CO O

NBS SPECIAL PUBLICATION 400-59

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

QC

100

.U57

NO.n00-59|

1979

. c>2 _

Semiconductor Measurement Technology:

Nondestructive Tests Used

to Insure the Integrity

of Semiconductor Devices,

with Emphasis on Acoustic

Emission Techniques
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities^ — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering^ — Mechanical

Engineering and Process Technology^ — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303.
OCT 2 2 1979

Semiconductor Measurement Technology:

Nondestructive Tests Used to Insure the Integrity of

Semiconductor Devices, with Emphasis on Acoustic

Emission Techniques

George G. Harman

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

Sponsored in part by: The Defense Advanced Research Projects Agency

1400 Wilson Boulevard

Arlington, VA 22209

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued September 1979
Library of Congress Catalog Card Number 79-600131

National Bureau of Standards Special Publication 400-59

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-59, 72 pages (Sept. 1979)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Stock No. 003-003-02116-4 Price $3.50

(Add 25 percent additional for other than U.S. mailing).

ir U.S. GOVERNMENT PRINTING OFFICE : 1 979 O-300-303
SemiaonduQtov Measurement Teohnology

Nondestructive Tests Used to Insure the Integrity of

Semiconductor Devices with Emphasis on Acoustic

Emission Techniques

TABLE OF CONTENTS

Abstract v

1.1 Introduction 1

2.1 Some Current Production Line Assembly Tests 2

2.2 Introduction to Microelectronics and Hybrid Packaging

Methods 2

2.3 Review of Typical Nondestructive Tests Used to Reveal

Devices with Mechanical Defects 6

2.3.1 The Nondestructive Wire Bond Pull Test 6

2.3.2 Internal Visual Inspection 9

2.3.3 Temperature Cycling 9

2.3.4 Package Seal Leak Tests (Hermeticity ) 11

2.3.4.1 Helium Mass Spectrometer Leak Detector Test 14

2.3.4.2 Bubble Emission Tests 15

2.3.5 Burn-In Test 16

2.3.6 Particle Impact Noise Detection (PIND) Test 19

2.3-7 The Statistics of Sampling for Special Production

Lots 22

2.4 Conclusions of Section 2 23

3.1 Passive Acoustic Techniques 23

3.2 Introduction to Acoustic Emission 23

3.3 Review of Acoustic Emission Applications to the Real Time

Nondestructive Tasting of the Mechanical Integrity of Electronic Components 30

3.4 Acoustic Emission as a Post-Production Screen for Bond Integrity in Microelectronics 33

3.4.1 Introduction 33

3.4.2 Methods of Applying a Mechanical Stress to the BeamLead System 34

3.4.3 Preparation of Controlled Bondability Substrates . . 38

3.4.4 Experimental Apparatus 39

3.4.5 Experimental Results 46

3.4.5.1 AE Results from Pulling Beam Leaded Devices 46

3.4.5.2 Tests That Apply Force Only to the Beams . • 50

iii
Page

3.4.6 Application of Acoustic Emission to Determine the Integrity of Tape Bonded Devices and Hybrid Components . 50

3.5 Conclusions of Section 3 .................. , 61

Acknowledgment 62

References 62

iv
ABSTRACT

The discussion is divided into two major sections. The

first consists of an introduction to device assembly

techniques and problems followed by a review of six important nondestructive tests used during and after device packaging to insure the mechanical integrity of

completed electronic devices. Most of these tests are called out in the military testing standard, MIL-STD-883

and are generally classified as screens. The first sec- tion concludes with a brief introduction to the economic

and other factors that result in the choice of one screen

over another and to production line statistical sampling

(LTPD) appropriate to special high reliability device

lots such as those used for space flight.

The second section begins with an introduction to acoustic emission, the status of theory as it can be applied

to microelectronics. Then the published papers that

have applied AE as a nondestructive test in electronics

applications will be reviewed. Finally, passive AE

techniques are applied to establishing the mechanical

bond integrity of beam lead, flip chip, and tape-bonded

integrated circuits as well as components in hybrid microcircui ts

.

Key Words: Acoxistic emission; beam lead devices; electronic devices; hermeticity ; Jiybrids ; nondestructive

tests; semiconductor; tape-bonded devices.

V

SemiaonduGtor Measurement Teahnology

Nondestructive Tests Used to Insure the Integrity of

Semiconductor Devices with Emphasis on Acoustic

Emission Techniques

George G. Harman

Electron Devices Division

National Bureau of Standards

Washington, DC 20234

1 . 1 INTRODUCTION

This paper reviews a number of important nondestructive tests used

frequently in the semiconductor industry to test the mechanical integrity of semiconductor devices. Many of these tests are not rigorously

quantitative, but rather, involve an element of human judgment or some

empirical comparison for interpretation. As such, the usage of some

of the tests is controversial even though they are specified in important military and other microelectronic standards. The scientist or engineer just entering the microelectronics field should look upon this

as an opportunity to develop better tests rather than be discouraged

by the lack of rigor.

The discussion is divided into two major sections. The first section begins with a brief review of device assembly techniques and problems. This serves as necessary background for all of the material that

follows. Next follows a review of six important nondestructive tests

that are used during and after device packaging to insure the mechanical

integrity of completed electronic devices. Most of these tests are called out in MIL-STD-883 [l], a widely used guide for device testing,

and are generally classified as screens. These tests, presented in the

order that they are usually performed are: (1) the nondestructive wire

bond pull test, (2) internal visual inspection, (3) temperature cycling

and shock, {A) package seal integrity ( hermeticity ) , (5) burn-in (removing early failures), and (6) particle impact noise detection.

The first section concludes with a brief introduction to some

factors that result in the choice of one screen over another and to

production line statistical sampling appropriate to special high reliability device lots such as those used for space flight.

The second section begins with an introduction to acoustic emission

(AE), and the status of its theory as it can be applied to microelectronics. Also, the published papers that have applied AE as a nondestructive test in electronics applications will be reviewed. Finally,

acoustic emission measurement techniques developed at the NBS are applied to establishing the mechanical bond integrity of beam lead.

1
flip chip, and tape bonded integrated circuits as well as components

in hybrid microcircui ts

.

2.1 SOME CURRENT PRODUCTION LINE ASSEMBLY TESTS

2 . 2 Introduction to Microelectronics and Hybrid Packaging Methods

Microelectronic assembly starts after the scribe-and-break or sawing operation that cuts the individual die (chips) from the wafer. Once

cut out they are usually die-attached to the package by gold-silicon

eutectic , a solder, or an epoxy and then conventionally interconnected

by wire bonding. Other attach technologies, such as flip chip, beamlead, and tape-carrier bonding can essentially combine die attach and

interconnection bonding into a single operation. The first, most familiar, and still overwhelmingly dominant method of interconnection is

to use flying wires, an example of which is shown in Figure 1. These

wires, typically 25- ym diameter aluminum or gold, are welded on to the

semiconductor bonding pads by thermocompression [2], ultrasonic [3], or thermosonic [4] (a combination of both) techniques.

The second method of interconnection uses gold beam leads, which

are made during wafer processing, in place of wire bonds. Figure 2 is an example of such a device. These beams are welded to a package,

usually a hybrid, by thermocompression techniques, in which the substrate is heated to 250 C and a heated bonding tool compresses the

leads against the substrate with forces > ICQ kg/cm^. The flip-chip

approach requires building up the bonding pads, usually with a solder

bump, and then reflow soldering the bumps to the package face down,

obscuring the joints thus they cannot be visually inspected, discouraging the use of this technology. Several typical flip-chips are shown

in Figure 3. Neither beam leads nor flip chips are used widely at present and their main applications are for large scale in-house production

and consumption.

The final technique of interconnection which is relatively new

is called tape bonding. For this, one normally builds up the semiconductor aluminum bonding pads into copper or gold "bumps" by plating techniques, then a special metallized tape with individual extended leads

is bonded to the bumps by a thermocompression or reflow solder process.

This is termed inner-lead bonding. Figure 4 gives two examples of chips

bonded to such tape. The tape in 4A is "so called" testable tape. Here

the individual insulated leads terminate on pads that may be probed

and the device electrically tested after bonding. The tape in 4B is

of all metal construction which cannot be tested before packaging. Once

the chips are attached to the tape, it is usually wound onto reels and

stored until final assembly into either an IC package or a hybrid. In

the case of some calculators and watches, the leads are bonded onto

a printed circuit board and the chip is protected by a drop of epoxy.

Figure 5 is a typical high technology thick film hybrid. In this

figure, A is a chip capacitor, and B is a chip resistor. Some hybrids

are very large (e.g., 5 x IQ cm) and the substrate may crack during

2
Figure 1. An example of 50-pra diameter aluminum ultrasonic wire bonds (flying wires) interconnecting a silicon microwave power transistor.

a b

Figure 2. (a) is a gold-beam lead device shown face up.

Interconnections are made by the beams extending over the

edge of the chip. (b) shows the device bonded into its hybrid microcircuit . The contact and metallization system is

platinum silicide, titani\im, platinum, and gold. The sealed

junction is obtained by depositing silicon nitride over oxide

layers.

3
b

Figure 3. (a) A solder bump flip-chip device shown soldered

onto a substrate (b) . Several devices shown face up. The

solder bumps are apparent, covering what would be the bonding

pads in a chip and wire device

.

4
a b

Figure 4. (a) is a 35-mm, three-layer (metal, adhesive, polyamide) testable tape-bonded device. (b) is an example of an 11-mm copper all-metal single-layer tape with an IC chip innerlead-bonded in the center. The leads are made of tin-plated copper. (Figs. 35, 38, and 39 give closeups of the bumped chips and

bonded leads .

)

a b

X /

Figure 5. A high technology five-layer-thick film hybrid

microcircuit . Arrows point to chip capacitors (a), and chip

resistors (b) . (Courtesy General Dynamics Electronics Division)

5
certain assembly operations such as thermocompression bonding. Some

methods of detecting such cracks will be described m Section 3.3.

Figure 5 is a photograph of such a large substrate. Once the hybrid

components are assembled, the substrate is usually put in a package

or epoxy coated. In the case of packages, there may be moisture leaks

if the lid seal and metal le^ds are damaged or improperly assembled.

The seal integrity of packages that contain numerous glass-to-metal

lead-throughs is a potential reliability problem. Such seals are shown

in Figure 7. Thomas [5] has stated that the glass seals are the major

source of hermetic moisture ingress and that these may open only at

high temperature and subsequently reseal , avoiding detection later in

a normal leak test.

2 . 3 Review of Typical Nondestructive Tests Used to Reveal Devices

with Mechanical Defects

There are a number of mechanical screens that may be applied to

devices that have specific reliability requirements. The decision to

implement one test over another can be based on such considerations

as cost, the specifying engineers' personal familiarity with one test,

or the availability of test personnel. It is therefore appropriate to

briefly review the most common nondestructive tests used on assembly

lines to verify mechanical integrity. These will be described in the

order that they are usually performed on a production line.

2.3.1 The Nondestructive Wire Bond Pull Test

The purpose of the nondestructive wire bond pull test (NDPT) is

to remove weak wire bonds having pull forces less than a designated

force value, while avoiding damage to acceptable bonds. An example of

a weak 25-ijm diameter aluminum bond revealed by the NDPT is shown in

Figure 8. This bond lifted at ^ 0.2-grams force (gf). The bond would

have passed any internal visual inspection (see 2.3.2) and its weakness

could only be revealed by a pull test or possibly temperature cycling

(2.3.3)

.

To perform the test, a hook, made of tungsten or steel wire, 2 to 3 times the diameter of the wire to be tested, is placed under the

loop and a specified pull force is applied vertically to that loop.

The bond and perhaps the device is rejected if the wire breaks. The

pull force is usually specified for a given wire diameter, but it can also be derived from appropriate equations [6] based on the results

of a sample destructive pull test and the metallurgical characteristics

of the particular wire being tested. A typical equation is:

F = 0.9(X - 3S ) X

where F is the nondestructive pull force (in gf )

, X and S^ are the mean

and standard deviation of the destructive pull force respectively. This

equation is appropriate for small diameter (25- to 50-ym) aluminum wire

used for ultrasonic bonding.

6
Figure 5. A large thin-film hybrid substrate. The ICs are beam lead devices bonded face down. The substrate is alumina

ceramic and interconnection metallizations are titanium, palladium, and gold.

Figure 7. A 5 by 5 cm sealed hermetic-type hybrid package

with numerous glass-metal seals (a) . A gross leak in the

package lid is also indicated (b)

.

7
Figure 8. SEM photograph of a weak bond resulting from poor

process control. A 25-ym diameter ultrasonic wire bond failure

resulting from a layer of glassivation or other contamination

on the surface of the bonding pad. The visually perfect bond

is shown in the upper part of the figure. The bond lifted at

'^0.2 gf.

8
This test is used rather extensively in large hybrids and in integrated circuits for space and other applications requiring very high

reliability. The NDPT has been reported to cost about half the price

of the original wire bonding [7]. Although there is considerable evidence that the test is indeed nondestructive (it has been used on

> 10^ wire bonds), it is nevertheless controversial. Some organizations

require it on a 100% basis and others absolutely prohibit its use. This

test can be both used and prohibited on different contracts for the same electronic system.

2.3.2 Internal Visual Inspection

The purpose of this test is to check the internal construction,

and workmanship of microcircuits for compliance with the requirements

of the applicable specifications. This test will normally be used prior

to capping or encapsulation on a 100% inspection basis to detect and

eliminate devices with visually detectable internal defects that could

lead to device failure in normal application. It may also be employed

on a sampling basis prior to capping to determine the effectiveness

of the manufacturer's quality control and handling procedures for microelectronic devices.

Most visual inspection criteria are accompanied by photographs

or sketches so that the inspector has a basis for comparison. An example

of one of these from MIL-STD-883B [l], Method 2010.3 is shown in Figure 9. Items generally covered by such inspection include placement

of wire bonds with respect to the pad, lead dress, device metallization

scratches, contamination, obvious passivation and diffusion faults,

and die attach faults. Sometimes a scanning electron microscope (SEM)

examination of devices on a sampling basis is specified to look for

defects such as metallization coverage of oxide steps and diffusion

window defects. This is done at the wafer level.

Visual inspection is a relatively expensive test in which much

is left to the judgment of the inspector. At best, it is 80% effective

in detecting faults and, at times, good product is rejected. If the same device is recycled through the same or a different inspector, different defects are frequently discovered and original ones may pass

unobserved. The test is of limited usefulness for large scale integrated

circuit chip integrity because of the high magnification and time

required. Nevertheless, visual inspection is considered useful and important for removing gross chip and assembly induced defects.

2.3.3 Temperature Cycling

After the package is sealed additional nondestructive mechanical

integrity tests are often performed. Some of these follow: The purpose

of this test is to determine the mechanical resistance of a part to

exposure to various thermal environments that may be encountered during

system life. Internal mechanical stresses result from the different

thermal expansion coefficients of different parts of the device. This

test is the only effective means of revealing very weak aluminum wire

9
Reject, semicircular crack having

chord greater than 75% of narrowest

iglassivated metallization separation

Reject, cracl< < 0.25 mil separation

from operating metallization

r

\

)

Reject, substrate crack

in active circuit area

Reject, crack > 1.0 mil

i ns i de scribe line

0 MIL

Accept, crack

< 1.0 mil i ns i de scr i be 1 i ne

Scr i be grid

or scr i be 1 i ne

Reject, crack > 3-0 mi

i n length

Figure 9. Figure 2010-25 from MIL-STD-883 [1] which is an example of visual inspection criteria for an integrated circuit. Interpretation can be subjective.

10
bonds after the package is sealed. The test is also effective in revealing poorly welded or soldered caps or damaged glass or ceramic to metal

lead throughs.

The apparatus usually consists of two chambers, one cooled and

one heated. Many devices are placed in a holder which is rotated or otherwise moved from one chamber to another. Heat transfer takes place

by forced air. Typically, the device is tested for 10 to 20 cycles of

from -55° to 125°C with 5 minutes equilibrium time at each temperature;

but these test conditions may vary depending on anticipated device

usage. A hundred or more cycles is usually considered destructive. Temperature cycling is considered an effective mechanical test and is inexpensive to perform. It is usually followed by an electrical test to

reveal failures arising during cycling. Temperature shock is a variation

of this test. The intent is similar to that of temperature cycling,

but shock is a more severe test. The temperature shock test consists

of liquid-to-liquid transfer. Although a typical temperature range would

be -55 to 125 C, the most severe specified condition in MIL-STD-883

is -195 to 200 C, where the low temperature liquid is liquid nitrogen

and the high temperature liquid is a fluorocarbon . The transfer time

from one liquid to the other is less than 10 seconds. The test is

usually carried out for 15 cycles, where one cycle includes one high

and one low temperature immersion. Temperature shock is useful on a sample basis to determine the integrity of package glass to metal seals.

Most operating devices will experience some temperature cycling

during system life due to system turn on and off (e.g., automobile

engine compartment electronics). Howevei', it is not reasonable to expect

a device or system to undergo liquid-to-liquid thermal shock in any

anticipated usage. Therefore, if a choice is given, temperature cycling

is usually the preferred test.

2.3.^ Package Seal Leak Tests (Hermeticity

)

Moisture ingress in electronic packages is the cause of many device

failures and, thus, the explanation of leak testing given here will be

longer than other nondestructive production line screens. In addition,

some of the moisture induced failure mechanisms are worth reviewing

since moisture is a major, if not the major, long-term reliability problem for semiconductor devices. The usual point of corrosion attack is

the exposed bonding pad aluminum, and subtle metallurgical and contamination induced interactions are common. An example of chlorine-moisture induced corrosion on an IC [8] is shown in Figure 10, and an example of

metal migration [9] in Figure 11. One of the less obvious sources of con- tamination is spittle which may be deposited in micron sized droplets as

a production line operator speaks to an associate [8] . The droplets contain contaminants such as sodium, phosphorus, sulphur, and chlorine. The

initial water dries quickly, but later after package seal, the deliquescent contaminants may be reactivated by moisture ingress through a leaking package

.

11
Figure 10. Aluminum chloride corrosion product on a glassivated IC chip starting at the gold ball bonds. (After Ebel

[8])

12
(c) (d)

Figure 11. Gold migrative resistive shorts. The (+) symbol is

the anode, and the (-) symbol, the cathode. (a) is an SEM view

of a portion of a microcircuit showing migrated gold between a T-shaped stripe and two neighboring stripes below the T-bar.

(b) is a closeup of the right half portion of the T-shaped

stripe. Dendritic or fern-like features between the stripes

have been identified to be gold by energy-dispersive analysis

of x-rays (EDAX) . (c) shows the center stripe as the cathode.

Dendrites are shown growing from both sides of the cathode (center stripe) and proceeding toward the neighboring anodes. (d)

is a closeup of (c) , illustrating the migrated-gold dendritic

features typically observed. (After Shumka [9])

13
The purpose of the various leak tests is to determine the hermeticity of the seal of microelectronic and semiconductor devices with

designed internal cavities. There are a number of methods such as bubble, dye, weight gain, halogen leak detection, helium leak detection,

and radioisotope leak detection. These have been critically reviewed

by Ruthberg [10] and the following treatment is derived from his. There

are variations on the methods of performing each test. Because of the

small volumes and package constructions, most of the test methods

require back pressurization , a process of driving a tracer gas or fluid

into the interior by pressurization, and then detection of the tracer

on reemission. The bubble, dye, and weight gain methods are appropriate

for the gross leak range, which is taken to be > 10~6 Pa. m3/s. The

leak detector and radioisotope methods are essentially intended for

the fine leak range (<_ 10"^ Pa-m^/s), but can be used for detection

into the gross leak range depending on the size of the package internal

volume

.

Dye penetration techniques are more appropriate to the destructive

testing of individual components for diagnostic purposes, where decapping or other physical alteration of the package occurs. Dye techniques

are used nondestructi vely , however for devices with transparent walls.

The halogen leak detector method is not frequently used for semiconductor components because of its corrosion potential. Bubble and the helium

leak detection are most widely used of all of the leak tests and will

be described in some detail.

2.3.4.1 Helium Mass Spectrometer Leak Detector Test

The use of the helium leak detector is well documented. This

instrument has the widest leak rate application range for general use. For this procedure, the packages to be tested are pressurized in a simple pressure bomb with helium gas, removed, transferred to the helium

leak detector, evacuated, and tested for effusing helium. A numerical

indication is obtained from the leak detector which can be related to

true leak rate if an appropriate theoretical relationship is available

to relate these two quantities. The correlation depends upon the regime

of the gas flow into the test object, the pressurization parameters,

internal free volume, the delay time between pressurization and readout,

and the flow mechanism for helium effusion from the test part. Since

enough of the helium must first be driven into the part to give discernible effusion, the pressurization times can be quite long for packages

of large internal volume when tested to package leak rates ^ 1 x 10"^

Pa*mVs. In most standards for helium leak detector use, the package

leak rate is determined from an expression based upon the m.olecular

flow regime. The equation is:

Units of flow rate are conventionally atm'cm /s or torr'l/s, but in

the International System (SI) of metric units the unit of flow rate

is the Pa'mVs. 1 Pa'mVs = 9.86925 atm'cm'^/s and 7.50064 torr*l/s.

14
where

R = machine reading for helium,

L = package leak rate under conditions of one atmosphere of helium

pressure upstream and zero pressure downstream,

= pressure of one atmosphere,

= bombing pressure,

V = internal free volume,

T = pressurization time, and

t = delay or dwell time between pressurization and readout.

The first exponential term describes the pressure rise of helium within

the package due to pressurization, while the second exponential term

describes the fall off of pressure due to effusion. Since this expression is based upon the molecular flow regime, it is in principle only

applicable to fine leaks; whereas in practice, it is applied to the

whole leak range. A double valuedness in leak rate (L) as a function

of machine reading (R) and package volume (V) is indicated by the calculation which predicts that a gross leaker may not be distinguishable

from a fine leaker without further manipulation of test variables. In

practice it is assumed that a minimum and maximum detectable leak rate

exists and the range of leak rates between these two limits will be

detected for any given bombing pressure (in atmospheres), internal

free volume (V) and minimum detectable machine reading (R . )

.

min

The failure criteria from MIL-STD-883 is as follows: "devices with

an internal cavity volume of 0.01 cc or less shall be rejected if the

equivalent standard leak rate (L) exceeds 5 x 10~^ atm cc/secHe . Devices

with an internal cavity volume greater than 0.01 cc and equal to or less than 0.4 cc shall be rejected if the equivalent standard leak rate

(L) exceeds 1 x 10"'' atm cc/secHe. Devices with an internal cavity volume

greater than 0.4 cc shall be rejected if the equivalent standard leak

rate (L) exceeds 1 x 10~^ atm cc/secHe."

2.3.4.2 Bubble Emission Tests

There are two classes of bubble tests. One is simply direct immersion of the test object into a hot, clear, inert fluorocarbon liquid

of low surface tension. If a leak is present, bubbles will appear as

the gas in the device expands on heating. The leak test range is narrow.

The second class is superior in test range and detection of leakers.

For this, the component is exposed first to vacuum and then back pressurized with a high vapor pressure fluorocarbon liquid so that if a leak is present the fluorocarbon is driven into the component. On immersion in a hot, low surface tension, indicator fluid, the fluorocarbon

bubbles out of a leaky device.

15
Although bubble size and frequency have been related to leak rate

under ideal conditions, the interpretation of bubble tests are subjective in practice. They are tedious, results are very dependent upon

the geometry of the package, and a gross leak comprised of several fine

leaks can be missed. The use of liquids requires that this test be performed after fine leak tests have been completed to avoid the plugging

of leaks; it also provides the possibility of residual contamination

in the accepted components having undetected leaks.

It has been observed that the tiny bubbles released during the test

emit acoustical noise that may be detected in the hundred kilohertz range

Thus it is possible that an acoustic emission type test (see sec. 3) may

be developed to eliminate the subjective nature of this test and possibly automate it.

2.3.5 Burn-In Test

The burn-in test is not considered to be a mechanical integrity

test (although it may reveal such problems), but it is included in the

present discussion because of its importance in assuring device reliability. The burn-in test is performed for the purpose of screening

or eliminating marginal devices, those with inherent defects or defects

resulting from manufacturing aberrations which cause time and stress

dependent failures. These are generally referred to as freaks. In the

absence of burn-in, these defective devices would be expected to result

in "infant mortality" or early lifetime failures under use conditions.

Therefore, it is the intent of this screen to stress microcircuits above

maximum rated operating conditions in order to reveal time and stress

dependent failure modes in a practical length of time. The procedure

involves placing the device in an oven at a fixed temperature for a specified number of hours (e.g., 125 C for 168 hrs) with an applied

bias, forward or reverse, depending on the stated test conditions. The

regression equations are based on a simple Arrhenius equation derived

for the burn-in time (t) where.

where A is a constant, is the apparent activation energy in eV (in

MIL-STD-883B it is 0.44 eV) chosen for freak population removal, T is

the absolute junction temperature, and k is Boltzmann's constant.

Figure 12 gives the burn-in regression curve from MIL-STD-883B [l].

This curve is applied to the overwhelming number of devices that are burn-in tested. However the temperature selected must not be high enough

to create failure modes not related to freak removal. One such failure

mode, known as purple plague (Au-Al intermetallic compound formation

[11]), may occur when gold wire bonds are made to aluminum bonding pads.

If burn-in is carried out for one hour at 400°C , the interconnects will

fail due to this new failure mode.

16
Various activation energies have been reported for freak populations, 0.25 to 0.7 eV, and main populations, 0.5 to 1.7 eV. These latter

vary according to the technology (bipolar 1.1 eV, C-MOS 1.3 eV,

beam-lead silicon-nitride sealed-junction 1.7 eV). However, the variations between manufacturers or even wafer lots are often larger than

this indicated range of values [12,13,14,15,16].

The freak population produces "infant mortality" and represents

devices with relatively gross manufacturing defects. Many different

defects may be involved such as cracked chips, nearly open interconnections, oxide pinholes and other oxide defects, mask alignment-induced

coverage (oxide or metal) defects, gross contamination and failures that occur earlier than expected from main population failure mechanisms.

Main population life may be limited by some of the above causes. It

is more often related to ionic drift in oxides, but it may include such

process related mechanisms as gold penetration through a barrier metal

(for a complex metallization system). Figure 13 vividly demonstrates

the different temperature-time dependence of freak and main populations

[15]. For this case both the freak population activation energy (0.8 eV)

and the main population activation energy (2.0 eV) are quite high,

indicative of what is expected from the beam lead type metallizationpassivation system.

It has been pointed out by Stitch et at. [15] that, in addition

to temperature, bias voltage is an important parameter in accelerating

failures. In this case the Eyring [16] modification of the Arrhenius

time equation is most applicable. It is:

where G, C, and D are positive constants, and V is the bias voltage.

This equation indicates that for a fixed temperature as the voltage

increases the median life decreases. In one case on increasing the

applied voltage from 5 to 15 V, the main population failure mode activation energy decreased by approximately 0.1 eV (apparent) [16]. The problem of directly applying this model results from the fact that in a complex integrated circuit it may be impossible to have the same voltage

across each junction, thus one can only speak in terms of voltage

applied to the overall device. Interpretation is further complicated

by some tests that are carried out at temperatures > 200 C.

The burn-in test is generally considered to be the only test that can reduce electrical "infant mortality" in electronic systems that

incorporate many devices. It is relatively inexpensive to perform and

is required on essentially all hermetic military microelectronic

devices. The use of this test on nonhermetic (plastic encapsulated)

devices can be undertaken only after consideration of such additional

factors as the plastic glass-transition temperature and decomposition

temperature. Most plastic devices are burned-in at lower temperatures

C + kl

17
-V

T'Tln^

- -

s

|-^-——: ill

A- Tl

W!

(HRS) = 6 454 I 10-4 ExP

IHRS) = 4 303 I 10"'' EXP

^

1

i 1 M

~|"

i"

1 ! Hi

5 10 6 7 8 Hill

1

—1 - 273 15 + TEMP ("C ) J [i|

1

1

^ l!|| 5106 78 ! 1

' '

1

1

T

* Vvn^^

—

its |^-^ltM"

u m i

. — — — ..

• —-

11 \ « '

, Ll;

i 'Ml

L

1

]

i :\ c\

1- --[

1 1-4

|i 1—

,

1

;i —

II

I" 1 il 1

1 iilllii i i ill III 1 -ttI 10 100 1000 10^ I05 106 I07

B URN IN TIME (HOURS }

Figure 12. Burn-in regression curve from MIL-STD-883B [1]

,

Figure 1015-1.

ISOMETRIC GRAPH OF SNM54L10 FAILURE DISTRIBUTIONS

Figure 13. Isometric graph of accelerated test failure distribution for a device with Au-Ti-W metallization showing freak

and main population distributions. (After Johnson [15])

18
than hermetic devices. A comprehensive review of accelerated testing

has been given recently by Reynolds [17].

2.3.6 Particle Impact Noise Detection (FIND) Test

The purpose of this test is to detect loose particles inside a device package. To perform this test, the device is attached to a piezoelectric transducer with an ultrasonic couplant material . The

transducer is attached to a shaker which vibrates the device and transducer, typically at 60 Hz, at accelerations in the 10 to 20 G level.

Various specifications may require different frequencies for different

package sizes in order to allow for the variation in time of flight

of loose particles with different characteristics. The transducer and

ultrasonic amplifier are usually peak tuned to 140 kHz. Most small

particles of 25-ym diameter or more will produce ample acoustic signals

at that frequency upon impacting with the walls of the device package.

Unfortunately, the hissing of compressed air escaping, clapping of the

hands and many other common noises may also excite the transducer leading to rejection of good, particle-free devices. Also, some particles

do not break loose from the package so that they can be detected during

the test. After the device is installed, these undetected particles

can cause a failure. Figure 14a shows a wire bond with low lead dress

and an arrow points to die attach eutectic particles which could break

loose and possibly be detected by a FIND test. Figure 14b shows a loose

eutectic particle short between the wire bond and the chip.

The detection systems usually consist of an oscilloscope and headphones or a speaker which are connected to the apparatus through a heterodyne system to reduce the frequency to the audible range. Figure

15 is a sketch of a typical equipment setup for this test.

Before and/or during the test, the device is tapped by a "15 cm solid copper rod 2.5 mm in diameter with rounded end or other apparatus

capable of imparting shock pulses of 200 to 1500 G to the device under

test" (Method 2020 MIL-STD-883B ) to loosen particles adhering to the

device or package. Both mechanical and electromechanical apparatus,

are available to shock the parts, but because of their differences thev

may contribute to variability in the test results. Also, different package types display different "personalities" by holding captive similar

particles under more or less shock than other package types.

From this brief description, it should be apparent that this test

is neither quantitative nor particularly accurate. In factj_ it is controversial. David considers it at best 50% effective [18]. The test

is costly to perform and made even more costly because of the ambiguous

results. It does not distinguish between metal particles which can cause

shorts, and insulator particles that are usually benign. Small particles

of aluminum are frequently undetectable. The limit of detection is typi-

*Hilten, J. S., Lederer, P. S., Mayo-Wells, J. F., and Vezzetti, C. F.

,

Loose-Particle Detection in Microelectronic Devices, NBSIR 78-1590 (1979)

19
b

Figure 14. (a) An IC having low lead dress and gold-silicon

eutectic particles formed during die attach. A typical particle is indicated by arrow. (b) , shorting gold/silicon eutectic particle under different lead on same IC. (After Ebel [8])

20
LAMP

I

I THRES HOLD

DETECTOR

INDICATOR

ULTRASONIC

AMPLIFIER

OSCILLOSCOPE

DUT COUPLANT

TRANS

DUCER

ISOLATOR ACCELEROMETE

/

Eh

SINE

GEN ERATOR

AND AMP

SHAKER

CHARGE

AMP

Figure 15. Schematic of typical loose particle detection apparatus from MIL-STD-883B [1], Figure 2020-1.

21
cally 0.02 yg, which represents an aluminum particle of approximately

25 ym in diameter.

Even though this test is of limited usefulness, loose metal particles have caused numerous field failures, and a high confidence level

test to detect them is still needed. As an alternate solution to the

problem, polymer conformal coatings have been used by some organizations

to immobilize the particles. This is highly effective, but at times,

these coatings can produce their own reliability problems.

2.3.7 The Statistics of Sampling for Special Production Lots

On a production line, all tests, whether destructive or nondestructive, are expensive, and the decision to use one must be a compromise

between cost and the reliability confidence level required by the end use of the devices. The costs of applying any individual test are not

always apparent to those who must make decisions concerning their use. Implementation of a test is sometimes based on intuition rather than

on the physics or statistics involved.

Once implemented, the obvious cost factors of a quality assurance

test are the initial expense of test equipment, operator time, and the

actual loss of product that results from the test. Some of the less

obvious costs are the test throughput time that may cause delays in

shipment (such as burn-in or lot-sample life-tests); training of operators; maintenance of equipment; data evaluation costs; record keeping;

and the electrical and other costs of running the test equipment (burnin, lifetests, temperature cycle, etc.). Time lost in making decisions

on border line cases (such as in visual inspection of expensive

devices), cost of rejecting good product due to incorrect decisions

or to such problems as faulty test equipment (e.g., electrical tester

with high electrical transients which damage devices or faulty probe

adjustments that damage bonding pads).

Because the cost of 100% testing is often prohibitive, various

statistical sampling plans have become a normal part of semiconductor

device quality assurance. In the past, such plans were generally based

on AQL (acceptable quality level), but recently, perhaps because they

describe more directly the protection to the consumer for individual

lots, LTPD (lot tolerance percent defective) plans are usually specified. MIL-IVI-38510D [19] contains LTPD tables that are extensively used

by the semiconductor industry. This sampling method is valid for special

product made in single lots. Parts for space application or other high

reliability usage often fall into this category. Also, many hybrids

are made and sold in unique lots of 10, 50, or 100 units. The statistics

used in controlling the production of parts such as these are not, in

general, the same as are appropriate for month after month identical

production of a given product, where mean plus standard deviation (X,

S ) control charts are used to build up confidence. LTPD sampling, as it is practiced in the electronic device industry usually requires segregating production over some period (e.g., 2 hours or a day) from one machine performing a particular assembly or test operation. A number

22
of devices (determined by the LTPD number) are then randomly selected

from the lot and tested. Usually, but not necessarily, if one unit fails

(acceptance number, C = 0) the entire lot is rejected or must be 100%

tested (if possible) before acceptance. Table 1 presents a portion of

the LTPD table from MIL-M-38510D [19]. It should be noted that LTPD

is based upon the number of parts tested and not upon the percentage

of the lot tested. (If the lot is small, relative to the number to be

tested, special calculations are required.)

Figure 16 is a plot of LTPD at defect levels that can be applied

to high reliability parts. The values are calculated from a paper by

Schilling [20] in which an appendix derives the LTPD equations. It can

be seen that a very large percentage of units must be tested to arrive

at a low defect level. In fact if the test is destructive, (such as

a bond pull test), then it becomes prohibitively expensive and an appropriate nondestructive test is often substituted on a 100% basis (such

as a nondestructive bond pull test)

.

As indicated, LTPD sampling is widely used in industry specifications, and it is almost as widely abused. LTPD plans were specifically

derived for protection with regard to single lots, and yet specifications call out the same LTPD values for both single lot and normal,

continuous production. Typically in the electronics industry, loose

LTPD values of 5 to 10 (% defectives) are specified for all cases. This

may result in product of much higher quality than indicated by the

specified LTPD when applied to continuous production where a high con- fidence level, based on X, charts, is built up over a period of time

for the production equipment and personnel. However, the same loose LTPD

values are also used indiscriminately for small single lots as well and

the user apparently expects lots which pass to have defect levels similar to space parts {"^ 0.1% defectives) .

2.3.7 Conclusions of Section 2

Several nondestructive tests (screens) called out in MIL-STD-883B

that are applied to establishing the mechanical integrity of electronic

devices have been reviewed. Some of these (e.g., the PIND test) result

in such a low test confidence level that they should be rarely used,

while others (e.g., burn-in) offer considerable assurance against infant

mortality or other failures and should be increased in usage. The statistical sampling basis (LTPD) often used in applying these tests was

also reviewed.

3.1 PASSIVE ACOUSTIC TECHNIQUES

3 . 2 Introduction to Acoustic Emission

Acoustic emission (AE) is generally defined as being a transient

elastic wave or stress w^ve generated by the rapid release of energy

within a material when that material undergoes fracture or deformation.

The classic example of this has been known for years to the metallurgy

23
o

2303

(0.002) 3891

(0.009) 5323

(0.015)

0.15 1534

(0.003) 2592

(0.013) 3547

(0.022)

rsi

1152

(0.005) 1946

(0.018) 2662 I

(0.031)

O

767 (0.007) 1296

(0.027) 1773(0.045)

ino

461

(0.01) 778 (0.045) 1065

(0.080)

o

ninimum

Sample

Sizes

328

(0.02) 555(0.06) 759

(0.11)

231

(0.02)

.

390 (0.09)

,

533

(0.15)

in

153

(0.03) 258

(0.14) 354

(0.23) 1

116

(0.04) 195(0.18) 266

(0.31)

76 (0.07) 129

(0.28) 176

(0.47)

in in r-i V • o

77 (0.46) 105

(0.78)

32 (0.16) 55 (0.65) 75

(1.1)

o 22 (0.23) 33 (0.94) 52

(1.6)

in 15 (0.34) 25

(1.4) 34 (2.24)

o (0.46) 18 (2.0) 25 (3.4)

O 8 (0.64) 13

(2.7) 18 (4.5)

O

in 5 (1.03) CO ^ 11 (7.4)

Max.

Percent

Defective (LTPD)

Acceptance

Number

(c)

(r

=c

+

1)

o

24
1.0

CQ

«z5 0.1

0.01

I I—I—I I I \

(N) LOT SIZE^IOOO UNITS \ X \

|N| LOT SIZE=10,000 UNITS '(ITS \\ \\

TO% OFBE

UNITS TESTEDFROM

TO GAIN

LOT SIZE 90%|N| \^

CONFIDENCE THAT A LOT CONTAINING

X°/o DEFECTIVES WILL BE REJECTED. ONE FAILURE REJECTS ENTIRE LOT |c=0)

J I I

' I I r I 1.0 10 100 % OF UNITS IN LOT TO BE TESTED

Figure 16. Plot of LTPD VS. sample size to be tested at

specified defect levels.

25
industry as "tin cry" where merely bending a piece of tin will result

in an audible sound. The first scientific report of sounds being emitted

from metals during deformation was given by Joffe [21]. In 1950, Kaiser

did the first comprehensive study of the phenomena [22]. His name is

associated with the generally irreversible nature of AE , in which little

or no acoustic emission occurs until previously applied stress levels

are exceeded. Literally, a deformation or crack will only produce more

AE if it is enlarged. The emitted stress waves may have frequencies

ranging from the audible into the megahertz region, but the maximum

energy is usually concentrated in the mechanical resonance modes of

the test specimen. Detection of these waves usually takes place with

ceramic piezoelectric transducers that are acoustically coupled to the

specimen; however, wide band optical [23,24] and capacitive [25] detection methods have recently been used.

Many of the early materials studies were carried out on metals

and correlations were made between various metallurgical properties

and the AE released after the elastic limits were exceeded in stressstrain type of studies. Dunegan [26], for instance, obtained an excellent

fit of a mobile dislocation model for 7075-T6 aluminum, but little cor- relation was obtained for other metals. The various sources of acoustic

emission that have been observed include: crack nucleation and propagation, twinning, grain boundary sliding, multiple dislocation slip, creation of multiple dislocations, solid-solid, solid-liquid, and liquidsolid phase transformations, and the Barkhausen effect (realignment

of magnetic domains).

In general, most microelectronic uses of AE , with the exception

of certain melt- type welding applications, are more concerned with crack

initiation and propagation than with structural dislocations or defects.

Crack propagation in brittle materials was first explained by Griffith

[27]. He postulated that elliptical cracks exist on the surface of brittle materials such as glass, lowering the tensile strength. Crack propagation occurs when the stress at thg ends of the crack exceeds a theoretical value, a^, . The stress (a ) at the ends of such a crack as th e modified by Orowan [27] is given as:

where:

c = the half length of an interior crack or the length of a surface

crack and

p = the radius of curvature of the ends of the crack ellipse.

For the crack to spread, the stress at the point of the crack must

exceed the theoretical breaking stress (a, ) of the material and b

Units for stress could be. grams force/cm^.

26
where

Y

= the specific surface energy and

E = Young's modulus.

It can be seen that as the crack length increases, the stress necessary

to keep the crack growing decreases . Thus, once started, complete fracture can occur. The maximum or limiting crack propagation velocity is

0.38 times the velocity of sound in the material.

For brittle cracks in poly crystal line materials, including metals,

Y is replaced by Yp which is the "effective" specific surface energy

and c is replaced by d, the average grain diameter. The equation now predicts that the strength of a polycrystalline metal which fails by

brittle cleavage should vary as the reciprocal of the square root of

the grain size. The energy (Eg) released during microcrack growth,

assuming all energy in the grain is released as the crack passes through

the grain, is Eg = a^a^ , where a is a constant depending on grain size.

Presumably, much of this energy is released in the form of stress waves

(AE), although there can be other mechanisms of energy loss. As the

crack continues to propagate, stress waves (AE) are propagated in all

directions from the crack tip. They include a white spectrum of frequency components up to many megahertz.

Rapid propagation as predicted by the Griffith crack theory, or

its various modifications, is assumed to be valid for cracks in silicon

chips as well as for glass- and ceramic-to-metal seals as found in semiconductor packages. However, for ductile-metal bonds made under nonoptimized or contaminated welding conditions, this theory is not necessarily

correct. As reported in Section 3.4, bonds and various interconnections

made under contaminated conditions usually consist of numerous individual microwelds (see Figures 22 and 37) which, in principal, can break

individually without propagating a Griffith-type crack. Still, the crack

will always propagate along the bond interface without major deformation

of the joined pieces. This tends to make crack propagation at least

somewhat "Griffith like" rather than what one might expect in an pure

ductile fracture. One might calculate breaking stress of such weak

"welds"; but the microweld dimensions and number, when made under contaminated conditions, are unknown and this could lead to unrealistic

conclusions. Perhaps the closest description of this type of crack propagation has been given for adhesive bond (e.g., epoxy) fracture. Some

workers [28] have modified the Griffith equations to include terms for

plastic energy dissipation, and others have used a thermodynamic

approach. Most are complex and require computer aided computations.

Anderson et al. [28] have included an excellent review of the theories

of adhesive fracture along with the computer techniques used to solve

them.

27
Most aspects of acoustic emission theory are either in a state

of controversy or are incompletely developed. Several papers have

attempted to develop models that correlate the amount of acoustic emission to the size of a dislocation source, the grain size (for stress

wave scattering), and the distance from the AE detector. However, as pointed out by Green [24,29], it is usually necessary to oversimplify

the model for purposes of calculation as illustrated in Figure 17a and b.

Typically, special point sources and uniform propagation velocities are

assumed; but in fact, the sources may have various nonsymmetrical shapes

and the emitted wavefront continues to change due to directional variation in wave velocities associated with linear elastic wave propagation

in anisotropic solids. Also, the wave amplitude decreases as it travels

from the source because of the expanding wavefront and preferential

attenuation caused by ther'moelastic effects, grain boundary scattering,

acoustic diffraction, and scattering from point defects. Some of these

attenuation and scattering effects are frequency sensitive. The signal

may be further complicated by internal reflections and interferences

that depend upon the specimen geometry. Thus, the detected signal amplitude, phase, and frequency may not be characteristic of the source. Therefore, most theory can only be verified on large, simple geometry

samples. In addition if the piezoelectric detector has high sensitivity,

it usually has a high Q (narrow frequency) response. The amplifiers

used for detecting threshold signals usually have a limited band pass

to reduce noise, so that all signals are somewhat similar in appearance

and ringing is normal. Green [24] has given comparisons between acoustic

emission signals from a typical piezoelectric-transducer tuned-amplifier

combination and signals from a laser interferometer and optical

detector - broad-band amplifier system. The preciseness of the optical

detector output is striking as compared to the ringing output of the

piezoelectric ceramic transducer system as shown in Figure 17c.

Hsu et dl . [25] have developed a theory to combine as much information as is known about a simple AE source, its propagation, and detection by a wide band (capacitive) transducer. The theory is based on

a Fourier inversion technique. Experiments to verify this theory were carried out on a well characterized system consisting of a 2.5 x 23 x 44 cm aluminum plate with a simulated AE source directly across the

smallest dimension from the detector. This system simplified the waveform interpretation and excellent agreement with theory resulted.

It is not even clear how a piezoelectric detector responds to an AE wave front. Harris et ai . [30] demonstrated that the detector

responds to the square root of the energy released during a given deformation process; whereas, Jon et at . [31] show that the voltage generated by the transducer is related to the rate of energy released by

the source. Thus, with all of the variables and uncertainties, it is

extremely difficult to quantitatively relate a microscopic theory of

stress wave emission with experimentally measured frequency and waveforms. In addition, it is not clear on a microscopic basis how cracks

or other mechanisms actually generate "white" AE

.

28
TRANSDUCER SURFACE

c

Figure 17. (a) Oversimplified model of acoustic emission

sources. (b) A more realistic simplified model of acoustic

emission sources. (c) Two successive acoustic emission

bursts due to stress corrosion cracking; upper trace piezoelectric signal; lower trace optical signal. (After Green

and Pond [29])

29
Many of the concerns of Green [24] and others are not of practical

importance to AE detection in samples of dimensions encountered in

microelectronics. The various mechanical resonant modes of a specimen

may be as high as several megahertz and multiple internal reflections

are inevitable. Investigators are usually constrained to attach a transducer, which may be larger than the specimen, in whatever manner

is possible (such as using tapered acoustic waveguides) and work with

whatever signal is received. Waveform signatures, of frequency and

amplitude, are recorded and empirically correlated with appropriate

mechanical stress tests (e.g., destructive pull tests) for interpretation. It is obvious that the interpretation of AE signals from typical

electronics applications is undeveloped and may never be amenable to

clear mathematical solution.

Two books are available that give theory, equipment and applications of acoustic emission to a variety of nonelectronic problems and

these should be read for more detail than is given above [32,33]. Also,

a recent critical review of the status of the AE field has been given

by Lord [ 34]

.

3 . 3 Review of Acoustic Emission Applications to the Real Time

Nondestructive Testing of the Mechanical Integrity of Electronic

Components

The application of passive acoustic measurement techniques is

in its infancy in the electronics industry. Although the potential

is great, it is yet to be exploited. Acoustic emission has been used

as a tool to study materials and monitor the physical condition of large

structures such as nuclear pressure vessels and bridges, but it has

only recently been used to evaluate electronic materials and assembly

processes. The largest effort is perhaps at the Western Electric Engineering Research Center, Princeton, New Jersey, and a majority of the

published papers have come from there. Most of the emphasis has been

on real-time, in-process evaluation of some electronic production

process and these will be reviewed in this section. These real-time

applications will be treated first, and since the present author [35] was responsible for the only study of post-production integrity screening, this subject will be given in greater detail at the end.

The first published use of AE in electronics was by Vahaviolos

[36]. He used AE to reveal substrate cracking during the thermocompression bonding of beam lead devices to gold metallization on hybrid

ceramic substrates. During this bonding process the substrate is subjected to a temperature of approximately 300°C and high forces greater

than 100 kg/cm^ . Any ceramic flaw or warping located under the bonding

tool may initiate a microcrack, which, if undetected can ultimately

lead to failure of the entire device from moisture related causes; or

if the crack propagates during later qualification screening or during

device life, it may open up a metallization interconnection. A beam

leaded substrate of the type and vintage used during that work was shown

in Figure 6. The large size and completed cost of the substrate made

30
it mandatory to stop assembly and reject the unit as soon as any crack

was initiated (this problem is even more acute on sophisticated multilayer hybrids).

Since the AE detection work of Vahaviolos required placement of

a transducer on the heated bonding stage, he had to make his own high

temperature detector mounts and chose piezoelectric sensors according

to his special needs. He used a modified lead zirconate titanate transducer which could detect both longitudinal and shear waves and had a curie temperature of 350 C. He also used a lead metaniobate transducer

which only detected longitudinal waves but has a curie temperature of

> 600 C so that it can be used in many thermal tests.

As in most real-time AE detection systems, the many extraneous

production noises that are present must be discriminated against. These

may have the same frequency and amplitude characteristics as the AE

.

For example, during beam lead bonding there is first the impact force

of the hot bonding tool against the beams and substrate which generates

noise similar to AE . As the beams and the substrate metallization undergo plastic deformation, they also emit AE . If the substrate cracks

during this time, a rather sophisticated electronic system is required

to distinguish crack generated AE from the extraneous signals. Therefore, most real-time tests are relatively insensitive to cracks 50 to

100 ym in length in brittle materials, whereas a less sophisticated

electronics system used to screen completed devices can easily detect

brittle cracks 5 to 10 ym long since there are no extraneous signals.

Saifi and Vahaviolos [37] have reported the use of AE for realtime nondestructive evaluation of laser spot welding of small insulated

wires to electronic terminal posts. For this, a pulsed YAG laser was

used (40 pulses per second, pulse length 3.5 ms , energy per pulse

'V 20 J). They determined the conditions required for insulation vaporization as well as for the ideal combination of wire and terminal-post

metallurgy (copper wires, monel terminal posts). Production noise was minimized by gating the detection system only during the process period

when welding occurred, and after all insulation was vaporized. Although

this work is not closely related to microelectronic devices, it could

be the beginning of reliable laser welding in smaller structures such

as electronic device packages.

Carlos and Jon [38] reported the detection of cracking in high

reliability, high voltage ceramic capacitors that were intended for

submarine cable use. In this paper, AE is used to detect cracks created

in the ceramic casings due to the thermal shock of soldering. Cracks

so generated could not be observed in a visual inspection because the

susceptible area was covered with solder; however, such cracks could

later cause reliability problems. The electronic system was designed

to operate continuously and to process each burst of AE from the sensor,

using pattern recognition, to determine whether any particular burst

resulted from cracking the ceramic or was from such extraneous noise

sources as the soldering iron scraping against the ceramic. The system

could detect brittle cracks in the order of 25 ym but most actual cracks

31
were nearer 1 mm in length. This AE system was described as the onlypossible real-time method of 100% nondestructive checking for cracks

in such capacitors.

Jon et at. [31,39] have described the use of AE for the non- destructive evaluation of the quality of several types of resistance

welds. In one case on tantalum capacitor leads, the material combination

in the vicinity of the weld was unique (Ta, Ta^O^ , Cu , solder, and

steel) and it presented considerable AE interpretational problems. For

materials of complicated geometry and metallurgical composition, it

is not easy to identify any individual intrinsic processes from the

total AE generated during the welding process. This identification difficulty is due to the frequent overlap of the generated AE due to different causes. For example, one element of the composition such as copper could begin melting while the already-liquid solder could be going

through expulsion from the compressed joint at the same time. Signals

generated in such a manner are usually very difficult to distinguish

electronically. In such a case, the best way to solve the problem is

to monitor the generated AE during a time period where only clearly

understood events are occurring. From a metallurgical point of view,

the cooling period is easy to identify experimentally. During this

period, weld nuggets start to solidify and generate AE signals because

of both the plastic deformation and the build-up of the residual

stresses. Both of these processes can be used to indicate the weld

strength because large nugget volume, a good physical indicator of weld

strength, will give rise to more plastic deformation as well as a higher

residual stress build-up. The result is to generate more AE signals

during this time frame. In order to avoid extraneous AE signals that

bore no relationship to weld quality the authors gated their AE detection system to be responsive only from 10 to 4-0 ms after the peak of

the weld current, during the nugget solidification. This period produced

AE signals that were directly correlated with weld quality.

Another evaluation of spot welding quality by the AE monitoring

of 500-ym diameter nickel wire welded in electronic components was carried out by Knollman and Weaver [-40]. The all nickel system was simpler and the wires larger than that described by Carlos and Jon [38].

In this case, a commercial AE weld quality control system was found

to be adequate. The authors found that weak welds could be detected

at a confidence level of better than 97%.

In 1976, Ikoma et al . [41] used acoustic emission to study dislocations in semiconductor materials. Many failures or shortened useful

life in GaAs light emitting devices have been shown to result from dislocations propagating into active regions of the device. Also, it had

been shown by Kotani et al . [42] that improperly controlled thermocompression bonding can create defects in GaAs devices and reduce their

reliability. Previously Sedgwick [43] had shown that AE was emitted

from dislocation loops in KCl and LiF. Thus, Ikoma designed experiments

to explore the possibility of observing similar AE in GaAs by intentionally creating defects. His experimental procedure was as follows. A

32
quartz rod with a smooth 1-mm hemispherical radius on its end and an AE transducer on the other was used as a pressure probe. The polished

GaAs sample was placed on a heat stage. A load of 100 gf was applied

in all cases to force the probe against the GaAs sample and the substrate was heated to various temperatures from 25° to 420°C. A study

of the dislocation patterns of samples was carried out on the area under

the probe by varying the temperature. At each temperature, the sample

was etched and the dislocation density and pattern determined. No dislocations were observed to occur below 150 C. At temperatures between

300 and 400 C clear "rosette" patterns were observed with "arms"

stretching radially along the <110>, <011> and <101> directions on a <rrr> surface. AE was only observed at and above 300 C. The AE signal

increased linearly with temperature from 300 to 400 C and then decreased above 400 C . The reason for the decrease was not understood

,

since dislocations were still generated. Since the AE-active temperatures and pressures were the same as those used in thermocompression

bonding, it would be logical to apply the results of this investigation

to controlling' the thermocompression bonding process. Without giving

any details, the authors stated, "This technique is now being applied

to the detection of dark-line-defects in GaAs-laser diodes and to the

real-time inspection of a thermocompression bonding orocess of GaAs

device fabrications."

Very recently, Ikoma et dl . [44] presented evidence that AE is

emitted from GaP light emitting diodes during electrical over stressing.

The devices were operated at currents up to three times their rated

values. The measurement apparatus consisted of a 4 . 7-mHz transducer

and high gain (69-db), low noise, tuned amplifier. The typical AE signals were very small ('^ 100 nV at the transducer) . The authors correlated increased AE with decreased light output. Diodes that showed no decrease in light output produced no AE . After the tests, the devices

were etched and those that had produced the most AE displayed the greatest dislocation density. There were large unpredictable variations in

degradation from diode to diode. At present, there is no obvious application of the AE correlation with decreased LED performance since the

decrease in light output can be more easily measured. However, the

authors have clearly shown that electrical and thermal stresses which

produce crystal dislocations in semiconductor materials can be revealed

by AE . Applications of these techniques for screening out failure prone

devices may be possible in the future.

3.4 Acousric Emission as a Post-Production Screen for Bond Integrity

in Microelectronics

3.4.1 Introduction

The previous section reviewed acoustic emission tests that could

observe a weak weld or a cracked substrate during the actual production

process (real-time). From the standpoint of production economics, this

is the ideal time to discover a defect so that no additional production

effort and money is lost in further assembly. However, there are numerous cases where such real-time detection is not practical , but instead

33
a screen applied at a later time is necessary to remove production

defects, as with tests in Section 1.1. For instance, in cases of simultaneous multiple bonding such as the 40 lead tape bonding of integrated

circuits, it is not possible to assure that all leads are well bonded

in real-time. Some stress tests must be applied later. Likewise, when

multiple devices are simultaneously soldered, such as in wave soldering,

acoustic emission signals are not interpretable . Epoxy bonded components

yield little or no acoustic emission during curing, and thus, cannot

be tested in real-time. Also, tests are often required as screens for

incoming inspection for individual parts such as packages. Thus, the

ability to test the component at a later time may be the only way to assure bond or package integrity. The next section describes acoustic

emission tests on completed units and will start with tests developed

to assiire beam lead bond integrity; then the techniques will be applied

to tape bonded devices and hybrid components [35]

.

Beam lead and other gold-gold thermocompression bonding is generally reliable, once an optimum bonding schedule is achieved; however,

as with any bonding system, contamination in the bond interface may

inhibit welding [45,46] on one or more beams in an unpredictable manner.

In addition, if the hardness of the gold varies for devices from different wafer lots or different manufacturers, a bonding schedule optimized for one lot may produce erratic bond reliability for another.

Thickness irregularities in thick-film bonding metallization may also

reduce bond adherence for one or more beams on a multibeam device. Thus,

it is desirable to have a simple 100% nondestructive test that will

detect one or two poorly bonded leads out of a large number of well

bonded ones and not require a subsequent visual inspection or electrical

test to reveal the results.

AE has been studied in a variety of materials by many workers.

However, there is only one known study of such emission from gold, the

material used in the beam lead bond system. Schofield [47] reported

that "the occurrence and behavior of AE in gold was undoubtedly the

most consistent and certainly the most active of any of the face centered cubic metals previously studied." He verified the Kaiser effect

[22] for the high frequency emission. However, he found that certain

"burst emissions" were reproducible without annealing. Schofield 's work

was on gold single crystals in two orientations; however, some specimens

that had been elongated during a first test were then annealed and

developed a relatively coarse grain structure. These polycrystalline

samples also emitted AE upon further testing. Thus, for the NBS investigation it was concluded that the gold in beam leads should be capable

of AE if poorly welded beams lead devices could be stressed adequately

to strain or break some of the microwelds in the few bonded areas.

3.4.2 Methods of Applying a Mechanical Stress to the Beam-Lead System

The problem of mechanically stressing a bonded beam-lead device

in a nondestructive manner is a formidable one. The most desirable

method is to lift the device upward. This would stress both the beam

34
anchors (the attachment to the chip) as well as the beam bonds. The

most obvious method of doing this would be to slip hooks under the corners of the boncjed beam-lead device and pull upward. However, since

the bugging height can be less than 25 pm and may vary from corner

to corner, such a grappling hook could crack the thinned silicon at

the edge of the chip, causing extraneous AE unless extreme care was exercised by the operator. Another method of pulling the device upward

would be to epoxy tiny hook-shaped wires to the top of the chip and

pull the device with a wire bond puller. A more convenient version of

this technique is to use a hot-melt-glue pull-test, applying a force

well below the pull-off level. However, the glue used for this purpose

would remain on the chip and that particular organic material is not

a desirable additive to a hybrid package. Also, the brittle hot melt

glue can develop cracks during pulling and emit extraneous, misleading

AE.

One simple alternative to the glue method is to apply a "dab" of

a silicone rubber (SR) to the top of the beam lead chip and let it cure overnight at a temperature of about 50 C. Then a sharply pointed hook- can easily pierce the rubber parallel to the chip as shown in Figure 18.

The hook is then pulled upward; about 40 gf can be applied to a 1 x 1 mm

chip before the rubber breaks."^ A tweezer type of device or a flatter

shaped hook could be used in place of the present hook if it is desired

to apply greater pull-forces. To verify that this method produced no extraneous AE , similar sized "dabs" of SR were bonded directly to the

substrate and pulled with the hook. The SR emitted no measurable AE

until the force that ruptured the rubber was reached. The silicone rubber used for this purpose was usually the stiff version of the

methanol-base resin that some organizations use to protectively coat

beam-lead devices. When the device is subsequently encapsulated, the new resin will seal the punctured rubber and fill the cavity under the

chip. Therefore, this pull method and its residue can be considered

nondestructive to both the device and the substrate. Either a 100% test

or a sample quantity of devices could be tested on each substrate as

a control. Alternately, a high strength silicone rubber can be applied

to the chip and this hook method can then be used as a destructive pull

test. Pull forces of up to 100 gf have been applied to 1 x 1 mm chips.

Another method of pulling a chip is to mold a vacuum cup out of

SR or another elastomer into the shape of the chip. When vacuum is

applied to a device, an upward force of about 0.5 gf per beam can be

obtained for a 14 or 16 beam device having an Electronic Industries

Association registered chip outline. A third method of forcing the chip

Defined as the vertical distance from the substrate metallization to

the bottom of the chip.

"''if the chip is not clean, this resin may pull free with about 20 to

30 gf. An adequate cleaning procedure is to immerse the bonded substrate in a fluorocarbon solvent and then air blow off the remainder

of the solvent.

35
upward and the beams outward is to inject the silicone protective coating resin under the chip and allow it to eyre. The material has a much

higher coefficient of expansion than the gold beams. Preferential heat

(such as infrared) applied to the chip will expand the rubber enough

to stress very weak bonds. This method works best with high bugging

heights which allow more resin under the chip.

An alternative method of stressing the bonds is to push downward

on top of the chip. Some of the force will be applied to the bond heels

in the shear direction. This is also the simplest method of applying

force to the beam lead system. Depending on the angle at which the beams

project from the chip, the uniformity of the bugging height around the

chip, variation in beam dimensions, and the gold hardness, a 15 lead

device will collapse to the substrate, accompanied by large bursts of

AE , with the application of about 30 to 50 gf. This collapse force may

be less if it is applied off-center or if the bugging height is not

uniform. An improved variation on the simple push-down technique is

to simultaneously push downward on top of the chip and, with an equivalent force, push horizontally along a diagonal of the chip. The force

is applied with a molded SR probe that fits over the chip. The advantage

of this method is that part of the horizontal force is applied to the

bonds in a peel direction. Care must still be exercised to prevent collapse of the bugging height.

It has been found that roughly three times as much force in a downward direction can be safely used if it is only applied to the beams,

leaving the chip free. A simple resolution of forces analysis based

on typical bonded beam dimensions indicated that with all beams uniformly stressed downward, 90% of the force is applied at the bond

heel in the shear direction and no net torque is applied to the beam

anchors (the beam attachment to the chip). However, when the beams along

only one side of the chip are stressed at one time, the chip is rigidly

held in place by the other beams and then essentially all of the applied

force appears as a torque on the anchors. Thus, it is possible to provide an AE test for both beams and anchors by appropriately applying

the stress. This assumes that the beams project horizontally outward

from the chip for about 50 ym before curving downward and bonding occurs

about 100 pm away from the chip. In some bonding situations, the beams

leave the chip at about 45 deg in a downward direction, and the bond occurs only 25 to 50 ym outward from the chip. No anchor torque is possible

in this case. Thus, in order for this test method to be applicable great

care in beam alignment during bonding is essential to obtain uniform

bugging height.

To determine the effectiveness of the chip push-down method, a simple technique was developed to determine the downward force necessary

to produce threshold deflection of the chip as well as of the bonded

beams. A 1-mW He-Ne laser with a focused spot diameter of ^ 25 ym was directed under the chip at a low angle between two of the beam leads

as shown in the sketch of Figure 19. The force was applied by the apparatus described in Section 3.4.4 that is normally used for such purposes

36
Figure 18. Scanning electron micrograph of a weakly bonded

beam lead device (A) with a silicone rubber "dab" (B) on top,

that has been pulled up by an electrolytically etched, ISO-ym

diameter tungsten hook (C) . (It is important that the hook be

very smooth and have a sharp point so that the rubber is not

torn while it is being pierced.) The hook carrier (D) is a section of a No. 22 hypodermic needle in which the hook had

been inserted and rigidly epoxied in place.

Figure 19. A simple method of measuring the threshold of

downward motion of a beam lead chip. The arrow points to

the static interference pattern (the dots between the beams)

.

When force is applied, the pattern changes.

37
in the AE test. The laser light was multiple-reflected back and forth

between the substrate and the chip and established a complex static

interference pattern that could be seen extending outward from the edge

of the chip for 125 to 250 ym. Downward or upward deflection of the

chip by only a small fraction of a wavelength produced changes in the

interference patterns that could be easily seen through a 40X binocular

microscope, even though no direct motion of the chip was discernible.

The threshold of observable motion of an individual unbonded beam could

also be seen by this method. Such motion of the chip occurs with an applied force as low as 3 to 5 gf . Essentially unbonded beams require

the application of about 10 to 15 gf to the chip before movement is

observed, and this requirement varies according to the angle at which

the beam lead approaches the substrate.

3.4.3 Preparation of Controlled Bondability Substrates

Various unpredictable bonding conditions can result in one or more

of the beam leads not having a strong weld. However, it is difficult

to deliberately obtain weak bonded leads. The usual method for obtainin

weakly bonded gold-to-gold leads by lowering the bonding temperature

is unreliable. The beam leads of such an intentionally weak bonding

series that are bonded first may increase in bond strength while other

devices are being bonded. Even substrate temperatures as low as 85

to 150 C for one hour can significantly increase gold-to-gold bond

strength on uncontaminated bonding surfaces [45,4-8], and higher tempera

tures require even less time for bond improvement. Such strengthening

of bonds has been verified in this study. Therefore, low temperature

methods of producing weak bonds are not desirable for use in developing

new measurement methods

.

In order to obtain weak bonds specified in both number and position, an effect that is normally avoided was used. It is well known

that chromium-locked-gold metallization must be kept at relatively

low temperatures or the chromium will diffuse to the surface, oxidize,

and severely decrease the thermal compression bondability [49]. Therefore, tantalum nitride-chromium-gold"'' substrates were heated to 310°C

for two hours to diffuse the chromium to the surface. A special photomask set was used to pattern the metallization, and a cerric ammonium

nitride etch [50] was used to preferentially remove the chromium oxide

Although qualitative in nature, this simple technique may be useful

in other types of visual inspection and should be a valuable aid in

the observation of the relative thermal expansion of components as well as for studying creep phenomena. In order to be effective, the

substrate must be coated with metal . A ceramic substrate under the

chip results in a confusing pattern of internal reflections and interference patterns

.

'''Efforts to use chromium-gold metal lizated substrates resulted in rapid

etching of the undiffused chromium, thus undercutting the gold. The reason for this is unknown.

38
in all but specifically designated areas. Figure 20 is a photomicrograph

of such a substrate bonded with beam lead devices. For clarity of

presentation, the chromium-oxide-covered areas have been darkened (normally they are only slightly darker than the rest of the metallization).

The four different patterns can be clearly seen. They include, from

left to right, a single poorly bonded beam on a corner location, (D), a single weak beam in the center, (B), two weak beams in the center,

and a control pattern for making all well bonded beams.

It should be pointed out that when using the chrome-diffused gold

bonding pads, some degree of control over the beam-lead bond-peelstrength (essentially all the bonds so prepared peel) can still be

exercised by varying the bonding parameters (force and temperature)

.

In this manner the peel force for an individual beam can be varied from

less than 0.5 to approximately 3 gf.

It was established that the chrome oxide method of producing controlled weak bonds still left a few areas that were welded and therefore

could produce valid AE signals when the lead was stressed. In addition,

it was desirable to determine the minimum AE signals that could be

detected with the available equipment. Devices were bonded following

various bonding schedules to substrates that had chromium oxide on the

surface. The devices were stressed and acoustic emission signals were recorded on digital pulse-capturing equipment. The poorly bonded beams

were then peeled back and examined for evidence of torn welded areas representing AE point sources. Figure 21a is an SEM photograph of the

chromium-gold coated ceramic substrate with a peeled-up beam in the

foreground. Examination of the beam lead bond depression in the substrate revealed tiny broken welded areas around the perimeter where

deformation is greatest. The largest of these are indicated by the

arrow. The beam is shown in Figure 21b. The tiny white dots near the

perimeter are the broken welded areas. The fact that the welded areas

lie around the perimeter is in agreement with the deformation theory

of thermal compression bonding by Tylecote [51]. A higher magnification

view of substrate weld breaks is given in Figure 22 . It should be noted

that the bonds made under such contaminated conditions consist of a large number of individual microwelds. Each one of these may be broken

relatively independently of distant ones, and thus, this type of bond

does not necessarily follow the crack propagation velocity as described

in 3.2. The arrows in the figure designate broken gold welds of one

to two micrometers width. Figure 22b shows part of the beam-lead,

clearly revealing pulled-off metal pieces from the substrate. In this

case, their size is two to three micrometers in width.

3.4.4 Experimental Apparatus

Most mechanical stressing experiments and AE measurements were

made using the apparatus of Figure 23. The force gage (A) can measure

either an upward or downward force. The entire gage-probe apparatus

can be rocked in front-to-back and side-to-side directions by the knob

(B) enabling a downward or upward force to be applied at an angle to

a single row of leads at a time. The top AE detector-force probe (C-D)

39
Figure 20. A patterned substrate showing bonded beam lead

devices. Chromium-oxide-covered areas for bond strength inhibition are stained black to increase visibility. The vertical row patterns are: (A) bonding controls (all good bonds)

,

(B) pattern containing one weak bond in the center of the span,

(C) pattern containing two weak bonds in the center, (D) pattern containing one weak bond on a corner. The chip dimensions

are 1 mm on a side.

40
Figure 21.- (a) is an SEM photograph of a chromium-diffused,

gold-coated ceramic substrate with a peeled-up beam lead in

the foreground. An arrow reveals some of the tiny broken

substrate welded areas. (b) is an SEM photograph of a peeled-up beam lead revealing the tiny white-appearing dots near the perimeter that were the welded areas.

a b

Figure 22. (a) is the SEM photograph showing pieces of the

substrate pulled off and sticking to the beam lead. (b) is

a high magnification SEM photograph of substrate weld breaks,

(These are indicated by the arrows.)

41
Figure 23. Apparatus used to apply upward or downward force on beam lead devices and to detect any resulting AE. (A)

force gage, (B) force angle control, (C) acoustic emission

detector, (D) acoustic waveguide and force probe, (E) substrate holder with vacuum hold down and substrate AE detector .

42
is screv;ed into the gage, facilitating rapid probe changes. The bonded

test substrate is held on a chuck (E) which contains the substrate AE

detector.

Close-up photographs of several top detector probes are shown in

Figure 24. Figure 24a shows a ceramic probe designed to apply a uniform

pressure on beam lead chips. Figure 24b shows a tungsten carbide conical

probe with a TS-ym diameter flat on the bottom. This probe is used to

stress individual leads. The probe in Figure 24c is made of tungsten

carbide and is essentially a beam lead bonding tool with small dimensions so that it only contacts the horizontal projection of the beam

near the silicon. The tips of all probes are coated with from 25 to

75 ym of SR both to increase the acoustical coupling to the leads or chips and to avoid metal-to-metal or metal-to-silicon contact since

such scraping can result in extraneous AE-type noise.

A substrate detector fixture is shown in Figure 25. In this particular fixture, the test substrate is held against the AE detector by

a cylindrical weight. (Other substrate detector fixtures such as the

one that was shov/n in Figure 23 use a vacuum hold down.) The detectors

are forced upward against the substrate by a spring. The surface of

the substrate detectors are coated with a thin film of very compliant

SR to facilitate acoustical mating with the ceramic substrate, in order

to avoid the use of various sticky organic coupling materials which

must be removed later. A textured SR surface is preferable. This is

obtained by pressing ground glass, treated with a mold release agent,

against the detector while the resin cures.

Several arrangements of signal preamplifiers, filters, and the transient recorder have been employed. However, the block diagram of the most

frequently used system is given in Figure 26. The total gain in each am- plifier channel is 80 dB. Each channel has a 24-dB/octave band pass filter, a tunable filter, or both. The special digital trigger circuit [52]

requires that a given number of cycles, selectable from 1 to 10 , of a separately specified positive and negative amplitude signal occur within

a total specified time frame for triggering the dual-channel transient

recorder. The overall system is capable of detecting AE signals barely

above the average noise level of the preamplifiers and considerably below various system and line transients. Most AE detector output signals

produced in the present experiments were in the range of about 10 to 100

yV and were easily captured by the above equipment. Because of the variety of gain adjustments possible (preamplifiers, pulse capturing equipment, and oscilloscope) , the vertical scale of most AE oscillograms will

not be specified. The only important consideration is the signal-tonoise ratio and this can be easily observed from the traces.

Experiments were run using various AE substrate detectors with the

tunable filter. It was found that the maximum AE output was obtained from

the thin, 2.5-cm square ceramic substrates at 350 to 400 kHz, and from

bonded 16-lead, beam-lead chips at approximately 1 MHz. The actual boundary conditions were unknown and could not be included in equations of

the mechanical resonances of the substrate and chip, and thus calcula43
Figure 24. AE probe-detectors. (1) Adaptor for attachment to

the force gage, (2) AE lead zirconate titanate type detector,

(3) acoustic waveguide and probe. Probe (A) is designed to apply uniform pressure on SR encapsulated beam lead chips. The

waveguide portion is ceramic and its tip is coated with SR or polyamide . Probe (B) is designed to probe individual beams.

The center tip is of tungsten carbide and has a 75-ym flat portion which is coated with SR. Probe (C) is a modified tungsten

carbide beam-lead bonding tool in which the inner walls are about 25 ym larger than the silicon chip on all sides.

44
Figure 25. Substrate holder using a weight to force the substrate against the detector. (A) AE detector, (B) simulated

substrate (glass), (C) removable brass weight, (D) spring to

force the detector against the substrate.

FORCE DETECTOR PREAMP

1 TUNABLE

FILTER PREAMP

2

SUBSTRATE DETECTOR

SELECTIVE TRIGGER

TUNED PREAMP

TRANSIENT RECORDER

[ \^— ^

OSCILLOSCOPE

Figure 26.

tus

.

Block diagram of the acoustic emission test appara45
tions were off by more than a factor of two. As a result of the measurements, the substrate preamplifiers and detectors were chosen to peak at

375 kHz and the chip probe equipment at 1.1 MHz. The AE experiments described here will all assume such frequency responses unless otherwise

stated. The digital trigger has been used in either the substrate or probe circuit; but in most cases, it was used in the probe circuit.

When the probe was in contact with the chip, there was essentially

no mutual response from the probe and substrate detectors resulting

from random AE sources remote to the beam lead device, regardless of

the operating frequencies of each detector. A crack in the substrate

would yield a strong signal in the substrate detector, but not in the

probe. A ceramic scribe-scratch on the tapered probe, which saturated

its detector preamplifier, was only negligibly registered on the substrate detector. However, stress waves generated by a failure within

the beam bond-anchor system resulted in a substantial signal in both

channels. Therefore, in experiments where both substrate and probe

detectors were employed, some AE output was required from each detector

in order to define a failure, although their relative amplitude as well

as the number of bursts recorded from each detector often varied considerably when the detectors were operated at different frequencies. The

purpose of the present study was to develop a specific test method;

however, monitoring a single AE source in two or more frequency bands,

as in the present experiments, may be a fruitful approach to understanding the nature and mechanism of stress wave emission.

3.4.5 Experimental Results

3.4.5.1 AE Results from Pulling Beam Leaded Devices

The silicone rubber-hook method of pulling beam-leaded devices,

as described in Section 3.4.2, was used to obtain quantitative information of various beam failure modes, since the pulling force is equally

distributed between all beams. For this work the apparatus of Figure 23 was used. The hook shown in Figure 18 was substituted for the probe

detector and all AE was picked up by the substrate detector.

In order to demonstrate the sensitivity of the AE method, all beams

except one were cut and the remaining one was pulled to destruction.

It broke at the bond heel. Figure 27 gives its AE pattern. Clipped waveform peaks indicate that the substrate detector output was significantly

greater than one millivolt peak to peak during the initial part of the

break. AE from such breaks generally continues erratically for several

times the 200 ys shown in the figure. Most AE signals in this work are much smaller and of shorter duration. For comparison a well bonded

device with weak anchors had all but one of its beams cut in a similar

manner to the above. Figure 28 gives the AE pattern of the single anchor

failure. The peak-to-peak detector output in this case was approximately

0.3 mV.

Pull tests were conducted on several well bonded devices that had

weak anchors (peel strength 3-gf/anchor ) , a series of short bursts

46
Figure 27. Oscillogram of the AE obtained from pulling off a single well-bonded beam lead with a force of "^3.3 gf . Horizontal scale is 20 ys/div. The peak-to-peak output of the detector was greater than 1 mV.

Figure 28. Oscillogram of the AE obtained from a single anchor

peeling off under a load of ^^^3 gf . All other leads on the device were cut. Horizontal scale is 16 ys/div.

Figure 29. A typical AE burst from a weak anchor. A pullforce of only 1.5 gf/beam was applied. The entire sweep is

200 ys in duration and the main AE burst is about 15 ys long,

47
was observed starting at '\' 1.5~gf/beam. Figure 29 gives a typical AE

signal from peeling anchors. Similar well-bonded devices with strong

anchors produced no AE until a force per beam of approximately 2.5 gf

was applied and the bursts in this case were longer and higher in

amplitude

.

Pull tests were conducted on strongly bonded beam-lead devices

to serve as controls for weakly bonded-beam experiments. A number of

devices from four different manufacturers were tested. The devices from

three of these manufacturers produced no detectable AE until stressed

to about 2.5-gf/beam, at which point the beam and the anchor system

began to deteriorate. However, devices from the fourth manufacturer

were quite different. Large bursts of AE were emitted when the devices

were stressed to only 1-gf/beam and these bursts increased with increasing stress. This result was verified on three different device types

and on lots purchased 18 months apart. Examination of these devices

after they had been stressed to the 1-gf/beam level revealed no obvious

problems; however, examination of them after stressing to the

2.5-gf/beam level (the point where well bonded devices from other

sources generally emitted their first AE bursts) revealed elongation

of the beams, separation of the relatively thick titanium layer, anchor

peeling, nitride separation from the beams or silicon, and chips of

silicon broken off at the anchor location (see Figure 30). Figure 31

gives a typical AE burst obtained by pulling a similar device to

1.2-gf/beam. Any of the mechanisms of beam system degradation shown

in Figure 30 could be responsible for bursts such as that of Figure 31.

It should be noted that a normal destructive pull-off or push test would

not have revealed any problem since these beams ultimately broke vith

forces similar to those from other sources.

Poor mechanical integrity could possibly lead to premature electrical problems resulting from thermal cycling if the device is encapsulated in silicone rubber (SR) . Dais [53] has calculated the forces on

the beam lead system during bonding, and although not explicit in his

calculations, it appears that forces high enough to produce beam system

degradation may occur during bonding. If so, the devices with poor

mechanical integrity could be damaged during the bonding process and

predisposed to relatively early field failure. It should be emphasized,

however, that there is no experimental proof of this possible result

of poor beam-anchor mechanical integrity, since no electrical tests

have been performed in this study.

In a large series of SR pull tests on devices bonded to the chrome

diffused substrates shown in Figure 20, it was found that a pull force

of between 1.0- and 1.5-gf/beam was required to produce AE from one or

two weakly bonded beams on an otherwise well-bonded device. A lower

force, of between 0.5- and 1.0-gf/beam, was often sufficient to produce

AE when all of the beams were poorly bonded (i.e., the device would

pull off at a force of 1.0- to 1.5-gf/beam).

48
Figure 30. SEM photograph of a beam lead from a device having

poor mechanical integrity. The device had been subjected to a pull force of approximately 2.5 gf/beam. (A) gold beam, (B)

separated titanium layer, (C) silicon nitride, (D) broken piece

of silicon, and (E) silicon chip.

Figure 31. An AE burst from a device having poor mechanical

integrity as shown in Figure 29. The pull force to produce

this burst was only 1.2 gf/beam. Horizontal scale is 20 ys/div.

49
3.4.5.2 Tests that Apply Force Only to the Beams

The two silicone rubber tipped probes, designed to avoid contact

with the chip, have been used to apply a downward force on the horizontal portion of the beam extending outward from the chip. As

previously stated , the simple resolution of forces analysis of a single

beam indicates that approximately all of that force is applied as a torque tending to peel the anchor. Thus, probing a single beam or a single row of beams along one side of the chip provides an anchor

adherence test. If all beams are probed at the same time, the torque

cancels out, and force is applied only to the bond system.

Individual beams were probed with the silicone rubber tipped

tungsten carbide probe shown in Figure 24b to establish AE patterns

for both anchor and beam failures. Figure 32 gives the twin AE oscilloscope traces resulting from applying a downward force of approximately

2 gf to a beam with a weak anchor. The anchor failed at an applied force

of 3.5 gf. A well bonded beam having a strong anchor would typically

collapse (curve downward until it touched the substrate) with a downward

force of from 6 to 10 gf depending on the beam curvature and the bugging

height. The beam-probe AE detector usually produces a larger signal

than the substrate detector for anchor failures. When a very weak bond

(failing at % 1-gf/beam pull force) is probed in a similar manner to

about 3 or 4 gf, the AE signal intensities are generally reversed, as shown in Figure 33. However, it should be emphasized that, while these

are typical AE patterns for their respective failure modes, those same failure modes may at times produce entirely different patterns.

Some experiments were performed using the single probe to try to

detect AE from silicon nitride breaks. In general, breaks in the thin

o

{"^j 2000 A) nitride skirt were not detected under normal circumstances.

This was believed to result from higher frequency emission as well as poor stress-wave coupling into the chip and substrate. This was verified

by coating the single beam probe with a viscous acoustic mating compound

and moving it sideways into an extended nitride skirt. A small AE burst

was recorded in the probe detector circuit (1.1 MHz) but not in the

substrate detector.

3.4.6 Application of Acoustic Emission to Determine the Integrity

of Tape Bonded Devices and Hybrid Components

There are a number of semiconductor device areas in which AE can

be used to insure mechanical integrity. One of the most straightforward

uses is in testing to assure the bond integrity of automated tape

carrier systems. To do this automatically, the mechanical stressing

of bonds may be accomplished as the carriers bend during winding on

a reel. A rubber coated detector could be pressed against part of the

inner bonded lead frame as the frame undergoes some maximum allowable

flexing or bending during or before being wound onto the reel

.

To show the feasibility of this procedure, some AE tests were performed on different types of automated gang-bonded integrated cir50
Figure 32. AE output from applying a downward force of '\>3.5 gf

to a single well-bonded beam that had a weak anchor. The horizontal scale is 20 ys/div. The upper trace is from the probe

of figure 23(B) (peak response is 1.1 MHz). The lower trace is

from the substrate detector (peak response is 375 kHz)

.

Figure 33. AE output oscillogram resulting from a downward

force of ^4 gf with the probe of Figure 24(b) on a single

very weakly bonded beam C^l gf pull force) . The scales are the same as in Figure 32

.

51
cuits. The first was of solder bump-Kovar inner-lead construction. A

slight bending applied 2 gf per lead to the uncut inner-lead frame and

produced the lifted lead shown in the upper illustration of Figure 34.

The lower illustration of Figure 34 shows the substrate detector

response to the AE resulting from that single bond lifting. Examination

of this device and others from this lot revealed a tendency for the

solder bump and its interfacial plating to separate from the aluminum

bonding pad. A second type of gang-bonded device having an aluminum

inner-lead construction was tested in a manner similar to that used

for the solder bump unit. One of these devices had several weakly bonded

leads which emitted bursts larger than that shown in Figure 34 when

they separated. Similar tests on several modern tape bonded devices

reveal no failures even though the visual appearance of one was quite

poor, see Figure 35. Destructive pull tests verified that both of these

leads were well bonded. Thus, it appears that an AE test can be used

to assure bond integrity on such gang-bonded systems. In addition, it

showed that a lead that would have been rejected in a visual inspection

was adequately bonded.

Failure was indicated by AE in the above cases when the lead

completely lifted up. Low stresses applied to weak bonds on similar

device structures often gave preliminary warnings of peel failures that

would later occur at higher forces. Figure 36 is the AE burst from a lead that partially lifted up at a stress of 6 gf. Later the lead com- pletely lifted at 14 gf. This is a demonstration that a catastrophical

Griffith-type break (see 3.2) is not necessarily valid for typical tape

bonded leads used in integrated circuits and that the microwelds are capable of breaking, perhaps in groups; but a peel (crack) may be

arrested part way into the bond and not propagate further until a higher

stress is applied. An investigation into the fracture mechanism of this

weakly bonded system revealed that the copper lead to gold plated bump

weld contained numerous individual microwelds similar to those of

Figure 22 for the gold-gold system. The only difference was the mode

of fracl^ure which was frequently the "cup fracture" type, characteristic

of OFHC copper [54]. Figure 37 shows two such copper cups remaining

on a gold plated bump after the lead had lifted at 14 grams.

A different metallurgical tape bonded system was shown to give

AE results that were not correlated with the bond integrity. This system

consisted of gold bumps on the chip and tin plated copper leads on the

tape. During the process of thermocompression bonding the tin melts

and may form intermetallic compounds with the gold bump, as is evident

in Figure 38. These compounds are relatively brittle, and if the lead

is stressed, they can crack at comparatively low forces. Such cracks

are shown in Figure 39. The upper lead cracked at 5 gf and was further

stressed to 10 gf with no additional AE . The lower lead was stressed

at 4 gf and cracked. It was further stressed to 7 gf with no additional

AE. Figure 40 is a close-up of the lower lead with the AE burst associated with the small double crack. Upon further stressing, these two

A pure grade of copper "oxygen free high conductivity."

52
Figure 34. The top illustration is an SEM photograph of a portion of an automated gang-bonded integrated circuit. The

bond on the left lifted up during minimal bending of the lead

frame. The lower illustration is the AE waveform resulting

from the lift up. The signal was picked up by the substrate

detector peak tuned to 375 kHz. The horizontal scale is 20

ps/div.

53
Figure 35. An SEM photograph of two bonds from a tape-bonded

integrated circuit. This device, including the visually poor

bond, remained intact and produced no^ AE even though the bonds

were stressed to approximately four times the value required

to produce the bond break in Figure 34.

Figure 36. The acoustic emission burst from a tape-bonded lead

that partially lifted at a stress of 6 gf . It gave 219 counts on

an AE counter. The lead later completely lifted at 12 gf.

54
a b

Figure 37. Examples of microwelds left on the gold-plated biimp

from the copper tape, indicated by arrows in (a). Some of these

failed by ductile "cup fracture." (b) Apparently a grain from

the tape lead was left welded on the bump.

Figure 38. SEM photograph of original unstressed tape-bonded

leads. The arrow on the upper lead points to the large tingold intermetallic compoiond lump. Part of the gold bump has

partially dissolved in the compound. Only minimal intermetallic compound is observable on the lower lead.

55
Figure 39. SEM photograph of the same two leads in Figure 38

after stressing the upper lead to 10 gf and the lower one to

7 gf. Cracks at the heel of the bond in both are obvious.

56
Figure 40. A close-up of the lower cracked lead and bump of

Figure 39. A small amount of intermetallic compound is evident filling the area as the lead leaves the bump as shown by

arrow. The double acoustic emission burst that occurred when

the lead was stressed to 4 gf is shown below. The horizontal

scale is 130 ys/div.

57
leads (and other similar ones) required 38 to 45 gf to break. Thus,

for this alloyed metallurgical system, it appears that the low-stress

AE, while indicative of brittle intermetallic compound formation, is

not related to the ultimate bond strength of the weld.

Flip-chip devices are another area where AE may be used to verify

the bond integrity. In this case visual inspection of the solder joints

is almost impossible as may be seen from the SEM photograph showing

a side-on view of such a flip chip in Figure 41. Therefore, it was decided to investigate the use of AE for this application. The usual

method of testing for bond strength is to measure the sheer strength

of the chip. This is typically in the order of 50 to 75 gf/bump for

good solder joints but decreases variously to 0 for bad ones. When an

AE sheer test was applied, it was found that, with rare exceptions,

there were no pre-break AE bursts. Although a large burst was detected

when the chip broke away, this could hardly be considered a nondestructive test. Further investigations using the stressing apparatus of

Figure 2 3 showed that poorly soldered flip chips did emit AE when the

probe was pressed down on top of the chip and orbited around. An example

of a very poorly soldered chip is given in Figure 42. The weak joints were revealed by AE during downward rotating force application. The

chip was then sheared off and photographed. The AE burst resulting from

applying a downward force of 42 gf is shown in the lower sector of

Figure 42. In numerous tests, the AE technique revealed all known poorly

soldered bonds. However, the difficulty of inspecting and verifying

the condition of flip chip bonds would require a long expensive study

to verify that the test indeed is a reliable screen.

Various discrete components such as chip capacitors bonded into

hybrids can be stressed by applying a small downward or shear force.

If weakly bonded, then AE should be detectable. One such capacitor was subjected to a downward orbiting force of 200 gf. It emitted the AE

signals shown in the lower portion of Figure 43. The capacitor was then

broken free and photographed. Only about 15% of the intended area was actually epoxy bonded as shown by the arrows on the right side.

Since propagating cracks emit stress waves, cracks in power device

chips should be detectable by current pulsing the device. Nonuniform

heating of the chip during such pulses should expand the crack and cause

the emission of stress waves. Cracks and flaws in hybrid substrates

should also be detectable. One such cracked substrate was detected in

the course of the present work while pressure testing silicone encapsulated devices. General package integrity should be assessable with AE

by stressing the package under pressure or with rapid heating. Such

conditions have been observed to destroy the hermeticity of potentially

defective packages [5]. AE detection equipment could be used in conjunction with the nondestructive wire-bond pull-test to assist in determining the maximum nondestructive force to be applied. It could then be

used to monitor that test to give ultimate assurance of its nondestructive nature. A limited evaluation of this was carried out as a preliminary to the present work and it appeared promising. Bonds that partially

lifted up during a nondestructive pull were easily detected.

58
Figure 41. The SEM photograph of a side-on view of a flip-chip

poorly bonded into its hybrid circuit. Arrows point to the poor

solder bonds. This chip was detected by AE probing. It is almost impossible to visually inspect the bonds on flip chips.

Figure 42. A poorly soldered flip-chip shown face up beside

its normal position in a hybrid microcircuit . The poor bond

quality was revealed by AE, and after removing the chip, it was evident that the bumps were only partially soldered.
Figure 43. The upper illustration is a photomicrograph of a 2-mm long chip capacitor removed from its hybrid circuit. The arrows point to the very small areas that had been conductiveepoxy bonded to the circuit. Below is the AE waveform resulting from applying an orbiting force of 200 gf with an AE probe

to the top of the capacitor before it was removed from the cir

cuit. Both 1.1-MHz probe detector and 375-kHz substrate detec

tor waveforms are shown. The horizontal scale is 20 ys/div.

60
3.5 Conclusions of Section 3

Previous studies of acoustic emission applied to real-time control

or evaluation of electronics assembly processes have been reviewed.

In addition, work directed towards developing AE-based tests to es- tablish the mechanical integrity of electronics devices as screens

after production are described. These studies have revealed considerable

differences in the mechanical integrity of beam lead bond-anchor systems

and demonstrated that AE testing offers a unique method of assessing

new beam lead-nitride-anchor designs and of maintaining quality control

on normal production. General deterioration of the beam-anchor system

begins at pull forces of from 1.0- to 2.5-gf/beam, depending on the manufacturer. Thus, no. test can be considered nondestructive thfl.t applies

forces higher than about 2.0-gf/beam to the mechanically strong beam

systems and perhaps 0.8-gf/beam to the weak ones. The maximum safe force

for each separate manufacturing procedure must be obtained experimentally. It was found that a pull force from about 1.0 to 1.5~gf/beam

was required to reveal a few poorly bonded beams in otherwise well

bonded devices; however, this force is equal to the beam-system deterioration force for devices with poor mechanical integrity. For such

devices, no meaningful nondestructive pull test is possible. The forces

applied to the beam-anchor system for all methods of stressing, except

the pull test, are dependent upon the shape of the individual beams

as they extend from the chip, as well as upon the uniformity of the

bugging height. Thus, to effectively use these tests, more operator

care is required than is usually achieved in typical production line

environments. The SR pull test is simple to employ and can be considered

nondestructive if the user does not object to leaving cured SR in the

package. The same material is, after all, often used as a conformal

coating. The silicone resin could be applied to chips with modified

epoxy die-attach equipment at either a 100% or some lower percentage

sampling basis. Of the methods studied, only the SR pull test could

reliably reveal weak bonds having equivalent strengths greater than

1 gf.

The main difficulty in the work with beam lead devices was encountered in the development of means of nondestructively stressing

delicate, irregularly extending beam leads. However, many other uses

of AE in electronics offer no such problems. Any system whose bond

strength normally is destructively tested by shearing or probing, such

as flip chips or capacitor chips in hybrids, can be nondestructively

tested by that same method at a lower force using AE as the failure

indicator. Both the inner and outer lead bonds on automated tapebonded integrated circuits can be probed or flexed (i.e., as on a tight spool or an axial twist) while monitoring for failures with AE

equipment to gain assurance that they are well bonded. The mechanical

integrity of large packages can likewise be assessed by rapid heating,

high or low pressure, or other means of stressing. Thus, it appears

that AE will have an increasing role in assuring reliability in microelectronics.

61
Acknowledgment

The author gratefully acknowledges valuable discussions on hermeticity testing with S. Ruthberg and on statistical sampling with Dr.

M. Natrella. The manuscript was prepared by Mrs. Kaye Dodson.

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Technical Report MDC-E 1208, January 24, 1975, McDonnell Douglas

Astronautics Co., East, Contract Report for NASA Goddard Space

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24-26, 1978, pp. 281-285.

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,

Brighton, England, June 1977.

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Damage in Aircraft Components, Air Force Office of Scientific Res.,

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505, pp. 238-249.

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for Nondestructive Testing, 3200 Riverside Drive, Columbus, Ohio,

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TAB and Hybrid Chip Capactor Bond Integrity, Proc. 14th Annual IEEE

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.

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Materials Using Stress Wave Emission, IEEE Trans. Parts, Hybrids,

and Packaging PHP-10 , No. 3, 152-159 (September 1974).

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(1976)

.

38. Carlos, M. F. , and Jon, M. C, Detection and Cracking during Rotational Soldering of a High Reliability and Voltage Ceramic Capacitor, Proc. 28th IEEE Electronics Components Conference, Anaheim,

California, April 24-26, 1978, pp. 336-339.

64
39. Jon, M. C. , Keskimaki, C. A., and Vahaviolos, S. J., Testing Resistance Spot Welds Using Stress Wave Emission Techniques, Materi-als

Evaluation 36_, No. 4, 41-51 (1978).

40. Knollman, G. C, and Weaver, J. L. , Evaluation of an Acoustic Emission Monitor for On-Line Quality Control in Spotwelding of Electronic Components, Proc. Third Acoustic Emission Symposium , Tokyo, Japan,

September 16-18, 1976, pp. 413-427.

41. Ikoma, T. , Ogura, M. , and Adachi, Y., Acoustic Emission From Single

Crystals of Gallium Arsenide, Ibid. , pp. 329-341.

42. Kotani , M. , Mitsui, S., and Shirahata, K. , Low Noise Gunn Diode Fabrication with Consideration of a Thermocompression Bonding Effect,

Trans. lECE Japan 58-C, 583-590 (1975)

.

43. Sedgwick, T. , Acoustic Emission from Single Crystals of LiF and

KCl, J. Appl. Phys. 39_, 1728-1740 (1968).

44. Ikoma, T. , Ogura, M. , and Adachi, Y., Acoustic Emission Study of

Defects in GAP-LEDS , an unpublished talk presented at the 20th

Electronic Materials Conference, University of California at Santa

Barbara, California, June 28-30, 1978.

45. Jellison, J. L. , Effect of Surface Contamination on the Thermocompression Bondability of Gold, IEEE Trans. Parts^ Hybrids^ and Packaging PHP-11 , No. 3, 206-211 (1975)

.

46. Holloway, P. H., and Bushmire, D. W., Detection by Auger Electron

Spectroscopy and Removal by Ozonization of Photoresist Residues,

Proc. 12th Annual IEEE Beliability Physics Symposium , Las Vegas,

Nevada, April 2-4, 1974, pp. 180-186.

47. Schofield, B. H. , Acoustic Emission Under Applied Stress, Technical

Document Report No. ASD-TDR-6 3-509 , Part II, pp. 1-29, May 1964,

AF Materials Laboratory, Wright-Patterson AFB, Ohio.

48. Wirsing, C. E., An Ultrasonic Bond Monometallic, Gold Interconnection Technique for Integrated Packages, Solid State Technology 16 , 48-50 (1973); also see Jellison, J. L. , Kinetics of Thermocompression Bonding to Organic Contaminated Gold Surfaces, Proc. 26th

IEEE Electronics Components Conference, San Francisco, California,

April 26-28, 1976, pp. 92-97.

49. Panousis , N. T. , and Bonham, H. B., Bonding Degradation in the Tantalum Nitride Chromium Gold Metallization System, Proc. 10th Annual

IEEE Reliability Physics Symposium, Las Vegas, Nevada, April

1973, pp. 21-25.

50. Holloway, P. H., and Long, R. L. , Jr., On Chemical Cleaning for

Thermocompression Bonding, IEEE Trans. Parts, Hybrids, and Packaging "^Vi^^-U^, No. 2, 83-88 (1975).

65
51. Tylecote, R. F. , Pressure Welding of Light Alloys Without Fusion,

Trans. Institute of VIelding , pp. 163-178 (November 1946) .

52. This circuit was designed and built by T. F. Leedy.

53. Dais, J. L. , Mechanics of Go Id- Be am- Leads During Thermocompression

Bonding, ProQ . 25th IEEE Electronics Components Conference, Washington, DC, May 12-14, 1975, pp. 43-51.

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Soc. AIME 218, 498-506 (1960)

.

66
N3S-114A 'REV. 9-78

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. SP 400- 59

2.Gov'L Accession No. 3. Recipient's Accession No,

4. TITLE AND SUBTITLE Senriaonductor Measurement Technology : Nondestructive Tests

Used to Insure the Integrity of Semiconductor Devices with

Emphasis on Acoustic Emission Techniques

5. Publication Date September 1979

6. Performing Organization Code

7. AUTHOR S) George G. Harman

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Project/Task/Wofk Unit No.

11. Contract/Grant No. ARPA Order 2397

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS rstree/, City. State. ZIP) ARPA

In part by: 1400 Wilson Boulevard

Arlington, VA 22209

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 79-600131

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200~word or less factual summary of most s i gni ft cant information. If document includes a significant bibliography or

literature survey, mention it here.)

The discussion is divided into two major sections. The first consists of an introduction to device assen±)ly techniques and problems followed by a review of

six important nondestructive tests used during and after device packaging to in- sure the mechanical integrity of completed electronic devices . Most of these

tests are called out in the military testing standard, MIL-STD-883 and are generally classified as screens. The first section concludes with a brief introduction to the economic and other factors that result in the choice of one screen

over another and to production line statistical sampling (LTPD) appropriate to

special high reliability device lots such as those used for space flight.

The second section begins with an introduction to acoustic emission, the status

of theory as it can be applied to microelectronics. Then the published papers that

have applied AE as a nondestructive test in electronics applications will be re- viewed. Finally, passive AE techniques are applied to establishing the mechanical

bond integrity of beam lead, flip chip, and tape-bonded integrated circuits as well as components in hybrid microcircuits

.

17. KEY WORDS Csix to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons) Acoustic emission; beam lead devices; electronic devices; hermeticity; hybrids;

nondestructive tests; semiconductor; tape-bonded devices.

18. AVAILABILITY iX^: Unlimited ^ For Official Distribution. Do Not Release to NTIS

Order From Sup. of Doc, U.S. Government Printing Office, Wasiiington, DC

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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 purpose

of the standards is to establish nationally recognized require- ments for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS administers this program as a supple- ment 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 consumer. Easily understandable language and

illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent of Documents, Government Printing Office, Washington, D.C.

20402. Order following NBS publications—NBSlR's and FIPS 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.

Register serves as the official source of information in the Federal Government regarding standards issued by NBS

pursuant to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 StaL

1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 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 distribution is handled by the sponsor;

public distribution is by the National Technical Information

Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

Hie following current-awareness and literature-survey bibli- ographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A litera- ture survey issued biweekly. Annual subscription: Domestic, $25.00; Foreign, $30.00.

Liquefied Natural Gag. A literature survey issued quarterly.

Annual subscnption: $20.00.

Superconducting Devices and Materials. A literature survey

issued quarterly. Annual subscription: $30.00. Send subscription orders and remittances for the preceding bibliographic

services to National Bureau of Standards, Cryogenic Data

Center (736.00) Boulder, Colorado 80303.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

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