Bond signature analyzer

A feedback signal of an ultrasonic generator (13) employed in a ball bonder (11) is sampled by an analog-to-digital converter (23) under control of a microprocessor (25) to provide a "bond signature" indicative of bond quality. The microprocessor (25) then compares the bond signature to limits representing a good quality bond to provide an indication of a good or failed bond. The indication may be visually provided by projecting the bond signature and limits on a display (27), as well as automatically provided by a programmed comparison.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The subject invention relates generally to electrical circuit fabrication 
and, more particularly, to a method and apparatus for determining the 
integrity of wirebonds formed by wirebonding apparatus. 
2. Description of Related Art 
Presently, wirebonding to integrated circuitry is performed by several 
methods, including ball bonding and wedge bonding. A typical ball bonding 
operation forms a "ball bond" to an IC and a "stitch bond" to a second 
bond site, followed by a "stitch tear." 
Various defects are known to occur in such ball bonding procedures, 
including lifted balls, misregistered balls, improper ball size, 
cratering, and defective stitches. The "lifted" ball bond does not 
adequately weld to the die bond pad or conductor. Contamination or a 
misapplication of the bond force component is usually the cause of lifted 
balls. The misregistered ball is placed disproportionately on the ball 
bond pad site and an adjacent material, typically either silicon dioxide 
glassivation or dielectric. Hence, only a portion of the entire ball bond 
area is actually welded during the bond cycle. Additionally, the ball bond 
size may be either too small or too large. 
A cratered die bond pad occurs explosively during the ball bond cycle. 
Adequate coupling takes place in the early stages of the ball bond cycle, 
then suddenly, high shear forces cause the underlying silicon to break 
away. The ball bond is thus left unsupported. 
Defective stitches include cut stitches and lifted stitches. With a cut 
stitch, the wire tail has already been released from the bond site due to 
overbonding. On the other hand, with the lifted stitch, the wire at the 
stitch has been underbonded and a greater force is required to separate 
the wire tail from the stitch end of the bond. 
Given the foregoing and other potential defects, wirebonds formed by 
wirebonding apparatus are typically required to go through laborious and 
expensive testing. For example, current military requirements specify 
internal visual inspection for determining the integrity of wirebonds and 
other microcircuit interconnections. Visual inspection of wirebonds has 
become a costly task, for example, requiring approximately eight 
inspection stations operating two shifts per day at some facilities. 
Production operators at these stations may review hundreds of thousands of 
initial build wirebonds and rework build wirebonds in a normal week. Some 
military requirements also require a 100% nondestructive wire pull test of 
all wirebonds. Wire pull takes longer than visual inspection and is prone 
to cause damage to the product. Another useful test is the simple 
determination of the presence of a wire in the bond tool during bonding, 
e.g., during wirelooping and stitch bond formation. 
All of the foregoing extensive testing results in detection of a relatively 
small number of defective wirebonds. For example, the quantity of 
defective wirebonds on initial-build hybrids at one typical facility was 
calculated at 0.24% of the total quantity of wirebonds installed. In this 
instance, the capability to identify bonds likely to be visually marginal 
would effectively reduce the quantity of wirebonds required to be 
inspected and dispositioned for rework from 16,250,000 bonds to only 
39,000 bonds on initial build hybrids 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an improved method 
and apparatus for detecting defective wirebonds; 
It is another object of the invention to reduce or eliminate the need for 
visual inspection of wirebonds; 
It is another object of the invention to reduce or eliminate or reduce the 
need for 100% wire pull tests of wirebonds; 
It is another object of the invention to provide an improved method and 
apparatus for detecting defective wirebonds which is automated and 
suitable for application in expert systems; 
It is another objective of the invention to provide an improved method and 
apparatus for use in wirebond process (bond schedule) generation and 
control; and 
It is another objective of the invention to provide an improved method and 
apparatus useful for maintenance and field service repairs. 
According to the invention, a signal containing information indicative of 
bond quality is tapped from the wirebonder control system and is sampled 
and digitally processed to determine bond quality. The tapped signal is a 
signal indicative of power coupled to the bond site. For example, the 
signal may be one indicative of changes in feedback required to stabilize 
ultrasonic energy used to vibrate a bonding tool during the bonding 
process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates Z axis tool motion in a typical ball bonding procedure. 
A first ball bond 101 is made to a component 106, the wire is pulled off, 
looped 109, and moved to a second bond site, e.g., a terminal lead. At the 
second bond site, the second or "stitch" bond 107 is made, and the wire is 
flamed off. The flame-off process includes the steps of pulling the tail 
after the second bond is made, closing the wire clamp and breaking the 
tail off the second bond 107, moving to the flame-off position, and firing 
the flame-off system. Generally, the ball bond 101 is made to an IC pad 
and the stitch bond 107 to substrate gold, but reverse bonding can be 
done, as known in the art. 
A wirebonder 11 employing an ultrasonic generator 13 for performing such 
wirebonding is shown schematically in FIG. 2. One such 
commercially-available wirebonder 11 is the Model HMC 2460 thermosonic 
wirebonder available from Hughes Aircraft Company. An ultrasonic generator 
which has been employed in that wirebonder is one manufactured by 
Orthodyne Inc. of Costa Mesa, CA. The wirebonder 11 of FIG. 2 further 
includes an ultrasonic transducer 15, an ultrasonic feedhorn 18, and a 
bond tool or capillary 17. Mechanical force and ultrasonic energy are 
applied to the capillary 17 to create a wirebond between a bond surface 19 
and a gold ball 20 having a diameter of, e.g., 0.7 to 2.0 mils. 
Wirebonding by the bonder 11 is controlled by a controller 12 which, in 
the HMC 2460 bonder, comprises a number of programmed microprocessors. 
In operation, the wirebonder 11 applies heat to the bond site via a heated 
stage located underneath the bond surface 19. Force is applied by pressing 
the capillary 17 down toward the bond surface 19. An ultrasonic signal is 
generated by the generator 13 and converted to a mechanical ultrasonic 
frequency vibration of the feedhorn 18 by the transducer 15. The capillary 
17 transmits the ultrasonic energy and downward force, effectively 
"scrubbing" the gold ball 20 against the bond surface 19. The combination 
of heat, force and ultrasonic vibration causes bonding between the bond 
surface 19 and the gold ball 20. 
As the capillary 17 presses the gold ball 20 into the bond surface 19, the 
amplitude of the ultrasonic vibration imparted through the feedhorn 18 is 
dampened according to the coefficient of coupling between the tool 17 and 
the gold ball 20 and between the gold ball 20 and the bond surface 19. 
This coefficient of coupling varies with the type of material, force, 
temperature, contamination and other variables. 
A feedback loop in the ultrasonic generator 13 detects any variation in the 
resonance of the ultrasonic feedhorn 18. The feedback system then controls 
the energy to the feedhorn to provide the desired ultrasonic oscillation. 
The feedback loop of the ultrasonic generator 13 generates a DC feedback 
correction signal V.sub.C, which is tapped to provide a direct indication 
of the mechanical coherence between the transducer 15, the tool 17 and the 
bond surface 19. 
According to the preferred embodiment, this voltage V.sub.C is converted 
into a sequence of digital values by an off-the-shelf analog-to-digital 
converter apparatus 23, such as Part No. DAS 16G available from Metrabyte 
Corp. of Taunton, MA. This voltage V.sub.C over time for a particular bond 
is referred to as the "bond signature" of that bond. In the preferred 
embodiment, the digital representation of the analog waveform V.sub.C at 
the output 24 of the A/D converter 23 comprises a series of digital 
samples which represent the signal V.sub.C over time. These samples are 
taken and analyzed under control of a BSA microprocessor 25 to determine 
bond quality. The BSA microprocessor is preferably an IBM personal 
computer (PC) or PC compatible clone. 
The signal V.sub.C is a representation of the power coupled to the bond 
site. This power varies with conditions at the bond site, i.e., the 
aforementioned "coefficient of coupling" and, hence, incorporates both the 
drive function provided to the tool 17 and the real world reaction to the 
drive function. In the preferred embodiment, both the ball and stitch 
bonds 101, 107 are made in what is referred to as the constant current 
mode. 
As illustrated in FIG. 4, in a constant current mode embodiment, the signal 
V.sub.C is generated from a signal produced by applying a sample of the 
transducer current taken by a transformer T2 to a synchronous detector 61 
to generate a D.C. level directly proportional to the amplitude level of 
the tool 17. The D.C. level is then applied to a summing amplifier 63 
which, in the constant current mode, subtracts the D.C. level from the 
nominal D.C. level set by a computer and outputs the correction signal 
V.sub.C. The nominal D.C. level is set in a data buffer 65, converted to 
analog form by a digital-to-analog converter 67, and applied through an 
amplifier 69 to the summing amplifier 63. An ultrasonic generator 
employing such circuitry is disclosed in U.S. Pat. No. 4,824,005 
incorporated by reference herein. 
Another mode of ultrasonic generator operation well-known to those skilled 
in the art is the constant voltage mode operation. The signal V.sub.C can 
also be derived in this mode. In general, those skilled in the art will 
appreciate from the foregoing that various signal points in various 
ultrasonic generators can provide a signal or bond signature 
"representative of the power coupled to the bond site," as that phrase is 
used in this disclosure. 
FIG. 3 shows an "ideal" bond signature resulting from the procedure of FIG. 
2. An ideal bond signature shows a positive slope at the ball and stitch 
bond indicating an increasing transducer voltage drive function 
requirement at these points. This is believed to be caused by (1) an 
increase in cross-sectional area of the bond and increased tool contact as 
the capillary force is applied downward, and (2) an increase in the 
rigidity of the bond caused by the welding action of the mating surfaces. 
The ideal stitch bond signature 113 has a voltage requirement 
characteristic similar to the ideal ball bond signature 111, but is 
usually of greater magnitude. 
An added signal on the stitch side of the bond sequence is the stitch tear 
spike 115 (FIG. 3). Following the formation of the stitch bond, the bond 
head raises up slightly, leaving a small wire tail. The wire clamping 
mechanism closes, and the bond head moves rapidly upward. This operation 
releases the wire tail from the stitch bond, while leaving a portion of 
wire hanging out of the capillary for subsequent flame-off and ball 
formation. This parting action between the wire tail and the previous 
stitch bond leaves a characteristic spike 115 in the bond signature. 
The bond signatures of various defective bonds differ from those of "good" 
bonds. The lifted ball bond does not adequately weld to the die bond pad 
or conductor, and, with some bonding tools, no additional transducer 
voltage requirement is seen in the bond signature. A misregistered ball 
may result in a flatter transducer voltage requirement because only a 
portion of the entire ball bond area was actually welded during the bond 
cycle. The insufficient/excessive size ball bond will take lower and 
higher transducer voltage requirements, respectively. 
With the cratered die bond pad, adequate coupling takes place in the early 
stages of the ball bond cycle, then suddenly, high shear forces cause the 
underlying silicon to break away. The ball bond is unsupported and the 
voltage requirement drops rapidly. 
A cut stitch has a different signature when compared to the normal stitch 
because the wire tail has already been released from the bond site due to 
over-bonding. On the other hand, the lifted stitch bond gives a different 
tear signature because the wire at the stitch has been underbonded and a 
greater force is required to separate the wire tail from the stitch end of 
the bond, which may not occur if the stitch does not stick adequately. 
Thus, by examining bond signatures one may distinguish a "good" bond from a 
defective one. The preferred embodiment implements analysis of bond 
signatures under microprocessor control, integrated with the bond sequence 
of the bonder 11 of FIG. 2 
A bond sequence (BS) employing the HMC 2460 bonder according to the 
preferred embodiment proceeds as follows. Except for Steps BS 3, BS 11 and 
BS 14, the following procedure is conventional. 
Step BS 1: The X/Y table of the bonder is driven to a first bond position 
and the bonding tool 17 is moved along the Z axis to the "search" position 
at high speed with medium force. A first bonding force is applied and the 
tool 17 continues its Z motion at a first bond search speed to the bonding 
surface, where bond 101 is to be formed. 
Step BS 2: Delay 10-15 milliseconds. 
Step BS 3: The BSA processor 25 is triggered to begin acquiring data. The 
HM 2460 control processor 12 ramps up the power level of the ultrasonic 
generator 13 to the programmed level and continues to apply ultrasonic 
energy for the programmed first bond time (typically 20-40 msec) during 
which the ball bond 101 is to be formed. During this first bond time, the 
BSA processor 25 has controlled the A/D converter 23 to cause sampling of 
the bond signature V.sub.C and has caused storage of the samples in random 
access memory (RAM) or on disk or both, as discussed in more detail below. 
A 10-msec sampling period, starting at the beginning of the first bond 
time, has been used with success, although other periods can be used, 
depending upon the analysis desired. 
Step BS 4: The bonder controller 12 turns off ultrasonic energy, reduces 
force, and moves the bonding tool 17 up off the first bond 101. The Bond 
Signature Analysis system is done capturing data before the end of BS 3. 
The system can be commanded to capture data into BS 4, but normally it is 
not. 
Steps BS 5-8: The X/Y/Z positioning system moves the tool through various 
programmable "loop" modes to shape the wire as it is fed out. The X/Y 
position is at the second bond position 107 at the end of these sequences. 
Step BS 9: As in Step BS 1, the tool is moved to search height at high 
speed with medium force. The second bonding force is applied and the Z 
axis motion continues to the bonding surface at search speed, where bond 
107 is to be formed. p0 Step BS 10: Delay 10-15 milliseconds. 
Step BS 11: The BSA processor 25 again begins acquiring data. Ultrasonics 
energy is again ramped up to the programmed level and applied for a 
programmed second bond time. 
Step BS 12: As in BS 4, ultrasonic energy is turned off. The BSA processor 
25 is done capturing data. The tool moves along the Z-axis to tail pull 
position. 
Step BS 13: The wire clamp is closed, followed by a short delay. 
Step BS 14: The BSA processor 25 captures tail-pull information as the tool 
moves to EFO (electric flame-off) position. 
Step BS 15: The EFO wand is energized and a delay (for settling) occurs. 
Step BS 16: The EFO is fired for a programmable amount of time. 
Step BS 17: Short delay. 
Step BS 18: Turn off the EFO wand. 
Step BS 3, implementing the preferred embodiment, will now be described in 
more detail in connection with FIGS. 5 and 6. FIG. 5 is a flow chart 
illustrating the bond signature analysis procedure, while FIG. 6 
illustrates an analog bond signature V.sub.C plotted as curve 41. 
In Step 51 of FIG. 5, the A/D converter 23 samples the V.sub.C curve 41, as 
indicated by the vertical hash marks along curve 41 in FIG. 6. As 
illustrated, V.sub.C typically ramps up, peaks, and then slowly ramps up 
again for the period of the ball bond 101. The initial ramp-up is 
coincident with the ramping up of ultrasonic energy. The A/D converter 23 
is controlled by the BSA processor 25 to sample a selected period of from 
one to hundreds of milliseconds at a sampling rate of from DC to 70 kHz 
with the Metrabyte A/D card, with a maximum of 256 samples normally being 
taken. Many more samples could be taken at the expense of speed. The 
processor 25 is programmed to supply a start signal to the A/D converter 
23, as well as signals indicative of the sampling frequency and sample 
number, as hereafter described in more detail. The processor 25 also 
selects an A/D gain setting, e.g., 1, 2, 4 or 8.times., to ensure the 
display fits on the CRT display 27. 
The acquisition of data, i.e., sampling of V.sub.C, is controlled as 
follows. The bonder controller 12 supplies one of three control or toggle 
signals T.sub.0, T.sub.1, T.sub.2 to parallel control lines 28. These 
control lines 28 supply the respective signals T.sub.0, T.sub.1, T.sub.2 
to the A/D converter 23, while control lines 29 allow communications 
between the controller 12 and the BSA processor 25. These signals indicate 
that the bonder 11 is preparing to perform the ball bond, stitch bond, or 
tail formation, respectively. The signals T.sub.0, T.sub.1, T.sub.2 cause 
sampling by the A/D converter 23 to begin. For example, when the signal 
T.sub.0 appears, the A/D converter 23 samples the ball bond signature. The 
sampling rate and number of samples is set by a command line in the BSA 
processor 25. For example, the symbol pairs b, x; s, y; and t, z; may be 
used to denote the respective sample number and sample frequency assigned 
to each respective control signal T.sub.0, T.sub.1, T.sub.2. Accordingly, 
selected sample numbers b, s, t and sample frequencies x, y, z are 
programmed for acquisition of each of the respective ball, stitch, and 
tail formations. 
In Step BS 3, the BSA processor 25 causes the A/D converter 23 to take each 
sample as described, and causes each sample to be stored in random access 
semiconductor RAM with the option to store to disk later, Step 53 of FIG. 
5. The processor 25 then begins a procedure to qualify the data, i.e., to 
determine if the samples of V.sub.C fall within acceptable limits. The 
processor 25 acquires and stores the samples in a high speed direct memory 
access (DMA) mode, well-known to those skilled in the art. 
In Step 55, the data is qualified. The qualification procedure is performed 
before Step BS 4, i.e., well before the second bond is made. Optionally, 
qualification can be performed between each wire. In this manner, it may 
be determined whether one wire was properly bonded before bonding of 
another wire begins. Failure information is supplied to the controller 12 
from the BSA processor 25 over serial communications lines 29. 
To qualify the data according to the presently preferred embodiment, 
sampled data is displayed on the CRT display 27, resulting in a display of 
points following, for example, the curve 41 illustrated in FIG. 6. These 
points represent changes in feedback within the ultrasonic system 13. The 
limits 45, 49 define a "window" within which acceptable results fall. The 
limits 45, 49 are drawn on the display screen 27 with a mouse according to 
conventional procedures, providing a visual indication of whether the bond 
was acceptable. In an embodiment presently intended for commercial use, 
the limits 45, 49 are determined heuristically. Limit line or curve 49 is 
typically spaced the same distance from the ideal bond signature line 41 
as limit line 45. The heuristic limit curves 45, 49 may be determined by 
looking at the limits within which a dozen or so good bonds fall, and may 
be further refined by looking at the signatures of a few failed bonds. In 
addition to the visual indication provided by the display 27, the BSA 
processor 25 automatically determines whether or not a sampled value of 
V.sub.C falls outside the limits 45, 49 by a standard compare operation. 
In an alternative embodiment, the BSA processor is programmed to learn the 
tolerance levels represented by limit lines 45, 49 through examination of 
the limits within which the samples of the signatures of a selected number 
of good bonds fall. The processor 25 again does a limit comparison to 
detect a failure automatically. A failure signal is generated and brought 
to the user's attention either audibly or visually or by stopping the 
bonding equipment. The processor 25 may also be programmed to implement 
other qualifying analyses, such as "area under the curve," slope angle, 
and so forth. A "failure mode" may also be defined programmatically by 
limits 45, 49 and tested for by the BSA computer 25. For example, counting 
of 30 points in a row outside the window defined by limits 45, 49 may be 
defined as a failure. One may then define how many such failures are 
required to generate a failure event and, hence, a failure signal. 
The procedure of BS 3 is again followed in Step BS 11 and step BS 14. 
The just-described procedure for controlling the A/D converter 23 and 
qualifying the data could be incorporated in the HMC 2460 microprocessor 
controller 12 rather than in a separate processor 25, as shown in FIG. 1. 
The procedure according to the preferred embodiment employing 
heuristically determined limits has resulted in detection of 98.2% of "no 
stick" failures, wherein the ball 20 does not stick to the substrate 19, 
and about 90% of all failure modes. The method according to the preferred 
embodiment is also applicable to wedge and TAB (tape automated) bonding 
and to various other welding and bonding processes. 
According to the preferred method, only those bonds with unsatisfactory 
signatures need be subjected to a pull strength test and visual 
inspection, greatly reducing cost. Also according to the preferred method, 
the absence of a wire is positively detected because the bond signature 
generated will be unacceptable, i.e., out of limits. 
Numerous additional features, modifications, and adaptations may be 
provided without departing from the scope and spirit of the invention. 
Various command switches can provide numerous user-selectable 
manipulations of the data samples. For example, a command switch may be 
provided to transfer any waveform out to a CRT or printer. A command 
switch may be provided to cause any large discontinuities to be smoothed 
out. A sensitivity check can be provided, for example, if the tolerance is 
within "5 millivolts" do not use smoothing. All programs are preferably 
such that the numeric limits can be changed. 
A command switch may be provided to perform a Bezier smoothing algorithm 
with a weighted forward and backward look, such algorithm itself being 
known in the art. An averaging capability may also be provided to do a 
prime average calculation. 
Whether a signature "passes" may be judged by several methods. For example, 
1,000 ball or stitch signatures may be captured and displayed all at once, 
giving a known readable high and low. Then a mouse can be used on the 
display screen to specify upper and lower control limits, similar to the 
approach described above. Another technique, readily implementable in the 
processor 25, is to use standard XR (X bar R) analysis with a window at 
"1/2 .sigma." or a ".sigma." off of the 1,000 samples. 
Additionally, since the A/D card 23 has 16 inputs and is a 16-channel 
switch, it may receive inputs from several bonders or several other inputs 
such as force, temperature, etc., from just one bonder. In such an 
application, the switches are closed by the microprocessor 25, according 
to its particular programming. 
The preferred embodiment thus admits of numerous advantages. It detects 
improper bonds, signifies failure in the absence of a wire in the bonding 
tool, and significantly reduces the need for visual inspections, pull, and 
strength tests, setup, process generation and control, maintenance, and 
field service. A menu of acceptable signals may be provided, resulting in 
a system which is user-programmable to allow for changes in materials, 
bonding tools and machine settings. Such a system may further disposition 
circuits for rework or visual bond inspection automatically based upon 
unacceptable bond signatures. 
The system of the preferred embodiment suggests itself for application in 
testing for unacceptable ultrasonic characteristics and signalling for 
tooling changes only when they are warranted. The approach also suggests 
itself for artificial intelligence applications and expert systems. 
As is apparent, the described preferred embodiment admits of numerous 
modifications and adaptations without departing from the scope and spirit 
of the invention. Thus, within the scope of the appended claims, the 
invention may be practiced other than as specifically described herein.