8WE8

kRTMENT OF

JBUCATION NBS TECHNICAL NOTE 786

Methods for Testing

Wire-Bond Electrical

Connections

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oo

575*

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U.S.

'ARTMENT

OF

OMMERCE

National

Bureau

of

Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress 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: (Da 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 consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

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'Headquarters and Laboratories at Gaithersburg. Maryland, unless otherwise noted: mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

« Part of the Center for Building Technology.
Methods for Testing

Wire-Bond Electrical Connections

National Bureau of Standard? APR 2 9 1974 Harry A. Schafft QCI60

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards Washington, D.C. 20234

*

; mg

V*u of ***

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued November 1973
National Bureau of Standards Technical Note 786

Nat. Bur. Stand. (U.S.), Tech. Note 786, 23 pages (Nov. 1973) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:786). Price 50 cents.
TABLE OF CONTENTS

Page

1. Introduction 1

2. Pull Test

2.1. Destructive, Double-Bond Test 2 2.2. Destructive, Single-Bond Test 7 2.3. Nondestructive, Double-Bond Test 7

3. Centrifuge Test 8

4. Mechanical Shock Test 10

5. Variable Frequency Vibration Test 11

6. Vibration Fatigue Test 12

7. Thermally-Induced Stress Tests 12

8. Summary 14

9. References 15

METHODS FOR TESTING WIRE-BOND ELECTRICAL CONNECTIONS*

Harry A. Schafft

Abstract

A significant fraction of the failures that occur in integrated circuits are

due to failures of the wire-bond electrical connections that are used. Therefore,

a critical area for reliability improvement is in the methods for testing and evaluating wire bonds. Several of these methods are surveyed. In particular,

analyses with regard to the stress that .the test imposes on the wire bond in the pull, centrifuge, mechanical shock, vibration, and temperature cycling tests are presented and used in discussing the capabilities and limitations of these methods.

Key Words: Bonding; electrical connection; failure (wire bond) ; integrated

circuits; microelectronics; reliability; semiconductor devices; testing (wire

bond); wire bond.

1 . Introduction

Many microelectronic devices use wire bonds to electrically connect the semiconductor

die and the package terminals. The diameter of the wire is typically 1 mil.t Unless special precautions are taken, a significant fraction of the failures that occur in these devices is due to wire bond failure. Because of the increasingly large number of devices

used in many present-day electronic systems, the reliability of the individual devices must

be increased, even from present-day levels. Hence there is considerable interest in methods for testing and evaluating wire bonds. The term wire bond, for the purposes of this paper, includes all the components of the die-to-terminal electrical connection: the wire, the metal bonding surfaces, and the adjacent underlying supportive material. It is customary to speak of the bond as that part

of the wire bond that is associated with the volume of the wire deformed at the weld or attachment point and to speak of the heet of a stitch or wedge bond as that part of the wire at either end of the wire span which is deformed by the edge of the bonding tool. The purpose of this paper is to review aspects of the following tests: pull, centrifuge, temperature cycle, thermal shock, mechanical shock, variable frequency vibration, and vibration fatigue. In particular, these tests are examined with regard to the stress that

*This paper was prepared for presentation at the Third Symposium on Reliability in Electronics, November 13-16, 1973, in Budapest, Hungary.

tThe data referenced in this paper that are not given in the International System (SI) of units are followed in parentheses by the values in the appropriate SI unit. General usage

dictates that three exceptions be made: (1) acceleration is given in units of gravity

(1 g = 9.8 m/s 2

) , (2) the wire diameter is given in mils (1 mil =25.4 urn) , and (3) the force exerted on the wire or wire bond is given in grams-force (1 gf = 9.8 mN)

.
the test applies to the wire bond and what that implies about the capabilities and limitations of the test. These and other test procedures, including visual inspection, have been

discussed in detail elsewhere [72S2]*.

2. Pull Test

2.1. Destructive, Double-Bond Test1

"

The destructive, double-bond test consists of pulling on the wire span by some means

(usually with a hook) with increasing force until a rupture in the wire bond occurs. The pulling force required to produce rupture is called the pull strength, and it is used as a measure of quality for the wire bond. In the test, pull strengths of a sample are taken to be representative of the group. Usually, the location of the rupture is recorded as well as the pull strength which is often expressed in grams, although grams-force= are implied. To facilitate analysis of the data, the distribution of pull strengths can be displayed in a histogram. The magnitude

of the pulling force at the peak in the distribution displayed in the histogram gives an indication of the general ruggedness of the wire bond while the spread of the distribution

indicates the uniformity of a group. The actual stress in the wire bond is the tensile force in the wire. If the geo- metrical variables are defined as in figure 1, wire tensile forces on the terminal side,

Fwtj, and on the die side, Fwt , are related to the applied pulling force by

cos(6 d - <(>) Fwt = F x (1) sin(e d + e t )

cos(9 t + <j>) Fwd = F x • (2) sin(6 t + e d

)

It is assumed that the pulling probe is in the plane of the wire loop but inclined at an angle <p with respect to a normal to the bonding surface. The angles 8 t and 6 d are the contact angles that the wire makes with the bonding surfaces of the terminal and die,

respectively. The ratio Fwt/F as a function of 6 t is given in figure 2 for <\> = and various fixed ratios of 9 d /8 t

. The ratio of Fwd /F as a function of 6 d may be obtained by

interchanging the subscripts d and t in figure 2. Although the geometrical dependence is easily visualized in terms of the contact

angles, these angles are difficult to measure. The contact angles depend on the height, h, of the wire span above the terminal contact surface, the height difference, H, between the

*Alphanumerics in brackets indicate the literature references at the end of this paper. t[66Rl], [67H1], [67S3], [68D2] , [69B5], [69K1], [70A1], [71B4]

.

§See footnote on page 1.
TERMINAL

P ad'

SEMICONDUCTOR DIE

(l-o) d

Figure 1. Geometric variables for the double-bond pull test.

U?K 20 -

Figure 2. Dependence of Fwt /F on 6 t for various ratios 6^ to 6

t

. The curves are for the

case <Ji = 0. Horizontal bars above and below the curves for 26 Q = e t and 8d = 26 t show the effect of changing <f> to plus and minus 5 deg, respectively. To obtain the dependence of

Fwt/ F i interchange everywhere the subscripts d and t and change the sign of <ji.
die and terminal contact surfaces, and the horizontal distances, ad and (1 - ct)d, from the bonds to the point at which the wire span is contacted by the pulling probe. Expressing

eqs (1) and (2) in terms of these quantities we have

F { f

7!^

*wd

n I h + H . (1 - a; cos <j> + I ;— | sin

h f

(1 - a) 2 d"

(h + H) 2 M) [

A h • a cos 4 - -t sin

d

h

and (3)

(4)

For a normal pulling force (<j> = 0), these equations simplify to:

L wt F < (1 - a) 1+ Mn RW and; (5)

b wd

1 +

1 +

aH F

(1 - a) 2 d^

(h + H) 2 (6)

For a normal pulling force applied at mid-span to a single-level wire bond (<j> = 0, H = 0, a = 1/2) these equations simplify further to:

Fw = Fwt = Fwd = T Vl + (d/2h) 2

. (7)

The dependence of F/Fw

and Fw

/F on d/h in eq (7) is graphed in figure 3. The experimentally determined dependence of pull strength of ultrasonic aluminum wire

bonds on the geometrical variables has been compared with the predictions of eqs (3) and

(4), where it is assumed that the tensile force to rupture the wire bond remains constant

as the variables are changed.* In general the experimental results on unannealed singlelevel wire bonds agreed with the theory. However, differences were observed in the dependence of pull strength on loop height, h, for wire bonds with loop height greater than 1/3

the interbond spacing, d. It is felt that this is caused by a weakening of the wire bond

because of bond peeling as a result of the relatively high loop heights. Because of the

*[71B1], [71B4], [72B1], [72B2], [72B3], [72B4], [73B1] , [73B2] — A summary of this work

by A. Sher will be published as an NBS Technical Note.
6 10 14 18 22 26

d/h

Figure 3. F/Fwand Fw/F as functions of d/h for a single-level, double-bond pull test.

elongation of the wire during the test, all annealed wire bonds studied had loop heights

greater than 1/3 the interbond spacing at the time of failure, and hence the experimentally

obtained values for pull strength differed from the predicted values. While the pull test is the most widely used test for wire bonds it is also perhaps

the most under-specified. To maximize the usefulness of pull test data, especially when

used to evaluate and compare wire bonds, it is important to specify fully the wire bonds

tested and the test conditions as well as the failure mode.

It is necessary to have information about the average shape of the wire span* because

it can significantly affect the pull strength measured. To indicate the magnitude of the dependence of the pull strength on the shape of the wire span in a double-bond pull test,

*In cases where the elongation of the wire during the test is large, such as in annealed

wire, it is necessary to take this into account when providing information about the shape of the wire span.
consider two single-level wire bonds for which the same tensile force in the wire is re- quires to produce rupture. Let the contact angles that the wire makes with each bonding

surface be 10 degrees for one wire bond and 30 degrees for the other. The pull strength

measured for the one with contact angles of 30 degrees will be about three times as large

as for the other (see figure 2).*

If information about the wire tensile strength is not provided the pull strength data

of different wire bonds cannot usually be used to compare wire bond quality. This is be- cause failure is usually in the wire and in this case the pull strength is dependent on the tensile strength of the wire. Wire tensile strengths may vary significantly. For example,

aluminum ultrasonic wire bonds may be made from wire with an initial tensile strength of from 12 to 20 gf ; depending on the thermal history of the wire the final tensile strength

of the wire may be as little as 20 percent of its initial value [70P1]. A useful way of normalizing the data from pull tests of different wire bonds with wires having different

tensile strengths is to employ a bond efficiency [67R1] defined as the ratio of the tensile

force in the wire at rupture to the tensile strength of the wire. There has been some concern about two aspects of the pull test procedure which possibly can affect the pull strength measured. One is the speed of the pulling stress

applied. Therefore, the rate of pull is occasionally specified for the pull test. It is given either in terms of the speed of the pulling element [66R1], [67S3], [69K1] , or the rate of increase of the force, as measured at the pulling element [70B8]. In some pull

tests, the only specification is that the force must be applied "slowly" [67H1]. Leedy

and Main [71B4] reported no dependence of pull strength on pull rates in the range of 1 to

77 gf/s (equivalent to a range of from about 0.4 to 30 mm/min) for single-level, unannealed

wire bonds with 1-mil diameter aluminum wire bonded ultrasonically to an aluminum film on silicon. The higher rate may be comparable with the speed of some pull test machines used

in the industry where the pull may be likened to a jerk. It should be noted that the wire

bonds used by Leedy and Main were constructed so that rupture occurred in the heel of the bond; the independence of the pull strength on pull speed in the range reported may not hold for wire bonds where the failure mode is rupture or peel at the bond interface or where the two bonds are on different levels [71B4], The other aspect of concern arises if the applied pulling force is directed at some

angle, a, out of the plane of the wire loop because of possible tearing of the bond heels. Studies by Leedy, Sher, and Main [72B4] on 1-mil diameter aluminum ultrasonic wire bonds have been shown that the measured pull strength decreases only slightly as a increases;

the decrease is more pronounced for wire bonds with greater bond deformation. If a is maintained at less than about 10 deg the pull strength will not be affected significantly

by variations in a, except when testing bonds with bond deformationst of about three wire

diameters or more.

*For wire bonds made on the same plane, 10 degrees and 30 degrees correspond to values for

the ratio of the bond separation to loop height (d/h) of about 11 and 3.5, respectively.

tBond deformation refers to the width, as viewed from above, of the wire deformed at the weld or attachment point.
2.2. Destructive, Single-Bond Test

The destructive, single-bond test consists of pulling on a wire at some angle with

respect to the bonding surface with increasing force until rupture occurs. For the test, either the wire bond is cut somewhere along its span or only one bond is made. The purpose

of a single-bond pull test is usually to afford better control of the angle of pull (with

respect to the bond and contact surface) and also thereby minimize flexing and hence weakening the heel of the bond. A means of gripping the wire is required in this test. Wasson [65W1] has described

the use of special tweezers to grip the wire. Methods to reduce the grip-stress on the wire have been described by Adams and Anderson [68A1] and by Harman [69B5]. The former described the use of a black wax and a separate heating element to liquify the wax at the end of a probe which engulfs the wire by capillary action. The ability to apply a tensile

force of 150 gf was reported. The latter described the use of a high-tensile-strength,

hot-melt glue with a discrete melting point, at the tip of a nichrome wire-loop probe. The glue is melted by passing current through the wire loop which was electrolytically

thinned so that most of the joule heating occurs at the tip. The tensile strength and adherence of such glues are sufficient to test 1-mil diameter aluminum wire if a length

of about 0.13 mm is bonded with the glue. The glues do not adhere as well to gold wire

so that a longer length of wire must be bonded.

2.3. Nondestructive, Double-Bond Test

The nondestructive test consists of pulling on the wire span until a predetermined

force is applied. The test is meant to be nondegrading as well as nondestructive to satisfactory wire bonds but destructive to those bonds that are unsatisfactory. Such a test has been suggested as a 100 percent screen test by Slemmons [69S1] and Ang et at. [69A1], To substantiate the claim that the test is nondegrading to those that pass, both papers show

that after a group of wire bonds has been so tested and then pulled to destruction, the bond strength frequency distribution is merely truncated at the preselected stress level. Polcari and Bowe [71P3] have reported the results of some preliminary evaluations of a nondestructive pull test. They concluded that while the nondestructive test could be a valuable reliability tool the proper use and adjustment of the tester would be of critical

importance. Furthermore, if the nondestructive pull test were to be implemented, they

recommended that the manufacturer control loop heights to minimize variations in the stress imposed by the test. There has been skepticism that the test is actually nondegrading. The idea of using

devices whose wire spans have been pulled and, in the process, altered in shape is new and disturbing to some. On the other hand, there does not seem to be a similar reluctance to use devices stressed in the centrifuge test described in the next section.
3. Centrifuge Test*

In the centrifuge test a constant centrifugal force stresses the wire bonds. Poorly

adhering wires are expected to rupture while wires which have a large loop or which have

been improperly placed are expected to shift so that they will make electrical contact with

adjacent parts of the device. The resultant open or short circuits are detected by subse- quent electrical tests. The stress applied to the wire bond is dependent on the maximum acceleration (expressed in gravity units, g'st), the shape of the wire loop, and the direction of the centrifugal

force relative to the wire loop. The typical range of centrifugal forces used is from

30,000 to 50,000 g's. The typical duration at the maximum stress level is one minute in any

one direction of applied centrifugal force. The directions are chosen so that the force is directed either away from, toward, or parallel to the bonding surfaces. The test is often

preceded by other tests intended to weaken unreliable wire bonds and thereby promote their

failure in the centrifuge test. For the case where the centrifugal force is directed away from the bonding surfaces,

the tensile stress in the wire at the heel of the bond on the terminal, Fwt , and on the die,

FW(j, are given in grams-force by the following relations where it is assumed that the cen- trifugal force is sufficiently large so that the wire loop takes the shape of a catenary:

Fwt = pir 2 G(a + h) (8)

rwd F = P"2 G(a + h + H) (9)

where p = density of the wire (g/cm 3

) , r = radius of the wire (cm), h = vertical, distance between the terminal contact surface and the peak of the wire

loop after the centrifugal forces have deformed the loop to describe the catenary

curve (cm)

H = vertical distance between the terminal and die contact surface (cm)

d = horizontal distance between bonds (cm)

G = centrifugal acceleration (in units of gravity),

and where a is given by the relation h + H + a = a cosh (D/2a) in which D/2 is the lateral

distance between the bond at the die and the apex of the wire loop. For d £ 2(H + h) , a good approximation for a is given by

d

2

4h(l + /l + (H/h)) + 2H

A graphical representation of eq (8) is shown in figure 4. Here, the tensile force in

the wire at the heel of the bond of a single-level, 1-mil diameter wire bond subjected to a centrifugal force of 10,000 g's is shown as a function of d for different values of d/h.

*[64U1], [65C5], [65R1], [66G1], [66L4], [66P1], [67G1], [68B1], [68D2], [6811], [68R2],

[69B2], [6901], [69S4], [70D3]. +See footnote on page 1.
1.0 2.0

d (mm)

Figure 4. Tensile force, Fw , in the wire at the heel of the bond of a single-level, 1-mil diameter wire bond subjected to a centrifugal force of 10,000 g's for different values of d/h. The left- and right-hand vertical scales are for gold and aluminum wire bonds,

respectively.

The tensile force for gold wire is shown on the left vertical axis and for aluminum wire on

the right vertical axis. As can be seen, Fw increases as d is increased but Fw decreases

as h is increased. Fw begins to increase with h for h a, d/3. To use the centrifuge test for accelerations much greater than about 50,000 g's re- quires that special fixturing be used to hold each device and incurs the risk of damaging

other satisfactory components of the device. Therefore, there are practical limitations on

the maximum acceleration that can be used. Because of this and the low density of aluminum,

only gold wire bonds can be screened satisfactorily with centrifuge tests. While the cen- trifuge test is widely used for this purpose there are some who believe that even for gold

wire the centrifuge test is only acceptable-for culling out grossly defective wire bonds

[67A1] . For example, using figure 4 and considering a gold wire bond with a separation of 0.15 cm and a loop height of 0.015 cm the tensile force in the gold wire for an acceleration of 30,000 g's will only be about 0.55 gf. As a basis of comparison, such a tensile
stress would be produced in a pull test if the hook, placed at mid-span, were pulled with

a force of 0.25 gf.

Judging whether a centrifuge test may be useful, as with many of the other tests, is dependent on the values of d and h of the wire bonds to be tested. When d is sufficiently

large and h sufficiently small the test may stress the wire bonds sufficiently in those

cases when the centrifugal force is directed away from the bonding surfaces. For tests where the centrifugal force is directed either into or parallel with the bonding surfaces

no calculations on the stresses imposed have been found in the literature. Again, however,

it would be expected that the larger is d the greater will be the stress applied to the wire bond simply because of the greater mass of the wire involved.

4. Mechanical Shock Test*

In a typical test the device is first accelerated either by free-fall or by pneumatic

means and then brought to a sudden halt on striking an impact pad. The test conditions

that may be specified are the maximum deceleration (usually a few thousand g's), the duration of the impact or shock pulse width (between 0.1 and 1.0 ms) , the direction of the stress (usually along one or more of the principal axes of the device package) , and the number of shocks per direction (usually less than ten)

.

In essence, the basic stresses imposed by a mechanical test to a wire bond result

from the amplitude and the number of vibrations induced in the wire loop by the mechanical

pulse. The number of vibrations depends on the damping of the wire loop. Thus, if the wire has been softened, through earlier exposure to high temperatures, fewer oscillations

with smaller amplitudes occur and the resulting flexure stress may be less. However,

irreversible changes occur in annealed wire at smaller bending and torsional stress than

in unannealed wire.

To discuss the test in terms of the magnitude of the induced deflection and stress,

it is useful to define a dynamic-to-static deflection or stress ratio, K:

_ maximum wire deflection or stress by a mechanical pulse

wire deflection or stress induced if peak acceleration

of mechanical pulse were applied statically

As a rule of thumb, the upper bound for K is 2 [48F1], [65R3, p. 368], [6711]. How much

less than 2 K is depends on the shape and duration of the pulse and on the lowest resonant

frequency of the wire loop. However unless the product of the shock duration and the resonant frequency of the wire bond is less than about 0.4, K is not less than 1 [6711]. The results of estimates made of the lowest frequency of wire bonds [72S2] indicate that this might occur for 1-mil diameter gold wire bonds with a bond separation, d, greater than about 2 mm and for 1-mil diameter aluminum wire bonds with d greater than about 3 mm. Changes in the shape of the excitation pulse will produce differences in K. Consequently the International Electrotechnical Commission (IEC) in its recommended test method

*[67I1], [68D2], [69B2], [69S4] , [70D3].

10
[6711] has required the specification of the shape of the shock pulse. For the purpose of reproducibility, the use of a saw-tooth shaped pulse is suggested because it produces the least variation in vibration amplitude with changes in pulse duration and resonant frequency. The IEC method also stresses the need to minimize any ripples on the shock pulse in order to maximize the reproducibility of the method; ripple is considered to be the most significant

distortion of the shock-pulse shape. Because K is no greater than 2, the stress imposed by a mechanical shock test will be less than twice the stress imposed by a centrifuge test at a constant acceleration equal to the peak shock-induced deceleration and directed in the same direction, neglecting the un- certain effect of the number of vibrations that are generated. Because the largest peak

acceleration used is generally no more than several thousand g's and considering the small stress that a centrifuge test at twice such an acceleration would impose, the mechanical

shock test does not greatly stress most wire bonds.

To make the test more severe by increasing the amplitude of the shock can result in deformation of the package at impact unless special supporting structures are used. In some cases, use of the method is hampered by the fact that some package designs do not readily allow means for support along one or more axes.

5. Variable Frequency Vibration Test*

In a typical test the device is vibrated sinusoidally through a range of frequencies

for one or more cycles to excite and rupture any wire bonds which have a resonant frequency

within the range swept. The selection of the frequency range is based on an estimate of the frequency components of the kinds of shock and vibrations the device may encounter in its life, subject to the constraints of test equipment available. The test conditions us- ually specified are the frequency range (typically from about 10 to 2000 Hz) ; the sweep

rate (order of minutes per octave); the duration or number of cycles (from a fraction to tens of hours or less than five cycles) ; the direction of the vibration (along each of the principal axes of the device); and the maximum acceleration (less than 100 g's), sometimes

with a limitation on the maximum amplitude at the lowest vibration frequencies thus re- ducing the peak acceleration there. The maximum vibration frequency used in most tests is 2 kHz. Except for gold wire

bonds with a bond separation greater than about 3 mm, the lowest resonant frequency of wire

bonds is greater than 2 kHz [72S2]. Hence, the dynamic deflection of the wire and the in- duced stress is at most twice as large as that induced if a constant acceleration equal to the peak acceleration were applied [65R3, pp. 368 and 370]. The maximum acceleration used

is usually less than 100 g's which produces vibrations in the wire with a negligible amplitude; hence the test does not stress wire bonds greatly. However if the maximum vibration

frequency is increased to 6 kHz [6611] wire bonds with bond separations smaller than about

3 mm may also be stressed significantly.

*[66L4], [6611], [68D2], [69S4], [70D3]

.

11
6. Vibration Fatigue Test*

The test involves vibrating the device at a fixed frequency, for many hours, along each of the three principal axes of the device package to determine if the wire bonds can withstand relatively long periods of low-frequency sinusoidal oscillations without metal

fatigue. The test conditions usually specified are the frequency (less than 100 Hz) , the duration (between about 30 to 100 h) , the direction of the vibration (along each of the principal axes of the device), and the peak acceleration (less than 100 g's). Because the vibration frequency is very much lower than the lowest resonant frequency

of any practical wire bond and the peak acceleration is essentially negligible, the vibration fatigue test is an ineffective way to cull out weak wire bonds.

7. Thermally-Induced Stress Tests"1

"

There is a variety of tests that involve heating the device and using the difference

in the coefficients of expansion in the component parts of the device to stress the wire

bond. Temperature cycling and thermal shock are two such tests which involve exposing the device alternately between two temperature extremes. The test is referred to as a thermal shock or a temperature cycling test depending on the transfer time between these two extremes. A transfer time for shock tests is of the order of seconds; for temperature

cycling tests it is of the order of minutes. The low- and high-temperature extremes of the range that is used in such tests vary, but the range usually extends at least as low

as -65°C and as high as 125°C. A power cycling test can also be used. In this test there is sufficient time during

the on- and off-power periods for the wire bond to approach thermal equilibrium. The stresses induced are less than in the first two tests because only the part of the wire nearest

the semiconductor die, where the power is dissipated, is heated appreciably. Phillips

[72B1] [72B3] has shown that the temperature gradient is linear along the wire between the die and the terminal of an unencapsulated device if joule heating in the wire is negligible.

Thermally-induced stresses can dislodge or shift poorly adhering bonds. To detect

such defective bonds an electrical continuity monitoring test is combined with a thermal

stress test, usually a temperature cycling test. Such tests are effective in detecting

intermittent open circuits in plastic encapsulated devices [71H2] [72F1]. In these de- vices, open circuits may appear when the device temperature is above or below a certain

temperature or in one or more ranges of temperature [71H2], The usual practice when using an electrical continuity test is to limit the peak test voltage to less than about 0.5 V. The reason for this is to avoid arcing, which would heal

some kinds of micro-openings in the wire bond. Such openings can occur, for example, in an annular ring of Kirkendall voids [70P2] [70R1] around a gold ball bond made on an expandedcontact aluminum metallized pad.

*[65C5], [68D2], [69S4], [70D3]

t[68D2], [70D3].

12
The electrically monitored tests are usually performed for only one cycle while the unmonitored tests often are continued for many cycles to determine if the wire at the heel

of the bond, where flexure is greatest, can sustain the flexing stress. The magnitude of

this flexing which is produced by changes in ambient temperature is dependent on the wire

shape and on the differences in the thermal coefficients of expansion of the wire and the package. The contact angle for the case where the wire loop is in the shape of a circular

arc is given by

sin Op

[i - Cflw - e s

) (t - t )], (10)

where 8, 9q = the final and initial contact angles, respectively,

8W , 6 S = the thermal coefficient of expansion of the wire and bonding surface material, respectively, (°C-1

), and

T, Tg = the final and initial ambient temperature, respectively (°C)

.

The extent of the wire span for a given contact angle can be characterized by the

loop height, h, and bond separation, d, with the use of the relation 8 = 2 tan-1 (2h/d)

.

The dependence, described in eq (10), of the change in contact angle 6 - 8 , on the ratio

of the initial loop height to initial bond separation, hg/do, is graphed in figure 5. As

u_c/> JOO

0.05 0.15 0.20 h /d

0.25 0.30

Figure 5. Change in contact angle, 8 - 8 , versus initial value of the ratio of the loop

height to bond separation, hg/do, ^ or a differential expansion between the wire and the bonding surface, (Bw - (3 S ) (T - T ), of .004. A scale parallel to the 8 - 8q is provided

to show the percentage change in hg/do f° r the change in contact angle.

13
may be seen, flexing increases for smaller values of hg/dg for a given change in temperature. Thus one may expect the wire bonds with relatively large values of hg/dg will not be stressed significantly by these tests. This observation extended to the design of the wire

bonds suggests that for devices intended for applications where the device must sustain a great many on-off power or temperature cycles the ratio hg/dg should be as large as practical.

8. Summary

The test stress to which the wire bond is subjected and hence the effectiveness of the test is dependent on the dimensions of the wire loop as well as on a number of other

factors. All too often the wire bond tested is insufficiently described to estimate the stress applied and hence to judge adequately the quality of the wire bond. This is particularly true of the pull test which is the most widely used destructive test. The method

is fast and easy to perform and it provides a number, the pull strength, for use as a measure of quality. However, the temptation is to use only this number without specifying

other data required to interpret the results of the test. In the centrifuge test the stress is significant only for gold wire bonds and even

then it is relatively small for the acceleration levels typically used unless the bond separation is large. Because of practical limitations on the peak decelerations, the mechanical shock

test is generally not an effective screen test for wire bonds. The stress to wire bonds

in a vibration fatigue test for any practical wire bond is insignificant because of the very small accelerations involved. The variable frequency vibration test, on the other

hand, may stress wire bonds significantly if their lowest resonant frequency is within the frequency range of the test. This may occur for gold wire bonds with large bond separations

and loop heights.

Thermally-induced stress tests may apply a significant stress to wire bonds with small

loop heights and where the differences in the thermal coef f icients-of-expansion of the con- stituent parts of the device are significant. This is especially true for tests involving many cycles

Acknowledgement

It is a pleasure to acknowledge and thank W. Murray Bullis who provided editorial

assistance and Marilyn L. Stream who typed the camera-copy draft.

14
References*

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64U1 FINAL REPORT FOR INTEGRATED CIRCUIT

STUDY, Contract No. NObsr 89341, Aug.

1964. AD 605432t

66P1 Partridge, J., E. C. Hall, and L. D. Hanley

THE APPLICATION OF FAILURE ANALYSIS

IN PROCURING AND SCREENING OF INTE- GRATED CIRCUITS, Physios of Failure

in Electronics, vol. 4, M. E. Goldberg and J. Vaccaro, Eds., RADC

Series in Reliability, 1966, pp. 95-

139. AD 637529

65C5 Cunningham, J. A. EXPANDED CONTACTS AND INTERCONNECTIONS

TO MONOLITHIC SILICON INTEGRATED CIRCUITS, Solid State Electron., vol. 8, pp. 735-745, April 1965.

65R1 Ruggiero, E. M. ALUMINUM BONDING IS KEY TO 40-WATT MICROCIRCUITS, Electronics, vol. 38,

pp. 98-104, Aug. 23, 1965.

65R3 Roark, R. J. FORMULAS FOR STRESS AND STRAIN, 4th

Ed., McGraw-Hill Book Co., New York,

1965.

65W1 Wasson, R. D. THERMOCOMPRESSION BOND TESTER, Proc.

IEEE, vol. 53, pp. 1736-1737, Nov.

1965.

66G1 Go, H. T., N. J. McAfee, and H. C. Jones MICROELECTRONICS RELIABILITY FROM A

SYSTEM MANUFACTURER'S POINT OF VIEW, Second Int. Symp. on Microelectronics;

Munich, Germany, Oct. 1966.

6611 BASIC ENVIRONMENTAL TESTING PROCEDURES

FOR ELECTRONIC COMPONENTS AND ELEC- TRONIC EQUIPMENT PART 2: TESTSTEST F: VIBRATION, IEC Recommendation, publication 68-2-6 (1966) and supplements 68-2-6A, 68-2-6B and

68-2-6C; 1966-1969.

66L4 Lombardi, J., L. McDonough, and H. Padden HIGH RELIABILITY SCREENING OF SEMI- CONDUCTOR AND INTEGRATED CIRCUIT

DEVICES, Final Rpt. , Contract NAS

5-9639, Sept. 1966. N67-16772.

66R1 Riben, A. R. , and S. L. Sherman MICROBONDS FOR HYBRID MICROCIRCUITS

PROGRESS REPORT, Rpt. 8 (Nov. 1, 1965-Jan. 31, 1966), Contract No. DA 36-039 AMC-03742 (E) , May 20, 1966. AD 633723

67A1 Arleth, J. M. , and R. D. Demenus NEW TEST FOR THERMOCOMPRESSION

MICROBONDS, Electron. Prod., vol. 9, pp. 92, 94, May 1967.

67G1 Gill, W. L., and W. Workman

RELIABILITY SCREENING PROCEDURES FOR

INTEGRATED CIRCUITS, Physics of Failure in Electronics, vol. 5, RADC Series

in Reliability, T. S. Shilliday and

J. Vaccaro, Eds., 1967, pp. 101-141. AD 655397

67H1 Higbie, T. E. THERMOCOMPRESSION BONDING OF GOLD WIRE FOR MICROELECTRONIC CIRCUITS, Report

No. NAFI-TR-1108, Oct. 1967. AD

671879

6711 BASIC ENVIRONMENTAL TESTING PROCEDURES FOR ELECTRONIC COMPONENTS AND ELECTRONIC EQUIPMENT PART 2: TESTSTEST EA: ; HOCK, IEC Recommendation,

publication 68-2-27 and supplement

68-2-27A; 1967-1968.

67R1 Riben, A. R. , and S. L. Sherman MICROBONDS FOR HYBRID MICROCIRCUITS,

Physics of Failure in Electronics

,

vol. 5, RADC Series in Reliability,

T. S. Shilliday and J. Vaccaro, Eds.,

1967, pp. 534-556. AD 655397

67S3 Schile, R. D., and G. A. Rosica

SIMPLE TESTER FOR THE RAPID

identification codes used for references are taken from a comprehensive survey paper on wire bonds [72S2],

^Reports available from the National Technical Information Service (NTIS) , Sills Building,

5285 Port Royal Road, Springfield, Virginia 22151, are followed by a number preceded by

the letters AD or PB, or the letter N. This is the NTIS Accession number which should be

used when ordering.

15
68A1

68B1

68D2

6811

68R2

DETERMINATION OF THE TENSILE STRENGTH

OF FINE FILAMENTS, Rev. Sci. Instr.,

vol. 38, pp. 1103-1104, Aug. 1967.

Adams, A. H. , and J. H. Anderson, Jr. MEANS FOR GRIPPING FINE WIRES DURING MECHANICAL TESTS, Rev. Sci. Instr.,

vol. 39, p. 1768, Nov. 1968.

Berry, W. P. M. Hall, and M. T. Harris THIN FILM TECHNOLOGY, Van Nostrand

Reinhold Co., Princeton, New Jersey,

1968, pp. 604-632.

TEST METHODS AND PROCEDURES FOR MICRO- ELECTRONICS, Military Standard 883,

May 1, 1968; Notice 1, May 20, 1968; Notice 2, Nov. 20, 1969.

BASIC ENVIRONMENTAL TESTING PROCEDURES FOR ELECTRONIC COMPONENTS AND

ELECTRONIC EQUIPMENT PART 2: TESTSTEST GA: ACCELERATION, STEADY STATE,

IEC Recommendation, publication 68-2-

7, 1968.

Ruggiero, E. M. ALUMINUM BONDING FOR HIGH-POWER IC's,

Miaroel&otronia Packaging, George

Sideris, Ed., McGraw-Hill, New York,

1968, chapt. 7.3, pp. 240-248.

69A1 Ang, C. Y. , P. H. Eisenberg, and

H. C. Mattraw

PHYSICS OF CONTROL OF ELECTRONIC

DEVICES, Proc. 1969 Annual Symp. on Reliability, Chicago, 111., pp. 73-

85, Jan. 1969.

69B2 Binelli, W. D. , and R. H. Soltau DEVELOPMENT OF QUALIFICATION TEST PROGRAM FOR MICROELECTRONIC DEVICES,

Final Rpt. (Nov. 1, 1968 to July 3, 1969), Contract No. NAS 1-8714,

Sept. 1969. N70-11544

69B5 Bullis, W. M., Ed. METHODS OF MEASUREMENT FOR SEMICON- DUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES, NBS Technical Note 488,

Quarterly Rpt. (Jan. 1 to March 31, 1969), July 1969.

69K1 Kashiwabara, M. , S. Nakayama, and

M. Suzuki SETTING AND EVALUATION OF ULTRASONIC BONDING FOR AL WIRE, Rev. Elec. Commun. Lab., vol. 17, pp. 1014-1021,

Sept. 1969.

6901 O'Connell, E. P. AN INTRODUCTION TO MIL-STD-883 TEST

METHODS AND PROCEDURES FOR MICROELECTRONICS , Proc. 8th Reliability

and Maintainability Conf., Denver,

Colorado, pp. 530-542, July 1969.

69S1 Slemmons, J. W. THE MICROWORLD OF JOINING TECHNOLOGY,

American Welding Society 50th Annual

Meeting and Welding Exposition;

Philadelphia, Pa., April-May 1969.

69S4 Shurtleff, W. 0. RELIABILITY HANDBOOK FOR SILICON MONOLITHIC MICROCIRCUITS VOLUME 2 —

FAILURE MECHANISMS OF MONOLITHIC

MICROCIRCUITS, Contract No. NAS 8- 20639, April 1969. N69-23226

70A1 Adams, M. A. AN INVESTIGATION OF THE STRENGTH OF ALUMINUM WIRE USED IN INTEGRATED CIRCUITS, NASA Tech. Brief 70-10275,

Aug. 1970.

70B8 Bullis, W. M. , and A. J. Baroody, Jr.,

Eds. METHODS OF MEASUREMENT FOR SEMICON- DUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES, NBS Technical Note 560,

Quarterly Rpt. (April 1 to June 30,

1970), Nov. 1970.

70D3 MILITARY STANDARD — TEST METHODS FOR

SEMICONDUCTOR DEVICES, Military

Standard 750B, Feb. 27, 1970.

70P1 Pankratz, J. M. , and D. R. Collins A COMPARISON OF 1% MG-AL AND 1% SI-AL WIRE INTERCONNECTS, 8th Annual Proc. Reliability Physics, IEEE Catalog No. 70C59-PHY, pp. 163-169, 1970. [also

published in IEEE Trans. Reliability,

vol. R-19, pp. 89-94, Aug. 1970]

70P2 Philofsky, E. INTERMETALLIC FORMATION IN GOLD- ALUMINUM SYSTEMS, Solid-State

Electron., vol. 13, pp. 1391-1399,

Oct. 1970.

70R1 Rossiter, T. J. AMBIENT EFFECTS ON GOLD-ALUMINUM

SYSTEMS, Reliability Physics, Las Vegas, Nevada, IEEE Catalog No. 70C59-PHY, 1970, pp. 186-190.

71B1 Bullis, W. M., Ed. METHODS OF MEASUREMENT FOR SEMICON- DUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES, NBS Technical Note 571,

Quarterly Rpt. (July 1 to Sept. 30, 1970), April 1971.

16
71B4 Bullis, W. M. , Ed. METHODS OF MEASUREMENT FOR SEMICON- DUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES, NBS Technical Note 592,

Quarterly Rpt. (Oct. 1 to Dec. 31, 1970), Aug. 1971.

71H2 Haberer, J. R. TECHNIQUES FOR DETECTING STRESS INDUCED INTERMITTENT FAILURES IN ENCAPSULATED DEVICES, IEEE Intern.

Conv. Digest, New York, Session 7CJ,

pp. 612-613, March 1971.

73B1 Bullis, W. M. , Ed. METHODS OF MEASUREMENT FOR SEMICON- DUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES, NBS Technical Note 754,

Quarterly Rpt. (July 1 to Sept. 30, 1972), March 1973.

73B2 Bullis, W. M. , Ed. METHODS OF MEASUREMENT FOR SEMICON- DUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES, NBS Technical Note 773,

Quarterly Rpt. (Oct. 1 to Dec. 31, 1972), June 1973.

71P3 Polcari, S. M. , and J. J. Bowe EVALUATION OF NONDESTRUCTIVE TENSILE

TESTING, Report No. DOT-TSC-NASA-71-

10, pp. 1-46, June 1971. N71-37516

72B1 Bullis, W. M., Ed. METHODS OF MEASUREMENT FOR SEMICON- DUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES, NBS Technical Note 717,

Quarterly Rpt. (July 1 to Sept. 30, 1971), April 1972.

72B2 Bullis, W. M. , Ed. METHODS OF MEASUREMENT FOR SEMICON- DUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES, NBS Technical Note 727,

Quarterly Rpt. (Oct. 1 to Dec. 31,

1971), June 1972.

72B3 Bullis, W. M., Ed. METHODS OF MEASUREMENT FOR SEMICON- DUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES, NBS Technical Note 733,

Quarterly Rpt. (Jan. 1 to March 31, 1972), August 1972.

72B4 Bullis, W. M. , Ed. METHODS OF MEASUREMENT FOR SEMICON- DUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES, NBS Technical Note 743 Quarterly Rpt. (April 1 to June 30, 1972) , December 1972.

72F1 Feldt, J. E. , and E. R. Hnatek

HIGH RELIABILITY CONSUMER IC's, 10th Annual Proc. Reliability Physics

Symp., IEEE Catalog No. 72CH0628-8-

PHY, pp. 78-81, 1972.

72S2 Schafft, H. A. TESTING AND FABRICATION OF WIRE-BOND ELECTRICAL CONNECTIONS — A COMPRE- HENSIVE SURVEY, NBS Technical Note

726, September 1972.

U. S. GOVERNMENT PRINTING OFFICE :1973— 542-650/60

17

form NBS-114A (1-711 U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS-TN-786

2. Gov't Accession No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Methods for Testing Wire-Bond Electrical Connections

5. Publication Date November, 1973

6. Performing Organization Code

7. AUTHOR(S)

Harry A. Schafft

8. Performing Organization

10. Project/Task/Work Unit No. 9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Addtess National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

13. Type of Report & Pefiod Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliogtaphy or litetatute sutvey, mention it here.)

A significant fraction of the failures that occur in integrared circuits are due to failures of the wire-bond electrical connections that are used. Therefore, a critical area for reliability improvement is in the methods for testing and evaluating wire bonds. Several of these

methods are surveyed. In particular, analyses with regard to the stress

that the test imposes on the wire bond in the pull, centrifuge, mechanical

shock, vibration, and temperature cycling tests are presented and used in

discussing the capabilities and limitations of these methods.

17. KEY WORDS (Alphabetical order, separated by semicolons) Bonding; electrical connection; failure (wire bond); integrated circuits; microelectronics; reliability; semiconductor devices; testing (wire bond); wire bond.

18. AVAILABILITY STATEMENT

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