measured

in presence of water vapor at some given temperature. It is assumed that a fraction of water from the vapor phase condenses on the surface of the IC

chip and can form an electrolyte. This test can be performed with a protected or unprotected chip. The conductance is defined in these

experiments by

G = a • y , (1)

s where Og is defined as the specific surface conductivity and y the

thickness of the adsorbed, condensed water layer.

The relation between the thickness of the condensed layer of water and the

relative humidity of the vapor in the test chamber is derived from the

thermodynamics of the condensation process of water vapor in contact with a solid surface. Most authors follow the expression derived first by Brunaer,

Emmet, and Teller [11], which is applicable in a certain range of relative

humidity (RH) and which relates the surface coverage as a volume fraction to

the relative humidity of the atmosphere. In this equation, the surface

conductivity = am, where a is the B.E.T. constant and m is the volume

adsorbed from the atmosphere [15]. a represents the conductivity of an infinitely thin layer on the surfaces. In this very simple form of Cg, all

the information on the state of the surface and the properties of the

nonwater-like layer is lost.

On the contrary, what we propose in this paper is to analyze Og as a function of the structure of adsorbed layers as well as a function of

temperature

This approach will also reflect its dependability on the topography of the

surface itself [16].

The specific conductivity of an electrolyte in contact with a metal is

defined as a function of thermodynamic activity a(T,AG) and the overall

mobility y(T,E), with AG representing the free energy difference between the

dissolved state and the solid state and with E representing the activation

energy for the ion mobility. Thus, we get for

O = n F a(T,AG) y(T,E) , (2)

s

where n is the ionic charge, F the Faraday constant, and y is the ion

mobility function.

The reaction activation function a(T,AG) describes the process Me- Me""*" + ne~. In the above expression, we will assume that AG is constant for a given

metallurgical system.

As shown in figure 1 [16], the proposed model consists of a real IC surface

on which there is an adsorbed immobile layer (either continuous or in

isolated islands), and a transition layer above it which separates the

immobile layer from the ordinary bulk water, the latter exhibiting

conventional properties.

151
The immobile layer of water may have a quasi-ice-like structure [6], with an appropriate activation energy for ion mobility; e.g., see figure 2.

Although some water may be trapped in the crevices due to high local

activation energy, that water will not participate in the ion transport,

since ion transport occurs in the transition and mobile layers of water.

Thus, the thrust of our attention will be on the ion mobility as a function

of the topography of the IC surface and of the structure of the adsorbed

layers of water. To incorporate this dependence of the mobility on the

structure of adsorbed water layers, we postulate that the activation energy E

has the form:

E = E (1 - (y - y. )/y ) for 0 < y < y

O 1 o o

(3)

E = E • y. /y = const. for y > y . o 1 o o

This means that at the interface of immobile or ice-like water and

transition (partially mobile) layer, E = Eq, whereas at the surface of the

solid, the activation energy has the constant value E = Eq (1 + yi/yo)«

As can be seen from this equation, the activation energy at the surface of

the solid may become approximately twice as high as the energy at the

immobile-transition interface. The quantity E = Eq • Y±/Yo

considered as the activation energy for ion mobility in bulk water (in the

layer free from surface interaction).

It is convenient to introduce the fraction m of adsorbed water on a solid

surface from a vapor phase based on the B.E.T. theory in lieu of the "y"

variable, e.g., y = am.*

The proportionality constant is obtained from the B.E.T. type [11] equation.

Since the mobile transition layer may be comparable with the size of the ion

surrounded by water molecules, bound by dipole forces to the ion, the ion

mobility is a nonlocal functional of the form

y(y) = y /K(y,y')exp - (E /kT) (

i - (y' - y. )/y ) • dy' , (4)

O O 1 o

where the distribution function K(y,y') describes how the mobility of an ion at a point y depends on the conditions at a point y' in its immediate

neighborhood.

If K(y,y') falls off rapidly outside a small domain, the integration limits can be extended to infinity. Then by expanding the function in the integrand

[7], we obtain in the first-order approximation:

y(y,T) = M (1 - y /y ) exp - (E /kT) (l - (y - y. )/y ) , (5)

O CO O 1 o

where y^. represents the kernel size or the average fraction of transition

* a is determined by measuring volume adsorbed as a function of relative

humidity or partial pressure of water vapor in the atmosphere.

152
layer characteristic of the hydrated ion. yj^ is the point at which the

water starts its transition from bound ice-like structure to transition

layer. The water is ordered by the hydrogen-bonding to the silicon surfaces

and y-i^ is the point where the water is free to move, but the motion is

affected by the surface. y^ is the point at which the adsorbed water

mobility is no longer affected by the surface and the water or ion mobility

in it is identical to that of bulk water.

Combining eqs (1) and (4), we get for the specific conductivity in the

adsorbed layer of water (or more precisely, in the condensed phase of

vapor

)

a (y,T) = a(T,AF) (1 - y /y ) exp - (E /kTP ( 1 - (y - y . )/y ) , (6)

s coo 1 o

with the thermodynamic activity a(same a(T,AF) as measured in bulk water).

To simplify the analysis, we will define a relative surface conductance in

terms of adsorbed fraction of water. G(m,T)/GQ, where Gq = G(m = m-j^ ; T; m^ = 0), i.e., Gq is essentially the conductance of the immobile

layer, with y = a, throughout the adsorbed layer (in general, a may not be

constant )

In this model, as defined in eq (1), the conductance is equal to the specific

surface conductivity multiplied by the thickness of the adsorbed layer, or G(m,T) = 1(T,F) (1 - mc/mo) exp - (E^/kT) ( 1 - (m - mi)/mo) • am. Then with Gq = a exp - (E^/kT) • am^, we get for relative conductance

G/Gq these expressions:

G/G - m/m. (1 -m/m ) exp E /kT (m - m. )/m for m < m

o i c o o l o o

and (7)

G/G = m/m. (1 -m/m ) exp E /kT (m/m ) O 1 CO o o for m > m

The initial slope of G(m,T)/GQ (figs. 4-7) is a function of ( 1 - m^/mQ) /mj^ and can be related to the size of the hydrated ion through

m^ and the structure of the adsorbed water layer through m-j^ and mQ.

3. Critique of the Proposed Model and Its Application to a Real IC Surface

A plot of eqs (6), (8), and (9) reproduces qualitatively the measurements of

surface conductance versus relative humidity made by several authors

[1,8-10,13-15]. Figure 5 is a plot of the surface conductance measured by

Koelman [15] of a thermal oxide as a function of relative humidity. The

predicted break in the relative conductance is observed. The break is the

result of the ion mobility changing from a jump or gap mobility to an ion in

solution mobility. As can be seen from the plots in figure 4, the break in

conductance is related to the form of the kernel, which is related to ion

type, effective cloud, activator energy, and the fraction of the condensed

water in the mobile layer.*

should be a constant for any particular ion type such as Na+, and

CI- etc.

153
The dependence of the relative surface conductance G(T,m)/GQ on the ratio

of m^/m^ means that the measured conductance will change slope at some critical tciq. The change in slope is caused by a change in the activation

energy for ion mobility and the mechanism of ion transport.

Christou and Wilkins [10], as well as Kawasaki and Hackerman [8], have also

observed such an effect in the surface conductance across an IC, In their

measurement for DC 96084 [10], up to 60-percent RH, E is about 0.95 to 1.1

eV, whereas at 80- to 90-percent RH, E is about 0.35 to . 0. 30 eV,

respectively. For DC 90702, E is 1.3 eV to 0.97 eV over the relativehumidity range of 20 to 95 percent.

Applying our model to the data of Christou and Wilkins [10], we generate the

plots in figures 6 and 7. If we compare these plots with the plots in figure

4, then according to our proposed model, the initial slope of the curves of

the encapsulated devices should shift over the unencapsulated device because

the encapsulant coating would decrease the mobile layer and/or the transition

boundary layer on the surface of the IC. This would change the activation

energy E and would also change the thickness of the transition boundary layer

m^. Furthermore, since we get similar relative surface conductance curves

versus relative humidity for DC 90702 and DC 96084, we can conclude that both

coatings produce similar interphases and should yield similar results in

actual service. The differences observed by Christou and Wilkins are probably related to the differences in the rate of diffusion of water through

coatings.

If we assume that the corrosion failure of IC devices is proportional to the

rate of transport of ions and that the surface conductance is directly

related to the reliability of the IC device, then it is important to

distinguish these changes in slope. This is a better indicator of

reliability than a test of longevity in an environment that is never met in

reality. It means that the acceleration factor, or factors, perhaps do not

relate to corrosion effects, but to some overall endurance properties.

Acceleration tests are only valid if the mechanism that causes the failures

is precisely the same as that observed in the real world. The action of an encapsulant layer on the mobility of ions in the adsorbed water layer is of

crucial importance. Corrosion can only be prevented by stopping or interrupting the ion mobility. If a protective compound can effectively

separate the immobile layer from the bulk water that may diffuse into a package, corrosion of the IC would be prevented, even though the potential

for corrosion is still there. Furthermore, the method permits a quantitative

rating of an encapsulant in a comparatively short time by investigating

changes in the slopes and permits quick screening of candidates for long-term

testing.

Improvement in corrosion protection could probably be achieved by

incorporating anion and cation "getter" into the encapsulant, as was previously proposed in 1974 by Taube. This would show up as a change in the

position of the knee of InG/GQ.

If the relationship between failure rate and surface conductance is not

affected by the encapsulant process or, in other words, the ion concentration

and bound layer is not changed by the encapsulant process, then the rate of

154
change in the surface conductance at a given humidity and temperature can be

used to predict the corrosion rate and can be related to the life expectancy

of the IC device. This method can be used to verify the encapsulation

process. Once the relationship between ln(G°/G) and relative humidity is

known, a test sample can be exposed to a given relative humidity. If it has

a higher ln(G°/G) than the control device, it is considered to have failed,

and the test can be related to the reliability of the device.

That the bound water layer does not adversely affect the reliability of an adhesive joint has also been discussed by Plueddemann [12]. In his paper,

Plueddemann argues that the bound or immobile water layer is crucial for good

bonding or adhesion to a rigid substrate when silane coupling agents are used

to bond the protective coating to a glass substrate. The immobile layer

functions as a sort of stress reliever.

The central fact in protecting an IC device is in preventing the mobility of

ions in condensed surface water. The function of the interphase layer

between the IC surface and the encapsulant boundary is to form an interphase

whose rheological and topographic properties are such that the mobility of

ions is greatly hampered or prevented.

In addition, this model may provide some insight into the local crystal

growth such as found on an IC surface, as shown in figure 3. If the water

layer forms islands, then at the pores or areas between these islands, a much

higher activation energy exists. Namely, at pore areas, E = Eq which can

be much greater than the activation energy E in adsorbed layers, and

therefore an increase in local activity occurs. This is due to additional

energy from local stress, defects, or crystal nucleus.

To balance the energy of the surface, a microelectrode cell forms, where ions

plate out of the islands of water and react with the substrate to form a crystal growth which releases energy into the water for transport of more

ions (see fig. 3)

References

1. N. L. Sbar and R, P. Kozakiewicz, IEEE Trans. Electron Devices ED-26 , 56-71 (1979).

2. C. E. Cerofolini and G. Rovere, Thin Solid Films 47 , 83-94 (1977).

3. J. H. Andersen and G. A. Parks, Soc. Chem. Phys. 72 , 3662 (1968).

4. A. Kiselev, Editor, Infrared Spectra of Surface Compounds (J. Wiley, New

York, 1975).

5. J. H. Anderson and K. A. Wickersheim, Surf. Sci. 2, 252 (1962).

6. S. R. Morrison, The Chemical Physics of Surfaces , p. 14 (1977).

7. C. B. Cvijanovich, Foundations of Physics 7, No. 11/12, p. 785 (1977).

155

371-525 0-82-11
8. K. Kawasaki and N. Hackerman, Surf. Sci. 10, 229 (1968).

9. F. N. Fuss, C. T. Harting, and J. M. Morabito, Thin Solid Films 43 , 189-

21 3 (1977).

10. A. Christou and W. Wilkins, IEEE 15th Annual Proc. Reliability Physics , Las Vegas, Nevada, April 1977, pp. 112-119.

11. S. Brunaer, P. H. Emmer, and E. Teller, J. Am. Chem. Soc. 60 , 309

(1938)

12. E. P. Plueddemann, Modern Plastics (March 1970); E. P. Plueddemann and

G. L. Stark, Proc. Annual Conf., Soc. Plast. Ind. (1973), 28; E. P.

Plueddemann, "Best of Conference" Award Paper, SPI 25th Annual Tech.

Conf. , reprinted in J. Adhesion , July 2, 1970, pp. 184-201.

13. A. Shumka and R. R. Piety, IEEE 13th Annual Proc. Reliability Physics , Las Vegas, Nevada, April 1975, pp. 93-98.

14. F. J. Grunthaner, T. W. Griswald, and P. J. Clendening, IEEE 1 3th Annual

Proc. Reliability Physics , Las Vegas, Nevada, April 1975, pp. 99-106.

15. H. Koelmans, IEEE 12th Annual Proc. Reliability Physics , Las Vegas,

Nevada, April 1974, pp. 168-171.

16. F. R. Eirich, "Factors in Interface Conversion for Polymer Coatings,"

Interface Conversion for Polymer Coatings , P. Weiss and G. D. Cheever,

Editors (Elsevier, New York, 1968).

17. D. Ewight, VPI & SU, private communication.

156
Air

Adsorbed Gas

157
Water: , ., Ice Like y

1

@ @ © @ @ ©

Q © © Q Q ©

0 0 6 6 0 0

(si) ki) ki> . . Sij

FIG. 2 (17)

Silicon Q ~ Oxygen O

Figure 2. Structure of water on a silicon-silicon oxide surface.

158
i
160
in i/i I— c >--t- .—

fO JZ ^

c D. (O <3-

.— -r- CJ UJ .— CTi O LiJ G; .—

I

^ llj c:

: r— O .—

1

3 r •- CD O ^

q: i-H 00 a.

rt)

c <u

a;

5. o

1—

eoue^onpuoo aoe^jns SAL^GLay ( 9/9) 5oi

161
ro3 - C t/1 ct •,- 3 to O ^ >>

+->+->

to LT) Q. -—

•r- 1—I r~- ^ LU +-> LJ LU r-H

*> ro C I— ^ O O) I >—1 ^ q: c\j

4- 3 U

O) O • S D. D.

ft

roCn

•T T

-t3T

e

E

>

-t-> ro

3Due:}Dnpuo3 aAL:jB[9y ( g/g) 6oi

162

u

•H
163
List of Notations

a (T,AF) = Thermodynamic activity

a = Porportional constant obtained from a B.E.T. equation

C = Normalization constant of kernel function K

E = Activation energy for ion mobility

E = Activation energy on the surface of the chip

P = Faraday constant

AF = Gibb's Free Energy

G = Surface conductance

G = Bulk conductance of surface layer

0

G/G = Relative surface conductance

0

K = Kernel function

N = Number of charges

m = Fraction of adsorbed water from the vapor

phase

m = Average size of the ion cluster around

an ion

m^. = Average thickness of the immobile layer

m^ = Boundary transition layer and bulk layer

T = Temperature in °K

Y = Vertical distance from surface

Y = Thickness of the immobile layer

y

Y^ = Interphase between the transition

1 ayer and bul k water

= Specific surface conductance

'^(T,E) = ion mobil ity

M = all metal

164
5.3 Microenvironments and Accelerated Testing

Michael G. Kovac

University of South Florida

College of Engineering

Tampa, FL 33620

(813) 974-2581

Abstract: This paper describes some of the problems associated

with accelerated testing of integrated circuits for moisture

related failures. The key factor is found to be maximizing

the residence time of moisture on the surface under all conditions of temperature cycling and biasing. Also discussed is

a new microelectronic chamber for establishing microenvironments.

Key words: Accelerated moisture testing; microenvironments;

moisture related failures; temperature effects on surface water.

1. Background

The deleterious effects of water trapped within the microelectronic cavity

are now well known [1] and have been reported by a number of investigators [2-8]

during the past few years. Moisture related failures include electrogalvanic

corrosion of metalization, disappearance of nichrome resistors, migrated

gold resistive shorts (MGRS) , MOS threshold shifts, etc. There is no doubt

that the reduction (or elimination) of moisture found within the cavity would

greatly reduce the reliability problems now seen. Failure analysis data collected in the past have suggested that various types of failure modes may have

a moisture threshold below which no reactions will take place. Accurately

establishing these threshold levels would greatly enhance reliability of

microelectronics. (Note: It is important to keep in mind that the effects

of contaminants, voltage, and temperature must be included in the moisturerelated failure mechanisms and thresholds.)

It is important to note that the quantitative establishment of thresholds

for failure is by no means a simple, straightforward task. For example,

studies (by Perkins, et al. [9]) have been conducted with parts that historically have had susceptibility to moisture related failures. However, these

controlled, accelerated cyclical bias-temperature tests were unable to cause failures due to moisture (in spite of their known susceptibility to it).

These results and others [10] like them suggest that either the test was too

benign to precipitate the failure, and/or other factors should have been

considered in the design of the accelerated test.

The complex nature of the role of water inside the sealed microelectronic

cavity must be better understood if progress is to be made toward establishing criteria for reliability. Among the factors that must be considered in

establishing thresholds for moisture related failures are the following:

understanding of the relative importance of "available water" versus total

water in a package. For example, due to plating porocity, water may be

absorbed in the walls of the cavity. UndeY high temperature, cycling this

water may become available for initiating failure mechanisms. A second

165
factor (discussed in more detail later) is the variation of the "sticking

coefficient" of surfaces (glassivation, bonding pads, gold plated walls,

etc.) with temperature and gas ambient mixture (e.g., hydrogen). Another

factor that must be characterized is the transportation of water and

contaminants within the cavity from one location to another. These factors

are currently being addressed and will be reported at a later date. This

present paper will discuss the problems associated with establishing accelerated tests and describe some experimental techniques that are currently

being used to investigate the problem of controlling the microenvironment

2. Acceleration Factor for Moisture

Related Failures

It is imperative that techniques be developed to allow accelerated testing

of circuits to determine susceptibility to moisture related failures. The

present bias-temperature tests [9] have been shown to be Inadequate in this

regard. The key factor in accelerating moisture related failure mechanisms

is to maximize the residence time of the moisture on the chip surface (and/or

at the site of impending failure - e.g., at a lead frame bond area). If the

water in the cavity does not preferentially condense on the chip surface,

there will be no acceleration of the failure mechanism and hence, the test

will be fruitless. As an example, if a package has a metal lid and is cooled

down in a bath or chamber, the inside surface of the metal lid would cool

faster than the ceramic parts and the chip. Therefore, preferential condensation will take place on the lid. If there is a limited amount of water

available in the cavity, all of it may condense on the lid before any has a chance to condense on the (warmer) chip surface. Thus, even if the chip is

subsequently brought below the "dew point," there may be no water available

to condense it. Obviously, under these conditions, no acceleration of

failures can take place.

A key factor in the maximizing of the residence time of moisture on the

surface is the minimization of the effect of self-heating. See Fig. 1. The

amount of self-heating for a particular chip is a function of the technology

used to fabricate that chip, the circuit design, the thermal paths of the

package, etc. As an example, the 8080 microprocessor chip (studied at USF)

as manufactured by INTEL, typically has a rise of 25-30°C over ambient. See

figures 2 and 3. This particular chip runs "hot." Even if care were taken to

cool the chip to below the dew point, exercising the chip under full power

will cause any condensed moisture to evaporate from the chip surface. This

type of phenomenon, of course, suggests that the constant bias or constant

exercising of a chip ( even under low temperature conditions ) may not be

optimal from an accelerated failure point of view.

It has been suggested that ramping the external environment temperature might

be a way of maximizing residence time. Basically the chip surface temperature behaves as a first order system. Its response to a ramp of the

environment temperature is shown in Fig. 4. The graph illustrates the fact

that a first order system with a ramp input will have a range response that

is delayed by Bti where 3 is the slope of the applied ramp and is the

thermal time constant for the path between the chip surface and the outside

of the package.

166
If internal power is now applied to the chip, the chip surface temperature

will quickly "cross over" the input ramp and therefore make any further

ramping useless. See Fig. 5.

Based on Figs. 4 and 5, a time/ temperature cycle can be designed for a given

package (i.e., for a given thermal impedance or, more specifically, for a given

thermal time constant between the input temperature ramp and the chip surface

temperature) . The period of the temperature cycle in both cases should be

adjusted so that a new cycle begins (i.e., cool-down phase) whenever the chip

surface temperature rises above that required for liquid water on the surface,

or (in the case of internal power being supplied) when the "cross over" begins

to take place.

Another approach to maximize the residence time of moisture on a surface is

to cycle the input power. Recently, Aj iki et al. [11] described a cyclic bias

temperature test tor power dissipating integrated circuits that demonstrated

the highly nonlinear nature of water residence time. In this experiment

(conducted using plastic encapsulated power transistors) , it was found that

an optimum acceleration of moisture related failures existed for certain

ratios of time-on to time-off at reduced power levels. Figure 6 shows these

results. Note that at a power dissipation level of 0.6 Watts, the failure

rate is nearly an order of magnitude higher than for the 2.0 Watt case.

The question of the minimum temperature differential between any two points

inside a closed cavity that will permit water to be pumped from one location

to the other must be answered. It should be noted that the optical dew point

hygrometer operates on a principle that causes water to be condensed and

re-evaporated into a flowing stream with a small fraction of a degree differential. This suggests that inside a package cavity, there may be sites that

are in extremely close physical proximity that may have different preferences

for water condensation due to a minute temperature differential. It is

therefore important to establish how the water can be pumped from one surface

to another within the cavity as a function of temperature gradients. If it

turns out that the minimum temperature differential required for a pumping

action to take place is less than a tenth of a degree C, it may be that even

on a large chip itself, there could be preferential pumping action. In the

long run, such information may be significant in the design of reliable VLSI

circuits

.

The question of pumping water from one location in a cavity to another takes

on even more important significance when we consider the following. Water

will preferentially condense at a nucleation site such as a crack in the

glassivation or some other defect. Contamination of the chip surface or at

the defect can thus be greatly enhanced by a water pump action. A model can be suggested that involves the transport of the contaminants to the defect

sites using moisture as the vehicle. It is very easy to visualize the

transport of species such as sodium (as Na"*") and potassium (as K"*") via this

model since all inorganic salts of sodium and potassium (example, NaCl , KCl,

etc.) are very soluble in water.

Figure 7a shows an experimental configuration that is currently being used

to determine the minimum temperature needed to pump water. Actually two of

these structures are located in the cavity. Once moisture has condensed on

the surface, the thick film heater will be used to warm the surface. Figure

7b shows a photo of the structures.

167
A dramatic example of the effect of subtle differences in temperature is

shown in Fig. 8a. The photo shows the trunk of an automobile with a pattern

on it. The pattern is due to preferentially condensed moisture. The automobile was outside in the environment overnight. During the night the

temperature dropped below the dew point. The pattern is thought to be due

to differences in condensation and nucleation due to minute temperature

differences on the trunk. These differences are caused by the heat capacity

of the underlying metal structure in the trunk. This is revealed in Fig. 8b.

3. Miniature Microenvironment Chamber

for IC Package

Figure 9 shows a sketch of the special chamber that has been developed at

USF for sealing known amounts of water vapor into integrated circuit

packages. A photo is shown in Fig. 10. The package shown represents the

results of a developmental effort in which several alternative types of

packages were investigated. The objective of the design was to have a chamber that could be easily attached to a wide variety of commercially

available integrated circuit packages, and at the same time, have a volume

that was not (markedly) larger than that of a standard package. The

package shown in Fig. 9 can be used on side-brazed packages with metal

lids, on Cerdip packages and on standard DIP packages.

The most demanding application was that of the Cerdip. The technique used

to attach the microenvironment chamber to a Cerdip is as follows. The Cerdip

package is placed in a sputtering (or thermal evaporation) system and a thin

layer of chrome-gold is deposited on the top surface. A ceramic coring

drill is then used to open a hole in the top surface. A specially modified

Cajon high vacuum nut is then soldered to the top of the Cerdip package.

All surface areas that are within the microenvironment are gold-plated

stainless steel. A specially modified gland is used as a mounting flange

for a TO-5 header containing an aluminum oxide moisture sensor. This

aluminum oxide moisture sensor is calibrated in a moisture calibration

facility. A high vacuum seal is achieved using a compressible metal gasket.

This configuration permits controlled amounts of moisture (or other contaminants) to be easily sealed inside a standard package with minimal effort.

Because of the metal gasket used in the design, a high vacuum seal is

assured. The device can be tested for hermeticity using the standard

helium leak test. The aluminum oxide moisture sensor mounted on the TO-5

header provides a method for continuously monitoring the moisture content

of the sealed environment. The package environment can be opened and

resealed as many times as required. In addition, the aluminum oxide moisture sensor can be recalibrated (if required) during the course of an investigation.

, 4 . Remarks

This paper has described so^me of the problems associated with maximizing

the residence time of mois'ture on an IC surface. This is the main

requirement for developing accelerated testing for moisture failure. Also

discussed above was a nq-vel chamber for establishing microenvironments for

individual chips.

168
5 . Acknowledgment

Portions of this work are supported by funds from Rome Air Development

Center under the Post Doctoral Research Program (F30602-78-C-0120) and

Contract #F30602-80-C-0168.

References

1. Thomas, R. W. , Moisture, Myths and Microcircuits , IEEE Trans. Parts,

Hybrids, and Packaging PHP-12 , 167-171 (1976).

2. Kolesar, S. C, Principles of Corrosion, 12th Annual Proc. Reliability

Physics , pp. 155-167, 1974.

3. Shumka, A., and Piety, R. R. , Migratad-Gold Resistive Shorts in

Microcircuits, 13th Annual Proc. Reliability Physics , pp. 93-98, 1975.

4. Paulson, W. M., and Kirk, R. W., The Effect of Phosphorus Doped

Passivation Glass on the Corrosion of Aluminum, 12th Annual Proc.

Reliability Physics , pp. 174-179, 1974.

5. Koelmans, H., Metallization Corrosion in Silicon Devices by MoistureInduced Electrolysis, 12th Annual Proc. Reliability Physics , pp. 168-

171, 1974.

6. Somerville, D. T. , The Role of Hybrid Construction Techniques on Sealed

Moisture Levels, 15th Annual Proc. Reliability Physics , pp. 107-111,

1977.

7. Thomas, R. W. , and Meyer, D. E. , Moisture in SC Packages, Solid State

Technology 17 , 56-59 (September 1974).

8. Cerofolini, G. F. , and Rovere, C. , The Role of Water Vapor in the

Corrosion of Microelectronic Circuits, Thin Solid Films 47, 83-94

(1977).

9. Perkins, K. , Licari, J., and Buckelew, R. , Investigation of Moisture

Effects on Selected Microelectronic Devices, Proc. 1978 Intl. Micro.

Symposium (ISHM), p. 125, 1978.

10. Murphy, E. R. , and Mitchell, D, E. , Investigation of RCA Test Results

for Hybrids with Epoxy Die/Substrate Attachment, Proc. 1978 Intl.

Micro. Symposium (ISHM), pp. 107-113, 1978.

11. Ajiki, J., Sugimoto, M. , Higuchi, H. , and Kumada, S., A New Cyclic

Biased T.H.B. Test for Power Dissipating ICs, 17th Annual Proc.

Reliability Physics , pp. 118-126, 1979.

12. Kovac, M. G., Performance Characteristics of Al^O^ Moisture Sensor

Inside Sealed Hybrid Packages, Proc. 1977 Intl. Micro. Symposium (ISHM)

pp. 249-252, 1977.

13. Kovac, M. G., Chleck, D. , and Goodman, P., A New Moisture Sensor for

'In-Situ' Monitoring of Sealed Packages, 15th Annual Proc. Reliability

Physics, pp. 85-91, 1977.

169
INTERNAL

MAXIMIZING WATER RESIDENCE ON CHIP SURFACE

Figure 1. Effect of self heating.

170
RESPONSE OF FIRST ORDER SYSTEM TO RAMP

TIME

Figure 4. Chip temperature response

to ramp of ambient (no Internal power)

.

NO INTERNAL POWER

TIME

Figure 5. Chip temperature re- sponse to ramp of ambient and

internal power.

1 CYCLE; H HOURS

_J I ! 100 1,000 10,000

HTF (HRS)

AFTER; AJIKI, et. al. 1979 IRPS

Figure 6. Mean time to failure of an IC as a function of time-on to time-off ratio.

17L

371-525 0-82-12
DIODE (TEIIP SENSOR)

Figure 7a. An experimental configuration to

determine the minimum temperature (difference)

to pump water.

Figure 7b. Photograph of two diagnostic devices for water pumping

within a test cavity.

172
!II1

173
HERI1ET1C mCROENVlRONMENT CHftMBER

Figure 9. Diagram for miniature microenvironment chamber for moist atmospheres,

Figure 10. External view of microenvironment chamber sealed to Cerdip.

174
5.4 Mo i s t ur e Failure Mechanism

George H. Ebel

Singer Company

150 Totowa Rd.

Wayne, NJ 07470

(201) 256-4000

Abstract: This paper reports on some observations that

were made as a result of a failure analysis on complex

hybrid mic r o c i r c u i t s . It suggests that failure modes

are different for various internal moisture level contents. In a range above 17,000 ppm semiconductors

failed. From about 6000 ppm to 17,000 ppm nichrome

resistors failed and below 1000 ppm of moisture no

failures occurred. Probably the most significant result of the observations is that, if the moisture content of the parts that had semiconductor failures had

not been measured, the failures would never have been

classified as moisture related.

Key Words: Dew point; failure; hybrid mic ro c i r c u it

;

moisture; nichrome resistors; semiconductor devices.

As a result of an investigation of failures in some complex hybrid mi c r o c i r c u i t s some interesting observations were made concerning moisture related failures. The investigation started as

a result of several hybrids that had failed on the first operational cold cycle at board testing level. These parts were found

to have the classic "disappearing nichrome resistor" problem.

Many more of these hybrids failed and an intensive investigation

was started to determine the cause of the failures.

A total of 25 hybrid m icr oc ir cu it s were selected for the investigation. Of these, 16 had failed during system test and 9 were

parts from the same date code series that had successfully passed

all systems tests. All 25 parts were tested for internal water

vapor content per MIL-STD-883 test method 1018. The parts were

then delidded and checked both visually and electrically. The

electrical testing showed no changes in the previous status,

i.e., 16 failures and 9 good parts. As expected, the 9 good parts

''-owed no visual abnormalities.

1. INTRODUCTION

2 PROCEDURE

175
Surprisingly, 8 of the 16 failures also showed no visual abnormalities. The remaining 8 parts all had considerable damage to

one or more nichrome resistors. Further testing of the 8 failed

parts with no visual abnormalities were all confirmed as semiconducuor failures. Gome of tliese parts cured themselves shortly

(the shortest time was about 48 hours) after they were opened.

The remaining parts that had semiconductor failures were restored

to working condition during high temperature reverse bias testing

Somejbut not all the semiconductor failures could be reinduced

when the parts were placed in a humidity chamber.

A review of the past histories on the 16 failures showed that

the 8 parts that had nichrome resistor damage failed on the

first powered cold cycle. The other 8 failed parts (semiconductor failure) dropped out randomly during system testing. Two of

these failed prior to the first cold cycle and one did not fail

until final system burn-in.

The data from the internal water vapor tests were rank ordered

according to moisture content. These data were plotted in figure

using different symbols for the three different categories of

failure mechanisms (no failures, resistor failures, and semiconductor failures). Figure 2 shows the dew point range for internal moisture content that produced nichrome resistor failures.

3. CONCLUSIONS

The major conclusion that can be

tigation is that unless internal

made a part of the failure analy

will not be properly reported a vations that could be suggested

drawn from this limited invesmoisture level measurements are

sis many semiconductor failures

moisture problems. Other obserby the investigation are:

1) If enough moisture is present in the hybrid

then the necessary voltage drop to cause nichrome disappearance may not be present.

2) If the moisture level is below the room temperature dew point, then the effect of the

semiconductor failure mechanism will be greatly reduced during room temperature storage.

3) Controlled tests should be run to determine

the physics involved in semiconductor failures

in the presence of moisture. Particular emphasis should be placed on understanding why

some semiconductor failures can be reestablished in the presence of moisture and others

canno t

176
Figure 1. Plot of 25

hybrid micro circuits

in rank order according to their internal

moisture content.

The failure mechanisms

are keyed on the plot.

Figure 2. The aewpoint range for internal moisture content

that produced nichrome

resistor failures.

177
5.5 Experiences in Microcircuit Moisture Problems

James R. Duffey

Hughes Aircraft Company

Centinela and Teale

Culver City, CA 90260

(213) 391-0711 x3317

Abstract: This paper contains information on two moisture related

experiences. Moisture sensors were placed in dummy hybrids to

measure moisture content. These hybrids were placed in a 125°C oven for 32 hours. The moisture sensors were monitored during high

temperature storage and after withdrawal. The behavior observed was

different than that previously observed by Kovac.(l) The presence

of a previously undetected leaker was evident by anomalous behavior

of the moisture sensor. This was later confirmed by residual gas

analysis. There appears to be no correlation between the moisture

value given by the sensor and that obtained by the residual gas

analysis. A high reliability thin film space hybrid that had been

extensively reworked was subjected to 125°C burn-in for 2500 hours,

being tested every 500 hours. After 1000 hours this hybrid developed

a leak. At the end of testing the hybrid wrs found to contain

154,000 pprn-^ of water. The hybrid still passed all electrical tests

and showed no visual signs of corrosion.

Key Words: Humidity; Hybrids; Microcircuits ; Moisture; Moisture

sensors; Reliability.

1. INTRODUCTION

Measurements of hermetically sealed package ambient atmospheres have been

conducted for 25 years (2) using mass spectrometric techniques. An excellent

review of this method applied to microelectronics was given by Thomas. (3)

In this article he mentioned the then recently introduced in-situ moisture

sensors (4) and predicted that with the use of these moisture sensors understanding of the complicated moisture climate in the microelectronic package

would be understood. The problem of moisture in microelectronic packages

has yet to be solved.

The work on moisture sensors presented here was undertaken in order to test

the water so to speak, that is, to familiarize the author with the use of

moisture sensors. The author's experiences with a very "wet" but functional

hybrid are also given. There is very little in the way of standard procedures

for a first time user of moisture sensors to follow. This situation is unfortunate because it can lead to confusing and conflicting results.

2 . PROCEDURE

Moisture Sensors

Five nonfunctional hybrids were fabricated. The hybrid consisted of a 1" x 2"

(2.54 cm X 5.08 cm) "HAC-PAC", a ceramic package with a soldered lid. A

ceramic substrate with thin film gold and nichrome metallization was attached to

the package with a strip of Ablefilm 550 and ten semiconductor chips were attach

178
to the substrate with Scotchcast 281 epoxy . The epoxies were cured for 2 hours at 150°C. A variable impedance moisture sensor, the Paname tries MiniMod HT, was mounted to the substrate with Ablefilm 550. The hybrids were

then baked for 16 hours at 125°C in a vacuum and sealed in a dry (less than

220 ppmv of water) nitrogen atmosphere. The time between curing and baking

in the oven was a half hour, much greater than the time required for the

epoxy to pick up enough water to saturate the package, but less than the

time required for the epoxy to saturate. (5) This procedure simulates the

production of a hybrid microcircuit except for the extensive (and unpredictable) handling and the accompanying contamination it would receive.

Unfortunately there is no way to quantify the contamination received in

such a manner. After sealing the hybrids were leak tested and found to have

no leaks greater than 1.0 x 10"'' std atm He cm^/sec.

During fabrication of the hybrids some difficulties were encountered in wirebonding. None of the methods tried gave results as good as can be achieved

with other commercially available semiconductor devices. The most reliable

bond was obtained with ultrasonically bonded 0.001" (.0254 mm) aluminum wire

with one percent silicon. Although the bond strength is low, it is still

acceptable under Mil-Spec standards. An optimum wire bond schedule and the

long term reliability of wire bonds made to moisture sensors are matters of

concern and will be addressed in the future.

After the hybrids were fabricated they were placed in a 125°C oven. The

hybrids were mounted in such a manner that the moisture sensor could be

monitored while the hybrid was in the oven. Measurements were made with a Panametrics model 771A hygrometer. The effects of the extra lead impedance

were small, but compensation was made for them. After sealing one sensor was found to be shorted. A sensor in such a condition indicates very wet.

After thirty hours in the oven the hybrids were removed from the oven and

allowed to cool. The moisture sensor reading during this process, and for

20 hours after removal from the oven, is shown in figure 1. After insertion

into the oven the moisture level rose, presumably due to evolution of water

by the epoxies. Panametrics (6) cautions that moisture sensor sensitivity is

reduced above 50°C so this reading may not be indicative of the true moisture

level in the package. Upon withdrawal from the oven, the moisture sensor reading dropped to the initial level except for sample B. Sample A dropped and

then rose again. An expanded view of the moisture immediately after withdrawal from the oven is shown in figure 2. The anomalous behavior of B is

more apparent here. A second leak test was given to B at this time; it showed

a leak rate of 1.5 x 10~^ std atm He cm^/sec.

The behavior of these samples is different than the behavior reported by

Kovac(l) on similar sensors. The reason for this is not entirely clear. Fong(6)

suggests extrapolating the moisture reading taken while the sensor is cooling

back to the time it was withdrawn from the oven to get an idea of the moisture

at the oven temperature. If this is done for figure 2, the moisture level will

be the same as indicated by the moisture sensor immediately prior to withdrawal.

The apparent reason for this behavior is that these samples were dry. It appears

from Kovac's(l) data that the desensitization effect is smaller for drier

ambients. This explanation is not entirely satisfactory because it does not

explain the behavior of the B sample.

179
After a week's storage at room temperature the moisture sensor reading in

Sample B had climbed to 3000 ppm-^ but the other samples remained at the low

value recorded earlier. Another leak test was done on Sample B; this time a leak rate of 3.0 x 10"^ std atm He cm-^/sec was obtained. All the samples

were then submitted to a residual gas analysis using mass spectrometrlc

techniques. The results of this analysis are shown in table I. Sample B is

definitely a leaker as is evidenced by the oxygen/argon ratio. There appears

to be poor correlation between the moisture sensor reading at 25°C and the mass spectrometrlc data at 100°C. This confirms what was found by Finn and

Fong(7) for dry samples.

Excessive Moisture

A high reliability space hybrid that had experienced a great deal of rework

during its manufacture was pulled from stores to be tested to see how the

rework had affected the hybrid's reliability. The test plan was to burn the

hybrid in at 1250C until failure. Tests were planned at 50, 125, 250, 500 and

1000 hours. The hybrid passed all electrical tests, but the leak continued to

increase each time. The hybrid was subjected to a residual gas analysis

after 2500 hours. The part contained 154,000 ppm^ water. The part was delidded

and given a visual examination. There were no signs of corrosion or plaque

formation. All nichrome resistors were measured and found to be within specification. All wirebond pull strengths were above specification. In short,

although there was an excessive amount of water in the hybrid, it continued to

function as it was designed to function.

3. CONCLUSIONS

The use of moisture sensors in hybrids needs more maturing before it can be con- sidered as an absolute measure of moisture. The moisture sensor does, however,

serve quick notice when the package in which it is contained leaks. Leak detection is an important use of the moisture sensor. This is critical, particularly

as hybrids become more complicated and are subject to increased handling during

testing

There are several questions that need clarification. Is the long term reliability of wirebonds made to the moisture sensor adequate for high reliability

hybrids? How does the die attach method affect moisture sensor readings? What

correlation between moisture sensor reading and mass spectrometrlc reading can

be made?

ACKNOWLEDGEMENTS

The author would like to acknowledge discussions with Bob Macko, Gene Anzivino,

and Dan Demeo as being useful in this study. He would also like to thank Bob

Vieth for his support of this project.

REFERENCES

(1) Kovac, Michael G., "Performance Characteristics of AI2O2 Moisture Sensor

Inside Sealed Hybrids", Proc. 1977 International Microelectronics Symp.,

Baltimore, MD, October 24-26, 1977, pp. 249-252.

180
(2) Bergsten, W. E. , and Mc Dowell, J. F., "The Internal Atmosphere of

Hermetically Sealed Components", 14th Annual National Relay Conference,

Oklahoma State University, Stillwater, OK, April 1955.

(3) Thomas, Robert W. , "Moisture, Myths and Microcircuits" , IEEE Trans, on

Parts, Hybrids and Packaging PHP-12 , 167-171 (September 1976).

(4) Meyer, D. E., "Miniature Moisture Sensors for In-package Use by the Microelectronic Industry", 13th Annual Proc . Reliability Physics, Las Vegas,

NV, April 1975.

(5) Vasofsky, Richard W. , Czanderna, Alvin W., and Czanderna, Karel K. , "Mass

Changes of Adhesives During Curing, Exposure to Water Vapor, Evacuation

and Outgassing, Part I: Ablefilms 529,. 535 and 550", IEEE Transactions

on Components, Hybrids, and Manufacturing Technology, CHMT-1 (1978).

(6) V. Fong , personal communication.

(7) Finn, J. B., and Fong, V., "Recent Advances in AI2O3 In-Situ Moisture

Monitoring Chips for Cerdip Package Applications", Reliability Physics

Symposium, Las Vegas, NV, April 1980.

181
+ A O B

C A 0

1000 PPMv

500

PPMv

I I I I I II I I I I I I I

I

I I I I 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 46 50 TIME

Figure 1 . Relative Impedance of the moisture sensor (measured with Parametrlcs

Model 771A hygroraeter as a function of time after Insertion In the oven The error bars are the 3' limits stated by the manufacturer. The approximate moisture level corresponding to the relative Impedance

la on the scale to the left, flolsture level assignments are accurate to * 25% as stated by the manufacturer.

0.10 —

0.08-

0.06-

0.04-

0.02-

0

CD A

Ll I III

12 4 5 6

I I I 20

I 30 TIME AFTER REMOVAL FROM OVEN (MINUTES)

Figure 2. The relative impeda^xe of the moisture sensor (as tneusured) by the Model 771A hygrometer) as a function of time after withdrawal front the oven. The approximate moisture level (accurate to 25Z) Is on the left. Note the rise In the B reading, later shown to be a leaker.

182
Table I. Residual Gas Analysis of Sample Hybrids

A Ru c D E

1 9nn 7n *^n J JU AnDU

Hydrogen m 7 r»U . mivJll n7'^ n n7'^ nVJ neiium 0 A . AD nu nnn uuu U

.

UUU U nnn . uuu

Methane nu nnn U . UU J nu nnn uuu U . nnn uuu nu . nnn uuu

Nitrogen o o oo . o QQ~ y O J QQy y . 79 QQ

uxygen u nnft 0 nnft uuo nn9 nu nnn

Argon 0 018 n 9 7 0 018 0

.

002 V V 0 019

0 000 0 005 0 002 0. 002 0 .004

Carbon Dioxide 0 65 0.116 0 051 0. 052 0 .080

Acetone 0 Oil 0-012 0 007 0 007 0 .009

Methyl Chloride 0 000 0.000 0 003 0 003 0 .002

Toluene 0 001 0.001 0 001 0. 001 0 .001

Xylene 0 .001 0.001 0 .001 0 001 0 .002

Methyl Ethyl

Ketone

183
5.6 Thermodynamic and Kinetic Considerations of Moisture

Sorption Phenomena

J. Gordon Davy

Westinghouse Defense and Electronic Systems Center

P.O. Box 746

Baltimore, MD 21230

(301) 765-3619

Abstract: Moisture sorption phenomena are predominant effects in

microelectronic packages, and intuition about processes occurring

inside may not be valid. In particular, the concept of dew point

can be misleading, and reporting moisture levels in terms of relative abundance (ppm^) can also be misleading. Instead, use of relative humidity and mass of water in a particular state per unit of

package volume is recommended. Models of absorption and adsorption are presented and applied to microelectronic packages to determine RH and mass as a function of temperature. The use of con- ductance/capacitance cells and alumina capacitors as in-situ moisture sensors is discussed with reference to sorption processes.

Finally, the possibility of condensation at an RH of less than 100

percent is considered for two special cases - ionic impurities on

the surface and fine cracks in the passivation layer.

Key Words: Absorption; adsorption; dew point; hygrometer; kinetics;

microelectronic package; moisture; moisture level; relative humidity;

sorption; thermodynamics.

1. INTRODUCTION

This paper will show how thermodynamics and kinetics can be used to help in

the understanding of moisture in small packages. In some cases, the results

are different from those that seem intuitive; here, the problem is that intuition based on experience with larger systems is not directly applicable.

For the packages of current interest, sorption phenomena are not minor effects,

but are the predominant effects, and a proper appreciation of what is happening

inside a package cannot be had unless they are considered.

The reason that sorption must be considered is that such very small amounts of

water are involved, as indicated in table 1. A package containing 5000 parts

per million by volume (ppmv) of water vapor and a total pressure of 1 atm contains only about 3 fig per cubic centimeter of internal volume. If this much

water were condensed into one hemispherical droplet, it would be almost impossible to even see without magnification. If the water were spread out on the

surface of a single chip (IC or moisture-sensing) , its thickness would be less

than the wavelength of light. And if it were adsorbed uniformly over the interior of the package, the thickness would only be one to ten molecular layers

thick. In such a case, the intuitive concept of condensation loses all utility, as does the idea of a frost or dew point. There simply is not enough

water present to form a condensed layer.

184
Three micrograms of water can also be absorbed by a small amount of epoxy or other organic material used inside the package. Water which is adsorbed or absorbed is water which is not in the vapor state; a great deal of confusion

in trying to compare moisture levels under different conditions comes directly

from a failure to adequately distinguish between the vapor and sorbed states.

This will be discussed at length below.

2. UNITS

Another source of confusion has been the units by which moisture levels are reported. A totally unambiguous unit of measurement is the mass of water per

unit of unoccupied package volume (e.g., yg cm-~3) . This expression, though not

often used, can be used to refer to water in any state, as long as it is made

clear to which state it refers. (For discussing the amount of sorbed water,

other suitable units would be the mass per unit of surface area or thickness

of coverage in nanometers or monolayers.) To facilitate comparisons of amounts

of water distributed among vapor, absorbed, and adsorbed states, this paper

will use micrograms per cubic centimeter of package volume for the mass of

water in each state. This unit has the dimensions of a density; the symbol p is used to represent it, but it should be understood that for absorbed and

adsorbed water, p does not represent an ordinary density.

In the literature of moisture in electronic packages, the unit mosc commuuj-y

used has been parts per million by volume, ppmv This unit has been used because the first measurements were done by mass spectrometry which measures

relative abundances of the various gases detected. The amount of each species

is reported by comparing the sum of the abundances of all gases. Mass spec

systems also have provisions to determine what the total internal pressure of

the package was before it was punctured. Thus, it is possible to calculate

what the partial pressure of each gas would have been (in the absence of sorption) , as well as what the partial pressure would have been if the internal

pressure had been 1 atm.

It is important to understand that the measure of the tendency of water vapor

to condense is its actual partial pressure in the package, not what its partial

pressure would have been if the total pressure were 1 atm, and not on the ratio

of the partial pressure to the total pressure (the relative abundance in ppmv).

This point can be emphasized by considering the relative abundance of water

vapor in the mass spectrometer system before the package is opened. The mass

spec operates at high vacuum, i.e., a residual pressure of 10~9 atm or less.

Ordinarily, the principal gas contributing to the residual pressure is water

vapor and, hence, its relative abundance exceeds 500,000 ppmv Of course, in

no way does this high value imply the presence of condensed water in the system since the partial pressure is less than 10"^ atm.

Similarly, a package might be sealed under vacuum, leaving water vapor as the

principal residual gas and, again, a moisture level in excess of 500,000 ppm^.

Nevertheless, condensation upon the cooling of this package would not begin

above a temperature for which the partial pressure of the water vapor inside

the package had risen to the saturation level.

185
As a final example, a package with a partial pressure of water vapor of 3xlO~3

atm and a total pressure of all gases of 0.5 atm contains a relative abundance

of 6000 ppmv of water. In the absence of sorption, condensation would begin

not at 0°C, but at that temperature for which the saturation vapor pressure of

water is 3x10"-^ atm, i.e., about -S^C for ice or -9°C for undercooled water.

The purpose of discussing the ppm^ unit so extensively is to demonstrate that,

while it is meaningful for reporting relative abundances of various gas species

in a package, it is inappropriate - despite its widespread usage - for reporting water vapor because the absolute value of its partial pressure is the

quantity of concern.

Another method for measuring the moisture level in a package is the conductance/capacitance cell. This device will be discussed later; here, it is only

necessary to know that the measurement involves an increase in the conductance

or capacitance of the sensor as the package is cooled and the frost or dew

point is reached. A fundamental problem with measuring the frost or dew point

of a closed system is that as water is taken from the vapor and condensed on a surface, the partial pressure drops, so the temperature must progressively

drop to get any further condensation. This situation, which is true for any

closed system even in the absence of sorption, is pronounced because of the

high surface-to-volume ratios for microelectronic packages. One's intuition

is derived from open or large closed systems; thus, while the concept of the

frost or dew point may seem obvious, for the case at hand, it is, in fact, am- biguous. Measuring, by any means, that temperature at which condensation, as opposed to adsorption, begins becomes a virtual impossibility, except for very

wet packages, and the temperature so identified depends on the experimental

details

Another approach often used is to calculate the frost or dew point from a knowledge of the partial pressure of water at some temperature. As usually

done, the calculation ignores the decrease in partial pressure due simply to

cooling; the further decrease due to sorption is not ordinarily known. This

is discussed further in the next section. Thus, the calculated value is only an upper limit and may be much too high. For these reasons, the frost/dew

point, whether measured or calculated, is an inappropriate unit to report the

moisture level of a package.

A unit which is particularly appropriate for reporting the amount of water

vapor present, because it also suggests the likelihood of condensation, is the

relative humidity (RH) , which is simply the ratio of the partial pressure of

water vapor in the package at some temperature to the saturation vapor pressure of water at that temperature. The utility of RH will become apparent in

the sections that follow.

3. SORPTION

The processes of adsorption and absorption have already been discussed in a general way; in this section, these concepts will be made more quantitative.

186
Absorption Is a bulk phenomenon; i.e., the amount of water absorbed is proportional to the mass of absorbent and is independent of its shape, although the

rate is very shape dependent. The water molecules may be regarded as dissolving

into the absorbent's molecular lattice. Most solids absorb measurable quantities of water; only metals and inorganic crystalline substances (including

oxides) do not. Glasses and all organic compounds (including materials that are hydrophobic) absorb water to some extent. Glass may present a problem of

desorption if and when it is used to seal the package, as discussed by Shukla

and by Lowry at the Workshop. However, the amount of water absorbed in any

glass in the package does not change significantly between the highest storage

temperature and the lowest service temperature, and it is not measured by any

of the analytical techniques applied between these temperatures.

Organic materials are a different matter. In particular, epoxy used for mounting chips and substrates in hybrid packages absorbs water; the moisture level

in such packages will show significantly different temperature dependence

than it will in packages containing no organics. The amount of water which

dissolves into the plastic is approximately proportional to the relative humidity; depending on the particular epoxy, the maximum weight percent varies from

about 0.01 [ 1 ] to 1 [ 2] (i.e., the mass fraction varies from 10"'^ to 10~^

under saturation conditions).

Because absorption involves the solid-state diffusion of water molecules, it

is a slow process. This is particularly true for the limited exposed surface

of the adhesive used for substrate mounting because of the long diffusion

distances involved. For the same reason, desorption is slow. This slowness

can be a major factor in the (nonequilibrium) moisture level that can be established if a package is held for an extended period at a high temperature and

then chilled. For some number of hours afterward, the ambient partial pressure of water vapor will be significantly above the final value, and, indeed,

condensation may occur upon returning to a temperature at which initially

there was no condensed water.

The behavior of desiccants (silica gel, molecular sieve, etc.), while frequently referred to as adsorption, is mass-related and relatively slow, so that by the criteria given, it is more conveniently regarded as absorption.

(It should follow from what has been said that organic material in a package

behaves like a desiccant.)

True adsorption is a surface phenomenon; i.e., the amount of water adsorbed is

proportional to the surface area of the adsorbent. It is also very fast, as discussed by Bossard at the Workshop, so that a package without absorbents

should show no hysteresis in the partial pressure of water upon heating and

cooling, apart from the effects of package temperature nonunif ormities . All

surfaces adsorb water to some extent.

The variation with RH of adsorption is not as linear as absorption. The general behavior is that the coverage of the surface is a strong function of RH

at low and high values, i.e., for coverages up to one monolayer and for conditions near condensation, where coverage may be as high as ten monolayers. It

is less strong at intermediate values.

187

371-525 0-82-13
In order to demonstrate the effects of adsorption and absorption on the ambient

moisture level (i.e., the RH) in microelectronic packages, very simple models

have been constructed.

Sorption Models

The model for absorption is given in figure 1. The mass fraction (fab^ °f

water absorbed is taken to be equal to 10"-^ times the relative humidity (expressed as a decimal fraction), independent of temperature. The hybrid package (organics are rarely used with discrete devices) is taken to contain 30 mg

of absorbent. Thus, the mass of water sorbed, in micrograms, is 30 times the

RH.

The model for adsorption is given in figure 2. The coverage is taken to have

a strong linear dependence on RH up to one monolayer coverage (i.e., up to one percent RH) and a weaker linear dependence on RH up to ten monolayers at 100

percent RH.

In the absence of condensed water, the mass (m^) of water vapor can be related

to the RH through the ideal gas law:

m^ = RH X P° X V X M/(R X T) (1)

where P° is the saturation vapor pressure, V is the package volume, M is the

molecular weight, R is the gas constant (82.06 atm cm^ mole~l K--*-), and T the

absolute temperature. Since the mass of water in the package is invariant,

summing its three components, all expressed as a function of RH, gives a means

of calculating the RH and from that, other interesting parameters.

Examples

The models were applied to three specific cases; in each case the package was assumed to be sealed at 200°C with an internal pressure of 1 atm. The package

was also assumed to contain 3 fig cm~3 of water. To enable comparisons, figure

3 presents the saturation values for the pressure and density of water vapor.

Figure 4 plots the total pressure, partial pressure, relative abundance of

water vapor, and RH as a function of temperature for a (hypothetical) package

in which sorption does not occur. For this case only, the relative abundance

and density of water vapor are independent of temperature and package volume.

Figures 5 and 6 show how the density of absorbed, adsorbed, and vaporous water

vary with temperature. At room temperature, nearly 20 percent of the water in

the hybrid package and over 90 percent of the water in the discrete package are

on the walls; when 30 mg of epoxy is included in the hybrid package, nearly 50

percent of the water is absorbed into it at room temperature. Another 13 percent is on the walls, so that less than 40 percent of the water is present as vapor. If more epoxy were used (e.g., as a substrate mount), virtually all of

the water present would be dissolved in it. Figure 7 shows the RH as a function of temperature for the various cases and, because moisture levels are so frequently reported in ppm^, figure 8 shows how this value varies.

188
It is important to note that any attempt to correlate the amount of water vapor

present in a package at two different temperatures will be complicated by variations in the construction details of the package, particularly if the package

contains any organic materials.

It is also important to note that different methods of measuring moisture can produce different results, because they measure different things. Mass spectrometry measures water vapor plus adsorbed water (which begins to desorb as soon as the package is opened into the vacuum) . Whatever absorbed water is

present is not detected because its rate of desorption into vacuum is much

slower. Alumina capacitors measure RH, usually at room temperature, and con- ductance/capacitance cells measure the temperature at which those parameters

rapidly increase (presumably due to condensed moisture)

.

The model applied to the hybrid package without organics predicts a water vapor

content of about 6100 ppm^ at lOO^C and, thus, a calculated dew point of O^C;

however, when cooled to that temperature, there is no condensation because the

relative abundance of water vapor has dropped below 4000 ppmy. The model predicts that the temperature at which the RH will reach 100 percent is absolute

zero and that there is no condensation because all the water is adsorbed.

There is no dew point predicted for the other two packages. Variations in the

model assumptions, or spot cooling of the package, could, of course, change

the results somewhat, but the most important result is an estimate of the extent to which the moisture level at lOO^C can overestimate frost/dew point.

The frost/dew point can be underestimated, too. For packages containing or- ganics, it is worthwhile to introduce the concept of maximum available moisture, i.e., the maximum amount of water vapor that could be made available for

condensation. Maximum available moisture excludes adsorbed moisture, but includes all the absorbed moisture that can be desorbed at the maximum storage

temperature since, if the package were chilled quickly, it would not have time

to be reabsorbed before condensation began. The maximum available moisture

then depends on the amount of organic material present and the maximum storage

temperature, as well as the moisture content. Its value can be estimated by

soaking the package at the high temperature long enough for equilibrium to be

reached and chilling to remove the moisture lost by adsorption. If the hightemperature soak is too short, the available moisture and the maximum possible

frost/dew point will be underestimated. From what has been said about the

speed of adsorption, it should be clear that for a package without organics,

the partial pressure or vapor density at a low temperature should not be affected by a high-temperature soak.

The concepts of sorption process that have been developed for packages can be

extended to in-situ hygrometers; this is done in the next section.

4. IN-SITU HYGROMETERS

Conductance/Capacitance Cell

Figure 9 represents top and side views of an interdigitated structure designed

to have a long length of closely spaced electrodes with an idealized layer of

189
condensed water on the surface. For maximum sensitivity, the electrode spacing (d) should be small, but current fabrication technology limits it to about

1 // m. The height (h) of the metallization is typically no greater than d.

As presented in table 1, a typical thickness of condensed water is much less

than 1 urn, even if all of the water in the package can be made to condense on the sensor chip. Moreover, the condensation will exist as a uniform film only

if it occurs as water on a clean oxide surface; water on a hydrophobic (organiccontaminated) surface or frost on any surface will develop as separate islands

instead.

The consequences of nonuniform condensation depend on the type of measurement

being made. With sufficiently sensitive equipment, it is possible to detect

increases in capacitance caused by adsorption as well as by any form of con- densation. The size of the increase will depend on the change in the local

dielectric constant e due to the presence of the condensate; as suggested by

the equation in figure 9, this is likely to be less than a picofarad.

The dielectric constant for water is virtually independent of the frequency u), and drops somewhat with increasing temperature (due to increasing thermal ran- domization of the water dipoles) . The dielectric constant of ice is not at all

constant with frequency or temperature, as shown in figure 10 [3], because of

the hindered rotation of the water molecule in the solid lattice. Consequently,

measuring capacitance (or admittance) as a function of frequency can be used

to develop clues to the nature of the condensate.

The interdigitated structure can also be used for the measurement of the con- ductance. This measurement will be sensitive to the distribution of the con- densed layer and to temperature. The conductivity of pure water is given in

figure 11. The concentration of charge carriers (protons) increases exponentially with temperature because of the ionization energy. Superimposed on this

behavior is the conduction (possibly much greater) due to charge carriers from

ionized impurities in the liquid. In fact, the height of the conductance peak

at a given temperature could be used as a rough gauge of the ionic contamination on the sensor surface.

Below QOC, water can be detected, if present, as an undercooled liquid, but the

conductance is insensitive to ice or frost because the metal electrodes make

blocking contact with ice; i.e., they cannot inject charge [4]. On the other hand,

the technique, with sufficiently sensitive equipment, can detect adsorbed

water, which can also carry current, as discussed by Merrett at the Workshop.

A significant feature of measuring conductance in a chilled package is that

this is the measurement that determines, without inference, whether a device

can develop enough moisture on its surface (under any conditions) to carry between electrodes a current that is large enough to lead to its degradation.

The optimum method of performing the test is to chill to the minimum service

temperature in such a way as to keep the sensor surface cooler than the rest

of the package interior. (Packages containing organics should be soaked at

the maximum storage temperature first.) If a significant amount of water is

190

/

/
present, the conductance will rise with increasing surface coverage and then

drop, either due to the diminishing conductivity of water or the vanishing of

conductivity as the condensate freezes.

Since cooling the package is somewhat tedious, simpler means of measuring

moisture are to be desired, if the necessary correlations (with a safety factor) can be established. Merrett, at the Workshop, discussed a simple room-temperature test of capacitance between adjacent metallization lines at two frequencies on an IC chip. The alumina capacitor also operates without cooling.

Alumina Capacitor

The first practical in-situ sensor for measuring low moisture levels in microelectronic packages was the alumina capacitor, in which the dielectric constant

of the porous alumina increases with adsorbed water. The capacitance as a function of moisture level for a typical sensor is shown in figure 12 for

three different ambient temperatures [5].

It has been widely (if tacitly) assumed that these sensors respond to the partial pressure of water vapor, which in an indirect way is given by ppm^ at 1 atm. The failure of this assumption is evident by the wide spread between the curves for the different temperatures. In fact, the dielectric constant of

the alumina depends on the amount of adsorbed water. For a constant adsorbed

surface coverage over a range of temperature, that part of the dielectric constant (at a fixed frequency) that is due to the adsorbed water (and, hence,

that part of the capacitance) can be expected to show a temperature dependence

similar to that for ice (fig. 10).

At frequencies low enough to give time for complete dipole rotation of the adsorbed water, this dependence may become quite small. The amount of adsorbed

water is more directly related to RH than to partial pressure, as discussed in

section 3 and further in the Appendix. In support of this argument, when the sensor capacitance from figure 12 is so plotted (fig. 13), the results for the

three temperatures are much closer together. The fact that the capacitance

increases with temperature at a constant RH suggests that the frequency of

measurement (770 Hz) was too high to allow the adsorbed molecules to reorient

fully in the oscillating field.

There seems to be no inherent reason why alumina capacitors could not be used

to measure RH at temperatures higher than AQOC (e.g., 100°C would enable com- parisons with mass spec results) , but the data do not seem to have been published .

5. CONDENSATION AT LESS THAN 100 PERCENT RH

There are two special cases of moisture condensation that should be considered

because they represent a particular threat to the integrity of a device. For

these two cases, condensation can occur before 100 percent RH is reached and

both are in particularly undesirable areas for it to occur: in the presence

of ionic impurities and at cracks in a device passivation layer.

191
Condensat ion can occur at Ibss than 100 percent RH because the saturation vapor

pressure of a salt solution is less than that of pure water at the same temperature. For some salts, this effect is so pronounced that condensation occurs under ordinary room conditions; they are referred to as deliquescent or hygroscopic, but in humid (RH > 75 percent) [6] weather, the same phenomenon is observed with its caking in the salt shaker. Such a tendency is, of course, unscopic, but in humid (RH > 75 percent) [6] weather, the same phenomenon is observed with its caking in the salt shaker. Such a tendency is, of course, un- desirable because, if a package cools slowly, a significant fraction of the

water present could condense at the site of an ionic impurity to form a salt

solution before condensation occurred elsewhere; this localized solution would

pose a much greater threat of corrosion than would a more uniformly distributed

layer of pure water [7].

Condensation in a crack in the passivation layer is possible at an RH less than

100 percent because of the surface tension and concave curvature of the liquid

surface: the vapor pressure is reduced for any liquid with a curved surface

according to the Kelvin equation [ 8 , 9

]

In — = ^^-^( — + —), where (2) o RT

P

P is the vapor pressure of water in the crack,

P° is the vapor pressure of water with a flat surface,

7 is the surface tension (or surface free energy)

,

v is the molar volume of water (18 cm^ mole"-*-) ,

R is the gas constant (8.31 x 10'' dyne cm mole"-'- K"-'-) ,

T is the absolute temperature, and

r-j^ and r2 are the principal radii of curvature of the meniscus.

For the particular case of a crack, the width d can be taken to be twice one radius and the other radius can be set equal to infinity. Condensation will occur when the partial pressure of water vapor rises to the point where it

would be in equilibrium with liquid water in the crack. This will occur at an RH of less than 100 percent, according to the expression

The value of RH for which crack condensation will occur is given for two temperatures as a function of the crack width in figure 14. It can be seen from

this figure that only very fine cracks in the passivation layer (< 10 nm) are

of concern. It should be recognized that 1 nm is only about three water molecules thick and that the derivation of equation 2, which was based on macroscopic, continuous properties of matter, may not extend to such small dimensions

192
At any rate, such extremely fine cracks would be completely undetectable by

light microscopy and would be at or beyond the limits of resolution of the

scanning electron microscope (SEM) . It is not known how likely cracks of this

size are to occur.

There is another method of detecting cracks, developed by Der Marderosian and

presented by him at the Workshop. This method has proved to be quite practical

for screening ceramic packages. Cracks too small to be seen by a light microscope are made visible with no magnification at all by condensing the vapor of

a liquid with low surface tension (FC-77)

.

The detection limits of this method are unknown; it is possible that cracks of

1-10 nm width on device surfaces (if, indeed, they are found to occur) could

be detected. Of course, wider cracks surely could be detected under appropriate experimental conditions and, while these do not promote condensation much

below 100 percent RH, they do increase the risk of failure. It might well be

worthwile to institute a screen for cracks of all high-reliability devices by

the condensation method.

6. CONCLUSION

There is one very practical means of avoiding the complications discussed in

this paper and that is always to produce very dry packages. If this is done,

it will not matter if the frost point has been overestimated, because the estimated value will be low enough, anyway. On the other hand, for those people

who are concerned with working out correlations between test methods, those

who are considering the proper use of organics in packaging, and those who are attempting failure analysis of a package which was not very dry, it is hoped

that the thermodynamic and kinetic concepts discussed in this paper will help

in the presentation and interpretation of experimental results of moisture

measurements

.

ACKNOWLEDGEMENTS

The author is grateful to a number of people for stimulating his thinking in

this area and extends particular thanks to:

• Mike Kovac, for raising the question of what alumina sensors respond to,

• Bob Merrett, for discussions on surface conductivity,

• Bob Thomas, for raising the spectre of crack condensation, and

• Aaron Der Marderosian, for raising the prospect of a solution.

193
REFERENCES

1. Czanderna, A.W. et al., ISHMProc. 1977 International Microelectronics

Symp., p. 197.

2. MIL-HDBK-700 [MR], Military Standardization Handbook Plastics , p. 67

(1965).

3. Smyth, CP. and Hitchcock, C.S., J. Am. Chem. Soc , 54 , 4631 (1932), presented

In Klttel, C, Introduction to Solid State Physics , 2nd Edition, p. 175

(John Wiley and Sons, Inc., New York, N.Y., 1956).

4. Sosnlak, J. and Unger, B.A. , Moisture Determination In Hermetic IC Packages by the Dew Point Method Using AC Capacitance and Conductance Measurement, Proc. International Electron Devices Meeting , Washington,

D.C., Dec. 4-6, 1978, pp. 104-107.

5. Hasegawa, S., Proc. 1980 Electronic Components Conf

.

, San Francisco,

California, May 1980, pp. 386-391.

6. International Critical Tables , Vol. 1, Washburn, E.W., Ed., p. 67,

(McGraw-Hill Book Co., Inc., 1926).

7. Der Marderoslan, A., The Electrochemical Migration of Metals, Proc. 1977

Reliability Physics Symposium , April 1977, pp. 134-141.

8. Sanfeld, A., Thermodynamics of Surfaces, Physical Chemistry, an Advanced

Treatise Vol. I Thermodynamics , W. Jost, ed . , pp 246-249, 285 (Academic

Press, New York, N.Y., 1971).

9. Adamson, A.W., Physical Chemistry of Surfaces , p. 58 (Interscience Publishers, New York, N.Y., 1967).

194
10-3

'ab'

Mass

Fraction

Absorbed

0

f3b = 10-3RH

= Mass of Absorbent x fg^^

For 30mg Absorbent, m^^ (^g) = 30 RH

c 10- No.of

Monolayers

9.09(RH + 0.1) 1

100 RH ^ 1% RH 100%

nriajj = No. of Monolayers x A x Monolayer Thickness x Density

mad (/ig) = 3A X RH (RH < 1 %

)

niad (Mg) = 0.273A(RH + 0.1)

Hybrid (/jg) Discrete (ng) V = 1 cm3, V = 10-3 cm3, A = 10cm2 A = 0.1 cm2

30 RH 300 RH

2.73(RH +0.1) 27.3(RH + 0.1)

Figure 1. Absorption Model,

100 85 70 55 40 25 10 0 -10 -20

Figure 2. Adsorption Model.

1,000

100 -

IJjg cm

2

80-1 134 V-1

1.0

0.9

0.8

0.7

0.6

P-|-, atm

- TOTAL PRESS., P-,

PI 0.5 \

0.4 10^P°, atm

0.3

0.1

0

'flELATIVE ABUNDANCE-^

WATER VAPOR PRESS., P

-\

0.2 _ \

\

RH

10,000

9,000

8,000

7,000

6,000

5,000ppm^

4.000

3,000

2,000

1,000

Figure 3. Saturation Values for Water

Vapor Density and Pressure.

0

0 20 40 60 80 100 ^200

t°,C 80-1134-V-2 Figure 4. Temperature Dependence

of Total Pressure and Moisture

Level in a Package Without

Sorption.

195
3.0

HYBRID

DISCRETE

3.0

0 20 40 60

i,°C

p.

jUg cm

ABSORBED

VAPOR

-^osoRBeo \

J L J L

0 20 40 60 80 100 200

80-1 134-V-4

Figure 5. Distribution of Moisture

Between States in Two Packages

Without Absorption.

Figure 6. Distribution of Moisture

Between States for a Hybrid

Package Containing 30 mg of Epoxy

.

10.000

0.001

HYBRID, WITHOUT

ABSORPTION

HYBRI D, WITH ABSORPTION

DISCRETE

0 20 40 60 80 100 200 t°C 80-1134-V-5

—//—

0 20 40 60 80 100 200

t," C 801134 V-6

Figure 7. Plot of RH vs Temperature Figure 8. Relative Abundance, in

for Four Cases. ppnivj of Water Vapor for Three Cases.

196
N = No. of Fingers. Total Length =i{N-1)

^nuhunil

Assumed Conditions: d > 1 fjm h = d N »1

7//7/7////T

Capacitance : C(pF) = 0.1 £(c,j,t,T) N i(cm) Conductance : G(otim ') = o(a),T) Ni t/h for water

0 = 0 for Ice Admittance : Y = G,+[(cjC)'2 + Gf^]''^'

deer T

-

Figure 9. Interdigitated Cell,

90

80

70

60

50

40

30

20

10

0

/

cc^/ill 1

60,000 - 70 - 60 - 50 -40 - 30 - 20 - 10 0 T(°C)

Figure 10. Dielectric "Constant"

of Ice as a Function of Temperature

and Frequency / lir

, Hz).

,10 ' (ohm cm)

Frost/Dew Point, °C

-10 0 10 "1

lO-'/T

10' ppm V at 1 atm

Figure 11. Electrical Conductivity

of Water,

Figure 12. Capacitance of a Typical

Alumina Capacitor.

197
100 1—I \ \—

r

Table 1 How Much Water in Electronic Packages?

Package Type Hybrid Discrete

Internal Volume 1 cm^ 10"^ cm^

Internal Surface Area lOcm^ 0.1 cm^

Mass of Water at 5000 ppm^ and 1 at m 3ng

Diameter of Hemispherical Droplet 220 (im 22 ^4m No. of Monolayers if Uniformly Condensed 10 1 Thickness of Layer if Condensed on 0.1 cm^ Chip 0.3 ^m 0.3 nm

Mass of Epoxy to Absorb Water at 10% RH 30 mg

198
APPENDIX

Maintaining Constant Adsorbed Surface Coverage

The question of the relationship among the adsorbed surface coverage, the partial pressure of water vapor, and temperature needs to be considered. It should

be clear that a temperature increase promotes desorption, and an increase in

partial pressure promotes adsorption, so the question can be posed: how much

must the partial pressure be increased when the temperature is increased so as

to maintain a constant adsorbed surface coverage? Thermodynamics can be used

to reach an answer.

Consider a surface with some known coverage of adsorbed water in equilibrium

with water vapor at a partial pressure of P^ and a temperature of Tj^. Suppose

the pressure and temperature are changed to P2 and T2 in such a way as to

leave the surface coverage unchanged. Since, under both conditions, the system

is at equilibrium, the change in free energy is equal to zero (A G = 0) for

the process vapor — adsorbed, so AHj^ = T]^AS]^ and AH2 = T2AS2, where the

deltas refer to the change in the state functions between vapor and adsorbed

water. We now want to use these relationships to determine the relationship

of pressure and temperature. We can consider the net process to consist of

these simpler processes.

Process H2 - S2 - S]^

1. Adsorbed (T^, P ^ T2, P2) (adsorbate) AT C^ (adsorbate) In (T2/T2^)

2. Vapor (T^, P^ - T^, P^ ) P2

(isobaric) C^ (vapor) AT C^ (vapor) In ^2/1^

3. Vapor (T^, P^ _ T^, P2)

(isothermal) 0 R In (P^/P2)

Net Process AH2 - AH^ AS2 - AS^

(ads + vapor) (T^, P^ _ T2 , P2) Ac AT R 1^ (P^/P2) +

AC In (T./T.)

p 2 1

To get an estimate of the relative magnitudes of the various terms, we may take

C (ads) = C (ice) = 9 cal mole"''' deg"'^ (A-1)

? P

C (vapor) « 9 cal mole ^ deg '' (A-2)

P

199
so ACp<l cal mole ^ deg ^< R =2 cal mole ^ deg ^ (A-3)

Then if Ti and T2 are taken as 273K and 373K (0°C and 100°C) , AT = 100 deg

and, since we expect the pressure to rise exponentially with temperature.

In (T2/T1) < I In (Pi/P2)| (A-4)

With these conditions,

AC AT < 100 cal mole~"^« AH ^ > 12 kcal mole""*" (A-5)

p ad

AH, = AH_ = AH , (A-6)

1 2 ad

ACp In (T2/T^)«|R In (P^/P2)| (A-7)

Thus we can write

AS2 - As^ = R m (p^/pp = AH^d(l^ - Y;) (^-s)

or

P2 rAH — = exp

1

Equation A-9 has the same form as the expression for the vapor pressure ,. except

for the enthalpy. As such, it can be concluded that if the enthalpy of adsorption were the same as the enthalpy of vaporization, the partial pressure of

water vapor in equilibrium with adsorbed water would have to increase with

temperature in exactly the same way to maintain constant coverage, as does the

partial pressure in equilibrium with condensed water. Expressed another- way,

if AH^jj = AHy and P° is the saturation vapor pressure, then

P P

li = _2

P° P°

1 2

RH (const.) (A-10)

and constant coverage is maintained by maintaining a constant RH. In general,

AHad ^ AH,^, so the RH must be maintained or raised somewhat with increasing

temperature. In any case, RH is a much more appropriate parameter than the

partial pressure in considering temperature effects. A similar argument applies for absorbed water.

200
5.7 The Detection of Cracks in Ceramic Packages by Vapor

Condensat ion

Aaron Der Marderosian

Raytheon Company

528 Boston Post Road

Sudbury, MA 01776

(617) 443-9521 x2791

Abstract: This paper details a novel approach to the detection of cracks in ceramic semiconductor packages. The technique is extremely fast and has been designed to be a cost

effective method of performing crack detection for high

volume production applications as well as for traditional

failure analysis. The test does not require any special

lighting nor any optical magnification. It is capable of

detecting cracks as fine as one-tenth of a micron in width.

Although developed for a specific situation, the test may

be useful for a wide variety of other applications.

Key Words: Ceramic crack detection; ceramic cracks; ceramic

fissures; crack detection; fissure detection; fissures;

vapor crack detection.

1. INTRODUCTION

In the course of normal 30X visual examination of ceramic semiconductor packages, subtle cracks in the body of some of these

devices were noted. All of the parts had passed standard hermeticity tests prior to this visual examination. Subsequent

leak tests verified their integrity. Suspect parts were examined several times by various organizations with different results noted by each of the inspectors. Some of the cracks were

visible at low magnification ('^lOX) while others were only

visible at 200 to 400X. Dye penetrants were used at that time

as a "referee" test to verify the presence or absence of cracks

(see figure 1 for a typical crack).

A detailed analysis of these failures revealed that the cracks

had initiated in the base of the package during the lead frame

embedment process. An examination of a cross-section of a faulty part showed that the die attach glass reflows into the

crack and usually seals it (see figure 2). Those that were not

sealed were in all probability removed by the hermeticity tests

performed at the manufacturer's plant. Subsequent design changes

at the package house remedied the problems; however, several thousand parts had made it through the manufacturing cycle before the

fix could be implemented. At this point it had become obvious

that a rapid, cost effective and non-destructive crack detection

screen had to be developed as soon as possible. Parts had to be

tested at two stages of the manufacturing cycle: 1) after the

lead embedment process (minimize total cost impact by screening

out a bad product prior to die attach) and 2) on finished, sealed

devices

.

201
The following report discusses in detail the development of a test specifically designed to screen out the cracked parts in

a manner consistent with the program schedule and cost constraints.

2. INITIAL STUDIES

Early work which attempted to apply established crack detection

techniques (e.g., SOX visual and dye penetrants) to this problem

was inadequate for production quantities for one or more of the

following reasons: 1) high cost, 2) poor repeatability, 3) opera

tor sensitive, 4) clean-up required, 5) potentially corrosive, 6)

cumbersome and slow, and 7) lack of adequate sensitivity. In an

attempt to overcome some of these difficulties, a variety of

methods were evaluated. They are briefly discussed in the following . Ultrasonic Acoustic Emissions

It was thought that a cracked part may emit ultrasonic sounds

when stressed. With this in mind, a standard particle impact

noise detector (FIND) was studied for its potential as a crack

detector. The part to be evaluated was placed on the base of

the ultrasonic sensor and stressed in a variety of ways. No

"sounds" were detected on even the most severely cracked devices.

Several parts were studied during this evaluation. None were detected successfully and as a result the approach was finally

ab and oned . Solvent Dip

Experience has shown that the crack of a damaged part will retain excess liquid during the drying phase, thereby highlighting

the crack. The standard dye penetrant test relies, to a great

degree, on this phenomenon. It was felt that other common liquids (without dyes) may be useful for crack detection. Alcohol,

acetone, freon T.F., water, and f luorocarbons (FC-77, 78, etc.)

were studied. Surprisingly, wetted distilled water solutions

gave the best results and were used for a short time to detect

cracks on sealed parts (see figure 3) in the initial stages of

the screening program. The technique proved to be reasonably

sensitive (at least as effective as the visual and dye penetrant

methods) yet could not be used for unsealed packages due to its

corrosive nature and an excessive test time of 30 to 45 sees per

part. Because of these negative factors this test was discontinued along with the basic solvent dip approach. It should be

noted, however, that this technique does have merits and may be

useful for certain applications where speed and/or potential

corrosion problems are not a major concern.

3. RECENT EXPERIMENTS

During the solvent dip studies, a variety of methods were used

to apply the different fluids to the test specimens. One of the

202
techniques employed a condensation method similar to a vapor degreasing operation. On occasion, the test highlighted the cracked

areas on each of the parts when the ambient light was reflected at

a particular angle. Attempts to repeat this, to produce consistent results, failed. The principle, however, was explored further. A distilling apparatus was assembled using a simple flask,

rubber stopper, and tubing. In addition, a stream of nitrogen gas

was directed into the flask through an impinger (porous stone).

Figure 4 is a schematic representation of the set-up. With this

configuration, the technique of condensing a thin film of liquid

onto the surface of a cracked package proved to be an extremely

effective way of highlighting the cracks. A study of the effectiveness of the method as a function of fluid type resulted in

our choosing FC-77 (a 3M fluid used in a variety of electronic

applications) as the best overall material. The visual effect

produced by condensing FC-77 vapors onto the surface of a cracked

device is shown in figure 5, while the equipment is shown in

figure 6. The inert physical properties of the FC-77 enabled us

to utilize this fluid as our test medium for both sealed and unsealed parts, thereby eliminating concern for potential damage

due to corrosivity. Added desirable features include low toxicity, low surface tension (forms a sheet of liquid rather than

individual droplets), and no need for cleaning the parts before

or after the test. Figures 7-9 are photographs of typical test

set-ups at the base assembly, component, and module levels. The

following paragraphs detail the nature of a variety of tests that

were performed in order to define further and understand the principal mechanisms involved in the detection of cracks using this

new method

.

Sensitivity

A large sample of cracked parts, screened out of the production

lots, was used to determine the sensitivity of the test. The

parts were examined optically and on a S.E.M. and categorized

according to crack width (approximately 0.2 to 50 microns). The

length of the crack (always, in this case, on the base of the

V X h'' (0 . 635 cm x 0 . 635 cm) flatpack) varied between .05"

(0.127 cm) to .25" (0.635 cm). The cracks were in either the

shape of a "Y" or a single line. Typical optical photographs

are shown in figures 10 and 11. It should be noted that these

parts had previously been screened out using the "vapor" test

and that many of these parts had not been detected using the

other techniques employed at the start of our investigation,

e.g., visual at 30X, dye penetrants, and the wetted water method.

The sensitivity test was performed in a normal room ambient

lighting arrangement (overhead fluorescent tubes mildly diffused

with semi-transparent acrylic shades). The operator adjusted

the angle of the part such that the overhead lighting would reflect directly back into his eye when the vapor was condensed

on the test specimen. No optical magnification was used for this

study. Those parts that did not appear to have cracks extending

across the package were studied closely. It was felt that these

203

371-525 0 - 82 - 14
parts had cracks, near one end, which were fine enough to escape

detection. These parts were carefully scribed at the point where

the crack appeared to end. The device was then examined closely

in the S.E.M. The crack was followed from the wide end (edge of

package) to the point where it was no longer possible to track

its path. This point ('^ 2/3 the way across) coincided closely

with the scribe mark made earlier. The width of the crack at

this point was 0.1 to 0.2 microns (see figure 12). Optical

examination of this part @ 200 to 400X could only trace the crack

about % the way across the package. It became obvious at this

point that the vapor test was considerably more sensitive than

traditional crack detection methods and that a technique had been

developed which could safely test production quantities of parts

in a cost-effective manner.

Mechanism of Detection

The visual effect produced using this method of detection is

quite interesting in that there is an apparent "magnification"

of the width of the crack when the part has been exposed to the

FC-77 vapors. The magnification is in the order of 10 to 1000 times

the width of the crack (the wider the crack the less the apparent

magnification and vice versa). Examination of this process under

a microscope indicates that the vapors (^28°C) condense on the

surface away from the crack and rapidly build up in thickness to

10 to 40 microns typically. At this same time, the liquid is being absorbed rapidly into the crack such that the area immediately

adjacent to the crack is depleted of fluid until the crack fills.

It is during this period of fluid absorption that the test operator must examine the device for cracks. This period is variable

and depends on the condensation rate of the vapors. When properly adjusted, this observation period is typically from 1-3

seconds. See figure 13 for a pictorial summary of the mechanism.

Testing Rate

The parts to be tested were divided into two categories: unassembled (bases and lead frame) and completely packaged including the plastic flatpak carriers. The unassembled parts were

held in thin plastic dividing trays (50 to each tray) while the

finished parts were packaged in tubes typically containing about

40-60 pieces. The individual test time for each part in the plastic trays averaged 1.5 seconds while those in the carriers averaged 2 seconds each. The average output for an 8-hour shift per

person for the base assemblies was 10,000 pieces. The average

output for the completed parts was 8000 pieces. These values

include the handling of the parts such as unpacking, repacking,

etc , Test Interferences

On occasion the surface of the package may be scratched or be

contaminated with lint, hair, etc. On casual observation, these

items may appear somewhat as a crack does. Closer p-x-am i na t ion

.

204
however, will reveal the differences fairly easily. Dust, hair,

etc. can be blown off thereby eliminating the source of interference. In addition, after the package has dried, the hair or dust

will still be quite visible while a crack will typically disappear

The same is true for a surface scratch, in that it too will remain

after the part has dried. These interferences are somewhat rare

and are not a serious problem in practice.

4. DISCUSSION AND CONCLUSIONS

This paper has discussed briefly, the salient characteristics of

a novel method of crack detection utilizing the condensation properties of an inert fluorocarbon (FC-77). Other liquids will work,

however, for our purposes we found this one to be most suitable

for reasons mentioned earlier. Prior to the introduction of this

technique large-scale crack testing was simply not considered due

to a variety of reasons such as expense, difficulty in interpretation, slow, etc. This test, however, has been found to be

ideally suitable for large production quantities of parts, as well as for traditional failure analysis purposes.

During the course of our study we had determined that those parts

which were cracked (also hermetic) had sufficient strength remaining to pass the rigors of the environment. This was most fortunate in our case, however, it may not be true for other part

types in similar or more stringent environments. It is also

possible that other part types could develop cracks in more sensitive areas and hence be more vulnerable to subsequent processing.

Assuming that this could happen, then it is possible that parts

with these defects, if not screened out, could get into deliverable hardware and become latent failures (die cracks, loss of

hermeticity, etc.) . At this point in time, it is not clear how

many of the parts manufactured each year by the semiconductor

industry have cracked lids and/or bases, simply because no specifications, other than visual examination, (hermeticity indirectly) have been generated which seriously address this situation.

We have also learned in this study that a visual examination is

probably the least reliable method of crack detection as well as the most expensive if done thoroughly. Because of this and the

usual demands of production schedules, it would seem logical that

very few, if any, in-depth studies have been made to examine the

extent of the problem in the industry.

In view of this situation, it would appear most prudent for those

of us associated with high reliability programs to seriously consider instituting the vapor condensation crack detection test

method into an incoming inspection operation. Once established

in place, statistical data would then become available on the

extent of the problem in the industry. Assuming that a problem

exists, steps could then be taken to correct the problem.

Although this study has confined itself to faulty ceramic packages

205
it is felt that the vapor condensation method of crack detection

could prove useful in a variety of other applications in which

it is desirable to highlight the presence or absence of narrow

channel fissures.

206
FIGURE 1

B

Cracked package (typical) as noted in early 30X High Mag. photo of crack shown above ~ lOOX. visual inspection at vendor's facility.

FIGURE 2

120x

White Streak is Zn Rich

Cross section of base of package showing crack

(S.E.M. photo at 120X).

High Mag. S.E.M. photo (l.2KX)of same package

highlighting the reflow of the die attach glass.

Package was resealed as a result of the reflow.

207
FIGURE 3 FIGURE 4

Photo depicting visual effect of a cracked package

when exposed to "wetted water" crack detection test. Schematic diagram detailing the principle of operation of the vapor condensation method of crack

detection.

FIGURE 5

Optical photo of cracked package as normally viewe, Optical photo of same package as normally viewed

when highlighted by new vapor condensation test technique.

208
FIGURE 6

Photo of vapor generator used to perform new crack detection test.

FIGURE 8

Photo showing typical set-up for production crack testing of Flatpaks in their carriers using the vapor test.

FIGURE 7

Photo showing typical set-up for production crack testing of base and lead frame assemblies using the vapor test.

FIGURE 9

Photo showing typical set-up for production crack testing of modules using the vapor test.

209
FIGURE 12

N I ;

i ) 1 1 \

Photo of a very fine crack in a package used to S.E.M. photo of widest part of crack (near edge) determine the resolution of the vapor test (300X). at 750X.

210
FIGURE 13 VAPOR CRACK DETECTION

MECHANISM

SIDE VIEW OF PACKAGE

Diagram detailing mechanism of vapor crack detection test. The fluid on the surface immediately

adjacent to the crack is absorbed into the fissure,

thereby highlighting the defect.

211
5.8 Thresholds for Corrosion

Aaron Der Marderosian

Raytheon Company

528 Boston Post Road

Sudbury, MA 01776

(617) 443-9521 x2791

Paper not submitted.

212
6. SESSION V PACKAGING

6.1 Moisture Content of Solder Glasses

Rama K. Shukla, J. SinghDeo, Nirmal K. Sharma and Richard Blish

Intel Corporation

3065 Bowers Ave.

Santa Clara, CA 95051

(408) 987-6256

Abstract

Cavity moisture content in Cerdips is strongly affected by the

moisture evolution from physically adsorbed, chemisorbed and bulk

moisture and is further aggravated by oxidation of residual carbonaceous specious in the glazed glass. At Intel, we have characterized these variables by using various moisture evolution measurement methods. RGA and MEA (Moisture Evolution Analysis) help in

understanding the high temperature moisture evolution from solder

glass while the Karl Fischer method helps in characterizing surface

adsorbed water. We have thus shown that organic impurities burnout can lead to as much as 30 percent excess moisture evolution

from the solder glasses, this contribution being vendor dependent.

Using these data, we are optimizing the Cerdip manufacturing process to get dry Cerdips.

Key words: Cerdips; desorption, mass spectrometry, moisture evolution analysis; water sorption phenomenon.

1. Introduction

Water vapor in Cerdip cavity arises from various sources. These are, solder

glasses (used to solder seal the two parts of Cerdip), ceramic composite

body, ambient (humidity of processing environment) and other (e.g., epoxies,

if used). Control of cavity moisture in Cerdips requires a thorough

understanding of the kinetics and thermodynamics of the water sorption

phenomenon from these sources which can lead to ultimate process controls to

minimize the problem of free moisture occurence in Cerdip cavities.

Solder glasses are known to be one of the primary sources of water vapor in a Cerdip. Moisture evolution from these can be studied by one or a combination

of various techniques, e.g., mass-spectrometry , MEA (Moisture Evolution

Analysis), Karl Fischer Titration, and thermogravimetry . We have used a variety of such techniques, which are somewhat complementary in nature, to

measure moisture evolution rates and identify the various stages which occur

in moisture evolution as a function of temperature.

2. Procedures

Mass-Spectrometry

Table 1 shows a typical mass spectrometrlc , result of gases evolved from a vitreous solder glass (XS1175-M1), which were outgassed upon heating the

213
glass in an evacuated chamber for 10 min at each test temperature. The

samples for this analysis were prepared by chipping off glass from the glazed

piece parts (Cerdip) from a particular vendor lot.

Large amounts of water and other carbonaceous products are evolved as temperature goes up. Evolution of large amounts of and H^O indicates

that the organic impurities (which occur from the vehicle and binders used in screen printing of these glasses) oxidize upon heating the sample to high

temperatures in air. This phenomenon would produce moisture in addition to

moisture released from the bulk glass by desorption at higher temperatures.

In order to separate these two effects, moisture evolution analysis,

discussed next, was carried out.

Moisture Evolution Analysis (MEA)

In this method, a sample is heated in a quartz tube with a dry carrier gas

flowing over it (fig. 1). The evolved moisture is carried over by the

carrier gas to an electrolytic P2^5 sensor cell, which electrically

integrates the amount of moisture flowing through it.

The contribution to moisture evolution from the bulk glass versus the

oxidation of organic outgassed products can be separated by choosing N2 versus ©2 as a carrier gas. In N2 atmosphere, oxidation of organic gases is

suppressed.

Using stepwise heating of the glass samples in MEA apparatus, we have

identified three distinct regimes of moisture evolution from these. The

first low temperature evolution around 200°C, corresponds to H2O desorption

from the surface of the specimen and is surface area dependent. At

intermediate temperatures, moisture evolution occurs due to chemi -desorption

and due to oxidative evolution of organic impurities in the glass. As

mentioned before, the two effects contributing to moisture evolution in this

second stage can be measured independently, since the organic burn-out gets

suppressed in the N2 atmosphere. The last stage of moisture evolution occurs when the glass is molten, and is due to the desorption of H2O from the bulk

glass. Lower viscosity of the glass at these high temperatures enables OH

bonds to break and resulting H2O to diffuse out rapidly.

Using MEA data with N2 and ©2 as the carrier gas, we measured the total

moisture which evolved from the glazed glass by different vendors. Figure 2 shows these data graphically. We see that the organic burn-out varies

significantly among vendors. This is illustrative of the glazing process

variations used in Cerdip manufacturing.

Karl Fischer Titration

The Karl Fischer method relies on the specificity of the Karl Fischer reagent

to water. The reagent contains pyridine, sulfur dioxide, iodine, and an organic solvent. The reaction between this reagent and water is specific and

quantitative. Water adsorbed by the reagent from the samples is determined

automatically by a titration method.

214
We have studied water adsorption characteristics of solder glasses and

ceramic bases by using this method. Data show parts physically adsorb

moisture until saturation is achieved in about 1 h (fig. 3).

3. Conclusions

Using mass spectrometry, MEA and Karl Fischer titration, we have shown that

moisture evolution from the solder glasses is vendor/atmosphere dependent due

to the presence of organic impurities in the glass.

These techniques also provide kinetic information about the moisture

evolution rates as a function of time and temperature.

Basic understanding of this functional dependence is the key to controlling

moisture evolution in Cerdips.

215
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TABLE I

* MASS SPECTROMETRIC ANALYSIS OF OUT-GASSED PRODUCTS

GAS

OF SOLDER GLASS

25°C

OUT- GAS TEMPERATURE

110°C 220°C 430°C

HYDROGEN 0 0

METHANE 0.5 40

WATER 401 4137

NITROGEN 0 0

OXYGEN 0 4

HYDROCARBONS

AS BUTANE 20 200

CARBON DIOXIDE 176 4020

BENZENE 0 0

PHENOL 73 142

0

87

5678

325

110

381

6237

0

142

21

1116

23127

5365

191

633

8098

2505

1450

*(I) DATA BY WEST COAST TECHNICAL SERVICE. ALL VALUES ARE

IN PPMV.

(II) DWELL TIME AT EACH TEMPERATURE BEFORE ANALYSIS WAS

10 MINUTES.

219

371-525 0-82-15
6.2 Dry Sealing Glasses - A Summary of Research

Robert K. Lowry

Harris Semiconductor, Products Division

Melbourne, FL 32901

^

, (305) 724-7566

Abstract: Ceramic dual- in-line (Cerdip) packages utilizing low temperature (less than approximately 500°C) sealing glasses have been

widely deployed for packaging integrated circuits, but simultaneously

condemned for high reliability applications due to high moisture content

in the sealed cavity. Chemical and physical properties of the sealing

glasses determine the quantity of water vapor which will be present in

the sealed package. Extensive testing has established that vitreous

glasses, when properly processed, contribute substantially less water

vapor to a package cavity than the devitrifying glasses originally used.

Vacuum microbalance and scanning electron microscopic studies are reported for a family of vitreous sealing glasses to help define processing parameters designed to assure dry Cerdip packages.

Key words: Cerdip; integrated circuit packaging; internal water vapor;

moisture evolution; package reliability; sealing glass.

) . 1. INTRODUCTION

Cerdip Packaging

Among assets of the Cerdip are its sturdy construction and good resistance to

thermal and mechanical shock. It is not likely to contain conductive particles

inside the die cavity. Chips packaged in the Cerdip benefit from an additional

high temperature bake of 400 to 520°C during sealing temperature residence time.

This is equivalent to 2 X 10'^ hour stabilization bake at 150°C (leV activation

energy) . The Cerdip package can be economically produced in high volume

quantities

.

A drawback of Cerdip packaging has been device failure rates attributed to

electrogalvanic corrosion of metallization on the chip. A necessary condition

for such corrosion is the presence of water vapor within the die cavity. If

cracks or pinholes exist in the protective glassivation covering the die, this

water vapor may migrate to the chip surface. If the dewpoint temperature of

the cavity ambient is reached, condensation will occur, producing liquid or frozen water which may contact metallized portions of the die surface. The

result is a conductive pathway for ion transport. With the device under bias,

stray currents across such pathways may corrode metal components resulting in

device failure. Corrosion failure modes have been and continue to be widely

studied [l-4]

.

The principal contributor to cavity water vapor is the solder glass used to form

the package seal. Assuming a worst case scenario where no water is adsorbed on the walls of a package cavity, only 0.15 microgram of water will provide 5,000

ppmv of water vapor to the 0.04 cc volume of a 16-lead Cerdip package [Sj . In a

220
previous paper, the moisture sorption and outgassing properties of sealing

glasses were reported [6^. The results showed that water was the principal

volatile component evolved during heating, indicating that precautions during

fabrication of the package need to be taken to keep the sealed cavity dry.

Developments in the fields of mass spectroscopy and in-situ moisture sensors have provided means for measuring package moisture levels. In this paper, these

methods are applied with other analytical investigations to identify inherent

moisture contents for the two major types of solder sealing glasses. Data are presented showing that packages sealed with devitrifying glass may contain

water vapor in excess of 5,000 ppmv, while otherwise identical packages sealed

with a vitreous glass are substantially drier.

Vitreous Sealing Glasses

The moisture evolution properties of a family of vitreous sealing glasses were

evaluated to assess process approaches for producing hermetic Cerdip packages

with greatly reduced levels of encapsulated moisture. Four types of commercially

glazed glasses of the KC-series, as supplied on Cerdip lids, were heated in

vacuum at 430°C for 3 hours. During this period the identity, quantity, and

evolution rate of gaseous components were measured. The outgassing studies show

that the quantity and rate of moisture evolution from KC-1 and KC-IM sealing

glasses depend in part on the way the glass was initially glazed on the alumina

lid: the quantity of water outgassed from glass glazed in the "standard manner"

is greater than the amount evolved from the glass glazed using the "super dry"

process. However, the quantity of water outgassed during the first 10 minutes

of heating at 430°C from all four sample types, KC-1 (standard), KC-1 (super dry),

KC-IM (standard), and KC-lM (super dry) was sufficiently high to indicate that

if sealing occurred during this initial 10-minute period, the moisture content

inside the sealed cavity could exceed 5,000 ppmv. Hence, a preseal bake of

vitreous-glazed package piece parts can contribute substantially to producing

dry cavities. The most important factor in determining duration of a preseal

bake is rate of moisture evolution from the glass.

2. EXPERIMENTAL PROCEDURES

Vitreous Versus Devitrifying Glass

Solder glasses for IC packaging have been widely characterized [7-9] . Devitrifying glasses are essentially thermosetting materials. They contain nucleating

agents which induce formation of a crystalline phase as the glass is heated.

The properties of the crystalline phase are uniquely a function of heating times

and temperatures. As devitrifying glass is heated, its viscosity drops and it

melts in the region of 350 to 650°C. During this low viscosity period, crystal

growth begins at a finite number of nucleation sites. Then, at higher temperatures, usually between 400 and 750°C, the viscosity rises, further crystal

growth ceases, and the glass "sets". The result is a rigid glass material

with different properties and a different Melting point than the starting

material.

Vitreous glass, on the other hand, simply softens and flows when heated above

its melting point. The glass becomes rigid and the viscosity rises only when

it is cooled below its melting point. Vitreous glass melts and flows at the

221
same temperature each time it is processed.

The major components of both vitreous and devitrifying glasses are oxides of

lead and boron. All low temperature solder glasses are necessarily formulated

with 70 to 90 percent PbO so that their thermal expansion coefficients will

approximate that of the alumina piece parts (3 - 7 X 10~^/°C). B2O3 is

present in both glass types as a flux, or glass-former.

Differences in metal oxide composition affect the properties and performance

of these glasses. Therefore, the glasses studied here were characterized by

DC arc optical emission spectroscopy to identify significant compositional

differences. The devitrifying glass contained significant amounts of zinc

and zirconium, the oxides of which are present to serve as nucleating agents.

Lesser amounts of aluminum and practically no lithium were found. On the

other hand, the vitreous glass had no significant concentrations of heavy

metals because it does not require nucleating agents. It did contain

significantly higher amounts of lithium and aluminum than the devitrifying

glass, with lesser amounts of zirconium and practically no zinc. This

vitreous glass contains a filler, such as lithium aluminum silicate, to

provide the required coefficient of thermal expansion. Table 1 summarizes

the qualitative compositional differences in the glass types.

Moisture Contents of Sealed Packages

As the problems caused by high package moisture contents became more evident

early in the 1970' s, an ongoing program of monitoring sealed package ambients was established in conjunction with a continuing package reliability testing

program to understand device failure modes and assure a high-quality dependabl

product. The following moisture information was gathered during testing over

a two-year period on packages sealed with a particular vitreous glass or two

particular types of devitrifying glass. These data provide an excellent

cross section of results on a wide variety of manufacturer piece part lots

and assembly production lots over that period. The data are comprised of

measurements taken both by mass spectroscopy [lO] and via in-situ surface

conductivity sensor [ll] . These measurement methods were applied to packages

sealed with both types of glass.

Devitrifying Glass

Figure 1 shows the moisture distribution for packages sealed with devitrifying

glass. This 42-sample group exhibited a mean dewpoint value of -6°C. Seventeen percent of these packages contained less than 500 ppmv water, while 46

percent contained more than 6,000 ppmv water.

Vitreous Glass.

Figure 2 shows the moisture distribution for packages sealed with vitreous

glass. This 65-sample group exhibited a mean dewpoint value of -37 C.

Ninety-one percent of these Cerdips contained less than 500 ppmv water.

Sealing Glass and Moisture

From these data it is evident that vitreous solder glass is a key to producing

222
dry package ambient. A number of factors contribute to these reduced moisture

levels

.

One is that raw vitreous glass with lithium aluminum silicate or similar

compounds contains roughly an order of magnitude less desorbable water than

devitrifying glass. Aluminosilicate glass has been determined by pressure

rise and mass spectrometry measurements to contain about fifteen times less

bound water than borosilicate glass [l2] . This smaller amount of desorbable

water in vitreous glass could be due to two factors. The aluminosilicate

may be reducing the number of bonding sites for hydroxyl groups in the

PbO-B203 structure. Or, it may be binding water of hydration within its own structure much more tightly than the Pb0-B202 network would alone.

Whatever the mechanism, bulk glass which includes aluminosilicate or related

compounds will contain or release fewer water molecules per unit volume than

other glasses.

A second factor is the preseal conditioning bake of the vitreous glass. This causes the glass to desorb much of its native water prior to the sealing

operation. When this bake is followed by direct introduction of parts to the

sealing furnace, the glass arrives at seal formation without opportunity to

rehydrolyze ambient moisture. This method insures for the sealing operation

a dry glass which has been preoutgassed of much of its initial water content.

The sealed glass of the finished package thus has substantially less water

to release to the cavity ambient during the life of the part. Devitrifying

glass cannot effectively be preseal baked since any premature devitrification

could result in hermeticity failure of the finished part.

The primary factor contributing to the higher moisture levels of devitrifying

glass package ambients is the events which occur at devitrification. On the

atomic level, a very energetic situation prevails as the Pb0-B203 system

undergoes the nucleation process with ZnO. At this time many chemical bonds

are broken and reformed as atomic rearrangements occur and the glass assumes

a more highly ordered lattice network. These events, which occur just at the

critical time the hermetic seal is being formed, free water molecules originally

bound in the glass. These molecules evolve and many of them are then trapped

within the package cavity. Because of this process, packages sealed with

devitrifying glass will always tend to have die cavities with higher moisture

contents

.

Still another contributing factor is the greater potential for water desorption

from devitrifying glass during the operating lifetime of the part. In contrast,

the aluminosilicate component of the vitreous glass in this study imparts a continuing dryness property which reduces the tendency to evolve moisture with

time. This effect is suggested by the activation energy for water desorption

from borosilicate and aluminosilicate glasses, which has been reported as 21

and 49 kcal/gram-mole , respectively [l3]

.

Further Characterization of Vitreous Solder Glasses

More specific information on moisture outgassing behavior of vitreous glass was obtained with the gravimetric-residual gas analyzer (RGA) apparatus. This

instrument consists of an automated vacuum microbalance [14] for measuring

magnitude and rate of mass change of a sample, as well as a UTI-IOOC quadrupole

223
mass spectrometer for identifying the molecular species associated with the mass changes. Details of the design and operation of this apparatus are available [l5]. In this study mass measurements of samples weighing up to

5.9 grams were made to a precision of 0.2 microgram.

Commercially glazed KC-1 and KC-IM sealing glasses were analyzed as received

on 16-lead black Cerdip lids. The glasses were glazed by the manufacturer

using two different processes: the "standard" and "super dry" process

variations of both KC-1 and KC-IM solder glasses (four samples total) . Each

sample for analysis consisted of several glazed lids which were tied together

with 6 mil gold wire to make a sample for the outgassing measurements; the

samples were suspended from the microbalance with a quartz fiber. After

suspending a lid-bundle from the microbalance, the gravimetric RCA apparatus

was evacuated to 10~^ Pa. The outgassing experiments were then conducted by

heating the sample in vacuum at a rate of 31°C per minute to reach a sample

temperature of 430°C for three hours. Both the microbalance and quadrupole

operated continuously during an outgassing experiment. Once the experiment was complete, the sample was allowed to cool in vacuum to 25°C before it was removed from the apparatus.

In all cases the major constituents outgassed initially from the samples were

water vapor, carbon dioxide, and a mass peak at amu 28 (carbon monoxide and/or

nitrogen) in the residual gas spectra. However, after approximately 6 to 10

minutes of heating, the amount of carbon dioxide and amu 28 became negligible.

During the remainder of the experiment, water vapor was the primary volatile

component outgassed. The mass losses due to outgassing of the four glass

samples at 430°C are shown in figure 3 and listed in table 2. As can be seen,

the outgassing profile of a given glass is dependent on the glazing process

used. Both the rate and quantity of gases evolved differ markedly between

the samples. For example, the initial rate of mass loss from KC-1, glazed in

the "standard" manner, corresponded to 2.77 micrograms lid"-'- min~l. In addition,

the total quantity of gases evolved during the three- hour heating were 24.6 and

14.3 micrograms per lid for KC-1 (standard) and KC-1 (super dry), respectively.

Similarly, the mass loss from KC-IM (standard) was greater compared with the mass loss from KC-IM (super dry) . The data presented in figure 3 suggest that

a prebake of the glasses to outgas water before sealing is desirable. During

the first 10 minutes of heating at 430°C, the mass losses from KC-1 (standard),

KC-1 (super dry), KC-IM (standard), and KC-IM (super dry) correspond to 16.10,

9.24, 6.44, and 7.14 micrograms per lid, respectively. If sealing occurred

during this period, the moisture content inside the sealed cavity may well

exceed 5,000 ppmv since e.g., only 0.15 microgram of water vapor is needed to

supply 5,000 ppmv of water vapor to a 0.04 cc sealed cavity. The rate of mass

loss will influence the prebake time selected for production of dry parts. For

the sake of this discussion assume (1) that the glass must remain at 430°C for

10 minutes during the seal operation and (2) that no more than 0.15 microgram

of moisture can be permitted to evolve from the glazed lid during sealing. The

data in figure 1 show that prebaking KC-1 (standard), KC-1 (super dry), and

KC-IM (super dry) in vacuum at 430°C for 80, 60, and 90 minutes, respectively,

will reduce the rate of mass loss to less than 0.015 microgram lid""*- min"-'-.

Hence, the quantity of water evolved from these three samples during a subsequent

10-minute heating will be less than 0.15 microgram, making production of dry

sealed cavities more probable. On the other hand, even after three hours of

heating, the rate of moisture evolution from KC-IM (standard) is relatively high.

224
a 0.049 microgram lid min , indicating that this glass may have to be

prebaked for much longer time to yield dry packages.

One of the glazed lid samples, KC-IM (super dry), was stored in the laboratory

environment for seven days after the initial outgassing experiment to determine

the effects of storage in air on the subsequent outgassing properties of the

glass. Comparison of the initial and subsequent outgassing properties of KC-IM

(super dry) are shown in figure 4. As can be seen, the quantity of water evolved

was significantly less, 1.26 micrograms per lid (curve B) , during a subsequent

heating compared with the initial amount outgassed, 14.42 micrograms per lid

(curve A) , even though the sample was exposed in the interim to a gaseous

environment containing water. Furthermoire, the maximum rate of mass loss

decreased from 1.12 micrograms lid"-'- min"! (A) to 0.056 microgram lid"-'- min"-'-

(B) , and the time required for the moisture evolution rate to Orop below 0.015

microgram lid" min , shortened from 90 to approximately 10 minutes of heating

at 430 C. These data suggest that the evolution of moisture at 430°C, at least

from this particular glass, baked under these conditions, is an irreversible

process, so that air storage after prebake becomes possible.

Scanning electron micrographs were taken of the sealing glasses in an attempt

to correlate outgassing properties of the glass with physical structure.

Representative samples of the four lid types, KC-1 (standard), KC-1 (super dry),

KC-lM (standard), and KC-lM (super dry) were studied.

Micrographs of the external surface of the sealing glasses prior to heating at

430°C are compared in figure 5. The surfaces of KC-1 (standard), KC-1 (super

dry), and KC-IM (super dry) are similar, having a relatively smooth nonporous

appearance. The surface of KC-IM (standard) is rougher with the possibility

of a few pores present. Comparison of these data with the outgassing data

discussed in figure 1 gives no correlation between the appearance of the external

surface of the glasses and the quantity and rate of mass loss from them during

the evolution of moisture.

In figure 6 a cross-sectional view of the internal structure of the glasses

prior to heating is shown. Voids or pockets were observed in all samples.

KC-1 (standard) contains the greatest number of voids. The quantity of gas

evolved from this glass sample was also greatest (see figure 1) , suggesting

that the moisture in the glass was trapped in these voids. Ramsey has observed

voids in sealed Cerdip packages; the concentration of the voids varied from

5 percent to 60 percent.

Cross-sectional views of KC-1 (standard) before and after heating the glass in a vacuiim for three hours at 430°C are compared in figure 7. After vacuum heating

there were fewer but larger voids present in the glass. In some instances

openings from the voids to the glass surface were observed, showing pathways

by which gases escaped. The presence of voids after heating is probably a consequence of the experimental conditions; there was no external force

available to compress the glass (other than its own weight) , and eliminate

the pockets once the gas had escaped from the voids.

SEM data also show that there are compositional differences between KC-1 and

KC-IM glasses. Dark and light regions are present in the micrograph of KC-1

(super dry) in figure 4. The light regions indicate that a larger quantity of

225
elements with a higher atomic number are present in this region when compared

with the dark regions. The micrograph of KC-IM (super dry) is also composed

of light and dark regions, but the dark areas are much less abundant. Differential scanning calorimetry data also show evidence of compositional differences

between the two glasses; a sharp endothermic peak is observed at 492°C in the

thermogram of KC-IM while no such peak is observed for KC-1. However, these

compositional differences do not appear to influence the moisture evolution

properties of the glasses significantly since the outgassing curves in figure 1

for KC-1 (super dry) and KC-lM (super dry) are nearly identical.

Moisture Contents of Sealed Packages

Observations of outgassing behavior and structure properties of KC-1 (standard)

and KC-IM (standard) glasses are in accord with earlier moisture measurements

made on these glasses during engineering studies of dry Cerdip processing [l6] . Production qualification tests performed on various glasses included determination of moisture content of sealed specimen packages to assess dryness

property of the glasses under study. Moisture data are given in table 2.

The values were obtained via in-situ surface conductivity moisture sensors

and confirmed by mass spectrometry. These engineering tests used different

preseal bake conditions (150*^C in air) than the outgassing analyses at 430°C

in vacuum. Nevertheless, the moisture contents found in actual packages

correspond with the data of figure 1. A two-hour preseal bake, even as low as 150°C, does in fact produce an acceptably dry package with KC-1 (standard)

glass, as the outgassing behavior predicts. KC-IM (standard) glass yields a relatively wet package because even after two hours preseal baking it still

contains significant quantities of water vapor which, upon sealing, are incorporated into the package cavity. The large volume and number of pores

in KC-1 (standard) glaze (figures 4 and 5) and the opening of many of these

pores to its surface may explain why it loses much water rapidly when baked.

It may be that KC-IM (standard) glass tends to retain closed-pore structure and

thus releases water much more slowly, so that much longer preseal baking is

necessary to make it an acceptably dry glass, as borne out by the moisture

values in table 2 and figure 1 outgassing curves.

Storage and handling of KC-1 (standard) glazed parts between bake and seal

affect ultimate package dryness. Advantages of preseal baking KC-1 (standard)

glass are negated in proportion to humidity of the storage ambient plus residence

time of baked parts therein prior to seal. Close coupling of bake and seal is

optimum. This is contrary to the results in figure 2 for KC-IM (dry) glass,

which takes much longer to outgas its water but does so irreversibly when baked

in vacuum.

It is interesting to note that results of the glass qualification testing in

table 2 indicate that it is not necessary to preseal-bake glass above its

softening temperature to accomplish moisture release and thus condition the

glass to yield a dry package cavity.

3. CONCLUSIONS

Even though the quantity of water outgassed from the "super dry" processed

glasses is less, the data in figure 1 and table I show that a correctly designed

226
preseal bake may still be required to produce dry cavities. Both the "standard"

and "super dry" processed glasses should be prebaked until the rate of moisture

evolution decreases to acceptable levels. For instance, baking KC-1 (standard),

KC-1 (super dry) , and KC-IM (super dry) for 90 minutes at 430°C in vacuum

reduces the moisture evolution rate to less than 0.015 microgram per minute,

making production of dry packages more probable. On the other hand, the data

in figure 1 show that the moisture evolution rate of KC-IM (standard) remains

relatively high (0.049 microgram per minute) even after a 3-hour bake, so that the corresponding moisture content of sealed packages will be higher.

This is consistent with the higher moisture contents of sealed packages observed

(table 2) when KC-lM (standard) glass is used.

Moisture content measurements of sealed packages indicate that a preseal bake

for 2 hours at 150°C, even though well below the glass softening temperature,

may still be adequate to produce dry cavities, when accompanied by good practice

Cerdip assembly procedures.

SEM studies suggest that the water is trapped in voids present in the glass.

The presence of these voids may also affect the mechanical strength of the

hermetic seal.

Once the water is removed from KC-IM (super dry) by heating at 430°C in vacuum,

the lids can be stored in an air environment without significant water uptake

(figure 2). On the other hand, KC-1 (standard) reabsorbs water after baking at

150°C in air. Because KC-1 (standard) was not heated to its softening point,

the voids formed during glazing still remain even though the trapped gases in

the voids may have escaped during heating. Reabsorption of moisture may be

related to the large internal surface area still available.

ACKNOWLEDGMENTS

The vacuum microbalance and SEM portions of this study were carried out by

R. W. Vasofsky at Rome Air Development Center. Additional assistance of

L. A. Miller and J. M. Bird of the Harris Semiconductor Reliability and Assembly

Engineering Departments respectively is gratefully acknowledged. Special thanks

are due to Shirley Harkins for typing manuscripts of all Harris contributions

to this publication.

REFERENCES

1. Eisenberg, P. H. , Brandewie, G. V., and Meyer, R. A., Effects of Ambient

Gases and Vapors at Low Temperatures On Solid State Devices, 7th New York

Conference on Electronics Reliability, (May 1966).

2. Thomas, R. W., Moisture, Myths, and Microc ircuits , IEEE Transactions on

Parts, Hybrids, and Packaging PHP-12 (3) (September 1976).

3. Kolesar, S. C. , Principles of Corrosion, J. Electrochem Soc. 123,

155-167 (1976).

227
4. Koelmans, H. , Metallization Corrosion in Silicon Devices by

Moisture-Induced Electrolysis, J. Electrochem Soc. 123, 168-171 (1976).

5. Vasofsky, R. W. , Czanderna, A. W. , and Czanderna, K. K. , Mass Changes of

Adhesives During Curing, Exposure to Water Vapor, Evacuation and Outgassing

Part I: Ablefilms 529, 535, and 550, IEEE Trans. Comp. Hybrids, Manuf

.

Technol. CHMT-1, 405-411.

6. Vasofsky, R. W. , Water Vapor Sorption of Package Sealants, Proc. IEEE

Intl. Rel. Phys. Symp. , San Francisco, CA, April 24-26, 1979, pp. 91-96.

7. Simpson, W. M. , American Glass Review , pp. 10-11, (October 1976).

8. Hogan, R. E. , Chemtech , p. 42, (January 1971).

9. Rabinovich, E. M. , Neorganicheskie Materialy , 7 (4), 545-560 (1971).

10. Thomas, R. W. , Microcircuit Package Gas Analysis Techniques, Proc. IEEE

Rel. Phys. Symp., Las Vegas, NV, April 20, 1976.

11. Lowry, R. K. , Miller, L. A., Jonas, A. W. , and Bird, J. M. , Characteristics

of a Surface Conductivity Moisture Monitor for Hermetic Integrated Circuit

Packages, Proc. IEEE Rel. Phys. Symp., San Francisco, CA, April 24-26, 1979,

pp. 97-102.

12. Garbe, S. and Christians, K. , Vacuum-Technik , 11 (9), 1962, The Properties

of Glass Surfaces , L. Holland, Ed., p. 212 (John Wiley and Sons, New York,

1964).

13. Ibid., p. 225.

14. Czanderna, A. W. , Kollen, W. , Biegen, J. R. , and Rodder, J., Photoelectrically

Automated, Bakeable, High-Load Ultramicrobalance, _J^ Vac. Sci. Technol.

13, 556-559 (Jan-Feb 1976).

15. Vasofsky, R. , Czanderna, A. W. , and Thomas, R. W. , UHV System for the

Ultramicrogravimetric Study of Samples Loaded in a Controlled Environment,

J. Vac. Sci. Technol. 1^, 711-715 (March-April 1979)

.

16. Lowry, R. K. , VanLeeuwen, C. J., Kennimer, B. L. , and Miller L.A.

,

A Reliable Dry Ceramic Dual-in-Line Package, Proc. IEEE Rel. Phys. Sjnnp.

,

San Diego, CA, April 18-20, 1978, pp. 207-212.

228
FIGURES

A= < 200

8=200-500

C=500-l,000

=1,000-2,000

E=2,000-4,000

F=4,000-6,000

G=6,000-8,000

H=8,000-10,000

I = > 10,000

ABTCDEFGH I Moisture Content, ppmv

A= < 200

B=200-500

C=500- 1,000

0=1,000-2,000

E=2,000-4,000

F=4,000-6,000

G=6,000-8,000

H=8,000-10,000

1= > 10,000

A BtC D E F G H

Moisture Content, ppmv

Figure 1. Distribution of Moisture

Contents in 42 Cerdips sealed with

devitrifying glass.

Figure 2. Distribution of moisture

contents in 65 Cerdips sealed with

vitreous glass.

OUTGASSING OF KC-1 AND KC-1M

0 005

v> 0.010 -

i 0.015

430OC30

t ^ Tug/LID i_

/-KC-1M (DRY PROCESS)

^KC-1M (STANDARD PROCESST

^KC-1

(DRY PRnCESSI — .KC-1 (STANDARD)PROCESS)

1 1 i 1 , / .

TEMPERATURE PROFILE

1 1 1 1 60 90 TIME (MIN)

120 150 180

0.005

S

430OC

OUTGASSING KC-1M (DRY PROCESS)

20

A. INITIAL OUTGASSING

B. SUBSEQUENT OUTGASSING

AFTER 7 DAY EXPOSURE TO AIR

TEMPERATURE PROFILE

30 40 50 TIME (MIN)

60 70 80 90

Figure 3. Moisture outgassing profiles

of four KC type glasses.

Figure 4. Moisture outgassing profile

of KC-IM glass after one week post-outgas

exposure to room air.

229
a. KC-1 Standard b. KC-1 Dry

Figure 5. Surface view of glazed sealing glasses as received and prior

to heating (200X).

230
Figure 6. Cross-section views of glazed sealing glasses as received and

prior to heating (400X)

.

231
Figure 7. Cross-section views of KC-1 (standard) glass before and after

heating at 430°C in vacuum (160X). Note substantially reduced pore density

after heating.

TABLES

Table 1 - Major Differences in Elemental Composition of Vitreous and

Devitrifying Glass As Determined By DC Arc Optical Emission

Spectroscopy

.

Vitreous Devitrifying

>1.0% Al Zn, Zr

0.01-1.0% Zr, Li Al

<0.01% Zn Li

232
Table 2 - Mass Losses at 430°C.

KC-1 KC-1M

KC-1 (DRY) KC-1M (DRY)

1ST 10 MIN.

*ig/LID 16.10 9.24 6.44 7.14

MAX. MASS

LOSS RATE, 2.77 0.92 0.76 1.12 Mg/LID/MIN.

TOTAL MASS

LOSS, 3 HRS.

24.6 14.3 18.8 14.4

0.15 /iig WATER VAPOR s BOOOppmv WATER

IN A0.04CC CAVITY

Table 3 - Typical Moisture Contents of Standard Process

KC-1 and KC-IM Glasses With and Without

Pre-seal Bake

.

KC-1 (STD. PROCESS)

SEALED AS PRE-SEAL

GLAZED BAKE

KC-IM (STD. PROCESS)

SEALED AS PRE-SEAL

GLAZED BAKE

4500ppmv 500ppmv 7600ppmv 3200ppmv

4800 200 3500 1500 6000 900 7600 2900

5500 700 5800 2200

5200 200 8000 2700

4200 400 5800 2000

5900 800 4200 1300 4800 600 5400 2400

5800 3800

7600 1600 5000 1700 PRE-SEAL BAKES WERE FOR 2 HOURS AT 150OC

233
6.3 What's Wrong With Cerdips?

Robert W. Thomas

Rome Air Development Center

RBRE

Griffiss AFB, NY 13441

(315) 330-3730

Abstract

Recent mechanical failures of Cerdip type packages during board

assembly operations prompted a study to determine the sensitivity

of Cerdip packages to thermal shock. The results confirmed the

potential hazard of exposing Cerdips to even minimal thermal shock

(135°C to 25°C). The primary reason for the field failure was traced back to improper solder dipping operations in which the

package was submerged in the solder bath and then guickly cooled in

water during flux removal. Some of the packages treated in this manner failed incoming leak testing while in other cases the lids

fell off during shipment. Although electrical failures traceable

to loss of hermeticity have not been found, it was recommended that

these mechanically damaged parts not be used in military systems.

Key words: Cerdip; glass sealed; integrated circuit; packages;

quality control; thermal shock.

1. Introduction

Recent progress made by the manufacturers of Cerdip packaged components in

drying out the internal ambient has been particularly encouraging [1]. It

has been demonstrated that it is routinely possible to produce Cerdips on a manufacturing line using the new pre-dried vitreous sealing glasses with

water content of less than 1000 ppm^. This has been a significant achievement since three years ago when the average moisture content of Cerdips fell

between 5,000 and 10,000 ppm^ with the high outliers typically in excess of

15,000 ppm^.. Unfortunately, along with the improved dryness came an unexpected weakness to thermal and possibly mechanical shock. During a detailed

investigation as to the cause of this particular part failure, several other

problems associated with the sealing glass were uncovered which need further

investigation to assure optimum performance of the sealing glass.

2. Procedure

2.1 Recent Field Experience

The first sign of a problem was noted by a systems house when incoming shipments from the device manufacturer began failing the incoming fine and gross

leak test [2]. In some cases the lids would pop off with very little applied

force. A mechanical handling loss was also noted during the insertion of

these components into an automatic tester board. The only point in common among the failed or weakened packages was that each of the parts had been

solder dipped and that the solder coating extended all the way to the glass

seal at the lead frame. Torgue testing of parts from the suspect date codes

2 34
indicated a wide distribution of torque strength. A close inspection of the

seal area revealed microcracking of the glass seal. Fractures tended to

start at the corners of individual leads and quite often extended across the

entire glass surface between the leads. Dye penetrant studies were performed

which clearly demonstrated that in some cases the surface microcracks had

penetrated into the package interior. After studying a large number of packages produced during a four-year period, it became clear that the variability

in the solder dipping depth (the commercial process specification limits the

depth to 50 mils below the package bottom) and another variable which the

manufacturer later verified was an improper flux removal process in which the

packages were washed in water immediately after the dipping operation rather

than air-dried as specified in the process specification were responsible for

the part failure. After identifying the process and operator error, the

manufacturer brought his solder dipping operation back on-line except in this

case, the parts were dipped using an automated dipping process in which the

immersion depth could be carefully controlled and care was taken to insure

that the parts were properly cooled in air before the flux was removed.

These parts demonstrated a highly uniform torque breakage strength. Microcracking was considerably reduced and the packages now had no trouble in

passing the fine and gross leak test. In short, the manufacturer had solved

his production problem and was back on line. To insure that the process

remained under control, the manufacturer voluntarily instituted a sample

torque test on all solder dipped parts and issued a bold face type warning to

all solder dip operators that quick cooling of the parts may destroy them.

Further testing of the failed parts, however, produced some interesting data

on the thermal and mechanical properties of glass-to-metal seals.

2.2 Thermal Shock Threshold Experiments

Good parts were taken from the previous tested devices and subjected to decreasing levels of thermal stress starting from 260°C into water at 25°C and

proceeding downward to 1 00°C to 25°C. Torque testing of the parts which saw

the 260°C to 25°C stress had widely varying breakage strengths, including

several failures, microcracks, scattered loss of hermeticity and gas analysis

results which indicated leakage of air into the cavity. Since the parts are sealed at 450°C, the pressure inside the package is reduced on cooldown to

between 0.5 to 0.7 atmospheres depending on the outgassing properties of the

glass. Pressures measured by the mass spectrometer technique of greater than

0.7 atmosphere generally are indicative of hermeticity failure at some point

after the seal was made. Leakage such as this quite often is not detected by

fine and gross leak testing; small leaks tend to plug easily and are often

missed during gas constituent analysis since the Cerdip package is often

sealed in an air atmosphere containing both oxygen and argon. Another interesting observation concerning these thermally damaged parts was that dye

penetrant often showed penetration of the cavity even though fine and gross

leak tests had been performed successfully (fig. 1) . Apparently the wicking

action of a glass fracture will draw in the dye penetrant, whereas the helium

atom will not diffuse along the interface at nearly as high a rate. This

phenomenon has been previously observed by increased moisture penetration

along glass fibers embedded in epoxy as opposed to bulk diffusion through the

epoxy. Helium has not been observed to penetrate the polymer seal in either

case. This could explain the observance of microcracking and dye penetration

without leak test failures or gas ambient changes in all the stressed parts.

235

371-525 0-82-15
Thermal cracking of the glass seal was observed in all cases where the stress

exceeded 1 35°C to 25°C shock. One additional observation is also illuminating. Thermal shock of the same part types from 260°C to 25°C in oil instead

of water did not produce thermal cracking. It is postulated that the formation of stream initially lowers the temperature of the metal leak frame more quickly than the nonvaporizing oil and that there may be a secondary mechanism involving the enhancement of glass microcracking and fracture penetration rates in the presence of water vapor. This phenomenon is well known in

the glass cutting and fabrication industry.

2.3 Glass -to -Metal Adhesion

On closer examination of the sealing glass -to-Kovar interface, several observations can be made (fig. 2). The glass adhesion to the Kovar lead frame is

especially poor. During torque testing of mechanically weak packages, it was noted that the separation occurred at the glass -to-Kovar interface and almost

entirely within the seal glass material. The stronger packages, without

exception, would separate at the glass-to-ceramic interface indicating that

the shear strength of the sealing glass-Kovar seal area was stronger than

glass-to-ceramic bond. Additionally, the glass adhered extremely well to the

aluminum coating placed on the upper side of the lead frame for bonding purposes, whereas the lower surface of lead frame often separated without any

adherence, many times forming a perfect replica of the lead frame bottom

surface. Although the aluminum was deposited for an entirely different

reason, it considerably improved the glass adhesion. This was to be expected

since silicon dioxide and aluminum have been the most successful dielectricmetal system used to fabricate integrated circuits for the past twenty years.

Glass bonds extremely well to the naturally occurring aluminum oxide. It is

somewhat amazing that someone has not thought of using aluminum on both sides

of the lead frame to make a really strong chemical bond of the glass to the

lead frame. Poor adhesion can also be due to excessive amounts of filler

added to the sealing glass in order to alter the thermal coefficient of expansion. The filler material remains in a solid state during the sealing

process and therefore does not participate in the adhesion mechanism. Some

sealing glasses were found to contain up to 50 percent filler either by defect or design. Since this variable is difficult to measure quantitatively,

let alone on piece part lot acceptance, it may be one of the unknown variables affecting day-to-day sealing yields as well as long term thermomechanical performance of the Cerdip sealing system. Thermal coefficient mismatches

between the glass and Kovar of as great as 1200 ppm^ have been privately

reported. More emphasis has been placed on the matching of the glass -toceramic temperature coefficient than the temperature coefficient of the

glass-to-metal seal. As a result, there have been documented cases where the

glass -to-Kovar interface acts as a leak valve during thermal excursions in

which large temperature gradients can exist for short periods of time at the

interface [3]. These same parts will pass fine and gross leak checks under

conditions of thermal equilibrium [4].

3. Conclusion

It has been demonstrated that the sealing glasses used to seal Cerdip components are susceptible to even mild thermal shock (i.e., 1 35°C to 25°C water).

Water was shown to be a more damaging medium than oil presumably because of

236
its higher vapor pressure. IDye penetrant can be used to trace the path of

the leak and will sometimes uncover leaks missed by fine and gross leak methods. Extreme care should be taken by the manufacturer to insure that if any

of the package assembly (i.e., ceramic, metal, glass) is ever raised in temperature above 1 35°C it is done slowly allowing the parts to remain in near

thermal equilibrium during both the heating and cooling cycle. Finally, it

is important that the manufacturer use a dye penetrant to check the thermal

shock sensitivity of his packages and clearly warn customers that the rugged

inexpensive and dry Cerdip does have some thermal shock limitations.

References

1. Lowry, R. K. , Van Leeuwen, C. J., Kennimer, B. L., and Miller, L. A., A

Reliable Dry Ceramic Dual In-Line Package (CERDIP) , Proc. 15th Annual

Reliability Physics Symposium, Las Vegas, Nevada, 18-20 April 1978, pp.

207-212.

2. Troup, P. B., and Coleman, D. L. , Government Industry Data Exchange

Program (GIDEP) Alert L7-A-80-01, 2 April 1980.

3. Kokini, K., Perkins, R. W. , and Libove, C, Thermal Stress Analysis of

Glass Seals in Microelectronic Packages Under Thermal Shock Conditions,

Rome Air Development Center Final Technical Report, RADC-TR-79-201 (July

1979) .

4. Shumka, A., and Piety, R. R. , Migrated-Gold Resistive Shorts in Microcircuits, Proc. 13th Annual Reliability Physics Symposium, Las Vegas,

Nevada, 1-3 April 1975, pp. 93-98.

237
Figure 1. Dye penetration along lead frame into package cavity

(shown as the white areas between the lead frame). Package passed

fine and gross leak test.

Figure 2. SEM micrograph of single lead frame showing lack of

adherence (arrow) of the glass to the bare Kovar while adhering

tenaciously to the aluminum-coated portion of the lead frame

towards the inside of the package.

238
6.4 Moisture Impermeable Polymers

Philipp wh Schuessler

IBM Corporation

Owego, NY 13827

(607) 687-2121

ABSTRACT

By the late 70' s the microelectronic manufacturers had made significant

gains in the area of packaging with pol3nneric materials. Unfortunately,

several fleet and field incidents occurred which rapidly brought polymer

seals and encapsulants into disfavor with the military. In 1979, the

Navy funded an effort with IBM FSD to investigate if indeed a hydrophobic

polymer could be commercially available. This report relates the progress

made to date in the identification and testing of water "impermeable"

polymers. The two primary contributors to moisture permeation, solubility,

and diffusion are addressed; included are insights to how permeation may

be reduced via molecular reconfigurations and atomic substitution,

unfortunately not to the limits presently in effect for the non-polymeric

seals

.

Key Words: Diffusion; hydrophobic; moisture permeation; polymeric

materials; solubility.

1. INTRODUCTION

In 1976, the U.S. Navy released its Manufacturing Technology Program

for FY 1977-1981. In that document project funding was tentatively

identified for the fabrication of a non-metal packaging material

for hybrid microcircuits which had repair and resealing capabilities.

IBM FSD responded to this proposal and in 1979 commenced a Phase I, R&D

activity to identify and, if possible, optimize hydrophobic polymers

to meet the above objectives. This report briefly describes the

theory, approach, and preliminary results of this effort. (1)

Underlying Aspects of Hydrophobicity

It has been long recognized that hydrophobic or hydrophilic qualities

exist in the variety/ of polymeric materials known to industry.

Polytetraf luoroethylene, the polyalkylenes in general, and some aromatics, such as the parylenes, are readily identifiable as hydrophobic materials due to the high contact angle that exists

between them and v-ater. In addition, water solubility constants

for these materials are correspondingly low, typically less than

0.01%. However, it was also recognized that moisture rapidly

diffuses through these materials as evidenced by their subsequent

permeation rates which range between 1.5 and 4.5 x 10~9 g^-^

atm* cm-^/cm2 • sec . cm Hg for the respective polymers. (2)

239
The hydrophobic quality is imparted to these structures via the use

of hydrocarbon and/or fluorocarbon chemistry. Conversely, hydrophilicity

is improved by increasing the number of unsaturated bonds and free

electron pairs in the material. The net of this is the fewer

electron rich sites left in a polymer, the fewer places the highly

polar water molecule can be electrostatically attracted to.

As one surveys the available literature on solubility limits of

moisture in polymers or generates the data, when previously not

available, it becomes obvious that moisture tends to diffuse into a polymer and eventually saturates it. It is understood that the

diffusion process exists with ease for those materials with low

crosslink densities or low crystallinity, as water molecules can squeeze between the polymer molecules. As the crosslink density

and/or crystallinity increases, the moisture diffusion rates tend

to decrease. The question then arises,, can we ever lower the

permeation rates to approximate those of glasses and metal seals,

i.e., the truly hermetic seals.

PROCEDURE

Several techniques have been reviewed for the determination of

moisture solubility and diffusion in the candidate materials. For

a variety of reasons not to be reviewed here, the technique of

sensing moisture levels via the established moisture sensors and/or

the also recognized phosphorus pentoxlde (P20^) detector as sketched

in figure 1 were not incorporated into this study. In their place,

the historically established McBain balance was used to sense moisture

at microgram levels in situ for optimum temperatures and humidities . (3)

The McBain balance is simply a vertical quartz coil spring at the

lower end of which is the sample to be tested. As the material takes

on or gives off moisture, a change in the spring's extension results.

This change in length or displacement is monitored by means of a cathetometer as indicated in figure 2. Candidate materials for the

study included a variety of hydrophobic structures as indicated in

figure 3 which include:

f luorcepoxies polys iloxyimide

f luoroacrylics f luorosilicones

polyphenylene acetylenes

acrylated epoxy novolar

Although solubility characterizations for these materials have just

started, none have or are expected to have a moisture solubility

constant of 10~2 or less. This predication is based on data in

hand, available literature, and theory.

Results to Date and Forward Plan

The preliminary findings of this study indicate that regardless of

how hydrophobic, i.e., nonpolar, one makes a material, it ultimately

will have some low level of moisture solubility. This solubility,

in association with the materials moisture diffusion constant,

ultimately results in moisture breakthrough (permeation) in a polymer

sealed device.

240
This moisture breakthrough is accomplished via an intermolecular

phenomenon whose limitation or elimination remains to be addressed

in the follow-up studies. This phenomenon, London dispersion forces

or Van der Waals forces, exists to an extensive degree between all

molecules regardless of polarity. (^) As a result, intermolecular

attractive forces are relatively high and diffusion and solubility

result. One of the approaches to meet this objective is to limit

diffusion via "plugging" the spaces that exist between the polymer

chains. This could be accomplished via plasma deposition using a non-polar species (5) or the formation of interpenetrating polymer

networks (IPN) . (6) Literature sources tend to indicate that the

latter approach would be more successful. (7) Testing in this area

will be done as outlined in the experimental section on samples

provided by the various experts in the fields or plasma deposition

and IPN polymers

.

CONCLUSIONS

IBM FSD has been studying available polymeric materials which can maintain their hydrophobic qualities when used in a microelectronics

packaging application. Both contributors to permeation, i.e.,

diffusion and solubility, are being addressed in this effort. It

is suspected, but remains to be proven, that (1) the solubility

constants cannot be lowered sufficiently such that when factored

against, (2) the lowest possible diffusion rates the resultant

moisture breakthrough, i.e., permeation, will still remain

unacceptably high for processable polymeric sealants when mil-spec

moisture limits are in effect.

241
REFERENCES

^^^Nav Air Sys Comm. Contract N00019-79-C-0665 "Hydrophobic Resins".

(2)

^ '^Tobolsky, A.V., and Mark, H.F., "Polymer Science and Materials",

1971, J. Wiley & Sons.

(3) McBain, J.W. , Bakr, A.M., "A New Sorption Balance", J. Am. Chem.

Soc. 48 (1926) , page 690.

(4) Fowkes, F.M. , "Forces at the Interface", Polymer Conference Series,

University of Utah, June 24-28, 1974.

^^^Czornyj, G. , "Plasma Pol3nnerization of Crystalline Fluorocarbon Films"

53rd Colloid and Surface Science Symposium, University of Missouri,

Rolla, June 13-15, 1979.

Sperling, L.H. , "Interpenetrating Polymer Networks: A New Class of

Materials", Materials Engineering, September 1980.

^^^Frisch, H.L. , Frisch, K.C., and Klempner, D. , "Examining the

Properties of Interpenetrating Polymer Networks", Modern Plastics,

May 1977.

242
T

Exit

Figure 1. In-situ analyses via thermistor and ^2^5 detectors,

243
/ Quartz

Spring

Sample

Cathetometer

1

Water Supply

At Constant

Temperature

Figure 2. McBain balance.

244
Materials Source

Fluoroepoxy

Fluoroacrylic

Polyphenylene Acetylene

Bis-Phenol-S-Epoxy

Fluorosilicone

Fluoroether Bibenzoxazole

Acrylated Epoxy Novolac

Polysiloxyimide

Oxi-lmide

Polyparabanic Acid

Hydrophilic Coupling Agents

Polyphenylene Sulfide

NRL

NRL

Hercules

Ablestik Labs

Dow & GE

Wright Patterson AFB

IBM

M&T

Dupont

Exxon

Dow Corning/Union Carbide

Figure 3. Candidate materials for study of hydrophobic structures.

245
6.5 Plastic Packaging

Dimitry Grabbe

AMP Incorporated

3705 Paxton Street

Harrisburg, PA 17111

, - (717) 780-6645

Paper not submitted.

246
7. SESSION VI HERMETICITY, FACT OR FICTION

7.1 A Method of Assessing the Surface Conductivity of Plastic

Encapsulated Integrated Circuits

Robert P. Merrett

British Telecom Research Laboratories

Martlesham Heath

Ipswich IP5 7RE England

0473 - 642309

Abstract: The moisture-induced surface conductivity of passivated

ICs, in plastic packages, has been measured using a technique previously used for ICs in hermetic enclosures. The method involves

measurement of the capacitance between two metallisation tracks, at

frequencies of 100 and 1000 Hz. By comparing the capacitances

obtained before and after storage of the packages for 24 hours in a saturated autoclave at 110°C, it is possible (I) to detect whether

water permeating the plastic has formed a conducting film on the

surface of the die, and (ii) to obtain an estimate of the surface

conductivity. The technique thus offers the prospect of the rapid

appraisal of two of the factors affecting the reliability of

plastic-encapsulated ICs in humid environments.

Key Words: Integrated circuits; plastic encapsulation; moisture

reliability; surface conductivity.

1. INTRODUCTION

Several types of accelerated life-test have been developed to assess the

performance of plastic-encapsulated semiconductor components in humid

environments [1]. They all involve determining whether an unacceptable

deterioration in performance has occurred during a period of stress, at high

temperature and relative humidity, which accelerates the effects of the less severe environments encountered in normal use. The most commonly used stress

being several hundred hours, with bias applied, in an environmental test

chamber at 85°C, 85% RH. It is, however, possible to reduce the duration

of the stress by employing unsaturated autoclaves to produce more strenuous

conditions (typically 110°C 90% RH) [2,3]. There has also been interest in

tests based on the degradation which occurs when unbiased components are placed in saturated autoclaves (typically at 120°C) [4].

An alternative approach which merits consideration is to determine whether

a short period (24 hours) in a saturated autoclave can produce the conditions

- at the die surface - needed to induce degradation. The advantage of this

approach is that it would not be necessary to ensure that the duration of

the stress was sufficient for these conditions to progressively produce a detectable degradation.

247
When a plastic-encapsulated semiconductor die is exposed to a humid

environment, water permeates the plastic and eventually reaches the surface

of the die [2]. The arrival of water at this interface does not, however,

constitute a reliability hazard unless the adhesion between the two surfaces

is poor enough to allow the formation of a film of water on the surface of

the die [5]. Such a film provides the conduction path required to sustain

moisture-induced failure mechanisms. The rate of degradation depends on the

thickness of the film, and on the degree to which its conductivity is

enhanced by ionic contamination leached from the plastic [5,6].

By measuring the surface leakage currents of unpassivated metallisation

patterns, encapsulated in a variety of plastics, it has been possible to

detect the accumulation of water on the die and to correlate the surface

conductivity with the tirae-to-failure [5,6]. These studies were not extended

to include passivated ICs because, as the water film was no longer linking

the metallisation tracks, there was no way to measure the surface

conductivity. It has, however, recently been demonstrated [7] that the

capacitive coupling between the water film and the tracks, can be used to assess the surface conductivity of passivated ICs in hermetic enclosures.

The objective of this study was to devise a method of (i) detecting the

accumulation of water on the IC die and (ii) of estimating the surface

conductivity. The paper first discusses the theoretical aspects of the

method and then outlines the experiments performed to demonstrate its

potential for the rapid assessment of plastic encapsulated ICs.

2. THEORY

The measurement technique is based on the observation [8] that a conducting

film on the surface of a passivated semiconductor die enhances the capacitance

between metallisation tracks. The effect of such a film can be explained

by reference to the generalised structure illustrated in figure 1, which

shows a cross-section through a passivated metallisation pattern lying on an

oxide grown on a conducting substrate. A film of water on the surface of the

passivation forms a conducting sheet which is capacitively coupled, by the

passivation layer, to the two metal tracks. This coupling enhances the

capacitance ( C) presented to an alternating voltage source connected between

these tracks. When quantifying the effect of the coupling, it is necessary

to take the appreciable resistance of the water film into account. Such an analysis shows [7] that the capacitance C can be expressed as the sum of two

tems ;-

C = C. + 0.0^ ... (1)

where C. is the inherent capacitance between the tracks in the absence

i

of water; its value is determined by the thickness of the oxide

and the dimensions of the tracks, and

0 is the fractional increase in capacitance when the film of water

forms.

248
The factor jD, which is called the capacitance-ratio, has a value given by

the equation:-

0 = ^Jf ••• (2)

is the surface conductivity,

is the frequency of the alternating voltage,

is the width of the metal tracks, and

is a factor which depends on the thicknesses of the oxide, and

the passivation [7]. For ICs these are typically 0.5 /xm and the

corresponding value of V is 28 m.(Q/s)T,

The important feature of this equation is that, as neither the spacing nor

the length of the tracks are involved, it is valid for any metallisation

pattern. The only restriction is that eq (2) is based on an approximation

[7] which requires that the term (cr/f) be small enough to ensure that 0 is

less than about 0.1.

A simple method of measuring the capacitance-ratio 0 can be devised by noting

that the contribution that water makes to the capacitance between tracks

decreases as the frequency of the alternating voltage increases. Thus, the

difference between the capacitance C(f-]_) measured at a low frequency f-^, and

the capacitance C(f2) at a higher frequency f2, can be attributed to moisture.

By combining eqs (1) and (2), it can be shown that the capacitance-ratio at

the lower frequency is given by the equation:-

The criteria which determine the choice of measurement frequencies have been

discussed elsewhere [7]. For most applications values of 100 and 1000 Hz

are suitable.

2. 1 Application to ICs

Many of the pins of ICs have direct connection to metallisation tracks, so the capacitance between a pair of such pins will have the same sensitivity

to moisture as the capacitance between the tracks of the generalised

structure. The tracks on the former are, however, linked by circuit elements

which can prevent the determination of capacitance unless the amplitude of

the measuring voltage is small enough (<35 mV rms) to stop these elements

from having a significant conductance [7,8]. Even when this precaution is

taken, it must be recognised that a fraction of the measured capacitance will

be attributable to circuit elements. To minimise the size of this fraction,

it is necessary to choose a pair of pins which are connected to long lengths

of track.

Even when such steps are taken, allowance must be made for the possibility

that the capicitance added by circuit elements may be frequency dependent.

In such a case, the capacitance-ratio obtained by using eq (3) will include

where o

f

W

Y

C(f^) - C(fg)

C(f2)

1 -

— I ... (3)

249
a contribution due to circuit elements (^^) in addition to that caused by

a film of water on the surface of the die. Fortunately, the value of 0^ can be determined by measuring the capacitance-ratio before subjecting the

IC to a stress which encourages a water film to form. Any increase in the

capacitance-ratio, as a result of the stress, can thus be ascribed solely

to moisture, and eq (2) can be used to determine the surface conductivity

needed to account for such an increase.

A simpler method of determining the surface conductivity would be to compare

the capacitances measured, at a single frequency, both before and after

the stress period. Any increase in capacitance would be attributable to the

formation of a film of water on the die surface, and thus used to calculate

the surface conductivity. However, this approach is impractical because

the difference in capacitance ( typically ^0. 1 pF) is likely to be comparable

with the changes in the parasitic capacitances associated with the measuring

system. Such changes do not significantly affect the method which relies

on a comparison of the frequency dependence of the capacitance.

2.2 Accuracy of the Estimation of Surface Conductivity.

In the derivation of eq (2) it is assumed that the conducting film extends

over the whole of the metallisation involved, and that the Inherent

capacitance (Cj[) does not include a contribution from circuit elements. As

these assumptions will not be valid for all ICs , it must be recognised that

the value of surface conductivity calculated using this equation is only an estimate. For example, if only half of the metallisation was under the

conducting film, or if circuit elements contributed half of the inherent

capacitance, then the calculated surface conductivity would only be a quarter

of the actual value. This inaccuracy does not detract from the ability of

the measurement technique to detect the formation of a conducting film, but

it may well complicate any attempt to correlate surface conductivity with

time-to-failure.

3. EXPERIMENTAL DETAILS

3.1 Specimens

Results are presented for specimens from two production batches of 741

operational amplifiers made by a single manufacturer. These batches,

which will be identified as A and B, had date codes which differed by four

weeks. The moulding compound used for the eight pin dual-in-line packages wa epoxy novolac, a compound in widespread use. Of the eight specimens which

were withdrawn from each batch, four were used in the main experiment and fou

in a control experiment as defined below.

3.2 Experimental Conditions

The effect of moisture on the capacitance-ratios of the ICs was observed by

comparing their initial values with those obtained after the ICs had been

stored in a saturated autoclave at 110°C. For the main experiment, the

specimens v.'ere subjected to this 'damp-heat' storage for a total of three day

but they were removed every 24 hours and a measurement of the capacitance- .

250
ratio made after allowing a 30-minute cool-down period. On completion of

the storage in damp-heat, the moisture in the plastic was driven out by placing

the specimens in an oven at 110°C for three days. As before, the specimens were removed every 24 hours from this 'dry-heat' to permit measurement of

the capacitance-ratio.

For the control experiment, the specimens were subjected to storage in

dry-heat before storage in damp-heat. As in the main experiment, the total

duration of each storage period was three days, and the interval between measure

ment of the capacitance-ratio was 24 hours.

Previous studies [6,9] have shown that moisture-induced surface conductivity

is an increasing function of temperature. Accordingly, at every measurement

interval, the capacitance-ratio was determined at a die temperature of 20°C

and then when a strip-heater, pressed against the top of the package, had

raised the die temperature to 75°C. The conditions needed to obtain the

higher temperature were established, in subsidiary experiments, by using a forward-biased junction to measure the die temperature.

3.3 Measurement of the Capacitance-Ratio

The determination of the capacitance-ratio was based on the frequency

dependence of the capacitance between the pins used for the ± Vcc supplies.

This capacitance, which had a nominal value of ~22 pF, was measured while

the ICs were in an electrically screened box purged by dry nitrogen. The

system used for the measurement of the capacitance has been described

elsewhere [7]. The amplitude of the measurement voltage was 15 mV rms , and

its frequency could be switched between 100 and 1000 Hz. Having determined

the capacitance of these two frequencies, eq (3) was used to calculate the

value of the capacitance-ratio at 100 Hz.

4. RESULTS

4. 1 Initial Values of the Capacitance-Ratio

Before the storage periods, the measured capacitance-ratio was very sensitive

to die temperature. At 20°C the average value of the capacitance-ratios of

the 16 specimens was 0.007, and it rose to 0.08 when the temperature was increased to 75°C. The values of the capacitance-ratio at 20°C, and the

temperature dependence, were comparable with those previously shown to be

due to circuit elements [7].

The effect of the storage periods on the specimens was found to be dependent

on the date of the manufacture, so the results for the two batches studied

will be given separately.

4.2 Specimens From Batch A

During the first storage period, there was a gradual increase in the

capacitance-ratios of the specimens subjected to damp-heat. The changes in

the value at 75°C being about six times greater than those observed at 20°C.

When the specimens were subsequently stored in dry-heat, the capacitance-ratios

251

371-525 0-82-17
gradually decreased, but by the end of both storage periods, they were still

significantly higher than their initial values.

For the control group, the capacitance-ratios exhibited a small decrease

during the initial storage period in dry-heat. When this group was subsequently subjected to damp-heat, there was a marked increase in the

capacitance-ratios

.

Although all the specimens in both groups responded to the three days in dampheat, some were more sensitive than others. The increase in capacitance-ratio

relative to the initial value, ranged from 0.018 to 0.033 at a die temperature

of 75°C, and from 0.003 to 0.005 at 20°C.

The gradual changes in the capacitance-ratio during the storage periods can

be characterised by giving the average increase (relative to the initial

values) noted at each measurement interval. The results for the main group an

the control group are presented separately in figures 2a and 2b, respectively.

4.3 Specimens From Batch B

In contrast with the behaviour described above, there was a small decrease

in the capacitance-ratios of the specimens which spent the first storage

period in damp-heat. This behaviour was continued ''uring the subsequent

storage in dry-heat.

However, the capacitance-ratios of the specimens in the control group changed

in the same way as those of the specimens in the corresponding group from

batch A. During the storage in dry-heat, there was a small decrease in the

capacitance-ratios, whereas the subsequent storage in damp-heat caused a considerable increase in the capacitance-ratios of all the specimens in this

group.

As before, the changes in the capacitance-ratios during the storage periods

can be characterised by giving the average increases for the specimens in

each group. The values, noted at the various measurement intervals, are presented in figures 3a and 3b.

5. DISCUSSION

These tests on nominally identical plactic-encapsulated ICs have shown that

for some, the capacitance-ratio could be significantly increased by storage

in damp-heat. There is no reason to doubt that all the packages were equally

permeable to water, so it must be concluded that the increase in capacitanceratio only occurred when water accumulated on the surface of the die and so formed a conducting film. Thus, by measuring the capacitance-ratio before

and after storage in damp-heat, it is possible to make a rapid appraisal of

the interface between the plastic and the die.

The storage time needed for such an appraisal will obviously depend on the

permeability of the plastic. For the epoxy novolac packages considered here,

a storage time of only 24 hours in a saturated autoclave at 110°C was sufficient to cause an unambiguous rise in the capacitance-ratio. This

252
observation is consistent with published results [5] for unpassivated test

structures encapsulated in either epoxy novolac or epoxy anhydride; it was reported that the surface conductivity began to rise rapidly after about a 10-hour exposure to a 90% RH at 110°C.

The accuracy of the determination of the amount by which the capacitance-ratio

has risen above its initial value will, in general, depend on the temperature

at which the measurement is made. The results presented in figures 2 and 3

show that the water film enhanced the capacitance-ratio more at 75°C than

it did at 20°C. However, it must also be noted that some of the advantage

gained, from increasing the response to the conducting film, was lost because

raising the temperature had an even larger effect on the initial value

(the contribution due to circuit elements). Thus, the optimum test temperature

will depend both on the resolution of the measuring system and on the size

of the initial capacitance-ratio. For the combination considered here, this

temperature was about 75 °C. The influence of the measurement temperature,

on the amount by which moisture enhanced the capacitance-ratio, can be shown

to be in accord with published data for surface conductivity [9]. For

unpassivated test structures, covered by a film of water whose thickness does

not change with temperature, the surface conductivity can be described by

the equation :-

a = exp - k.T ... (A)

where is a constant of proportionality, k is Boltzman's constant and T

the absolute temperature. The activation energy £ depends on the thickness

of the water film, its value is about 0.8 eV when the surface is covered with

a monolayer, and it reaches a limiting value of 0.4 eV when the film is more

than about 3 layers thick. By combining eqs (2) and (4) it can be shown that

the increase in the capacitance-ratio at 75°C should be exp(3.13£) times

that at 20°C. Thus, an activation energy of 0.57 eV is required to account

for the observation that the quotient of the increases in capacitance-ratios,

at these two temperatures, had an average value of 6.0.

By using eq (2) it was possible, as explained in sect. 2.2, to relate the rise

in the capacitance-ratio to the surface conductivity of the die. For the

ICs examined here, the width of the metallisation was 10 ^m and the oxide

and overglaze thicknesses were taken to be 0.5 ,um, so the relationship between

the surface conductivity and the rise in capacitance-ratio becomes:

11 2 o = 1.3 x 10 (increase in capacitance-ratio) . (5)

For the 12 specimens which responded to the three days in damp heat, the average

increase in the capacitance-ratio at 75°C, was 0.022. Substitution of this

value into eq (5) gives a surface conductivity of 6.3 x 10~15f2-l/Q^ The

conductivity at other temperatures can be estimated by using the activation

energy of 0.57 eV. For example, at 110°C the surface conductivity would rise

to 4 X 10~14 fi-l/D J a value near the middle of the range of surface

conductivities observed [5] when plastic-encapsulated test structures were subjected to a 90% RH at 110°C. Life-test data for the se structures indicated

that corrosion of the aluminium metallisation would cause failure within 100

253
hours if the surface conductivity was equal to the calculated value for the

ICS at 110°C.

Although the results given here are too limited to enable a comment to be

made on how successful manufacturers are at achieving good adhesion between

the plastic and the die, they do highlight one disturbing feature. Namely

that, for specimens from batch B, storage in damp-heat was only able to cause

an accumulation of water on the die surface if it had been preceded by

storage in dry-heat. Thus, this combination of storage periods must have

altered the interface between the plastic and the die. The changes at the

interface cannot be blamed solely on temperature-cycling; because the batch

B specimens, which were subjected to damp-heat during the first storage period

exhibited no response to water despite the fact that they experienced large

temperature changes when they were periodically removed for measurements.

A plausible explanation is that removal of residual water in the plastic,

during the storage in dry heat, followed by hydration of the plastic during

damp-heat, causes the plastic to contract, and then swell, sufficiently to

disturb the interface between the plastic and the die.

6. CONCLUSIONS

A simple method of detecting the formation of a film of water, on the surface

of the die of a plastic-encapsulated IC , has been devised by noting that the

effect of such a film, on the capacitance between metallisation tracks, is

frequency dependent. By measuring the capacitance at two frequencies, both

before and after a short period of storage in a saturated autoclave, it is

possible to determine whether a film of water has formed, and to estimate

the surface conductivity of the die. As these are two of the factors which

affect the reliability of plastic-encapsulated ICs operated in humid

environments, the capacitance technique could be used to assess the

suitability of these components for exposure to such conditions.

ACKNOWLEDGMENT

Acknowledgment is made to the Director of Research, British Telecom Research

Laboratories, for permission to publish.

REFERENCES

1. Peck, D.S., and Zierdt, C.H. , Temperature-Humidity Acceleration of

Metal-Electrolysis Failure in Semiconductor Devices, 11th Annual

Proceedings Reliability Physics Symposium IEEE, 1973, 146-152.

2. Lawson, R.W., The Accelerated Testing of Plastic Encapsulated

Semiconductor Components, 12th Annual Proceedings Reliability Physics

Symposium IEEE, 1974, 243-249.

3. Sinnadurai, F.N., The Accelerated Aging of Plastic Encapsulated

Semiconductor Devices in Environments Containing a High Vapour Pressure

of Water, Microelectronics and Reliability, 13, 23-27, 1974.

254
4. McGarvey, W.J., Autoclave Vs. 85°C/85% RH Testing - A Comparison,

17th Annual Proceedings Reliability Physics Symposium IEEE, 1979,

136-142.

5. Sim, S.P., and Lawson, R.W. , The Influence of Plastic Encapsulants and

Passivation Layers on the Corrosion of Thin Aluminium Films Subjected

to Humidity Stress, 17th Annual Proceedings Reliability Physics

Symposium IEEE, 1979, 103-112.

6. Sbar , N.L., and Kozakiewicz, R.P., New Acceleration Factors for

Temperature, Humidity, Bias Testing, 16th Annual Proceedings Reliability

Physics Symposium IEEE, 1978, 161-178.

7. Merrett, R.P., Sim, S.P., and Bryant, J. P., A Simple Method of Using

the Die of an IC to Measure the Relative Humidity Inside its

Encapsulation, 18th Annual Proceedings Reliability Physics Sjnmposium

IEEE, 1980, 17-23.

8. Bakker, N. , An In-Line Measurement of Moisture in Sealed IC Packages,

Philips Telecommunications Review, 37, 11-19, 1979.

9. Koelmans, H. , Metallization Corrosion in Silicon Devices By Moisture

Induced Electrolysis, 12th Annual Proceedings Reliability Physics

Symposium IEEE, 1974, 168-171.

255
metallisationsubstrate

film of water

passivation

oxide

w w

Figure 1. Cross-section through the generalised structure

o . c

+j . 'o . 03Q.

03 .

.—C ^^

03 a) ^ .>

en Qj

damp-heat j dry-heat

0.03

0.02 -

0.01

-0.005 L

• 75 °C

20 °C

I ^ 0 1

dry-heat | damp-heat

0 1

time in storage (days)

(a) (b)

Figure 2. The average changes in the capacitance-ratios as a function of the time in storage (in damp-heat and dry-heat) at 1 10 °C, for batch A specimens in

(a) the main group and (b) the control group .

256
CO

• QJ

OJ 3 O -= C TO

CO > M

O .2

CO '+-> Q.- =

CD .E =5

<^ o ?°

CO <u -C .> O !-«

CO 1- >

00

damp-heat i dry-heat

0.03 r

0.02

0.01

0

-0.005

dry-heat • damp-heat

75°C

20 °C

-A —

JL 0123456 012

time in storage (days)

(b)

Figure 3. The average changes in the capacitance-ratios as a function of the time in storage (in damp-heat and dry-heat) at 110 °C, for batch B specimens in

(a) the main group and (b) the control group.

257
.2 Internal Moisture Measurement of IBM Integrated

Circuit Memory Package

Henry C. Baron and F. Richard Moser

IBM Corporation

Essex Junction, VT 05452

(802) 769-0111

John Susko

IBM Corporation

Endicott, NY 13760

(607) 755-0123

ABSTRACT

Moisture measurements on IBM Integrated circuit

packages and package materials have been conducted with the aid of a DuPont moisture analyzer and a unique moisture extraction apparatus designed by Mr. John Susko of IBM. The data indicate the quantity or moisture reaching the interior of the package is many times greater than

the amount one would estimate solely from the

free volume vapor space and the solubility of

water in the various package materials at equilibrium. A case is made to support the hypothesis that a significant water monolayer buildup

occurs within such a package. The experimental

data also indicate that moisture ingress and

egress through the epoxy package backseal is

initially quite rapid and suggest a significant

water monolayer buildup occurs with a few days.

Key Words: Egress; ingress; integrated circuit

package; moisture; monolayer buildup.

INTRODUCTION

Since the presence of moisture is a necessary condition

for chip and/or substrate corrosion, a study was conducted

to determine the quantity, distribution, and kinetics of

water from the interior of IBM's memory packages.

This paper will describe the experimental apparatus used

to measure such moisture. The results will be discussed

in the context of previous works which were done.

A brief description of an IBM module package will follow

in order to give the reader an understanding of the configuration that is being measured.

258
IBM MODULE DESCRIPTION

A photograph of a fully encapsulated IBM module is shown

in Figure 1. The package is basically a crimped metal

can which covers an alumina substrate and is encapsulated

using an epoxy backseal material. A cross section of the

module's interior is depicted schematically in Figure 2.

The chip is attached to the substrate using solder connections. A top seal is used to passivate/insulate the

substrate. Since the backseal is not hermetic, moisture

permeates through the backseal and enters the module

between the can/substrate gap. The permeation rate is

a function of relative humidity and temperature and is

discussed in the literature.

MEASUREMENT SYSTEM

Figure 3 is a block diagram of the major components of

the measurement system. In principle the technique

involves diluting the moisture in the package with a

carrier gas and routing this gas to the moisture

analyzer where the measurement takes place.

The module moisture extraction chamber is shown in Figure

4. The module is placed into the base of the chamber as

is shown. After the module is mounted, and the chamber

is closed, the can is punctured with tv/o hypodermic needle

These punctures provide an entrance through which the dry

N„ gas can enter the module's interior and an exit from

which the moist gas can be routed to the moisture analyzer

"0-Rings" are used to provide a seal at the needle-can

interface. In order to drive off the moisture, the

chamber must be externally heated.

The DuPont 901 Moisture Evolution Analyzer is shown in

the photograph of Figure 5. Its principle of operation

is to convert the moisture of the carrier gas to an electrical signal. The system's electronics in turn measures

the signal and converts it to micrograms in the LED output

The electrical voltage is generated by passing the gas

through the electrolyte cell as is shown in Figure 6.

The cell is a U-shaped tube with two helically wound

electrodes separated by a phosphorous pentoxide coating.

As the gas traverses the tube, the moisture reacts with

the phosphorous pentoxide such that

2P^0^ + 6H^0 -> 4H^P0, .

259
The product E^FO^ is regenerated to ^2^5 ^-^ applying a reverse bias across the electrodes so tnat

4H,^P0^ + 12e~ -> 2V^0^ -.-^ + 30^^ . The charge that is created by the regeneration is integrated and displayed by the system's electronics. The

manufacturers specify sensitivity to within 0.1 yg

and accuracy to 2% of the reading or +20 yg H„0, whichever is greater.

The duration of each run is approximately one hour.

Before each set of runs, the MEA is calibrated by measurin

a 1980 yg H^O standard.

REVIEW OF PREVIOUS WORKS (HISTORICAL OVERVIEW ) Much of the work done by other investigators focused on

the measurement of the module's internal relative humidity

as a gauge of the total internal moisture.

In particular, theoretical work done by Memis and experimental measurements made by M. E. Sweet indicated that

the relative humidity within a module eauiiibrated

with the exterior environment in an exponential manner;

e.g.,

RH(t) = RH + (RH„ - RH ) (1 - e'^^^T ) .

RH modules interior relative humidity

RH module's interior relative humidity at t = 0

RH relative humidity of the environment (dessicator)

T RC times constant of backseal

This analysis treated the backseal analogies to an RC

electrical element and the time constant was found to

be

T = exp (.301 ev)/RT = .0047 hrs.

At 85°C/80% RH, t is 80.3 hrs. At this condition the

maximum water that the (0.86 cc) interior module vapor

space can hold is 2 46 yg. Converting the above equation

to mass of water. Curve A in Figure 7 indicates as a function of charging time how much water is contributed

by the vapor space alone.

Permeation studies by J. Susko et al. found that at

85°C the transmission rate of moisture through the epoxy

backseal is

P (85°C) = 5 X lO""^ (std cc) cm/cm^ (cmH ) sec.

260
For our module configuration, this figure translates to

21.7 yg/hr as is shown in Curve B of Figure 7. This value

represents the maximum rate at which moisture may be

transmitted through the backseal. If a higher rate is

observed, this would be indicative of a gross leakage

path.

EXPERIMENTAL RESULTS

In order to characterize the kinetics of ingress, a group

of modules were "charged" with moisture for a period of

time. The charging was accomplished by placing the modules

in a dessicator which was in equilibrium with a saturated

salt solution. The dessicator was placed in an elevated

temperature ambient. Depending on the salt used, constant

relative humidities could be obtained.

This study focused on a group of modules charged at 85°C/80%

RH. Members of this group were measured after various

charging times, and the results of these measurements are

given in Curve C of Figure 7. It can be observed from

this plot that initially the accumulated moisture falls

along the permearion plot (Curve B) . However, as the

partial pressure inside the module increases, the perm.eation rate is inhibited and the total module moisture

begins to reach saturation. This occurs around 160 hours.

Comparing the experimental data with the moisture contributed by the vapor space (Curve A) , we are led to

believe that other sources exist that contribute six

to seven times the water contributed by the vapor space.

It seems plausible to assume that these sources are water

absorbed and adsorbed in the module materials.

This hypothesis is supported by observing the real time

dilution of a charged module as is shown in Figure 8.

At a flow rate of 40 cc/min it would take 45 seconds for

dry N2 to sweep 99% of the module's vapor moisture

through the MEA. This contribution is contained in the

sharp peak in the right hand side of Figure 8. Close

examination of this region of the curve also reveals the

246 ug of vapor space moisture is but a fraction of the

total water that is "easily" extracted from our module.

This additional moisture is believed to be many monolayers

of adsorbed water v.'hich quickly desorbs into the vapor

space to maintain the partial pressure equilbrium as water

is being removed from this volume. The curve to the left

of the peak has a time constant of '^j 14 minutes.

An estimiate of the source of the moisture that was detected

is shown in Table 1. The values shown for the individual

module materials are based on measurements made on those

261
specific materials after being conditioned at a given

temperature and humidity ambient. The value shown for

the aluminum cap is quite high but is not inconsistent

with measurements made on this particular material.

Indeed, the amount of water associated with the aluminum

cap strongly suggests a significant monolayer buildup

(> 1000) on that surface.

One final caveat, this study indicates how the moisture

may be distributed but is not intended to be conclusive

due to a somewhat limited data base.

VI . CONCLUSIONS

This work endeavored to determine the quantity and distribution of moisture in an encapsulated IBM module. To

this end a measurement system which basically consisted

of a moisture analyzer and extraction chamber was assembled.

The moisture of modules and their component material were

measured after being fully charged with water. These

measurements indicated that the backseal tied up the

most water per unit volume. Moisture measurements taken

after various charging time indicate that the total

moisture is greater than what is contributed by the vapor

space alone by a factor of 6-7X. This discrepancy is

explained as due to the contribution of adsorbed moisture

from the module's surfaces and absorbed moisture from the

backseal. To support this argument, an analysis of a

typical moisture rate was made. This plot segregated

the contribution of the vapor space from the adsorbed

surfaces.

It has been concluded that, in modeling moisture-related

failure mechnaisms, the characterization of surface

water plays a more important role than has been previously

observed.

VII. REFERENCES

1) M. E. Sweet, "Assessment of Moisture Permeation of

an Epoxy Sealed Test Module," 18th Electronic Components Conference Proceedings, pp 33-37, San Francisco, California, 1978.

2) R. K. Traeger, "Hermeticity of Polymeric Lid Sealants, Sandia Laboratory, Albuquerque, New Mexico.

262
FIGURE 1 IBM MODULES

FIGURE 2 - MODULE CROSS SECTION

VAPOR SPACE

263
FIGURE 3 - BLOCK DIAGRAM OF MOISTURE MEASUREMENT SYSTEM

MOISTURE EVOLUTION ANALYZER

FLOVVMETER

GAS MODULE MOISTURE

EXTRACTION CHAf-lBER

HEATER

STRIP CHART

RECORDER

CELL POWER

SUPPLY

GAS VENT

ELEC CELL

V/F CO:r/ERTER

DIGIT.-iL OUTPUI

FIGURE 4 - MODULE MOISTURE EXTRACTION CHAMBER

264
FIGURE 5 - DuPONT 901 MOISTURE EVOLUTION ANALYZER
FIGURE 6 - ELECTROLYTIC CELL

266

FIGURE 8a - MOISTURE MASS RATE, 85°C EXTRACTION TEMPERATURE

MODULE CONDITIONED AT 85°C/80% RH/300 HOURS

60 50 40 30 20 10 0

TIME (MINUTES)

268
FIGURE 8b - MOISTURE MASS RATE, 126°C EXTRACTION TEMPERATURE,

MODULE CONDITIONED AT 85 C/80% RH/300 HOURS

TIME (MINUTES)

269
TABLE I. ESTIMATE OF INTERNAL MODULE MOISTURE DISTRIBUTION

QTY. H2O, iug

COMPONENT/AREA 85°C EXTRACTION 125^0 EXTRACTION

VAPOR SPACE 245 245

TOPSEAL 13 13

CERAMIC SUBSTRATE 6 6

EPOXY BACKSEAL 21 60

ALUMINUM CAP 445_ 1616

730 1940

270
7.3 A Method of Leak Detection and Location for Conformally Coated Packages

George H. Ebel and Richard ... De„^ Cristofaro

Singer Company - Kearfott Division

1225 McBride Avenue

Little Falls, NJ 07424

(201) 256-4000

ABSTRACT

During failure analysis of a hermetically sealed microelectronic

package, one important step is to

verify the package's hermeticity. Unfortunately, many defective field units

have been conformally coated which, it

will be shown, interferes with standard

hermeticity tests. Internal and external coatings can enter and seal a leak

path, allowing the device to erroneously pass a standard (He bomb) leak test.

One approach to this problem is to

attempt to remove the coating. This is

impractical for internal coatings, and

complete removal of external coating

is difficult to achieve in practice.

Another approach is to use a different

type of leak detection, which is less

affected by the coatings, such as the

use of dye penetrants.

Key Words: Hermeticity; hybrid; leak

test; methanol; silicone coating; UV

light.

STATEMENT OF "THE PROBLEM

Field failures have been observed

with internal moisture related problems

in devices which passed the fine and

gross leak tests during failure analysis. In one example, voids were found

in the solder preform, which is used

to attach the hybrid lid. The voids

formed a leak path, but examination

showed the voids were filled with conformal coating. A moisture problem

was suspected in another hybrid, coated

internally with Parylene^. After it

passed fine and gross leak tests, a residual gas analysis (RGA) was performed. The RGA results indicated

excessive moisture was present in the

hybrid (12,000 ppm moisture were detected whereas the maximum permissible

is 2000 ppm by SKD spec: MIL STD 5008

allows 6000 ppm). After delidding, a dye penetrant was introduced into the

open hybrid. Within a few minutes the

dye flowed through a cracked glass

bead confirming the loss of package

integrity. The Parylene coating prevented this leak from being detected

during fine and gross leak tests. A

related problem^ was observed when devices which were known leakers passed

a hermeticity test after thermal shock

tests. In this case it is believed

that the f luorocarbons used in the

thermal shock test were drawn into the

leak path by capillary action and at

least temporarily prevented the leak

from being detected. Thorough cleaning

with alcohol and acetone followed by a

1 hour bake at 150°C allowed this leak

to be detected. These examples demonstrate the limitations of standard fine

and gross leak tests. Material may

form a "pseudo seal" in the leak path

resulting in misleading test results.

METHODS

Two types of conformal coatings were selected from the MIL 1-46058 QPL

for use in these experiments. The

first is an acrylic coating (MIL type

AR) . The second is a silicone coating

(MIL type SR) which is applied over a primer coating.

Coatings were applied to sample

hybrids in the same manner in which

circuit boards are done. The thickness

of the cured acrylic coating ranged from

.001 inch to .003 inch. The cured silicone coating thickness is .005 +.003

inch

.

The leak test used is the Helium

bomb method. The device is exposed to

a helium atmosphere pressurized to 30

psig for 4 hours. At the completion of

this bomb (within 30 minutes maximum)

the rate of helium leakage is measured

with a Veeco model MS9ABC or MS17AB

(mass spectrometer) fine leak tester.

To detect a gross leak, the device is

then immersed in FC-43 ( fluorinert

liquid) at 125°C and observed for at least 30 seconds to detect any bubbles; none are allowed.

THE EFFECT OF COATING AND CLEANING

The sample hybrids were leak tested to establish a baseline leak rate

for each device. The rates ranged from

10"^ atm-cc/sec to gross leaks.

271
The hybrids were then coated with

either acrylic or silicone. In all

cases, the coating reduced the leak

rate significantly. With only one ex- ception, a former gross leaker, the

coating allowed the leaking hybrids to

pass the He bomb fine and gross leak

test. The results are shown in Table 1.

Attempts were then made to completely

remove the coatings from the hybrids.

Acrylic coating is soluble in solvents

such as Freon TF , 1, 1, 1-trichloroethane and acetone. Methanol loosens

the coating, allowing it to be peeled

off. For our tests, the hybrids were

soaked in methanol for at least 10

minutes after which the majority of the

coating was peeled off. The remaining

coating was removed with 1 , 1 , 1- tr ichloroethane by soaking and brushing. The

acrylic coating glows brightly when

illuminated by ultra-violet (UV) light

so this is useful in assuring that all

the coating is removed.

The silicone coating is more difficult to remove. This coating is not

readily soluble in Freon TF , 1,1,1 trichloroethane , acetone, or methanol.

However, soaking the hybrid in any of

these solvents softens the coating

enough to allow manual removal. For

our tests, the samples were soaked in

Freon, 1,1,1 tr ichloroethane or methanol

for approximately 30 minutes while the

coating was scraped and picked off with

wooden probes and tweezers. Ultraviolet

illumination was used to detect any re- maining coating but, since the silicone

coating is only dimly fluorescent, it was

not as useful a technique as for the

acrylic coating.

After cleaning, the hybrids were subjected to a leak test (Table 1)

.

The hybrids coated with acrylic showed

an increase in leak rate after cleaning,

but in general did not return to the

pre-coating leak rate. One hybrid still

passed the leak test after cleaning.

The hybrids coated with silicone did not

change significantly after cleaning.

These results indicate that coating a hybrid masks leaks and even rigorous

cleaning does not restore the original

leak rate. ,

TABLE 1

a) Acrylic coated hybrids

Leak rate (Atm-cc/sec)

Sample Before After

Number Coating Coating Cleaning

1 3 X 10"^ 8 X 10"" 2 X 10-"

2 2 X 10-^1.2 X lO-'^l. 9 X 10-"

3 6 X 10-'2. 5 X 10-" 2 X 10-^

4 3 X 10-"6. 5 X 10-" Gross

5 Gross 7 X 10-^ Gros s

b) Silicone coated hybrids

Leak rate (Atm-cc/sec)

Sample Before After After

Number Coating Coating Cleaning

6 2 X 10-" 9 X 10-"

7 1.9 X 10-"1.8 X 10-^2. 4 X 10-"*

8 Gross 8 X 10-"l. 7 X 10-"*

9 Gross 7 X 10-"2. 8 X 10-^*

Note: Measurements followed by an "*" were made with a Veeco MS9ABC;

all other measurements were made

with a Veeco MS17AB .

THE DYE PENETRANT TEST

Dye penetrants are useful in detecting leaks and additionally provide

a visual indication of the location of

the leak. In an example mentioned

earlier, a dye penetrant was introduced

into a delidded hybrid. The dye was drawn into a cracked glass bead by

capillary action readily identifying

the location of the leak. In hermetically sealed packages, a pressure bomb

with dye penetrants is used. This is an established practice to detect gross

leaks ^and has been used by Adams'* to

detect fine leaks. At first it would

appear that dye penetrants are not

sensitive enough to detect a fine leak,

but Adams showed that leak rates as low

as IQ-" or IQ-^ atm cc/sec were detectable using a pressure bomb with dye

penetrants (Magneflux Spotcheck SKL-HF,

60 psig for 1 hour) . Our tests with

conformally coated packages began with

the stripping of the coating by the

methods mentioned earlier. The devices

were then exposed to a vacuum for 1 hour

followed by a pressure bomb with dye

penetrants. The dye used in our tests

272
was Zyglo ZL-30A which exhibits a bright green fluorescence when illuminated with UV light. At the completion

of the pressure bomb, the dye was re- moved from the exterior of the package

(using Zyglo ZE-4A Emulsifier) and the

package was delidded. When the interior

of the device is illuminated with UV

light, any trace of dye can be identified by its green fluorescence. The

duration and pressure of the bomb depend on the package type and types of

leaks to be detected. In general, the

higher the bomb pressure, the greater

the sensitivity. The pressure must be

chosen carefully so as not to damage or deform the package. 30 psig was the

limit chosen for our sample of large

(1" X 2") hybrid packages. A one hour

bomb was sufficient to detect gross

leaks with acrylic coating. In one example a substantial amount of dye was observed inside the package. Fine leaks

will, in general, require a longer duration (or higher pressure) . 16 to 18

hours (overnight) was the longest time

considered convenient. With the longer

bomb time, fine leaks are more readily

detected. In acrylic coated hybrids,

this test proved sensitive enough to

detect fine leaks in the range of 10"^

atm cc/sec (Figure 1). In one case with a pre-coating leak rate of 6 X 10"^

atm cc/sec, a quantity of dye was observed inside the package. The results

of the samples coated with silicone

were not as clear. When the 16-18 hour

bomb was used, a

10""* atm cc/sec leak

was detected by a glass bead completely

saturated with dye. However, no dye

was observed in the package (Figure 2)

.

Another hybrid with a pre-coating leak

rate of 2 x 10~^ atm cc/sec was observed as a partially saturated glass

bead, a good indication of the leak

location, but not complete proof of the

leak. The sensitivity of dye penetrant

test is apparently dependent on the

type of coating. Leaks in hybrids

coated with acrylic were more readily

detected than those packages coated

with silicone. Therefore, to use this

method of leak detection and location

for other package/coating combinations,

each coating material, pressure bomb

combination must be evaluated in order

to assure confidence in the results.

Fig. 1. The results of a dye penetrant

test on a fine leaker coated with acrylic. The lead coming through the glass

bead is covered with dye.

Fig. 2. A gross leak in a silicone

coated hybrid was identified by this

saturated glass bead.

273
Fig. 3. A fine leak in an acrylic

coated hybrid was identified by the

presence of dye around a glass bead

coated with conductive epoxy (for case ground)

.

PHOTOGRAPHIC AND VIEWING TECHNIQUES

The Zyglo ZL-30A provides a bright

green fluorescence when illuminated

with UV light. In order to detect

small quantities of dye, a bright UV

light and a darkened room is usually

required. Black and white photos can

be used to show the leaks clearly and are compatible with most reproduction

methods. Using only UV illumination

provides the highest contrast and shows

the areas which contain dye most

clearly. Usually the glow of the dye

is bright enough to allow the details

of the surrounding area to be seen. Additional white illumination may be

added, but this reduces the contrast.

The photos included in this paper were made with Polaroid Type 107 ASA 3000

film illuminated only with UV light.

Color photos allow more detail to

be shown when white light is added to

the UV light. The dye is then identified by its color rather than its

brightness. A long exposure with UV

light followed by a short exposure with

white light (flash) provides good

results

.

CONCLUSION

1. Fine and gross leak tests

performed on conformally coated hermetic packages were inadequate because

the coating can "seal" a leaking hybrid, allowing it to pass standard

hermeticity tests.

2. Coating removal does not

guarantee meaningful leak test results.

3. Dye penetrant leak tests, however, were successfully used in detecting and locating leaks in conformally

coated hybrids.

REFERENCES

1 . Personal correspondence with

John Wojnas, Reliability Engineering

General Electric, Utica, N.Y.

2. General Package Procurement

Specification, Martin Marietta Aerospace, Orlando, FL

.

3. MIL-STD-883B 31 August 1977

Method 1014.2, SEAL, 2.4 Test condition D-penetrant dye gross leak.

4. Adams, G. E. "Package Hermeticity" 1973 International Reliability

Physics Symposium, pp. 95-97.

5. Licari, J. J., Plastic Coatings for Electronics 1979 McGraw-Hill

Inc

.

274
7.4 Improved RTV Silicone for IC Encapsulant

Ching P. Wong* and Donald E. Maurer

Western Electric Company

Engineering Research Center

P. 0. Box 900

Princeton, NJ 08540

(609) 639-2579

Abstract: There is a large body of evidence that indicates that

ionic contaminants affect the electrical reliability performance of

encapsulated devices. These contaminants are introduced into the

sys'CGm tlirounh three main sources: improper substrate cleaning, the

encapsulant, and the environment. Our research herein reported adds

further 5vidence that ionic contaminants do affect performance and

that compounds such as crown ethers and cryptates significantly

improve the electrical performance of encapsulated devices.

Keywords: Encapsulant; integrated circuit; RTV; silicone.

I. INTRODUCTION

It has long been suspected that ionic materials, whether from the

device surface, encapsulation materials, or environment, affect the

electrical reliability performance of encapsulated IC devices. Sbar

has shown that exposure to hydrogen chloride accelerates the

deterioration of electrical properties of silicone encapsulated biased

triple track conductors, and resistors (l). Michael and Antonen report

that salt atmosphere testing (MIL Std 883A, Method 1009.1)

dramatically increases the FIT* rate for silicone encapsulated devices

over devices not exposed to salt [2]. Brown and Borcsik demonstrated

that the presence of ionic contaminants increases the rate of dendrite

formation between oppositely biased conductors on PWB (3).

Experiments in our laboratory with silicone RTVs deliberately doped

with HCl showed that FIT rates also increased (4). From these

results, it is logical to conclude that ionic contaminants do indeed

cause an increase in FIT rate, especially under hot, humid conditions.

Sodium, potassium, and chloride are the most likely ions present in

materials and environment. Certainly, material specifications can be

made to limit the levels of these ions but this makes no provision for

the rei ntroduction of these ions from the environment. Since the source of ions cannot be eliminated, it was felt that if we could

incorporate a mechanism for trapping or immobilizing these ions the

silicone RTVs would demonstrate better reliability. Our investigation

of a number of different types of ion trapping compounds showed that

this was the case.

*Failure in time

275
II. HISTORICAL

In 1967, C. J. Pederson of DuPont DeNemours Co. synthesized

thirty-three cyclic polyethers from aromatic vicinal diols (s) . The

polyethers contain from nine to sixty atoms with anywhere from three

to twenty oxygen atoms distributed about the ring structure (fig. 1).

In solution, these crowns are extremely effective ligands from a wide range of metals with the size of the ring cavity and the ionic

radius of the metal the determining factors in the stability of the

complex. Tables 1 and 2 list the cavity diameters for the crowns and

the ionic radius of a number of metals (6-11) .

The crown ethers and the related cryptates (fig. 2) have been used in

a variety of synthetic procedures primarily because of their ability

to solvate ionic materials in organic solvents [12, 13). Their

catalytic effects have been studied as well as their use as models for

enzymes [14-17) . Recently, it has been shown that crown ethers in the

solid state form "sandwich" like complexes with most metals and that

the counter ion is also tightly bound [18] . It is this evidence that

recommended them for use as ion traps in a silicone RTV formulation.

III. EXPERIMENTAL PROCEDURE

A. Sodium and Potassium Analysis: The amount of these ions in

commercial silicone RTVs and silicone fluids were measured by several

independent analytical laboratories. Values of 20 ppm for sodium and

7 ppm for potassium were obtained for the best samples.

B. Chloride Analysis: Chloride analysis by the independent

laboratories was not satisfactory with results varying by an order of

magnitude on the same sample. We are presently developing a specific

ion electrode titration analysis for chloride that so far has given

consistent results in the 100 ppm range for chloride. This procedure

will be the subject of a future report.

C. Screening Procedure: It was decided to use a commercial silicone

RTV with well characterized electrical properties as a "carrier"

formulation. This was Dow Coming's DC-3145 Clear, a methoxy-cure

paste-like RTV. DC-3145 Clear was dissolved in xylene to such a viscosity that it was fl ow-coatable. A quantity of the ion trap was then added such that its molar concentration was at least that of the

combined sodium and potassium concentrations. B^ias h[umidity

Temperature (BHT) Testing was performed on the mixtures using tantalum

nitride metallized Triple Track Resistors (TTR) at 100*^0/96 percent

R.H./180 VDC.

2 76
IV. RESULTS AND DISCUSSION

The results of the BHT-TTR testing showed that the inclusion of crown

ethers and cryptates into a silicone RTV formulation dramatically

enhances the electrical reliability of our test vehicles (Fig. 3).

These data add further evidence that sodium and potassium ions

contribute to failure of devices inasmuch as the crowns with the

smaller cavities outperform those with larger cavities. The

12-crown-4, with a cavity diameter of 0.18 nm, is most suitable for

complexing NA"^ with an ionic diameter of 0,18 nm (6-11) . The

15-crown-5 at 0.27 nm is effective for K+ at 0.26 nm and, indeed, the

evidence shows that the 12-crown-4 formulation shows better electrical

performance than the 15-crown-5 system. Also of note is that the

addition of 15-crown-5 to DCQ3-6550 Gray, the best commercially

available encapsulant, resulted in a significant improvement in its

electrical properties. It must be noted that it is suspected that low

levels of the crown ethers are carcinogenic (19). While it may be

possible to use these materials, present research is directed toward

chemically bonding crown or crown-like materials to the pol.ymer

backbone, thus rendering them immobile, or by finding alternative,

less suspect ion traps.

REFERENCES

1. Sbar, N. L., IEEE Proc, 26th Elec. Comp. Conf. , 277 (1976).

2. Michael, K. W., and Antonen, R. G., "The Properties of

Sil icon/Epoxy Electronic Grade Molding Compound", presented at

Int. Microelectronics Conf., Anaheim, Calif. (1978).

3. Brown, M. J., and Borcsik, S. J., private communication, March 1980.

4. Wong, C. P., Maurer, D. E., and Looby, M. M. (unpublished results).

5. Pederson, C. J., JACS . , 89, 7017 (1967), Pederson, C. J., and

Frensdorff, H. K. , Angew. Chem. , Internat. Edit , 11, 16 (1972).

Izatt, R. M. , and Christensen, J. J., Synthetic Mul ti dentate

Macrolydic Compound . Academic Press, New York ( 1 978)

.

6. Bright, D., and Truter, M. R., J. Chem. Soc. B. , 1544 (1970),

Bush, M. A., and Truter, M. R., Chem. Comm. , 1439 (1970).

7. Dalley, Smith, Larson, Christenson, and Izatt, J.C.S. Chem. Comm. , 43, (1975).

8. Mallison, J.C.S. , Perkin , 261 and 266 (1975).

9. Neman, Steince, Van Remortere, and Boer, Inorg. Chem., 14, 734

(1975).

10. Hughes, J.C.S. , Dalton, 2374 (1975).

277
11. Harman, Hart, Hursthouse, Moss, and Raithby, J .C.S. Chem. Comm. , 396 (1976).

12. Liotte, C. L., and Harris, H. P., JACS , 96, 2250 (1974).

13. Sam, D. J., and Simmons, H. E., JACS , 94, 4024 (1972).

14. Eisenman, G. , Cezny, S., and Szabo, G. , J . Membrane Biol

.

, 1, 294

(1964), and "Syn. on Biol, and Artificial Membranes" Fed. Prac , (1968).

15. Dye, J. V., DeBacker, M. G. , and Nicely, V. A., JACS, 92, 226

(1970).

16. Brown, C. A., JACS, 95, 982 (1973).

17. Moss, and Pickiewicz, JACS, 96, S632 (1974).

18. Poonia, M. S., and Aja j , A. V., Chem. Rev. , 29(S), 389 (1979).

19. PCR Inc. Product Technical Information.

278
CHCH,

Cli,

Cti,

6'.

CH,

CM,

1S-CROVVrj-6

Figure 1 - CHEMICAL STRUCTURE OF T8-CR0WN-6. Figure 2 - CHEMICAL STRUCTURE OF 4,7 ,13 ,16-TETRAOXA-l ,10- DIAZA-CYCLOOCTADENCANE (KRYTOFIX® 22).

10 rC.Oi

(1) Con-mercial s (?) CopT"ercia"; s (3) Co.-:::ercial i (4) CoriMercioi s (5) Co'^j;'e'-cial s

licone encapsulant

licone encapsuUnt + 18-crown-6

licone encapscilant + 15-crown-5

licone encapsulant + 15-crown-5 (Monocyclo-)

licone encapsulant + 12-crown-^

1 —T~rT'nT -yr rrrn

ICO

TIME O'i TEST (;:•.$)

Figure 3 - RESULTS OF BIAS HUMIDITY TEMPERATURE (8HT) TESTING

AT 95% RELATIVE HUMIDITY, lOO'-'C 180V DC BIAS. TRIPLE TPJ^CK RESISTOR TESTING DEVICE -- TANTALURA

NITRIDE CONDUCTOR WITH 3 MIL LINE IN THREE MIL SPACING.

279
Table 1

Atomic and ionic radius of some important alkali and transition metals.

Atomic Radius (A) Ionic Radius (A)

Na 1.95 Na+ (0.95)

K 2.35 K+ (1.35)

Cu 1.28 Cu (I) 0.95, Cu (II) 0.69

Ag 1.44 Ag (I) 1.26

Au 1.46 Au (I) 1.37

Table 2

Structure and diameter of some crown ethers.

Crown ethers Diameter (A) Comments

dibenzo-12-crown-4

di benzo-14-crown-4

dibenzo-15-crown-5

di benzo-18-crown-6

1.8 - 1.9

1.8 - 1.9

2.7

4.0

Not coplanar

Coplanar & s^metrical

Coplanar & symmetrical

Coplanar & symmetrical

280
7.5 Leak Testing Electronic Components

Paul R. Forant

Varian/Lexington Vacuum Division

121 Hartwell Avenue

Lexington, MA 02173

(617) 861-7200

ABSTRACT

The making and testing of hermetic seals to prevent moisture or other contaminants from entering the device in the manufacture

of sealed parts is the subject of this paper. It is intended

that the emphasis will be on fine leak detection with some notes

on the overall leak detection process.

In order to make the point of hermeticity testing, the various

tests are to be covered in the order they should occur. First,

subassembly testing, then fine leak testing, followed by gross

leak testing. The test order is important to the validity of

the tests.

Subassembly tests are made prior to sealing the packages so that

glass- to-metal seals, f eedthroughs , etc., are proven prior to

assembly of the chip and closing the cover. This sorts out

leakers before the value added makes rejection too costly.

Fine leak tests are to be made after the final assembly is complete. In some cases the parts can be sealed in a helium rich

atmosphere. The usual method involves pressurizing the parts in

a helium atmosphere. The time of bombing and elapsed time of

test for leak rates of varying sizes will be discussed.

Gross leak testing can be conducted in a number of ways. It is

desirable to select a method that will give the desired results

with reliability and with the least possibility of product damage.

Key Words: Bombing; fine leak test; gross leak test, helium;

hermeticity; tracer gas.

1. INTRODUCTION

It is intended to provide a brief background on leak testing of

electronic components to prove hermeticity of the device. The

test sequence of devices from subassemblies to fine leak test

to gross leak test will be described with recommendations on

equipment, processes, and procedures to achieve valid test results. Errors in procedure and techniques and their effect on

test results will be discussed.

2. SUBASSEMBLY TESTS

It is desirable to test the envelope with its feedthroughs prior

to assembly of the chip. The envelope can be tested quickly

281
with reasonably simple fixtures. The value added by further assembly makes the cost of rejects too high.

In the case of an envelope with many electrical connections that

may be 1 inch square, the test on a helium mass spectrometer to 1 x 10"^ std cc/sec can be made in less than ten seconds. The part

is placed on a flat gasket and pumped out while spraying the

feedthrough seals with helium. The test will be reliable as long

as there is no helium carryover from the previous test. To

avoid this problem, the gasket is changed frequently, making sure

the part seats in the same track each time. If the part is not located precisely, the helium captured by the relatively porous

elastomer will be present at the next test. See figure 1 for fixtures designed to accomplish this test.

" 3. FINE LEAK TEST

It is important that the fine leak test precede the gross leak

test. The gross leak test involves immersion in liquids that

will plug small leaks, making the fine leak test impossible.

Sealed packages, such as semiconductor integrated circuits, are

tested by putting tracer gas inside and detecting it as it leaks

out.

In the radioactive tracer method, krypton 85 inside the leaking

unit emits gamma radiation. This is detected by a scintillation

counter.

In the helium method, escaping gas is picked up by the mass spectrometer leak detector, using the vacuum envelope technique.

Two fundamental problems in testing sealed packages are:

1. How to put the tracer gas inside the package, and

2. How to detect "gross leakers" (from which the tracer gas has

escaped prior to test)

.

One method of putting tracer gas inside is to make the final

assembly in a glove box filled with the tracer. With radioactive

tracers, this procedure is not practical because of safety considerations. Also, nonleaking packages would contain a radioactive tracer and would continue to emit gamma radiation.

Although helium can be sealed in at final closure, this is not

often done for two reasons:

1. Product handling in glove boxes is more difficult and more

expensive, and

2. Some electrical devices perform poorly with helium inside.

Even if the tracer is put in at final closure, detection of gross

leakers remains a problem since the tracer can dissipate prior to

282
test, especially if the leak is large and the internal volume is

small

.

The majority of small sealed products, such as integrated circuits, reed switches, etc., are given their charge of tracer gas

after final closure by pressurizing them in the tracer gas for a specified time. This process, known as "bombing" in semiconductor jargon, is selective, in that it puts tracer gas into

leakers only.

Typical pressure is 5 atm (about 60 psig, after first pumping

the air out of the pressure vessel). Typical time is one hour,

but longer times may be necessary, particularly if the leak is

small and the internal volume is large. The consequent small

amount of tracer inside leaking back out through the small leak

may result in marginal detectability

.

Since the concentration of tracer gas inside the product varies

with leak size, internal volume, bombing pressure, bombing time

(and even elapsed time before testing) , the indicated leak rate

will vary from the actual leak rate. The "actual" leak rate is

defined as the reading that would be given by the leak detector

if the package contained 100% helium at normal atmospheric pressure. The relationship of the above parameters is expressed in

the equation in figure 2. The relationships have been covered

in tables which apply whether the trace gas is helium or a radioactive gas.

There are two interesting and sometimes misunderstood aspects

of the flow of helium now captured in the part to the atmospheric pressure surrounding it.

1. Each gas flows through the leak as if the other gas were not

present in the other chamber.

2. Each gas flows in both directions in proportion to the

partial pressure of that gas behind this flow. Thus the

net flow is in proportion to the difference in pressure.

As far as "bombing" of sealed packages is concerned, the significance of the first statement is this: once tracer gas is

present inside a leaking part and leaking out at some particular

rate, it leaks out at this rate regardless of whether the environment is air (after removal from the bomb) or vacuum (during

test) . In fact, even during bombing, some tracer gas is leaking

out, but since the environment is also tracer gas — under

pressure -- it leaks in at a higher rate. Therefore, the net

effect is a continual increase in quantity of trace gas within

the part during bombing.

Now in the bombing process, the tracer gas flows first at a maximum rate into a leaking part (no flow into a good part) . As the

partial pressure of helium builds up inside, the net rate of flow

into the part decreases.

283
That is why it is incorrect to calculate the amount of tracer gas

within the part simply by multiplying the "actual" leak rate by

pressure and time.. This calculation does not take into account

the decrease in net inflow caused by the accumulation of tracer

gas inside the part. In some cases, the errors would be considerable.

Similarly, the rate of outflow of tracer gas after the part is

removed from the "bomb" is a logarithmic function. This is because the rate of flovz decreases as the residual amount in the

part decreases. Sometimes the test has to be made within short

time limits and at others the time at which the test is made is

immaterial. Figure 3 indicates some of the time constraints.

4. GROSS LEAK TESTING

Gross leak testing is performed after the parts have been accepted in the fine leak mode. As with the fine leak test methods,

there are many pitfalls in the method selection and interpretation of results. The following comments apply to the various

test methods.

1. Pressurize with water and detergent and give the part a functional test. This is not usually done as the functional test may

not be affected immediately.

2. Immerse in a liquid and watch for bubbles. The liquid may be

water with a detergent, a special compound or alcohol. The temperature of the bath can be increased to cause the internal

gasses to increase the pressure or a vacuum drawn over the surface of the bath will accomplish the pressure differential. All

of the above have disadvantages. The more sensitive bath materials may be difficult to remove from the product or present hazardous environmental control problems or lack sensitivity. All

tend to close small leaks and make further fine leak testing

difficult

.

3. Vary the immersion test with the use of a fluorescent liquid.

This is very effective in testing parts with glass envelopes as

the fluorescent dye shows up well under ultraviolet light. With

metal envelopes the parts can be cleaned and dried and placed

on an absorbent surface. Dye leaking out is easily detected.

4. Use weight gain method, found to be effective for gross leak

testing of devices. A fluorocarbon fluid is used in the bath

and is subjected first to a vacuum for a period of 1 hour, then

pressurized to 4 atmospheres for 2 hours. The parts are weighed

within a few minutes of removal from the bath. Weight change

of a milligram will be cause for rejection in small devices.

284
The gross leak tests are equally important as the fine leak tests.

Sensitivities can be as high as 10~6 std cc/sec giving a comfortable overlap with the fine leak tests.

Selecting and implementing the test procedure most compatible

with the device and the manufacturing process requires careful

consideration of the methods available to achieve quality levels

demanded of the part.

285

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5 X 10

Std

cc/sec

5

4 X 10

3x10

2 X 10

1 X 10

-5

Cavity

Fills in Less Than 1 Hr

(5 Atm Abs)

^500%

( 1 r 0 12 3 Bomb Time (Hr)

Volume: 10'^ cc 5

Leak: 10 Std cc/sec

Package cavity fills rapidly with

helium during "bombing" but test must be done quickly - if possible

to do at all. (Note: Extended bomb

time does not increase amount of helium in cavity.)

-Actual Leak = L

=

1 X 10'^ Std cc/sec

Indicated Leak S

= 3 X \(P Std cc/sec (at 233 sec)

T

Wait Time (Sec)

0 200 400 600 800 1000

(a) Example of Gross Leak

1 x 10

Std

cc/sec

3

0

L/V = 10'^ S^ = 5. 1 x lO'*^ Std cc/sec

I r r 1 r I t r T I T

-1

1 3 Bomb Time

Wait Time (sec)

200 400 600 800 1000

(b) Example of Typical Leak

Volume = 10 ^ cc -8

Leak = 10 Std cc/sec

Cavity fills to less than

1 atm. but depletion rate

is low, allowing ample

time for test.

Figure 3. Examples of Gross and Typical Leaks

288
8. WORKSHOP PARTICIPANTS

8.1 List of Speakers

Moisture Measurement Workshop

November 5-7, 1980

National Bureau of Standards

Allen R. Bailey

AMP Inc.

3705 Paxton St.

Harrisburg, PA 17111

(717) 780-6859

Henry Baron

IBM

IBM Burlington; E82 966/2

Essex Jet. VT 05452

(802) 769-7608

Peter R. Bossard

Bell Labs

600 Mountain Ave

Murray Hill, NJ 07976

(201) 582-3189

Prof. George Cvijanovich

AMP Inc.

3705 Paxton St.

Harrisburg, PA 17111

(717) 780-6645

J. Gordon Davy

Westinghouse Defense Electronic

Systems Center

Mail Stop G-16, P.O. Box 746

Baltimore, MD 21203

(301) 765-3619

Aaron Der Marderosian*

Raytheon Co

528 Boston Post Rd.

Sudbury, MA 01776

(617) 443-9521

James R. Duffey

Hughes Aircraft Co.

Centinela and Teale

Culver City, CA 90230

(213) 591-0711 x3317

Session Chairman

George H. Ebel

Singer Co

150 Totowa Road

Wayne, NJ 07470

(201) 256-4000, x4208, 5014

Victor Fong

Panametrics, Inc.

221 Crescent St.

Waltham, MA 02154

(617) 899-2719

Paul R. Forant

Varian/Lexington Vacuum Division

121 Hartwell Ave.

Lexington, MA 02173

(617) 861-7200

Rebecca J. Gale

Bell Labs

555 Union Blvd.

Allentown, PA 18103

(215) 439-6768

Dimitry G. Grabbe

AMP Inc.

3705 Paxton St.

Harrisburg, PA 17111

(717) 780-6645

John Hale

Texas Instruments

P. 0. Box 6448

Midland, TX 79701

(915) 685-6709

Michael G. Kovac*

Univ. of South Florida

College of Engineering

Tampa, FL 33617

(813) 974-2581

R. K. Lowry*

Harris Semiconductor

Box 883, Mail Stop 52-07

Melbourne, FL 32901

(305) 724-7566

289
Robert Macko

Hughes Aircraft Co.

Centlnela and Teale

Culver City, CA 90230

(213) 591-0711

Mr. D. E. Maurer

Western Electric Co.

P.O. Box 900

Princton, NJ 08540

(609) 639-2584

Dr. R. P. Merrett

Post Office Telecommunications

Research Dept.

Martlesham Heath, Ipswich

United Kingdom, IP5 7RE

44 473-642309

Benjamin A. Moore*

Rome Air Development Center

RBRE

Griffiss AFB, NY 13440

(315) 330-4055

Richard Moser

IBM Corp.

Dept. E82, Bldg 966-2

Essex Junction, VT 05452

(802) 769-7667

Kenneth L. Perkins

Rockwell Int'l

3370 Miraloma Ave.

Anaheim, CA 92803

(714) 632-2529

John C. Pernicka

Pernicka Corp.

450 E. Middlefield Rd.

Mt. View, CA 94043

(415) 969-0220

Wesley Poate

General Dynamics

P.O. Box 2507

Pomona, CA 91766

(714) 620-7511 x3597

Dr. Bruce Raby

Pernicka Corp.

450 E. Middlefield Rd.

Mt. View, CA 94043

(415) 969-0220

J. Michael Richtarsic

Martin Marietta AerospaceMicroelectronics

Sand Lake Road

Orlando, FL 32855

(305) 352-2685

Stanley Ruthberg*

National Bureau of Standards

Bldg. 225, Rm. B-310

Washington, D. C. 20236

(301) 921-3625

Phillipp wh Schuessler

IBM-FSD

580-102A

Owego, NY 13827

(607) 687-2121 x4219

Rama Shukla

Intel Corp.

3065 Bowers Avenue, MS 3-307

Santa Clara, CA 95051

(408) 987-6256

Saeed H. Siddiqui

Digital Equipment Corp.

75 Reed Road

Hudson, MA 01749

(617) 568-4573

Robert Thomas

Rome Air Development Center

RBRE

Griffiss AFB, NY 13441

(315) 330-4632

Albert F. Walcheski

Bell Telephone Labs

555 Union Blvd.

Allentown, PA 18103

(215) 439-6177

Malcolm L. White

Bell Laboratories

555 Union Blvd.

Allentown, PA 18103

(215) 439-7457

C. P. Wong

Western Electric Company

P.O. Box 900

Princeton, NJ 08540

(609) 639-2579

* Session Chairman

290
8.2 List of Attendees

Moisture Measurement Workshop

November 5-7, 1980

National Bureau of Standards

Muni Aggarwal

Monolithic Memories

1165 East Arques Ave. Sunnyvale, CA 94086

(408) 739-3535

James M. Amnions U. of South Florida, Dept.

of Engineering

4202 Fowler Ave

Tampa, FL 33620

(813) 974-2220

Charles Ansley

Micropac

905 E. Walnut

Garland, TX 75040

(214) 272-3571

Veijo Antikainen

Vaisala Oy

3970 Longwood Ave. Boulder, CO 80303

(303) 499-3505

Jim Arnold

Motorola Inc. 2553 N. Edington

Franklin Park, IL 60131

(312) 451-1000

Gregory K. Arslanian

Jade Systems Div.

1120 Industrial Highway

Southampton, PA 18966

(215) 322-9020 x88

Bernard A. Bang

Westinghouse Electric Corp.

P.O. Box 1521

Baltimore, MD 21210

(301) 765-7340

W. M. Beckenbaugh

Western Electric Co. P.O. Box 900

Princeton, NJ 08540

(609) 639-2428

Yen Benjamin

AMD

901 Thompson Place

Sunnyvale, CA 94086

(408) 732-2400 x2337

Dr. J. B. Bindell

Bell Telephone Labs

555 Union Blvd.

Allentown, PA 18103

(215) 439-6827

Robert Boschan

Hughes Aircraft Co. Bldg. 316, MS R-127

Culver City, CA 90230

(213) 391-0711 x303

Steven J. Brown General Electric Co. 100 Plastics Ave. Pittsfield, MA 01201

(413) 494-3058

Eugene E. Brull

United Technologies Research

Center

Silver Lane

East Hartford, CT 06108

(203) 727-7127

Tony Bumbalough

USAF AFWAL/MLPO

Wright-Patterson AFB, OH 45433

(513) 255-4474 -4098

Salvatore P. Carbone

IBM D/350, B/OOl, RTE 9 Poughkeepsie, NY 12602

(914) 463-8168

Richard 0. Carlson

General Electric Co. Corp. R&D, Box 43 37/2077

Schenectady, NY 12345

(518) 385-8351

Stephen Carvellas

Molinini-Gollob Analytical

Service Inc.

47 Industrial Rd

Berkeley Heights, NJ 07922

(201) 464-3331

Jim Cavanaugh

Medtronic, Inc. 6970 Old Central,

Minneapolis, MN

(612) 574-4000

P.O. Box 1453

55440

Harry K. Charles, Jr.,

The Johns Hopkins Univ. Applied

Physics Laboratory

Johns Hopkins Road

Laurel, MD 20810

(301) 953-7100 x2652

Dan Hian Cheong

Medtronic Inc

6970 Old Central, NE Fridley, MN 55432

Bruce Church

Medtronic Inc. 6970 Central Ave., NE Minneapolis, MN 55432

(612) 574-4486

Richard Ciochon

Signetics Corp.

811 E. Arques Ave. Sunnyvale, CA 94086

(408) 739-7700 x4979

Albert C. Cooper

Corning Glass Works

MP 8-5 Corning, NY 14830

(607) 974-7021

Jack D. Grouse

Motorola

52nd St. Phoenix, AZ 85008

(602) 244-6568

James Deacutis

Analog Devices

829 Woburn St. Wilmington, MA 01887

(617) 935-5565 x331 or 211

Wentworth 0. Denoon NASA-GSFC

Goddard Space Flight Center

Greenbelt, MD 20771

(301) 344-7437

Bruce Dinger

Analog Devices Semiconductor

829 Woburn St. Wilmington, MA 01887

(617) 935-5565

Gilbert Dix

Westinghouse

P.O. Box 746

Baltimore, MD 21203

(301) 765-6955

Robert Eklund

Texas Instruments

P.O. Box 225012, M/S 46 Dallas, TX 75026

(214) 995-4473

Robert B. Elo

American Microsystems Inc. 3800 Homestead Rd. Santa Clara, CA

(408) 246-0330

291
Ingbert Elvnert

L. M. Ericsson

S-12625

Stockholm, Sweden

(08) 7190000

Diane Feliciano

Oneida Research Services, Inc.

3 Allinwood Court

New Hartford, NY 13413

(315) 736-4722

Horace A. Firth

Burroughs Corp.

16701 W. Bernardo Dr. San Diego, CA 92127

(714) 487-3000

Bill Fitch < Motorola

4629 E. Sunset Dr. Phoenix, AZ 85028

(602) 962-2780

Richard Gerber

Aerospace Corp.-

P.O. Box 92957 Los Angeles, CA 90009

(213) 684-7594

R. Gilbert

Univ. of Florida

Bowler Ave. Tampa, FL 33620

(813) 974-2581 x246

Ra5nnond Goetz

NSWC (WDL)

Silver Spring, MD 20910

(202) 394-1395

Fred Gollob

Molinini-Gollob Analytical

Service Inc.

47 Industrial Rd.

Berkeley Heights, NJ 07922

(201) 464-3331

Carlo Grilletto

RCA Solid State Div. RTE 202

Somerville, NJ 08876

(201) 685-7052

Thomas F. Gukelberger

IBM Corp.

B/330-75C, Rt. 52

E. Fishkill, NY 12570

(914) 897-2662

John C. Harding Jr. General Eastern Corp.

50 Hont St. Watertown, MA 02172

(617) 923-2386

Lewis G. Harriman III Cargo Calre Engineering

79 Monroe St. Amesbury, MA 01913

(617) 388-0600

Sab Hasegawa

National Bureau of Standards

Bldg. Physics, Rm. B-252

Washington, D. C. 20234

(301) 921-2794

Keith John Hauer Mostek Corp.

1215 N. Crosby Rd. Carrollton, TX 75006

(214) 323-7357

Charles Hopkins

Cordis Corp.

14425 N. W., 60th Ave. Hialeah, FL 33014

(305) 557-4500 xl95

James S. Humphrey

AMP Inc.

P.O. Box 3608

Harrisburg, PA 17105

(919) 725-9222

Robert G. Johnson

Honeywell, Inc. 10701 Lyndale Ave. S. Bloomington, MN 55420

(612) 887-4502

William L. Johnson

Vitarel Inc. 3572 Corporate Court

San Diego, CA 92123

(714) 292-8353

Harold D. Joss

Coors Porcelain Co. 17750 W. 32nd Ave. Golden, CO 80401

(303) 277-4204

Paul Kerrigan

Gerald Rosen Co. Inc.

271 Worcester kd.

Framingham, MA 01701

(617) 879-5505

Jeff L. Kersey, Jr. Advanced Micro Devices

901 Thompson Place, MS 33 Sunnyvale, CA 94086

(408) 732-2400 x2652

John Kiely

Intel Corp. MS 2-258, 3065 Bowers Ave. Santa Clara, CA 95051

(408) 496-9549

Allen King

Northrop Corp/Electronic Division

2301 W. 120th St. Hawthrone, CA 92649

(213) 615-3063

Angelo Koudounaris

Hughes Aircraft Co. 500 Superior Ave. Newport Beach, CA 92663

(714) 759-2812

R. W. Lawson

Post Office Telecommunications,

Research Dept

.

Post Office Research Centre

Martlesham Heath, Ipswich

United Kingdom 1P5 7RE

44 473-642309

Kuo-Chin Lin

UTI

325 N. Mathilda Ave. P.O. Box 519

Sunnyvale, CA 94086

(408) 738-3301

B. R. Livesay

Georgia Institute of Technology

EES-Baker Bldg.

Atlanta, GA 30332

(404) 894-3489

Gregory A. Losito

Spectra Physics

1250 W. Middlefield

Mt. View, CA 94042

(415) 961-2550

Stephen J. Madlgan

ILC Data Device Corp.

105 Wilbur Place

Bohemia, NY 11716

(516) 567-5600 x282

292
Dr. Arlan W. Mantz Spectra Physics; Laser Analytics Div.

25 Wiggins Ave. Bedford, MA 01730

(617) 275-2650

Margie Marlow

Naval Surface Weapons Center

Silver Spring, MD 20910

(202) 394-1295

Alan S. McGuire

General Electric Co. Mail Stop A34 Neumann Way

Cincinnati, OH 45215

(513) 243-44991 (x4967/4968)

Richard Merrell

West Coast Technical Service Inc. 17650 Fabrica Way

Cerritos, CA 90701

(213) 921-9831

Robert J. Miller

IBM - T.J. Watson Res. Ctr .

Room 15-251

P.O. Box 218

Yorktown Hts., NY 10958

(914) 945-2801

Wayne D. Mooring

Martin Marietta Aerospace

P.O. Box 5837

Orlando, FL 32805

(305) 352-3047

Richard L. Morgan

Texas Instruments

P.O. Box 84 Sherman, TX 75090

(214) 892-7670

Hayden Morris

U.S. Naval Surface Weapons Center

Silver Spring, MD 20910

Wayne A. Mulholland

Mostek

P.O. Box 169, MS 407

Carrollton, TX 75006

(214) 323-6573

Jay Patel

RCA Corporation

Rt. 202 Somerville, NJ 08876

(201) 685-6556

Michael Petrow

Raytheon Co. 528 Boston Post Rd. Sudbury, MA 01776

(617) 443-9521

Elmer Potter Corning Glass Works

BB-1 Houghton Park

Corning, NY 14830

(607) 974-8345

Vince A. Pree General Dynamics

1675 West Mission

Pomana, CA 91767

(714) 629-5111 (x3655)

F. C. Quinn

Hygrodynamics Inc.

949 Selim Rd. Silver Spring, MD 20910

(301) 589-1727

Jerry L. Raznov

Solid State Equipment Corp.

1015 Virginia Drive

Fort Washington, PA 19034

(215) 643-7900

James H. Richardson

Aerospace Corporation

P.O. Box 92957 Los Angeles, CA 90009

(213) 648-5439

Giorgio Riga

Fairchild Camera and Instrument

464 Ellis Street M/S 20-709

Mountain View, CA 94042

(415) 962-2046

L. J. Rigby

ITT /STL

London Rd.

Harlow, Essex, U.K.

Jerome Roddy

NSA

9800 Savage Rd. Ft. Meade, MD 20755

(301) 688-7195

Jack Ronning

Medtronic, Inc. 6972 Old Central Ave.,N.E.

Minneapolis, MN 55432

(612) 574-4761

Harry P. Ross Western Electric Co., Inc.

555 Union Blvd.

Allentown, PA 18103

(215) 776-5471

Thomas J. Rossiter

Oneida Research Services, Inc.

3 Ellinwood Court New Hartford, NY 13413

(315) 736-4722

Graig Sloneker

Medtronic Inc. Minneapolis, MN 55432

(612) 574-4233

R. G. Spiecker

RCA/Government Systems Division

P.O. Box 588

Burlington, MA 01803

Maurice R. Stamler

General Electric Co. P.O. Box 8555

Philadelphia, PA 19101

(215) 962-2885

R. J. Steward

Honeywell

13350 Highway 19 P.O. Box 11568

St. Petersburg, FL 33733

(831) 531-4611 (x2676)

Milton Stoll

Research Instrument Co., Inc.

36 Mascolo Rd.

So Windsor, CT 06074

(203) 528-9625

W. E. Swartz Jr. Univ. of South Flordla

Department of Chemistry

Tampa, FL 33620

(831) 974-2344

Marvin E. Sweet

IBM Endicott

1701 North St. Endicott, NY 13760

(607) 755-8417

Basil C. Tasker

Fairchild Semiconductor

333 Western Ave.

So. Portland, ME 04106

(207) 775-8139

293
Leopoldo Valero

National Semiconductor Corp.

2900 Semiconductor Drive

M/S T 020

Santa Clara, CA 95051

(408) 737-6825

Lex E. Vinsel

McDonnell Douglas Astronautics

Co. 5301 Bolsa Ave., MS. 10-3,

Dept. 431

Huntington Beach, CA 92647

(714) 896-4057

David Walker

Sperry Gyroscope

Great Neck, NY 11020

(516) 574-1385

Curt Ward

Kyocera International. Inc. 8611 Balboa Ave. San Diego, CA 92123

(714) 279-8310 (x254)

Stephen Weisskoff

Phys-Chemical Research Corp.

36 West 20th Street New York, NY 10011

(212) 924-2070

David Wilson

Martin Marietta

P.O. Box 179

Denver, CO 80201

(303) 977-5508

David Zalar

Telectronics Limited

301 W. Vogel Ave. Milwaukee, WI 53207

(414) 481-5400

Conrad H. Zierdt, Jr. Bell Laboratories

555 Union Blvd.

Allentown, PA 18103

(215) 439-7500

LATE ATTENDEES

Heikki Kellomaki

Vaisala Oy

PL 26 00421

Helsinki, Finland

Christopher Yonclas

M&T Chemicals, PSI Venture Group

P.O. Box 1104

Rahway, NJ 07065

(201) 499-2427

294
NBS-n4A (REV. 2-ec) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1, PUBLICATION OR

RFPORT NO

NBS SP 400-72

2. Performing Organ. Report No. 3. Publication Date

April 1982

4. TITLE AND SUBTITLE Semiconductor Measurement lechnoiogy:

Moisture Measurement Technology for Hermetic Semiconductor Devices, II Proceedings of a Workshop held at NBS, Gai thersburg , MD

Nov. 5-7, 1980

5. AUTHOR(S)

Elaine C. Cohen and Stanley Ruthberg, Editors

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

National Bureau of Standards Rome Air Development Center

Washington, DC 20234 RBRE

Griffiss AFB, NY 13441

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 82-600503

[

' Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here) The Workshop, one of a series concerned with measurement problems in

integrated circuit processing and assembly, served as a forum to examine the

progress that has been made in the measurement and control of moisture in

hermetically packaged semiconductor devices. While moisture-induced failure

modes and mechanisms have been extensively documented, the lack of accurate

and reliable measurement of the moisture content itself has been a major

obstacle to meaningful efforts to limit and control this pervasive contaminant.

Manuscripts are provided of 36 presentations which detail the progress that

has been made in mass spectrometer measurements and calibration of internal

package moisture, in increased assurance with moisture sensors, in testing,

and in package control.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

analysis of moisture content; hermetically packaged semiconductor devices; mass spectrometer measurement; moisture; moisture generators; moisture sensors; quality

control; reliability of semiconductor devices; semiconductor devices.

13. AVAILABILITY

[jijj Unlimited

I I For Official Distribution. Do Not Release to NTIS

nS] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

[_j Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

302

15. Price

Announcement of Semiconductor Measurement Technology

List of Publications 72 - 1962-1981

Elaine C. Cohen

Information Assistant

Semiconductor Materi?.ls ?nd Processes Division

National Bureau of Standards

Bldg. 225, Room A305

Washington, DC 20234

Dear Mrs. Cohen:

Please send a copy of your latest "Semiconductor Measurement Technology,

List of Publications 72."

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U.S. GOVERNMENT PRINTING OFFICE : 1982 0 - 371-525

NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research of the National Bureau of Standards reports NBS research and develop- ment in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry,

engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement

methodology and the basic technology underlying standardization. Also included from time to time are survey articles on topics

closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete

citations to all recent Bureau publications in both NBS and non- NBS media. Issued six times a year. Annual subscription; domestic

$18; foreign $22.50. Single copy, $4.25 domestic; $5.35 foreign.

NONPERIODICALS

Monographs—Major contributions to the technical literature on

various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with in- terested industries, professional organizations, and regulatory

bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special publications

appropriate to this grouping such as wall charts, pocket cards, and

bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and

studies of special interest to physicists, engineers, chemists,

biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NBS under

the authority of the National Standard Data Act (Public Law

90-396).

NOTE; The principal publication outlet for the foregoing data is the Journal of Physical and Chemical Reference Data (JPCRD)

published quarterly for NBS by the American Chemical Society

(ACS) and the American Institute of Physics (AlP). Subscriptions,

reprints, and supplements available from ACS, 1 155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research results,

test methods, and performance criteria related to the structural and

environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's tech- nological marketplace. Order the above NBS publications from: Superintendent oj Documents. Government Printing Office, Washington, DC 20402. Order r/ie following NBS publication.^— Fl PS and NBSIR'.K—from

the National Technical Information Services, Springfield, VA 22161

.

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Slat. 1127), and as implemented by Executive Order 1 1717 (38 FR 12315, dated May II, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations),

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.
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