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T

Q

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PUBLICATIONS

NBS SPECIAL PUBLICATION 400

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

Semiconductor Measurement Technology:

Graphical Solution for the

Helium Leak Detector and

Radioisotope Methods of

Hermetic Test

Master Graphs and Instructions

uo

57

UG-73
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N

Semiconductor Measurement Technology:

Graphical Solution for the Helium Leak Detector

and Radioisotope Methods of Hermetic Test

Master Graphs and Instructions ^SSd^^^

D£C 1 m?

Stanley Ruthberg

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued November 1982
Library of Congress Catalog Card Number: 82-600626

National Bureau of Standards Special Publication 400-73

Natl. Bur. Stand. (U.S.), Spec. Publ. 400-73, 34 pages (Nov. 1982)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1982

For sale by the Superintendent of Documents, U.S. Government Printing Office, Wasfiington, DC 20402

Price (Add 25 percent for other than U.S. mailing.)
Table of Contents

Page

I. Introduction 1

II. Helium Leak Detector Method 2

A. Test Factors 2 B. Graphical Procedure 4

III. Radioisotope Hermetic Test Method 8

A. Test Factors 8

B. Graphical Procedure 10

IV. General Characteristics 14

References 16

Master Graphs 17

Appendix 29

List of Figures

1 . Characteristics and test lines for the helium leak detector method . 3

2. The graphical procedure for determining leak size with the helium

test method from the leak detector indicated response for given

conditions of Pg, T, t, and V 6

3. Characteristics for the krypton-85 radioisotope method 9

4. The graphical procedure for determining leak size with the radioisotope test method from the gamma count rate for given conditions

of Pg , T, t, and V 12

List of Master Graphs

1. Characteristics and test lines for the helium leak detector method . 19

2. Characteristics for the krypton-85 radioisotope method 23

3. Characteristics for any tracer gas 27

iii

SemiaonduGtov Measurement Technology

Graphical Solution for the Helium Leak Detector and

Radioisotope Methods of Hermetic Test

Master Graphs and Instructions

Stanley Ruthberg

Semiconductor Materials and Processes Division

National Bureau of Standards

Washington, DC 20234

A graphical procedure for solution of the molecular flow approximation for the back pressurization method of hermetic test makes use

of a set of characteristic curves and a test line. The characteristic curves are appropriate for both the helium leak detector and

the radioisotope methods of test, although the form of the test

line differs between the two methods. Master graphs of the characteristic curves and test lines are now provided in a scale and

format appropriate for producing suitable worksheets with a copier.

Step-by-step instructions are given for their use in obtaining

solutions for various examples relative to the test specifications

in acceptance standards such as MIL-STD 883B, etc. One set of

characteristics is provided specifically for the heliiim leak detector mode as expressed directly in terms of air leak rate; a second

set is provided specifically for the krypton-85 radioisotope mode

also in terms of air leak rate; and a third set is retained in the

original form for use with any tracer gas.

Key words: back pressurization; electronic packages; hermetic

test; leak testing.

I. INTRODUCTION

Both the helium and the radioisotope leak test methods for hermetically

sealed electronic packages are examples of the back pressurization procedure.

This procedure involves forcing a tracer gas (or liquid) into the package

interior and then measuring the quantity of gas that has penetrated through

an existing leak. With helium as the tracer gas, measurement is made by

extracting the gas back through the leak into a helium mass spectrometer leak

detector; with radioisotope krypton-85 as the tracer, an external gamma counter determines the activity of the gas remaining in the interior. Neither

technique gives the true leak size directly; rather, both require a knowledge

of the gas flow mechanism and the use of appropriate mathematical models.

Since the gas transport mechanism for leaks over the range of practical interest is complex and is not amenable to direct analytical solution, a number

of simplifying flow models have to be used.

Traditionally, the helium leak test method has been quantified by a molecular

flow model [1], and the test parameters set forth in such acceptance standards as MIL-STD 883, MIL-STD 750, NASA Certification Requirements, ASTM

Standards, and others have been so derived [2-6] . Nevertheless, the relationship between the measured leak rate and the leak size is transcendental

1
so that a numerical solution is necessary. On the other hand, the radioisotope method has usually been modeled by a laminar viscous flow approximation,

which in principle is relevant to the larger leak sizes [2-8]. While such

modeling leads to a simple relationship between count rate and leak size, the

correlation of the results between the two methods is obscured. Furthermore,

the test parametric values set forth in MIL-STDs 883 and 750 for the radioisotope method were not derived from flow models, but were established empirically through comparison of test results to those obtained using the helium

leak detector method [9].

In a previous publication [10], a general solution to the molecular flow

equation for the back pressurization method of hermetic testing was formulated into a normalized set of characteristic curves. Using these curves, a simple graphical procedure is followed to obtain quick and adequately precise

solutions of the molecular flow fomulation for any particular set of test

conditions and to provide a basis for comparing the results from both test

methods. For this graphical procedure only a single set of characteristic

curves is required along with the use of a test line. The curves and procedure are appropriate for both methods, although the form of the test line is

different for the two methods. All test parameters are readily obtained for

any package volume and leak test range, and the effect on test results due to

a change in parameters is easily found.

The purposes of this publication are: (1) to provide master graphs of the

characteristic curves and test lines from which worksheets can be made by

photocopying, (2) to put the solutions directly in terms of air leak rate,

and (3) to provide a manual of step-by-step instructions for obtaining leak

test solutions. The detailed derivations and explanations of the graphical

procedure are found in reference [10] (Appendix A).

II. Helium Leak Detector Method

A. Test Factors

Solutions for the helium leak detector method are found as intersections

between appropriate test lines and the characteristic curves describing the

interior gas pressure, as shown in figure 1 (also master graph 1). The internal fractional helium pressure, E, is a measure of the interior helium

pressure per atmosphere of external pressurization at any elapsed time, t,

after pressurization of the sealed packages in helium for a period of time,

T, and is given by:

E = ^ [l - exp[-R t]] exp (-R t) (1)

0

in which Rj,, the relaxation rate for helium, is related to that for air as

R = 2.69 R

^ . (2)

r r

The relaxation rate for air, R^^^, defines the ratio of leak size and

package interior free volume as

2
Figure 1 . Characteristics and test lines for the helium leak detector

method . E, the relative partial pressure of helium within the package per

atmosphere of external helium pressurization, as a function of the relaxation

rate for air R^.^ with pressurization time T and dwell time t as parameters.

Test lines are at -45 deg and correspond to E»Rj.^ = const. Intermediate

values for test lines, for T, and for t are indicated by the scale for each.

Tick marks include those at 1.25, 1.5, 1.75, and 2.5 with the remainder at

integral values for each decade on the axes for E»Rj.^, for T, and for t. Use master graph 1 for photocopying.

3
The test lines drawn in with a slope of minus one (-45 deg) on the log Elog R^^ plane represent constant values of the product E»Rj-^. The

equation for these lines, and the formalized solution of the molecular flow

model, is

R

2.69 P„VP

0 E

(4)

The requisite quantities are as follows:

L is the standard leak rate for air, or the leak size*

3. V is the package interior free volume

Pq is one standard atmosphere (1.01325 x 10^ Pa, ~ 14.7 psi

)

p„ is the absolute pressuriza tion value of helium

R is the leak detector response

T is the time interval of pressuriza tion

t is the dwell time or that elapsed from the end of pressuriza tion to

detection.

In figure 1 test lines are included at values for E'R^.^ corresponding to

each decade from 10"^ to 10~^^ s~^»atm~^. Lines at intermediate values can

be obtained with the aid of the test line scale. Characteristic curves for E

in the positive slope region at the left in figure 1 are indicated for pressurization times T in multiples of 1 and 3; intermediate values can be obtained with the T-scale. The characteristic curves are merged to a fewer

number in the negative slope region with t as the significant parameter. The

t-scale can be used for intermediate values.

B. Graphical Procedure

1 . Unknown Leak Size

Conditions

Given: pressuriza tion value

pressuriza tion time

dwell time

leak detector response^

package free volume

Find: leak size

Pg (in atm abs , helium)

T (h)

t (s)

R (atm'cmVs, helium)

V (cm 3)

L (atm«cm3/s)

* By definition the standard leak rate L for a given gas is that flow rate

obtained with 1 atm of gas pressure upstream to the leak channel and zero pressure downstream.

t R is an above background value; use values large compared to background or correct for background.
Procedure

a. Compute R/[2.69 PgVPg] . This is the value of the test line.

Construct this line on the worksheet parallel to those given

using the test line scale.

b. Find the two intersections of the test line with the characteristic curve of E for the given value of T and t. If that E is not included in figure 1 , construct a line parallel to those given passing through the required value of T on the T-scale (similarly for t).

c. Drop vertical lines through the points of intersection to the

Rj-^ axis.

Read values of R^.^ at the axis.

d. Compute as R^.^ p^v. These are the two possible leak

values.

Example - See figure 2.

Let ^ ^ helium, T = 0. 3 h, t = 3000 s, R = 5 x 1 0~ 9 atm 'cm Vs for

helium, V = 0.01 cm^. Find two possible values of L^.

a. R/2.69 PqVPe = 3.7 x 10"^. See test line a. b. The intersections are within the circled domains.

c. See lines b and c. R^.^ = 3.8 x 10"^ and 1.3 x 10"^ s~ ^. d. = 3.8 X 10"^ atm'cm^/s or 1.3 x io~^ atrn'om^s.

2. Selection of test parameters: Pressurization time and maximum detectable leak.

Conditions

Given: package free volume V

reject level (all leaks >)

dwell time t

response R

pressurization value Pg

Find: pressurization time T

maximum detectable leak L^(max)

Procedure

a. Compute Rj,^ by (3)

Draw a vertical line at that value.

b. Compute R/[2.69 P^VPg]

.

Construct this test line parallel to those given using the

included scale if necessary.

c. Locate the intersection of the test line and the R^.^ line.

Find the value of T for the E-characteristic curve passing

through this point. This is the pressurization value. If

the E-curve is not included in figure 1 (or master graph 1),

pass a line through the intersection parallel to those represented and extend it to the T-scale.

5
Figure 2. The graphical procedure for determining leak size with the helium

test method from the leak detector indicated response for given conditions of

Pg, T, t, and V. See example 1 of Section II. a - test line. b - first

solution for R^^ and hence the smaller possible value for L^. c - second

solution for R^.^ and hence the larger possible value for I^»

6
.'a

Figure 1 . Characteristics and test lines for the helium leak detector

method . E, the relative partial pressure of helium within the package per

atmosphere of external helium pressurization, as a function of the relaxation

rate for air R^^ with pressurization time T and dwell time t as parameters.

Test lines are at -45 deg and correspond to E»Rj.^ = const. Intermediate

values for test lines, for T, and for t are indicated by the scale for each.

Tick marks include those at 1.25, 1.5, 1.75, and 2.5 with the remainder at

integral values for each decade on the axes for E»R^^ , for T, and for t. Use master graph 1 for photocopying.

3
Figure 2. The graphical procedure for determining leak size with the helium

test method from the leak detector indicated response for given conditions of

Pg, T, t, and V. See example 1 of Section II. a - test line, b - first

solution for R^.^ and hence the smaller possible value for I^. c - second

solution for R^.^ and hence the larger possible value for

6
d. Locate the intersection of the test line and the negative

slope portion of the E-characteristic curve for the given

value of t. Drop a vertical line to the R^,^ axis.

Compute L^(max) from (3).

3. Selection of test parameters: Maximum allowable dwell time and

pressurization value.

Conditions

Given: upper leak limit required L^(max)

package free volume V

leak detector response R

Find: maximum allowable dwell time t

pressurization value P-g

Procedure

a. Compute Rj,^(max) as La(max)/PQV.

Draw a vertical line through this value.

b. The Rj.^ (max ) -line intersects characteristics corresponding

to a range of values of t. The information given is not

definitive; therefore, some choice is allowed.

Select a point on the Rj.^ (max ) -line which corresponds to a practical value for t. c. Erect a test line through this point at -45 deg.

Determine the test line value from the scale. Let this be a. d. Compute P^ = R/2.69 PgVa from (4).

e. If the values for Pg and t are practical, then Pg becomes

the pressurization value and t is the maximiam dwell time

allowed in order for the test range to include L^(max) for

a all parts tested.

If Pg is not usable, adjust the value for t and/or the test

line until satisfactory values are obtained.

4. Selection of test parameters: Minimum and maximum leak size limits

as a function of pressurization time.

Conditions

This is a general case that applies to all packages independent of specific conditions.

Procedure

a. Choose any test line and mark the intersection with an Echaracteristic curve for some value of T, e.g., test line

(a) in figure 2. Drop a vertical line to the Rj.^ axis, e.g., line (b) in

figure 2. b. Proceed up the test line to a second value of T, which is two

decades greater than T .

7
Drop a vertical to the R^^ axis.

Note that R^^ has decreased by one decade; i.e., Rj.^

changes as -T^^^.

c. Follow the test line to the negative slope portion of the Echaracteristic curve. Note that the E-characteris tic curves converge in this region,

and the dependence on the pressuriza tion time T vanishes.

The dwell time t is the major factor for the determination

of maximum leak size.

5. Leak Detector Response

Conditions

Given: leak size reject value

pressuriza tion time

pressuriza tion value

package free volume

Find: leak detector response

Procedure

a. Compute R^.^ by (3). Draw a vertical line at this value.

b. Locate the intersection of the Rj,^-line with the E-curve

corresponding to the given T value. If that curve is not

included in the representation, construct one parallel to

those given and passing through the given value on the Tscale.

Read off the value of E corresponding to the intersection.

c. Compute R = Rj.^ •E/PqVPe»

III. Radioisotope Hermetic Test Method

A. Test Factors

Although the molecular flow model is not normally employed for the radioisotope procedure, it is the appropriate model for fine leaks and for correlation of this test method with the heliiam leak detector method. Since detection in the radioisotope method is accomplished using the gas which still

resides within the package, without resorting to extraction back through the

leak channel, the graphical solution procedure is simplified.

Solutions for the radioisotope method require only the set of characteristics

shown in figure 3 (and master graph 2). The internal fractional krypton-85

pressure, E*, represents the interior gas concentration at any time, t, after

pressuriza tion of the parts in krypton-85/ni trogen for a period of time, T. Test lines for this method are now horizontal straight lines with numerical

values of E*. However, practical operational conditions tend to raise the

test line into the nonlinear region of the E*-characteristic curves. The

formalized solution is given by

L^ (atm»cm3/s)

T (h)

Pg (atm)

V (cm 3)

R (atm»cmVs)
Figure 3. Characteristics for the krypton-85 radioisotope method . E* , the

relative partial pressure of krypton within the package per atmosphere of

external krypton pressuriza tion, as a function of the relaxation rate for air

R^^ with pressuriza tion time T and dwell time t as parameters. Tick marks

include those at 1.25, 1.5, 1.75, and 2.5 with the remainder at integral

values for each decade on the axes, for T, and for t. Test lines are not

included since these are easily constructed as horizontal lines at values for

E = const. Use naster graph 2 for photocopying.

9
E* = (5) PqVSKP^*

where

V is the package free volume

Pq is one standard atmosphere

Pg* is the absolute pressuriza tion value for the Kr^^-N^ g^s mixture

R* is the gamma count rate above background

S is the specific activity of the gas mixture

K is the overall counting efficiency

and the relaxation rate Rj.* for krypton-85 is

R * = R .71 (6)

r r

for

• , L = 1.71 L, ^ (7) a krypton

where Rj,^ is the relaxation rate for air, eq (3), and is the leak

size for air.

B. Graphical Procedure

1 . Unknown Leak Size

Conditions

Given: pressuriza tion value Pg* (atm abs

)

pressuriza tion time T (h)

dwell time t (s)

count rate R* (min~M

package free volume V (cm 3)

specific activity S ( yCi/a tm 'cm ^

)

counting efficiency K (min~ ^ "yCi" ^) Find: leak size (atm •cm ^/s) a Procedure

a. Calculate E* = R*/ [PQVPg*SK] . Draw a horizontal line at that value of E*

.

b. Locate the point of intersection with the E* characteristic

corresponding to the given value of T. Drop a vertical line to the R^,^ axis. Read off Rj.^

.

Calculate L^ = P^VR^^. This is one possible value.

c. Repeat b for the given value of t. This is the second possible value.

10

1
Example - See figure 4.

Let Pg* = 5 atm, T = 0.3 h, t = 3000 s, R* = 1000 min~l, v = 0.01 cm^, S = 200 uCi/atm»cm3, and K = 10^+ min~ 1 • yCi" 1. Find possible values for L^*

a. E* = RVPpVPg*SK = 10-2 atm"^. See test line a. b. See the circled domain at T = 0.3 h characteristic. See line

Rj.^ = 1.6 X 10" ^ s~ ^. = 1.6 X 10-7 atm 'cm 3/s.

c. See line c.

Rj.^ = 2.5 X 10" 3 s"^.

= 2.5 X 10"5 atm«cm^/s.

2. Selection of test parameters: Pressuriza tion time and maximum detectable leak.

Conditions

pressuriza tion value p * dwell time t

count rate R*

package volume V

reject level ^a

specific activity S

counting efficiency K

pressuriza tion time T

maximum detectable leak L (max)

3. Procedure

a. Calculate E* = R*/ [PQVPg*SK] . Draw the horizontal test line

at that value.

b. Calculate R^.^ = Lq/Pq^* Draw a vertical line at that

value.

c. Locate the intersection of the two lines, E* and R^^.

Find the value of T corresponding to the E-characteristic

curve passing through this point. This is the pressurization value. If the E-characteristic curve is not included

in figure 2, pass a line through the point of intersection

parallel to the E-characteristic curve and extend to the Tscale.

d. Extend the horizontal test line to the negative slope region

of the characteristic curves until it intersects with that curve corresponding to the given value of t. Drop a vertical line to the R^^ axis.

Compute L^ from (3). This is L^(max).

3, Selection of test parameters: Maximum allowable dwell time and

pressuriza tion value.

11
Figure 4, The graphical procedure for determining leak size with the radioisotope test method from the gamma count rate for given conditions of Pg , T, t, and V. See example 1 of Section III. a - test line, b and c - two

solutions for R^^ and hence 1^'

12
Conditions

Given: package free volume V

count rate R*

maximum leak limit required L^(max)

specific activity S

counting efficiency K

Find: maximum dwell time t(max)

pressurization value Pp.*

Procedure

a. Calculate Rj.^(max) from (3).

Erect a vertical line at this value.

b. Note that the information given is not sufficient to derive a specific value for either t(max) or Pg** Some choice is

allowed.

Select a point on the Rj.^ (max ) -line which corresponds to a usable value for t. c. Draw a horizontal test line through this point.

Determine the test line value from the E* axis.

d. Compute P^,* from (5) or R*/P^VSKE*,

e. If Pg* is a suitable value, then t is taken as the maximum

dwell time allowed in order for the test range to include

L (max), and P„* is the pressurization value of test.

ci ill If Pg* is not a suitable value, adjust t and E*.

4. Selection of test parameters: Leak size as a function of pressurization time.

Conditions

This is a general case that applies to all packages independent of specific conditions.

Procedure

a. Choose a horizontal test line of some value E*, e.g., test

line (a) in figure 4. Locate the intersection with this and an E-characteristic

curve at some value of T,

Drop a vertical line to the R^.^ axis, e.g., line (b) in

figure 4.

b. Proceed along the test line to a larger value of T by one decade.

Drop a vertical line.

Note R^^ has decreased by one decade; i.e., R^.^ decreases

proportionately to the increase in T. c. Proceed to the other end of the test line.

Note that the E-characteristic curves may retain a significant

dependence on the pressurization time T for typical values

of the test line and dwell time t, e.g., at the intersection

13
of lines (a) and (c) in figure 4. This is different from

the usual helium leak detector test situations.

5. Selection of Test Parameters: Count Rate

Conditions

Given: leak size reject value

pressuriza tion time

pressurization value

package free volume

specific activity

counting efficiency

Find: count rate

Procedure

a. Compute Rj.^( reject) using (3). Draw a vertical line at

this value.

b. Locate the intersection of the R^^-line with the E*-

characteristic curve corresponding to the given T value. If

that curve is not represented, construct one parallel to

those drawn passing through the given value on the T-scale.

Read off the value of E* corresponding to the intersection.

c. Compute R* = SK«E* •P^VPg*.

IV. General Characteristics

The internal fractional gas pressure per atmosphere of pressurization with

any tracer gas is given by eq (1) when the relaxation rate R^ for the specific tracer gas is

R = (8) r PqV

and L is now the standard leak rate for the specific tracer gas. Characteristic curves for E as a function of the relaxation rate for any particular

tracer gas are given in master graph 3.

If the tracer gas is drawn back through the leak for detection, as with a mass spectrometer, the solution is of the form of eq (4), i.e.,

^ • ^r = p4^ ^9)

0 E

with R^ given by eq (8), Test lines are at -45 deg on the log E-log R^

plane with values of |e • Rj. | . Figure 1 and the solutions for the helium

leak detector are drawn from the general case by superposing the relaxation

rate for air relative to that of helium from eq (4) on the log R^ axis and

with test lines of value |e • R^^|.

L^( reject) (atm.cmVs)

T (h)

Pg*(atm)

V (cm 3)

S ( pCi/atm -cm^)

K (min~^»ijCi"M

R (min~l)

14
If the tracer gas is detected while in the package, the solution is of the

form

p-^=3.E (10)

0 E

where

R = 3*PV (11)

and is a measurement of some property proportional to the quantity of gas

(PV) at ambient temperature such as for another radioactive scheme, mass,

etc. Test lines are horizontal on the log E-log plane with values of

E. Figure 3 and the solutions for the krypton-85 radioisotope method are drawn from this by superposing the relaxation rate for air relative to that

of k.rypton-85 by eq (6).

15
References

1. Howl, D. A., and Mann, C. A., The Back Pressuriza tion Technique of Leak

Testing, Vacuum^, 347-352 (1965)

2. Test Methods and Procedures for Microelectronics: Method 1014.4 - Seal,

in MIL-STD 883B, 4 Nov 1980. (Available from U.S. Naval Publications

and Forms Center, 5801 Tabor Ave., Philadelphia, PA 19120.)

3. Test Methods for Semiconductor Devices: Method 1071.2 - Hermetic Seal,

in MIL-STD 750B, 27 Feb 1970. (Available from U.S. Naval Publications

and Forms Center, 5801 Tabor Ave., Philadelphia, PA 19120.)

4. Test Methods for Electronic and Electrical Component Parts: Method 1 1 2B

- Seal, in MIL-STD 202E, 16 Apr 1973. (Available from U.S. Naval Publications and Forms Center, 5801 Tabor Ave., Philadelphia, PA 19120.)

5. Test Methods and Procedures for Microcircuit Line Certification: Sealing, in NHB 5300.4 (3D), now superseded by (same title) MIL-ST9-977, 25

Jan 1982 which does not contain the sealing test,

6. Standard Recommended Practices for Determining Hermeticity of Electron

Devices with a Helium Mass Spectrometer Leak Detector, ASTM Designation

F134, in Annual Book of ASTM Standards, Part 43, 1981, pp. 565-572.

7. Cassen, B. , and Burnham D. , A Method of Leak Testing Hermetically Sealed

Components Utilizing Radioactive Gas, Int» J, Appt* Radiation and Isotopes 9, 54-59 (1960)

.

8. Ruthberg, S., Neff, G. R., and Martin, B. D., Radioisotope Hermetic Test

Precision, Pvoc. 1977 Int, Miovoetectvonies Symposium (ISHM), Baltimore,

Maryland, Oct. 24-26, 1977, pp. 131-137.

9. Banks, S. B. , McCullough, R. E., and Roberts, E. G. , Investigation of

Microcircuit Seal Testing, Air Force Systems Command, Rome Air Development Center Tech. Rep. RADC-TR-75-89, Apr. 1975.

10. Ruthberg, S., Graphical Solution for the Back Pressuriza tion Method of

Hermetic Test, IEEE Trans. Components Hybrids^ and Manufacturing Technology cmT-_4, 217-224 (1981).

16
Master graph 1. Characteristics and test lines for the helium leak detector

method . E, the relative partial pressure of helium within the package per

atmosphere of external helium pressuriza tion , as a function of the relaxation

rate for air R^^ with pressurization time T and dwell time t as parameters.

Tick marks include those at 1.25, 1.5, 1.75, and 2.5 with the remainder at

integral values for each decade on the axes, for E»R^, for T, and for t.

17
!
19

Master graph 2. Characteristics for the krypton-85 radioisotope method . E , the relative partial pressure of krypton within the package per atmosphere of external krypton pressuriza tion, as a function of the relaxation

rate for air R^^ with pressuriza tion time T and dwell time t as parameters.

Tick marks include those at 1.25, 1.5, 1.75, and 2.5 with the remainder at

integral values for each decade on the axes, for T, and for t.

21



Master graph 3. Characteristics for any tracer gas . E, the relative partial

pressure of the specific tracer gas within the package per atmosphere of

external pressurization in that gas, as a function of the relaxation rate for

that specific tracer gas with T and t as parameters. Test lines are inclined

at -45 deg with value • e| if the gas is extracted back though the leak

for measurement or are horizontal with value |e| if the quantity of gas is

measured while within the package. Transport mechanism is molecular flow.

25

27

APPENDIX

IEEE TRANSACTIONS ON COMPONENTS, HYBRIDS, AND MANUKACTURING TECHNOLOGY, VOL CHMT-t. NO 2, JUNE 1981 Graphical Solution for the Back Pressurization Method of

Hermetic Test

STANLEY RUTHBERG

Abstract—The back pressurization method for leak-testing her- metically sealed electronic packages requires gas-flow modeling to relate indicated leakage rates to true leak size. The molecular flow relationship which is appropriate for fme leak sizes is nonlinear and

requires a numerical solution, which in actual test application may

involve either many trial calculations or the use of approximations

that lead to limiting case values. A new graphical procedure is presented for complete solution of the molecular flow equation for any

given test condition and package volume through the use of a single set of characteristic curves and a test line. The effects of repetitive testing and of preflll with tracer gas are also considered. The characteristic curves are appropriate fur both the helium leak detector and the radioisotope methods of test, while the form of the test line dis- tinguishes between the two methods.

I. INTRODUCTION

THE BACK pressurization method for leak-testing hermetically sealed packages is accomplished by forcing a tracer gas into the package interior and then measuring the quantity of gas that has penetrated the leak channel. The two tracer gases most commonly used for semiconductor devices are helium and radioactive krypton. With helium as the tracer

gas. measurement is made by extracting the gas back through

the leak into the helium mass spectrometer leak detector; with radioisotope Kr*' as the tracer, an external gamma

counter determines the activity of the internal gas. Neither technique gives the true leak rale value directly; rather, both

require a knowledge of the gas transport mechanism and the

use of appropriate mathematical models for the determination of test parameters. As the gas transport mechanism for leaks under pressurization is that of transition flow [1], which combines the elements of molecular and laminar viscous flow in a manner not amenable to direct analytical solution

[2] , a number of simplifying flow models have been used. Traditionally the molecular flow approximation which is relevant for very fine leaks has been used to relate the helium

leak detector indication to the leak size, but even here a transcendental relationship is obtained which is doublevalued in leak size for each machine indication [3] . Thus

numerical calculations are required to select pressurization parameters and to determine leak size. If precise values of

leak size are required as in test evaluation or comparison,

it is necessary either to use successive approximations or to construct a family of curves representing solutions for each

parametric variation [4] . If an approximate correlation is sufficient, extensive tables of computer solutions for discrete

Manuscript received July 14. 1980; revised December 5, 1980. The author is with the tlectron Devices Division, National Bureau

of Standards, Washington, DC 20234.

values of parameters have been available [5] . Finally, if only

approximate values for the minimum detectable leak size and/or the maximum detectable leak size are sufficient, as for screening purposes, graphs are available for a limited range of parametric values [6]

.

The radioisotope test method is usually modeled by the laminar viscous flow approximation [7] which is relevant

in principle to the larger leak sizes. The assumption is made

that the internal gas pressure remains small and increases

linearly with time under pressurization. The loss of gas back

through the leak is also neglected [8] . While such modeling

leads to a simple relationship between gamma-ray count rate and leak size, correlation of the results between this method

and the helium leak-detector method is obscured.

In this paper a graphical procedure is presented for complete solution of the molecular flow equation for any given

test condition. Only a single set of characteristic curves is required along with the use of a test line. The selection of

parametric values for pressurization, time of pressurization, and lapsed time to achieve a given leak-test range for any

package volume is readily made, and the effect on test results due to variation in parameters is easUy visualized. The pro- cedure is applied to both the helium and the radioisotope

methods, and a comparison of operational behavior is derived for the fine leak range for which molecular flow is appropriate. In addition the case of repetitive testing is considered. Either prior testing or a prefill of packages with

tracer gas can affect test results considerably, yet this situa- tion has not been approached previously on a formal basis. Correction of results is particularly appropriate for test evaluation and interlaboratory comparisons.

II. INTERNAL FRACTIONAL PARTIAL PRESSURE

A. Exact Solution The gas transport into and out of a package due to a molecular flow leak is described by

V — = F(P,-P2) (1) dt

where V is the internal free volume of the package available to gas collection, V dPjdt is the flow rate into (or out of) the package at ambient temperature, F is the molecular-flow conductance of the leak channel [9] , and and Pj are the

partial pressure of the tracer gas at each end of the leak channel. By definition the standard leak rate L for a given

gas is that flow rate obtained with 1 atm of gas pressure upU.S. Government work not protected by U.S. copyright.

29
stream to the leak channel and zero pressure downstream

so that from (1)

(2)

for Pq equal to 1 atm [10]. It then follows from (1) and

(2) that when a previously unexposed package has been im- mersed in a tracer gas at a pressure for a period T, the

internal partial pressure P is

P=Pe 1 — exp I T

When the pressurization ceases the tracer gas effuses back

through the leak so that the interior tracer gas pressure decays

exponentially. Then for any given lapsed or dwell time t

P=Pr 1-exp r exp (3)

If the interior partial pressure of the tracer gas is not zero but is initially P' due to a previous test or to a prefiU of the package, the resultant pressure is

P = P'+ \Pe~P'\ 1-expl-—

r

exp,-— r (4)

A family of projections for £" as a function of the relaxa- tion rate, with pressurization time and dwell time as parameters, is shown in Fig. 1 as derived from (6) for a range of

parametric values sufficient for most semiconductor device packages; i.e., characteristics are included for a geometric

series of values of pressurization time ranging from 0.1 to 300 h and values of dwell time ranging from 5 min to about

3 h. The initial and final segments for these curves can be obtained readily from limiting solutions for small leaks and

for large leaks as follows.

B. Fine Leak Approximation

For relatively small values of R^, from (6)

E=RrT/Po. (7)

Therefore with T as a parameter the E, characteristics are

initially linear projections at a 45° slope on the log-log plane. A projection for any value of T can be erected quickly by

taking a coordinate point and passing a line of 45° slope through the point, i.e., by calculating E for a given R^ or an Rr for a given E with T as a parameter in (7). The locations of

lines of intermediate values of T are indicated in the figure by

tick marks for T = 1-10.

C. Large Leak Approximation

For relatively large values of R^

1 E = — exp (-Rrt)

P0

The pressure change within the package is thus determined by a time constant

(5)

or the relaxation rate R^ which is the reciprocal of the time

constant

Rr =

P<.V

(5a)

Rr = -- \niPoE).

t (8)

Thus the relaxation rate increases directly as the dwell time

diminishes, the pressurization time is not a significant factor, and the relaxation rate changes little with large variation in E for any given dwell time. This downslope segment for any

E{t) can be approximated readily by considering numerical

values of E of the form

1 £ = — X 10^ Now the bracketed expression in (3) is a measure of the

relative increase in the internal gas concentration, and this value normalized by 1 atm pressure is defined here as the quantity

whence

2.303«

(9)

1

P'o

[1 -exp(-/?,r)] exp (-7?/) (6)

which represents the internal fractional tracer gas pressure per atm of pressurization. This quantity E will be used below

to characterize the back pressurization method of hermetic

test for both the helium leak detector and the radioisotope

procedures.

and by joining with a straight line segment two points cal- culated from (9) for the selected value of t and two successive values of n in the R^ region of interest.

111. HELIUM LEAK DETECTOR METHOD

A. Test Equations

When the tracer gas is helium, the How rate of helium from

the package interior back through the leak conductance and

30
10' 10' 10' 10' 10' 10' 10' 10' 10'

RELAXATION RATE. R, |s''|

Fig. 1. £", the relative partial pressure of tracer gas within package in- terior free volume per atmosphere of external tracer gas pressurization, as function of relaxation rate with pressurization time T

and dwell time t as parameters. Transport mechanism is molecular

flow.

into the helium leak detector will by (1) give rise to an in- dicated leak rate R given by

R=F'P (10)

where the pressure within the leak detector is much less than P. Most tests for package leakage are done soon after assembly so that the initial interior helium concentration is zero. For such packages the leak detector indication is then from (2) and (3)

R=Pe: -L - E

or from (5a) R

=R,-E. (11) PoVPe

Thus the solution is characterized by the two factors E and

Rr, and all solutions relating leak detector response to standard leak rate may be derived from the characteristic projections for E along with an appropriately placed test line.

B. Test Lines

It is apparent from (11) that the locus of all solutions for any test sequence will lie on a line of 45° negative slope on

the log EAogRr plane; for, with a given volume, indicated leak

rate and pressurization the product is a constant. Fig. 2 includes test lines ranging from 1 X 10^^ (atm • s)"' to

1 X 10"'° (atm • s)~'.' A test line for any other value can be constructed similarly as a line of 45° negative slope

throijgh a coordinate point represented by Rr • E = constant.

' Although the SI system of metric units is now preferred, present engineering practice uses units of atm-cm^/s for leak rate and Ibf/in^

for pressures near or greater than 1 atm. Converstion factors are 1 Pa = 1.451 X 10-* lbf/in2, 1 Pa-m^/s = 9.869 atm-cm^/s, and 1 atm = 1.01325 X 105 Pa.

10' 10'

10' 10" 10' 10 ' 10' 10' 10' 10' 10'

RELAXATION RATE. R, Is ']

Fig. 2. Test lines for helium leak detector method. E and are for helium gas. Test line values are for R^-E = constant with indicated

lines ranging from 10-5 to 10-'° 5-) ^atm-'.

Then specific solutions are defined by the intersection of the

test line with that E{T, t) characteristic corresponding to the

particular test values of T and /. Solutions to all modes of

test operation may be so obtained.

1) Unknown Leak Size: In one mode of operation it may

be desired to determine the leak size of a specimen of internal volume V which has been subjected to an arbitrary set of

back-pressurization parameters Pg, T, and t for which an in- dicated leak rate R was obtained. To solve this, the appropriate test line value is first computed from RI{Pq • V • P^). The intersection of this test line with the upslope portion

E{T) of the curve corresponding to the given pressurization time and with the downslope portion E(t) corresponding to the given dwell time determine the two possible solutions

for Rr from which the standard leak rate for helium L is determined. The leak size, which is defined as the standard

leak rate for air, isL/2.69.^ Example: Consider a volume of 0.1 cm^ pressurized to

5 atm abs (~60 psig) in pure helium for 2 h and then put on the helium leak detector 50 min later to give a reading of 5 X 10~* atm • cm^/s. The true leak size is required. The appropriate parameters follow. K = 0.1 cm^

5 atm • abs r = 2h.

t = 3000 s. R = 5 X 10"* atm • cm^/s. The steps are as follows.

a) Compute R^ • E = RUPq • V Pe) = ] X

10^'' (atm •

b) In Fig. 2 find the mtersection of the 1 X 10 ^ (atm •

s)~ ' test line with the T = 2 h characteristic. Since r = 2 h is not shown in Fig. 2, lay a scale parallel to

2 Leak rate for helium/leak rate for air = (molecular weight air/ molecular weight helium)l/2 = 2.69, where A/air = 28.98 |11].

31

I
the Rf, E characteristics and through the 2-h tick mark. Determine the value of /?, at the intersection. Rf =

3.6 X lO^^s- Therefore by (5a) L the standard leak

rate for helium is 3.6 X 10^^ atm • cm-'/s from which

the leak size is 1.3 X IC^ atm • cm^/s.

c) Follow the 1 X 10"'' test line to the t = 3000 s E(t)

characteristic. Determine the value of R^ at the intersect. Rr = 2.8 X 10" ^s"'. Therefore/, = 2.8 X lO"^" atm • cm^/s and L^^ = 1 X

10""* atm • cm^/s.

d) The leak size is either 1.3 X

10"'' or 1 X lO"'* atm • cm^/s. Discrimination is made with a followup reading on the leak detector at a later time. Any significant change indicates that the leak rate is the larger value.

2) Pressurization Parameters: By far most hermetic test activity is for screening purposes. This requires a selection of

parametric values so that all specimens with leak sizes greater than some specified value be rejected. The upper limit of the

test is determined, however, by the shortest dwell time that can be obtained, and it is hoped the test range can be made

broad enough to overlap that of the gross leak test that would

follow.

First the reject level relaxation rate is calculated from (5a) from the package-free volume and the specified standard leak

rate for helium, and a vertical line is erected on the chart at that value. The test line is calculated from the left side of

(11) with the selected value of leak detector signal R, the package volume V, and a convenient pressurization value P^. The intersection of this test line with the R^ line sets the choice for T. If the first test line selected produces an in- convenient value for T one can move away from the first intersection to choose more satisfactory values for both T

and a test line. The only constraint is that the leak detector

reject signal has to be greater than the minimum detectable

signal by a suitable amount [12]. Then the corresponding

maximum value of Rr that can be detected is easily picked

off by extending the test line to the E{t) characteristic cor- responding to the attainable dwell time. Alternatively the major factor in the test may be the maximum detectable

leak size. Then the maximum value of Rr is calculated and

a vertical line extended from this value. The intersection with a suitable E{t) characteristic determines the lower end

of the test line which is then extended at a —45° slope to a suitable £"(7^ characteristic. Example: Consider an IC package of 0.01 cm^ nominal

free volume for which a reject leak size of 5 X 10"* atm • cm^/s of air is required [8]. Suppose the background signal

is found to be 1 X 10"^ atm • cm^/s of helium and a signal- to-background value of five is selected. Also the tests could

be completed within 50 min after release from the pressuriza- tion chamber. Consider P^ = 5 atm • abs. The appropriate parameters follow: V =0.01cm^

^air reject = 5 X 10~^ atm • Cm-'/s, ^reject = 5 X 10"^ atm • cm^/s (helium),

t = 3000 s, = 5 atm • abs.

ThenL = 1.35 X 10"'' atm • cm^/s, Rr = 1.35 X 10-5 5-1^ R^eiectlPoVPE = 1 X 1 0"

^ atm" ' • s" '

.

The intersection of test line 1 X 10"^ and of 1.35 X

10-5 is at r = 0.15 h (see Fig. 3). But T = 9 min is too

short for convenience. Consider T = 0.3 h instead. Doubling

the value of T at constant Rr doubles E, as seen in (7), and

hence the value of the test line; thus the detector reject level

is increased to 1 X 10~* atm • cm^/s. A —45° test line established at r = 0.3 h and Rr = 1.35 X lO'^s"' (RrE = 2 X

10"'') intersects the t = 3000 s characteristic at Rr = 3.3 X

10~^ so that the maximum standard leak rate for helium

Z, = 3.3 X 10^5 atm • cm^/s and then Lair = 1.2 X 10"

atm • cm'/s. However 1.2 X 10"' is borderline for any gross

leak test, and the only way to increase the present limit significantly is to decrease the dwell time. Decreasing dwell time to the 1000-s (~17-min) characteristic would lead to an Rr of 1.1 X 10"^ and hence an L^i, ~ 4.1 X 10"' atm • cm^/s for some improvement in overlap between the fine and gross leak test. Note that an increase in dwell time beyond

~1 h would pull the intersection point off the straight line region of £'(7^. It should also be noted that P^ could be dropped to 2.5 atm pressure and the original value of leak detector reject limit of 5 X 10"' atm • cm^/s be retained without changing the pressurization time test limits. Only the

ratio of signal to background would be affected. Since a leak size of 5 X 10"* atm • cm^/s in a 0.01-cm^ volume produces a time constant of only 2 days, consider a decrease in the reject leak size by a factor of 5 to give 1 X

10"* atm • cm^/s. Then/?;, becomes 2.7 X 10"* s"' and the

intersection of this value with the 2 X 10"^ test line shows T = 7.5 h, a 25-fold increase in pressurization time; i.e., with i7)Rr-E~Rr^TIPo.

3) Repetitive Measurement: Often the package may in- clude an initial partial pressure of helium because of prior

testing or prefiU, and this initial concentration will affect the test results. In practice specimens are conditioned in vacuum or allowed to stand for some time until an initial leak detector reading is relatively small, but when such a long

wait time may not be suitable or where some accuracy is required a formal correction procedure is desirable. The specimen is first placed in the inlet of the leak de- tector and an initial measurement is made of the effusion. The effusion rate is related to the interior partial pres- sure/', and to the standard leak rate through (2) and (10) as L

After another delay period of t" , the specimen is subjected

to hermetic test, and a final measurement is made. The initial partial pressure P' just prior to pressurization, after further

possible effusion during the delay, is R P

i>' = -i-° -exp (-/?/'). (12)

32
10' 10' REUUTION RATE. R, Is

'

Fig. 3. Selection of pressurization parameters for helium leak detector method: an example. Reject leak size (air) of 5 x 10-8 atm-cm^/s

with V = 0.01 cm^.P£ = 5 atm.A = 3000 s, and minimum detectable

signal of 5 X 10-' atm-cm^/s for signal-to-noise of 5. Line (a) repre- sents an /J ^ of 1.35 X 10-5 s-1 for helium. Then/?/(/'o^'^£)= 1 x

10-'' s-i-atm-i. First intersection of 10-'^ test line and (a) at T ~ 0.15 h is not satisfactory, so 7" = 0.3 h is chosen. Line (b) be- comes the test line. Intersection of (b) at / = 3000 s leads to leak

size of 1.2 X 10-5 atm-cm^/s of air. Line (c) for intersect at t = 1000 s leads to L^i^ 4.1 x 10-5 atm-cm^/s.

The measured leak rate R2 obtained after pressurization is equal to LP/Pq where P is now described by (4). With (4), (6), and (12), it follows that Rj =Ri exp (-R/') [exp(-Rrt2)-Po£2] + PoVPERrE2 (13)

where the subscript 2 denotes the quantities related to the pressurization sequence. As a general approach, the solution

is obtained by successive approximation as follows.

a) Compute an approximate test line value from

R. = Rr-E2

PoVPe

b) With this test line find the approximate values for R^

and £"2 2t £'(7*2 , ^2 ). here designated as Rr and £"2 • c) Compute the value of exp (-Rr'tj) from the value of

tj used in the test. d) Compute the value of exp {-Rr't") from the measured

value of t"

.

e) With these approximate values from b. c, d

R2-R1 expj-R/t") [exp{-R;t2)-PoE2']

Rr'E2.

(14)

This then is a corrected test line from which the two possible solutions may be obtained at the intersects for E{T2) and

E(t2). Further iteration can be made.

If however the first approximated test line crosses the E(T^

characteristics in the region of straight line projections, then

exp (-/?/')- 1

exp (—/?r?2) ~* 1

£2-^1,

so that by (14)

-Rr • E2 (15)

as one might expect. At the large leak end of the test line, the solution is some- what insensitive to variation in test line value since R^ is primarily a function of t2, i.e.,

exp (-R/') < 1

exp (-Rrt2)^0

so that

R,

PoVPe

^Rr'Ej, (16)

and the prereading does not produce a significant correction. Thus one only need see where the approximated test line

falls to determine how much further iteration is required. If

it falls in the fine leak area of straight line projections, simply

take the difference between the final leak detector value and

the prereading and calculate the test line value. The "true"

value for Rr is either at £"(7'2) or £"(f2)- If only the large leak value is desired, simply calculate the test line from /?2. If the approximated test line falls in the intermediate Rr range, the iteration procedure is appropriate.

IV. RADIOISOTOPE METHOD

A. Test Equation

Since the ~0.5-MeV gamma radiation emitted by Kr^'

easily penetrates most semiconductor package walls the amount of pure Kr*' that has passed through a leak channel and remains within the package cavity after pressurization in a Kr^^-Nj gas mixture can be measured without resorting to extraction back through the leak channel as in the use of

the helium leak detector. Thus the gamma count rate R*

becomes

R* = {PV)-A -K (17)

where P is the internal partial pressure of Kr*^, PV is the quantity of Kr^' at ambient temperature within the cavity.
A is the activity of pure Kr^' (/LiCi/atm • cm^),^ and K is the overall counting efficiency of the detector for the particular package type at a particular location within the crystal detector well (count rate/^jCi). The interior partial pressure is given by (3), but the external partial pressure is

Pe = --Pe* (18) A

where 5 is the specific activity of the Kr^'-Nj gas mixture

(^(Ci/atm • cm^) and P^* is the pressurization value for the

gas mixtufe. Thus

R*V-Pf = SK'E* (19)

where the solution is characterized only by the internal fractional partial pressure for Kr^', here designated asE*. IjFine Leak Approximation: For relatively small values of relaxation rate with krypton R^* by (7) and (19), R* Rr*T*

P,,V-Pp SK

with T* denoting pressurization time with krypton. Thus

Pe*SKT*

(20)

where Z,* is the standard leak rate for Kr^'. Package volume

is not a factor, and (20) differs from the traditional recipe

[7], [8] in that P^* is now raised only to the first power

rather than to the second power as in the laminar viscous flow model.

B. Test Lines For a given volume, indicated count rate, and pressuriza- tion in a particular gas mixture the locus of all solutions is now on a horizontal line of value E* rather than the 45° slope of the helium leak detector, and there is now no need to construct a set of such lines superimposed on the E, R^ plane. Specific solutions for the test sequence are as before at the

intersection of the test line with the E(T, t) characteristic specified by the parametric values of T and t.

1 ) Pressurization Parameters-Example: Consider again the 0.01-cm^ package to be leak tested to 5 X 10~^ atm • cm^/s

of air with a dwell time of 1000 s. Background count rates are generally of the order of 500 counts/min, a typical value

for 5 is 200 juCi/atm • cm^, and a typical value of K is 10* min~ ' • idCi" '

. Assume a signal-to-background ratio of 5 for the reject level count rate and a 5-atm pressurization. It is desired to establish the required value of Fand the maximum

detectable leak size Z-air- The appropriate parameters

follow.

3 Although the SI system of metric units is now preferred, present engineering practice uses units of curie for disintegration rate; 1 Ci =

3.7 X IQlO Bq (events/s).

reject V

E,Ro*

R* reject S

K

t*

= 0.01 cm\

= 5 X 10-* atm = 500 min~

= 2500 min"' = 200/iCi/atm • = 10" min = 1000 s. rKrSS

cm^/s

/iCi

Then with (M^'' IM^'y^ = 1/1.71, one obtains

Z,Kr85 reject = 5X10"** —rr-

= 2.92 X 10"* atm • cm^/s

whence

7?^* = 2.92 X 10"* $-

R*/(PoVPe*SK) = E* = 2.5 X 10"^ atm"'.

As shown in Fig. 4 a line is erected at R^ = 2.94 X 10"^

s"', and a horizontal test line is placed at E* = 2.5 X 10"^

.

At the intersection, T = 2.4 h. At the intersection with the

t = 1000 s segment, R^* max = 3.7 X 10"^ s"' such that Lair ~ 6.3 X 10~^ atm • cm^/s. Note that leak rates are for Kr*^ and hence R^ values are smaller than for helium for a given air leak. Were the reject limit lowered to an L^„ of 1 X 10~*

atm • cm^/s, R^* would be 5.88 X

10"'' s"' so that T is 11.8 h; that is, the pressurization time changes linearly with Rr, as evidenced in Fig. 4 and in (20) for the straight line pro- jection region of E(T).

2) Repetitive Measurement: An initial count rate made

on the specimen relates to the quantity of Kr*^ already within by (17). After a lapsed time t" the specimen is pres- surized. The interior pressure just before this is equivalent

to that expressed in (12) or

P' =

R,

V'A -K

exp (-R*f).

The final pressure after processing is as in (4), and the final count rate is again determined by (17) as

R2*-Ri* exp i-R/') [exp (-/J/rj )-Po£'2 *]

SK -E-,* (21)

which is of the same form as (14). In the small leak range where exp (-7?^*/") and exp (-/J^*':) are ~1, and as E2* < 1 , the initial count rate is a direct correction or

= SK •E2*. (22)
10' 10' 10' 10' 10 ' 10' 10' 10' 10'

RELAXATION RATE. R, [s 'l

Fig. 1. E, the relative partial pressure of tracer gas within package in- terior free volume per atmosphere of external tracer gas pressurization, as function of relaxation rate with pressurization time T

and dwell time t as parameters. Transport mechanism is molecular

flow.

into the helium leak detector will by (1) give rise to an in- dicated leak rate R given by

R=F-P (10)

where the pressure within the leak detector is much less than P. Most tests for package leakage are done soon after assembly so that the initial interior helium concentration is zero. For such packages the leak detector indication is then from (2) and (3) R=Pe -L - E

or from (5aJ R

=R,'E. (11)

Thus the solution is characterized by the two factors E and

Rr, and all solutions relating leak detector response to stand- ard leak rate may be derived from the characteristic projections for E along with an appropriately placed test line.

B. Test Lines

It is apparent from (11) that the locus of all solutions for any test sequence will lie on a line of 45° negative slope on

the log E-logRr plane; for, with a given volume, indicated leak

rate and pressurization the product R^ ' E is z constant. Fig. 2 includes test lines ranging from 1 X 10~' (atm • s)"' to

1 X 10"'° (atm • s)"'.' A test line for any other value can be constructed similarly as a line of 45° negative slope through a coordinate point represented by R^ ' E = constant.

' Although the SI system of metric units is now preferred, present engineering practice uses units of atm-cm^/s for leak rate and Ibf/in^

for pressures near or greater than 1 atm. Converstion factors are 1 Pa =

1.451 X 10-'» lbf/in2, 1 Pa-m^/s = 9.869 atm-cm^/s, and 1 atm = 1.01325 X 105 Pa.

10' 10'

10' 10' 10' 10 ' 10' 10' 10' 10' 10'

RELAXATION RATE. R, Is '

I

Fig. 2. Test lines for helium leak detector method. E and are for helium gas. Test line values are for R^'E = constant with indicated

lines ranging from IQ-^ to 10" '0 s"' - atm"'

.

Then specific solutions are defined by the intersection of the

test line with that E{T, t) characteristic corresponding to the

particular test values of T and t. Solutions to all modes of

test operation may be so obtained.

1) Unknown Leak Size: In one mode of operation it may

be desired to determine the leak size of a specimen of internal volume V which has been subjected to an arbitrary set of

back-pressurization parameters P^;, T, and t for which an in- dicated leak rate R was obtained. To solve this, the appropriate test line value is first computed from RUPq • V • P^). The intersection of this test line with the upslope portion

E{T) of the curve corresponding to the given pressurization time and with the downslope portion E(t) corresponding to the given dwell time determine the two possible solutions

for Rr from which the standard leak rate for helium L is determined. The leak size, which is defined as the standard

leak rate for air, isL/2.69.^ Example: Consider a volume of 0.1 cm'' pressurized to

5 atm abs (~60 psig) in pure helium for 2 h and then put on the helium leak detector 50 min later to give a reading of 5 X 10~* atm • cm^/s. The true leak size is required. The appropriate parameters follow. V = 0.1 cm^

?£= 5 atm • abs r = 2h.

t = 3000 s. R = 5 X 10"* atm • cm^/s. The steps are as follows.

a) Compute R, • E = RI(Pq • K ?£) = 1 X 10" (atm • s)"'.

b) In Fig. 2 find the intersection of the 1 X 10"^ (atm •

s)" ' test line with the T = 2 h characteristic. Since r = 2 h is not shown in Fig. 2, lay a scale parallel to

2 Leak rate for helium/leak rate for air = (molecular weight air/ molecular weight helium)!/^ = 2.69, where Afa'"' = 28.98 (11].

31
the E characteristics and through the 2-h tick mark. Determine the value of Rr at the intersection. Rr =

3.6 X 10"^s~ '. Therefore by (5a) L the standard leak

rate for helium is 3.6 X 10"^ atm • cm^/s from which

the leak size is 1 .3 X 10~ ^ atm • cm-'/s.

c) Follow the 1 X IQ-'' test line to the t = 3000 s E(t)

characteristic. Determine the value of Rr at the intersect. Rr = 2.8 X 10- ^s-'. Therefore L = 2.8 X lO""* atm • cm^/s and = 1 X 10"* atm • cm^/s.

d) The leak size is either 1.3 X

10"'' or 1 X lO"" atm • cm^/s. Discrimination is made with a foUowup reading on the leak detector at a later time. Any significant change indicates that the leak rate is the larger value.

2) Pressurization Parameters: By far most hermetic test activity is for screening purposes. This requires a selection of parametric values so that all specimens with leak sizes greater than some specified value be rejected. The upper limit of the

test is determined, however, by the shortest dwell time that can be obtained, and it is hoped the test range can be made

broad enough to overlap that of the gross leak test that would

follow.

First the reject level relaxation rate is calculated from (5a) from the package-free volume and the specified standard leak

rate for helium, and a vertical line is erected on the chart at that value. The test line is calculated from the left side of

(11) with the selected value of leak detector signal R, the package volume V, and a convenient pressurization value P^. The intersection of this test line with the Rr line sets the choice for T. If the first test line selected produces an in- convenient value for T one can move away from the first intersection to choose more satisfactory values for both T

and a test line. The only constraint is that the leak detector

reject signal has to be greater than the minimum detectable

signal by a suitable amount [12]. Then the corresponding

maximum value of Rr that can be detected is easily picked

off by extending the test line to the E{t) characteristic cor- responding to the attainable dwell time. Alternatively the major factor in the test may be the maximum detectable

leak size. Then the maximum value of Rr is calculated and

a vertical line extended from this value. The intersection with a suitable E(t) characteristic determines the lower end

of the test line which is then extended at a -45° slope to a suitable E(T) characteristic. Example: Consider an IC package of 0.01 cm^ nominal

free volume for which a reject leak size of 5 X 10~* atm • cm^/s of air is required [8] . Suppose the background signal

is found to be 1 X 10~^ atm • cm^/s of helium and a signal- to-background value of five is selected. Also the tests could

be completed within 50 min after release from the pressuriza- tion chamber. Consider Pg = 5 atm • abs. The appropriate parameters follow: V =0.01cm\

^air reject = 5 X 10"^ atm • cm^'/s, ^reject = 5 X 10"^ atm • cm-'/s (helium),

t = 3000 s, Pe =5 atm • abs.

ThenL

Rr

= 1.35 X 10" = 1.35 X 10'^ atm cm^/s,

RreiecJPoVPE =1X10-7

atm" The intersection of test line 1 X 10~^ and 7?^ of 1-35 X

10"' is at r = 0.15 h (see Fig. 3). But 7 = 9 min is too

short for convenience. Consider T = 0.3 h instead. Doubling

the value of T at constant /?, doubles E, as seen in (7), and

hence the value of the test line; thus the detector reject level

is increased to 1 X 10~^ atm • cm^/s. A —45° test line established at r = 0.3 h and Rr = 1.35 X lO'^s"' (RrE = 2 X

10"'') intersects the t = 3000 s characteristic at /?, = 3.3 X

10" so that the maximum standard leak rate for helium

Z, = 3.3 X 10^' atm • cm^/s and then = 1.2 X 10"' atm • cm^/s. However 1.2 X 10~' is borderline for any gross leak test, and the only way to increase the present limit significantly is to decrease the dwell time. Decreasing dwell time to the 1000-s (~17-min) characteristic would lead to an Rr of 1.1 X 10"^ and hence an L^i^ ~ 4.1 X 10"' atm • cm^/s for some improvement in overlap between the fine and gross leak test. Note that an increase in dwell time beyond

~1 h would pull the intersection point off the straight line region of E{T). It should also be noted that Pe could be dropped to 2.5 atm pressure and the original value of leak detector reject limit of 5 X 10"' atm • cm^/s be retained without changing the pressurization time test limits. Only the

ratio of signal to background would be affected. Since a leak size of 5 X 10~* atm • cm^/s in a 0.01-cm^ volume produces a time constant of only 2 days, consider a decrease in the reject leak size by a factor of 5 to give 1 X

10"^ atm • cm ^/s. Then/?;, becomes 2.7 X 10~^ s~' and the

intersection of this value with the 2 X 10~' test line shows T = 7.5 h, a 25-fold increase in pressurization time; i.e., with {l)Rr'E~Rr^T/Po.

3) Repetitive Measurement: Often the package may in- clude an initial partial pressure of helium because of prior

testing or prefiU, and this initial concentration will affect the test results. In practice specimens are conditioned in vacuum or allowed to stand for some time until an initial leak detector reading is relatively small, but when such a long

wait time may not be suitable or where some accuracy is required a formal correction procedure is desirable. The specimen is first placed in the inlet of the leak de- tector and an initial measurement is made of the effusion. The effusion rate i?, is related to the interior partial pres- sure /•] and to the standard leak rate through (2) and (10) as L^0 After another delay period of t" , the specimen is subjected

to hermetic test, and a final measurement is made. The initial partial pressure P' just prior to pressurization, after further

possible effusion during the delay, is

, R\Po

/>' = • exp i-Rrt"). (12)

32
REUUTIM RATE. R, {s"l

Fig. 3. Selection of pressurization parameters for helium leak detector method: an example. Reject leak size (air) of 5 x 10-* atm-cm^/s

with V = 0.01 cm'^.P^ = 5 atm, t = 3000 s, and minimum detectable

signal of 5 X 10-' atm-cm^/s for signal-to-noise of 5. Line (a) repre- sents an Rr of 1.35 x 10-5 j-i for helium. Then/?/(Pol''^£-) = 1 x

10—'' s-'-atm-'. First intersection of 10-'^ test line and (a) at

7" ~ 0.15 h is not satisfactory, so 7" = 0.3 h is chosen. Line (b) be- comes the test line. Intersection of (b) at f = 3000 s leads to leak

size of 1.2 X 10-5 atm-cm^/s of aii. Line (c) for intersect at f = 1000 s leads to L^^j = 4.1 x 10-5 atm-cm3/s.

The measured leak rate R2 obtained after pressurization is equal to LPIPq where P is now described by (4). With (4), (6), and (12), it follows that R2 =Ri exp(-R/') [exp (-.R^^) --^0^2

1

+ PoVPeR^2 (13)

where the subscript 2 denotes the quantities related to the

pressurization sequence. As a general approach, the solution

is obtained by successive approximation as follows.

a) Compute an approximate test line value from

^2 = Rr-E2.

PoVPe

b) With this test line find the approximate values for R^

and £2 3t EiTi , tj), here designated zsR^' and E^'

.

c) Compute the value of exp (-R/ti) from the value of

t2 used in the test. d) Compute the value of exp (,-R/t") from the measured

value of t"

.

e) With these approximate values from b, c, d

R2-R1 exp(-Rr't") [exp(-R;t2)-PoE2']

PoVPe

^Rr'E2

(14)

This then is a corrected test line from which the two possible solutions may be obtained at the intersects for E(T2) and

£"(^2 ) Further iteration can be made.

If however the first approximated test line crosses the E(T}

characteristics in the region of straight line projections, then

exp (—Rrt") -* 1

exp i-R,t2)-* 1

so that by (14) R2-Ri

= Rr'E2 (15)

as one might expect. At the large leak end of the test line, the solution is some- what insensitive to variation in test line value since Rr is primarily a function of tj, i.e.,

exp (-/?/')< 1

exp i-Rrt2)^0

so that

Ri

PoVPe = Rr-E2 (16)

and the prereading does not produce a significant correction. Thus one only need see where the approximated test line

falls to determine how much further iteration is required. If

it falls in the fine leak area of straight line projections, simply

take the difference between the final leak detector value and

the prereading and calculate the test line value. The "true"

value for R^ is either at E{T2) or E(t2). If only the large leak value is desired, simply calculate the test line from R2. K

the approximated test line falls in the intermediate R^ range,

the iteration procedure is appropriate.

IV. RADIOISOTOPE METHOD

A. Test Equation

Since the ~0.5-MeV gamma radiation emitted by Kr*'

easily penetrates most semiconductor package walls the amount of pure Kr*^ that has passed through a leak channel and remains within the package cavity after pressurization in

a Kr*'-N2 gas mixture can be measured without resorting

to extraction back through the leak channel as in the use of

the helium leak detector. Thus the gamma count rate R*

becomes

R* = {PV)'A • K (17)

where P is the internal partial pressure of Kr*', PV is the quantity of Kr^' at ambient temperature within the cavity.

33
A is the activity of pure Kr*^ (/jCi/atm • cm-'),^ and K is the overall counting efficiency of the detector for the particular package type at a particular location within the crystal detector well (count rate//iCi). The interior partial pressure is given by (3), but the external partial pressure is

Pe = --Pe* (18) A

where S is the specific activity of the Kr^^-Nj gas mixture

(/jCi/atm • cm^) and Pg* is the pressurization value for the

gas mixture. Thus

Pq 'V-P^ = SK-E* (19)

where the solution is characterized only by the internal

fractional partial pressure for Kr^^, here designated ^sE*. IjFine Leak Approximation: For relatively small values of relaxation rate with krypton R^* by (7) and (19), R* R*T*

PoV'Pe*'SK Po

with T* denoting pressurization time with krypton. Thus

R*Po

Pe*SKT*

(20)

where L* is the standard leak rate for Kr*'. Package volume

is not a factor, and (20) differs from the traditional recipe [7], [8] in that Pe* is now raised only to the first power

rather than to the second power as in the laminar viscous flow model.

B. Test Lines For a given volume, indicated count rate, and pressuriza- tion in a particular gas mixture the locus of all solutions is now on a horizontal line of value E* rather than the 45° slope of the helium leak detector, and there is now no need to construct a set of such lines superimposed on the E, R^ plane. Specific solutions for the test sequence are as before at the

intersection of the test line with the E{T, t) characteristic specified by the parametric values of T and t.

1 ) Pressurization Parameters-Example: Consider again the 0.01-cm^ package to be leak tested to 5 X 10~* atm • cm^/s

of air with a dwell time of 1000 s. Background count rates are generally of the order of 500 counts/min, a typical value

for S is 200 ^iCi/atm • cm^, and a typical value of is 10"* min~ ' • nC\~ '

. Assume a signal-to-background rat^o of 5 for the reject level count rate and a 5-atm pressurization. It is desired to establish the required value of T and the maximum

detectable

follow.

leak size Lair- The appropriate parameters

V =0.01cm^

reject = 5 X 10"^ atm • cm^/s

Rq* = 500 min"'

R* reject = 2500 min""

S = 200A(Ci/atm • cm^

A' = lO'* min-' • /iCi~^

t* = 1000 s. Then wiih (M^'" IM^'^^Y = 1/1.71, one obtains

Z,Kr85 reject = 5 X lO"** —

= 2.92 X 10"^ atm • cm^/s whence

/?,* = 2.92X 10"* s~

R*I(PoVPe*SK)=E* = 2.5 X 10"^ atm"-'.

As shown in Fig. 4 a line is erected at Rr = 2.94 X 10~^

s~', and a horizontal test line is placed at £"* = 2.5 X 10~^. At the intersection, T = 2.4 h. At the intersection with the

t = 1000 s segment, R^* max = 3.7 X 10"^ s~' such that

Z,air = 6.3 X 10"^ atm • cm^/s. Note that leak rates are for Kr*^ and hence R^ values are smaller than for helium for a given air leak. Were the reject limit lowered to an Lair of 1 X 10~^

atm • cm^/s, R^* would be 5.88 X 10"'' s"' so that T is 11.8 h; that is, the pressurization time changes linearly with

Rr, as evidenced in Fig. 4 and in (20) for the straight line projection region of E(T).

2) Repetitive Measurement: An initial count rate made

on the specimen relates to the quantity of Kr*^ already within by (17). After a lapsed time t" the specimen is pres- surized. The interior pressure just before this is equivalent

to that expressed in (12) or

P' =

Ri

V'A -K

exp (-Rr*t").

The final pressure after processing is as in (4), and the final count rate is again determined by (17) as

/?2* -i?,* exp i-R/') [exp {~R*t:,)-PoE^ *] PqV-Pe*

= SK -E-, (21)

which is of the same form as (14). In the small leak range where exp (-^r*'") and exp {-R*ti) are ~1, and as

* 1 , the initial count rate is a direct correction or

3 Although the SI system of metric units is now preferred, present /Jj* — /?i

* engineering practice uses units of curie for disintegration rate; 1 Ci = = SK •£'2*.

3.7 X 1010 Bq (events/s). PqV • Pe*

(22)

34
TABLE 1 comparative values for helium leak detector and

Radioisotope testing of hermetic Packages'

3011s :

10' 10" 10' 10' 10 ' 10' 10' 10' 10' REL^UTION RITE. R, ' Is 'l

Fig. 4. Test line and example of test range for radioisotope method.

Reject leak size of 5 x 10-8 atm-cm^/s of air with V = 0.01 cm^

/>£» = 5 atm of Kr-Nj mixture, S = 200 wCi/atm-cm3, K = ID" min~' "MCi"', and a minimum detectable count rate of 2500 min~' for a signal-to-noise ratio of 5. Line (a) represents of 2.94 X 10-6 s-l for Kr85. R" I(PqVPe*SK) = E* = 2.5 X 10-2 atm-1 is test line (b). The intersection of (a) and (b) sets T= 2.4 h. At the intersection of (b) and the f = 1000 s, line (c) represents /?/ = 3.7 X 10-3 s-i whichleadstoiair = 6.3 X IQ-S atm-cm3/s.

In the large leak range where exp (—Rr*t")< I,exp{ -Rr*t2)^

0, and E2 ^ \ . the prereading is not significant. In the inter- mediate range, (22) is solved by iteration just as for the helium

leak detector with the exception of the horizontal test lines and/?,'* values for Kr*' rather than for helium.

V. COMPARISON OF HELIUM AND

RADIOISOTOPE METHODS

Within the molecular flow regime test results obtained

with the two methods can be compared with the use of

characteristic leak-test curves. The capability of the helium

leak detector method is oriented toward the larger leak sizes and smaller volumes, for the pressurization time increases rapidly along the helium test line as the relaxation rate di- minishes. The radioisotope method requires a longer pres- surization time with intermediate leak sizes because the

leak rate for Kr*^ is some 4.6 times smaller than that for He

in the same leak channel, however the pressurization time

increases but linearly as the relaxation rate diminishes. The

radioisotope test line tends to lie in the nonlinear portion of

the characteristics, particularly at the large leak end, but in

this range the 45° slope of the helium leak detector method

tends to both the smaller values for E and the larger values of Rr where straight line segments give reasonable approximation. However in the conversion to air leak rates, both methods produce the same approximate upper limit when the same dwell times are used. The radioisotope method does

allow shorter dwell times in use and therefore can in principle extend to a larger upper limit of leak size. The short dwell time obtainable with the radioisotope method also allows two cycle testing for extension to gross leak sizes [13]

.

(cm3) (atm-cm3/s) (s-i) T/T* n*/n

0.01 10-6 2.7 X lO-'* 255 s 0,0016 10-7 2.7 X 10-5 0.71 h 0.016 10-8 2.7 X 10-6 7.1 h 0.16 0.16 10 10-6 2.7 X 10-7 255 s 1.6 10-7 2.7 X 10-8 0.71 h 16 10-8 2.7 X 10-9 7.1 hl61 161

1 Derived from (23). Pressurization times T/T* for equal signal-to- background ratios, and signal-to-background ratios n*/n for equal pres- surization times. 5 = 200 MCi/atm-cm^, K = 10^ min-l mQ -', Rq -

5 X 10-10 atm-cm3/s helium, Rq* = 500 min-t.

A direct numerical comparison can also be obtained for the fine leaks by using (19) for the radioisotope method,

(11) for the helium method, the approximation of (7), and

the detector outputs in terms of signal-to-background, for the same pressurization value, as

n*Ro* SK T* — = (23) nRo 4.61R, T

where the ratio R^* to R^ is equal to the ratio of the standard

leak rates for a given leak size which is (M^^/M^'y or

1/4.61, n is the signal-to-background ratio, and Rq is the threshold signal. Some results are listed in Table I. Note that the radioisotope pressurization times are independent of volume per (20). A comparison of signal-to-background

ratios is listed in the last column for equal pressurization

times.

VI. SUMMARY

The molecular fiow formulation for the back pressuriza- tion method of hermetic testing has been condensed into a normalized set of characteristic curves from which specific solutions can be obtained by means of a test line. The characteristics are appropriate for both the helium leak detector and

and radioisotope methods of test, while the form of the test

line distinguishes between two methods. Minimum detectable and maximum detectable leak sizes can be determined simul- taneously from the one set, and the selection of suitable test conditions is facilitated. Procedures are also derived for ap- plying corrections due to prior testing or prefill. REFERENCES

|l| S- Rulhberg. in Semicnnduclor Measuremeni Technology. ARPA/ NBS Workshop II Hermelicity Testing for Integrated Circuits. H. A Schafft, Ed . NBS Special Publication 400-9, pp. 27-32. Dec. 1974 (Available from U S Govt. Printing Office, SD no CI3, 10:400-9 1 |2| S Dushman and J M Lafferty, Scientific Foundations of Vacuum

Technolony. 2nd ed.. section 2.5, New York: Wiley, 1962, p. 104

(31 D. A. Howl and C, A Mann, "The back-pressurization technique

of leak testing," Vacuum, vol. 15, pp. 347-352, 1965.

|4| S B Banks. R E McCullough. and E. G Roberts. "Investigation

35
(it microL la ult seal Icsiiny.' Air t-orce Systems Command, Rome

Air Development Center. Tech Rep RADC-TR-75-89, Apr. 1975 W. E. Briggs and S. G. Burnett, "Helium mass spectrometer leak

testing of pressure—"bombed" sealed enclosures," Varian Vacuum

Division/NRC Operation. Newton Highlands. MA, Oct. I, 1968 Standard Recommended Practices for Determining Hermeticity of Electron Devices with a Helium Mass Spectrometer Leak Detector. ASTM Designation Fl 34. 1979 Annual Book of ASTM Standards, p 545-552.

B. Cassen and D. Bumham. "A method of leak testing hermetically sealed components utilizing radioactive gas," Int. J Appl

Radiation and Isotopes, vol. 9. pp. 54-59. 1960. Method 1014 2-Seal, Military Standard 8838. in Test Methods and

Procedures for Micrdelecironns, Aug. 31. 1977. (Available

from U.S. Naval Publications and Forms Center, 5801 Tabor Ave.,

Philadelphia, PA 19120 ) (9) S. Dushman and J. M Lafferty, Scientific Foundations of Vacuum

Technology . 2nd ed. . section 2.2. New York: Wiley, 1962, p. 81. |I0| Glossary of Terms Used in Vacuum Technology. Committee on

Standards, American Vacuum Society Inc New York: Pergamon,

1958, Definitions No. 20 and 236.

1 1 1 1 S Dushman and J M Laffeny, Scientific Foundations of Vacuum

Technology. 2nd ed , New York Wiley. 1962. p 16 |12| AVS Helium Mass Spectrometer Leak-Detector Calibration (2 1- 1963). J. Vac Sci Technol . vol 3. pp 229-235. 1966

|I3| S Ruthberg. G R Neff. and B D Martin. "Radioisotope

hermetic test precision." in Proc 1977 Ini Microelectronics Symposium. Baltimore, MD, Oct 24-26, 1977, pp 131-137.

36
NBS-n4A (REV. 2-8C) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO,

SHEET (See instructions) NBS SP 400-73 October 1982

4. TITLE AND SUBTITLE

Semiconduator Measurement Technology: Graphical Solution for the Helium Leak

Detector and Radioisotope Methods of Hermetic Test

Master Graphs and Instructions

5. AUTHOR(S)

Stanley Ruthberg

6. PERFORMING ORGANIZATION (l{ joint or other than NBS. see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 82-600626

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bibliography or literature survey, mention it here) A graphical procedure for solution of the molecular flow approximation for the back

pressurization method of hermetic test makes use of a set of characteristic curves

and a test line. The characteristic curves are appropriate for both the helium leak

detector and the radioisotope methods of test, although the form of the test line

differs between the two methods. Master graphs of the characteristic curves and

test lines are now provided in a scale and format appropriate for producing suitable

worksheets with a copier. Step-by-step instructions are given for their use in

obtaining solutions for various examples relative to the test specifications in

acceptance standards such as MIL-STD 883B, etc. One set of characteristics is

provided specifically for the helium leak detector mode as expressed directly in

terms of air leak rate; a second set is provided specifically for the krypton-85

radioisotope mode also in terms of air leak rate; and a third set is retained in the

original form for use with any tracer gas.

12. KEY WORDS (Six to twelve entries; olphabetica/ order; capitalize only proper names; and separate key words by semicolons)

back pressurization; electronic packages; hermetic test; leak testing.

13. AVAILABILITY

I xl Unl imited

I I For Official Distribution. Do Not Release to NTIS

[X] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

03] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

34

15. Price

^U.S. GOVERNMENT PRINTING OFFICE; 19 8 2-380-997/2319 USCOMM-DC 6043-P80

Announcement of Semiconductor Measurement Technology

List of Publications 72 - 1962-1981

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Dear Mrs. Cohen:

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Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717(38 FR 12315, dated May II, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.
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