Vibration-induced Fatigue Failures in Bonding Wires Used in Stacked Chip Modules

Henning Leidecker Scott Hull Goddard Space Flight Center 
Code 562
 Greenbelt, MD 20771 
301-286-9180 (voice) 
301-286-1695 (FAX) me@leidecker.gsfc.nasa.gov
Abstract
An instrument recently constructed at Goddard Space Flight Center involved stacked chip memory modules having about six thousand 1 mil gold wires, each between 4.6 mm and 6.1 mm long. Four bond wires fractured during the final box-level random vibration test, which had spectral power to 850 Hz. We determined that these wires have fundamental resonant frequencies in the range from 1.4 kHz to 2.3 kHz, with quality factors up to 300, when the bonds at each end are rigid. However, a nearly severed top bond decreases the lead's fundamental frequency into the range from 600 Hz to 750 Hz. The lowered mode is strongly excited by the box-level vibration test, leading to fatigue-fracture of that already weakened bond during the several minutes of the vibration test However, such a lowered mode would not be excited by previous shock and vibration tests, since these happened to have no spectral power in the region from 600 Hz to 750 Hz. In separate tests, an unfractured 1 mil gold wire was still intact after 12 days (1.4 billion oscillations) at 1.4 kHz at a root-strain of 0.2%, while several (deliberately) nearly fractured wires failed after less than a minute (less than 41,000 oscillations) at 680 Hz at the same drive stress.

Key words: resonance, fatigue, fracture, wire bond. Introduction
Goddard Space Flight Center is developing solid state recorders to augment and possibly replace mechanical tape recorders used in spacecraft. A solid state recorder is a computer with 10 or more gigabits (Gbit) of random access memory, interfaced to the spacecraft's   data-collection,   transmission,   and command centers.   DRAM of good commercial heritage is used, and special attention is given to ensuring radiation-tolerant behavior; this includes robust error sensing and correcting circuits. The DRAM is a stacked chip design that has steadily evolved in complexity as experience has given increased confidence in the basic design. The units chosen for a recently developed solid state recorder (hence called the SSR) achieve 320 megabits (Mbit) per module; this involved increasing the height of each of the two die stacks per module to 12 die. The memory chips used in the modules are each 16 Mbit, giving 384 Mbit per module; this allows defective elements (if any) to be masked out and still attain the specified 320 Mbit, increasing the yield. Higher density chips are available, but their radiation

resistance is lower at this time. There are 72 modules in the SSR, for a total of 23 Gbit.
The modules passed space flight quality screens, including a steady acceleration of 5,000 G's. The SSR passed a box-level sine burst test: 20 G's at 20 Hz for 1.6 seconds. However, the SSR no longer functioned after a random vibration test of 6.0 Grms with spectral power extending to 850 Hz.  This random vibration test is an accurate predictor of some of the stresses imposed on the SSR during launch. Thus, it was necessary to identify the root cause of the failure, and to fix it.
Investigation
The failure was traced to four of the seventy-two DRAM modules: three had failed with an electrical signature pointing to an open bond wire that is supposed to connect a pad at the top of the stack to the substrate bonding platform of the module. The fourth demonstrated an intermittent electrical defect during random vibration only. Inspection after de-lidding confirmed that each had a

stack wire that was ruptured at the top bond: the stack wire was electrically disconnected at the top end. The intermittent behavior was caused by sporadic shorting of a stack wire against a neighbor. The local geometry around the other three broken stack wires precluded shorting.
Each module contains 80 stack wires (40 per Stack) of 25.4 µm (=1 mil) gold wire, bonded to gold pads on a tungsten subpad.  See Figure 1 for a cartoon; not all the wires are shown.

Figure 1. A cartoon of the DRAM module
While there are other wire bonds within the module, they were all flaw-free and will not be discussed; in the following, "wire" refers to the stack wire only. There are a total of 5760 wires in the SSR modules.  The wires used in these modules are unusual in two respects. Most crescent bonds used in microcircuit applications are located at the package-(or substrate bonding platform-, if a platform is used) end of the wire, and the ball bond is located at the chip. However, the conventional bonding order was reversed in order to provide extra clearance at the package lid for these memory module stack wires: the ball bond is at the package and the crescent is at the top of the die stack. In addition, because of the height of the stack, the wires are much longer than in typical microcircuit applications - approximately 5.0 mm here, versus the typical 1.0 to 1.5 mm. Thus, the ratio of length to diameter is approximately 200 here, versus the typical 40 to 60. This breaks the usual rule that the ratio of the length to the diameter should not exceed 100. This extreme length happened incrementally as a result of gradually extending a successful design by adding additional die to the stack.

A scanning electron microscope was used to examine a total of about 100 crescent bonds, some from each of the four modules. The bonding pads were oval in shape, with a very rough granular surface: the valley-to-peak distances were roughly a quarter to a half of the diameter of the wire. Near the center of each pad was a cavity created by incomplete filling of the via between the top layer and the inner layers. Every examined bond showed a bond tool impression on the pad adjacent to the crescent bond, which is evidence of over compression.   Further, many of the crescents were wider and thinner than those regarded as ideal. This thinning was localized to the inside edge of the #2 chip stack in each module. Consultation with the company that carried out the bonding established that the wire bonding machine had been slightly out of alignment at the time, and did not apply the same pressure to all pads. Inspection of the alignment is now part of their daily procedure.
There were seven crescent bonds (7% of those examined) that were either partly lifted from the bonding pad, or both lifted and torn for about a quarter to a third of the width of the crescent. Tearing happened when three situations coincided:
(a) the crescent bond happened to be placed directly over the via-cavity, (b) the roughened surface also placed an irregularity at one edge of the crescent, and (c) the bonder's pressure was higher than usual. Thus, there is a degree of randomness to the extent of a tear in the crescent bond
Computing the resonant frequencies requires mapping the shapes of a number of these jumpers. See Figure 2 for a side view of typical shapes.
                 
Figure 2. Side view of wires.

We used a 200x microscope with a 3-axis micrometer stage. The bonding pads on the top die caps alternate in position: the first is near the edge and the second is more distant (etc.). This results in alternating long and short wires. Further, some of the short wires drop quickly, barely clearing the edge of the die stack, while the rest of the short wires have a generous loop. Thus, there are three classes of wires, based on length and shape. There is no analytic treatment possible for the shapes of these wires, but there is a complete analytic treatment for the nth resonant mode of a straight beam, under various end conditions.  See [1]. For example, the nth modal frequency is fn = [(k_n L)2][d/L2][Y/ρ]½ , where d is the diameter of the wire, L is its length, Y is the Young's modulus, ρ is the density, and (k_n L) is a dimensionless number that depends on the mode number n=l,2,3,..., and especially on the end conditions. We are only interested in the case of a rigid bottom bond: this is called a "clamped end" in beam theory. The number (k1 L)=4.73 when the top bond is also rigid, while (k1 L)=3.93 when a fracture is so nearly complete that it acts as a hinge: this called a "hinged end" or a "pinned end." Hence, the frequency of the lowest mode of a straight beam with a hinged top is 69% that of the beam with a rigid top bond. (The decrease is larger for the shapes of the bond wires.) This equation acts as a guide even when the beam is bent. For example, it illustrates that the modal frequency is proportional to the wire's diameter, and, thus, will vary over a family of bonds in the same way as the wire's diameter, some ± 5%, using a typical wire specification. On the other hand, a 5% increase in the length will decrease the modal frequency by 10%.   Since there is no analytic treatment possible for shapes like the ones of Figure 2, we used finite element analysis (FEA) which allowed computation of the frequencies and shapes of the lowest several modes within a few seconds; the most time consuming part was detailing the shapes. See Figure 3 for a perspective view of the fundamental mode shape, shown at its extreme right and left positions during a cycle of oscillation; the heavy line is the equilibrium position of the wire. For simplicity, the bottom platform is shown, but the chip stack is not.
We made direct measurements of the resonant frequencies of a number of the wires. A particular wire was selected and a small magnet was positioned directly in front of it. Microprobes were placed onto the top and bottom pads, and connected to an audio oscillator, to pass a current I=I0 sin(2π f t) through the wire. This imposed the force F = L x B x I(t) on it. The direction of the force is perpendicular

to the wire and to the magnetic field, and so was parallel to the long axis of the stacked chip array. This bends the wire sideways, and will excite its fundamental mode when the frequency is correctly adjusted. In effect, this turns the wire jumper into a linear electric motor, a kind of miniature loudspeaker. The magnet produced a field of about 500 gauss at the wire, while the current ranged from 1 mA to 50 mA.  This produced forces comparable with the weight of the wire.

Figure 3. The envelope of the fundamental.
The wire was observed using a 40x microscope, and the frequency of the oscillator was adjusted until the wire was seen to oscillate. The agreement between the computed and measured frequencies was within 15%, which is within the expected margins, given the variability of wire diameters and especially the uncertainty of measurements of wire shapes. The longest wires resonated at 1.4 kHz, the wide-looped short wires at 1.9 kHz, and the tight-looped short wires at 2.3 kHz.
The quality factor of a vibrating system is defined in various ways. One definition is a measure of the sharpness of the resonance, Q = f/Δf, where Δf is the full width between the half power frequencies. Another is a measure of the time t that the vibration takes to decay to 1/e of its original amplitude when the exciting force vanishes: τ = π Q/f Still another is a measure of the amplification that the amplitude of the oscillation shows at resonance Ares, over its value at much lower frequencies Ao: Q = Ares/Ao We directly measured the full-width between the half-power points, finding Q ≈100 to 300. This predicts that the wire continues to vibrate for 20 ms to 60 ms, which is consistent with observations: delays in the onset of oscillation when the current is suddenly applied, and in its cessation when the current is

suddenly terminated were just noticeable.   The source of this damping is the viscosity of the air in which the wire moves. These are hermetically sealed parts. If the air were removed, the quality factor would increase dramatically since the next source of energy dissipation is "Zener damping" which is much weaker. The damping imposed by air on a wire vibrating with an amplitude small compared with the diameter is well-known: Stokes' damping, see [2]. But the potentially damaging oscillations- of present interest have amplitudes that are many times the wire's diameter. An approximate treatment shows that, in this case, the effective Q is inversely proportional to the velocity of the motion, and so Q decreases both as the amplitude increases and as the resonance frequency increases. See [3]. Thus, the resonance is non-linear, and the above definitions of Q are not equivalent. We observed "resonance shifting" and "jumping" as we approached a resonance, and we observed that the maximum amplitude at resonance did not increase with the drive current, but more slowly.
The FEA computation shows that the magnetic forces we applied to the wire should cause a displacement of about 0.01 to 0.1 wire diameters when f « 1 kHz: this is below our detection limit using the 40x microscope.  This displacement is expected to increase at resonance by the quality factor (amplification factor) into the range of 1 to 30 wire diameters.    We observed peak-to-peak displacements up to 15 wire diameters, which is consistent with these estimates.
We set one wire into oscillation with a peak-to-peak amplitude of 10 wire diameters and checked its motion every few minutes for two hours. The initial resonance frequency was 1.40 kHz.  This dropped to 1.38 kHz at the end of the two hours, after which we terminated the driving current. We had established that these wires could endure the stresses applied during a vibration test that excited resonance;
these l.0xl07 oscillations had not induced fracture or even much fatigue. We repeated this study using a different wire, with an initial resonance frequency of 1.41 kHz, and a peak-to-peak amplitude of 10 wire diameters. The resonance frequency dropped to 1.36 kHz over a twelve-day period, after which we stopped the observations. The wire had sustained 1.4xl09 oscillations without fracturing, and without much fatigue.
Figure 4 is a photomicrograph looking down onto three bond pads at the edge of the chip stack, which is the dark gray object on the left of the image. There is no wire attached to the top bond pad. The

bottom wire is at rest. The wire attached to the middle pad is being driven into resonance, and only the extreme positions are apparent in this image. The vertical distance between the two extreme positions, versus the horizontal distance from the crescent bond, is plotted in Figure 5, along with a fit to the FEA mode shape.
            
Figure 4. Top-down image of two wires.
Figure 5 shows a top-down schematic view of a vibrating wire as observed when it was at one of its two extreme positions, and as fit by an FEA-derived shape. Note that the horizontal scale is about 6 times larger than the vertical one. The root-strain was 0.2%.
     
20         40         60        80        100
Displacement from pad (in wire diameters)
Figure 5. Top-down Schematic view of a wire.

This experiment was repeated using a wire whose crescent bond was deliberately cracked (the width of the crack was 50% of the width of the crescent): the resonant frequency dropped by a few percent, and the wire sustained several hours of vibration without breaking. This was repeated using a different wire, with the same result. The fatigue lifetime of a cracked crescent is expected to be substantially shorter than that of an uncracked bond. Some 80% to 90% of a given cycling history is spent creating an infant crack, while the remaining 10% to 20% is spent extending that infant crack into a complete rupture. However, a lifetime of even 10% of 2 billion cycles will allow the wire to pass the box-level test. This demonstrated that wires with even

half the crescent uncracked would not fracture due to stress, or to fatigue, during a vibration test which lasts for minutes (not hours).
We then deliberately induced a near fracture (less then 5% of the area remaining) in a crescent bond. This lowered the resonance frequency to 680 Hz, and, with the usual drive current, the fracture developed into a complete rupture within one minute. We repeated this using a second wire and found a similar result. Therefore, a nearly severed crescent bond reduces the rigidity of the top bond to the point that it acts as if it were hinged (not clamped); and the lifetime under the imposed vibration stresses is less than a minute for the stresses we imposed.
We used FEA to compute the fundamental frequency of a wire that had such a large crack that it was effectively hinged (not clamped). For the longer wires, this was some 600 Hz, in general agreement with the observations. It was up to 750 Hz for the shorter wires. (While the crack will absorb energy from the mechanical vibrations while it is growing, this is a small effect, and it will not affect the damping. The main energy loss is controlled by the viscosity of the air, and this loss drops with the frequency; thus, Q may double as the frequency halves.) Such a' cracked wire could be driven into resonance during any vibration test having enough spectral power in the range from 600 Hz to 750 Hz, and the wire would not survive long since it would be nearly cracked through already; rather, the crack would rapidly extend into a complete rupture. This is consistent with our observations of the behavior of the pair of deliberately cracked wires.
Interpretation
We suppose that the four wires (out of 5760) that broke during the box-level random vibration test were initially nearly fractured. Their fundamental resonance was in the range 600 Hz to 750 Hz, their quality factor was larger than 200, and they absorbed sufficient energy during the box-level test to rupture during the duration of that test. The presence of such cracks is suggested by the observation of partial cracks in 7% of the crescent bonds later inspected. We did not see any deep cracks, but we suppose that this is because all the pre-existing deep cracks were broken by the vibration test. We would have to inspect some 10,000 crescent bonds in unstressed modules in order to be sure of finding between 4 and 8 such deep cracks.

We now show that deep cracks would not have been detected by the tests the modules passed previously.
•   The resistance increase ΔR of a rectangular conductor (which is a model for the wedge section) with a width a and thickness b, due to a crack of depth c normal to one side, relative to the resistance R of the rest of the wire (of length L and diameter d) is ΔR/R = - [(d2)/b L)] ln[cos(π c/2 a)]. See [4]. For any wire 200 diameters long with a wedge that is 1/4 of a diameter thick, this relative increase remains less than 10% so long as there is 0.4% of the wedge left uncracked. That is, the crack can extend for 99.6% of the width of the wedge bond, and only increase the total resistance of the wire by 10%. This happens because (a) the crack only changes the current flow through the wire within about a diameter's location of the crack, and so only the resistance of that segment is changed, while the resistance of all the rest of the wire is not changed, and (b) these wires are some 200 times longer than their diameter. Thus, the presence of wires with initial cracks extending nearly the entire width of the wedge bond would not interfere with electrical behaviors.
•   These modules had survived a steady acceleration of 5,000 G's. The force was applied in such a way as to lift any improperly attached die stacks. Most (about 90%) of the force was carried by the bottom ball bond, all of which were made correctly. Therefore, each ball bond could sustain 5 to 10 grams of force (450 to 1,000 dynes). This is the force developed by 105 G's acting on a 5 mm length of wire. This is twenty times the applied acceleration, and so the ball bonds will carry this load safely. The remaining force (about 10%) is carried by the wedge bond: this force is about 25 dynes at 5,000 g's acceleration, or 0.03 grams of force. This can be carried so long as at least one percent (roughly) of the cross section of the wedge bond is intact.
•   The SSR operated correctly after a sine burst test in which 20 Hz at 20 G's was applied for 1.6 seconds. We do not have proper fatigue lifetime data for the gold wires used for these wire bonds.  Therefore we cannot compute the maximum size for a crack that would just survive this test. However, we

note that the frequency used in this test is much lower than any resonance, and so it is effectively a static test for the wires. Therefore, the amplification factor was unity for this test, and the sine burst test was at least bm order of magnitude less severe than the random vibration test.
We found that essentially identical multichip modules had been created and were being used elsewhere.  These modules had identical failures during vibration testing that was similar to ours. These broke in the same way during a similar random vibration test.
Conclusion
The initial failure of the SSR was caused by two factors, neither of which would have necessarily produced a failure if acting alone. The bonding process occasionally (about 7 per 10,000) produced a nearly cracked wedge bond when pad-features coincided with an especially thinned wedge bond. This dropped the resonant frequency into the range in which the box-level random vibration test spectrum excited the fundamental resonance, and thus applied an unusually high stress to the nearly cracked wedge bond, quickly extending it into a complete rupture.
These nearly cracked wedge bonds may not rupture during "normal" practice. This depends on the stresses encountered in that "normal" practice and on the service life. However, such bonds are not as reliable as spaceflight practice calls for, and it is prudent to eliminate any such bonds. The vibration test did this on the original version of the SSR, and we repeated it on the repaired version. We took care to ensure that this test used levels that were distinctly higher than what the instrument would see during launch and during service, so that this test would serve as a "proof test," eliminating any nearly cracked wedge bonds, but not creating any incipient failures. The instrument has been in service since February 1997, and is operating correctly.
George Harmon brought to our attention that several workers have proposed that wire bond integrity be tested by inducing wires to vibrate in a magnetic field of known intensity, using a known current. See [5,6,7]. We concur.
References
[1] McLachlan, N. W., "Theory of Vibrations", Dover Publications, Chapter 7, pp. 109—124,1951.

[2] Stokes, G. G., "On the effect of the internal
friction of fluids on the motion of pendulums", Trans.
Camb. Phil. Soc., Vol. 9, Part II, pp. 8—106,1851.
[3] Stuart, J. T. and L. Woodgate, "Experimental
Determination of the Aerodynamic Damping on a
Vibrating Circular Cylinder", Phil. Mag. Vol. 46,
pp.40—46, 1955.
[4] Gray, D. E. (editor), "American Institute of
Physics Handbook", McGraw-Hill Book Company,
third edition. Chapter 5, p. 5-25,1972.
[5] Tustaniwskyj, J. I., et al., "Progress Towards A
Cost-effective 100% Wire Bond Quality Screen", 37th
Electronic Components Conference, IEEE, pp. 557—
565,1987.
[6] J. I. Tustaniwskyj, "Method of testing wire bonds
for defects using a magnetic field", U.S. Patent
4,677,370, June 30, 1987.
[7] Lall, P. and M. Pecht, "Development Of An
Alternative Wire Bond Test Technique", 1992
International Electronics Manufacturing Technology
Symposium (IEEE/IEMT), Baltimore, Maryland,
September 28—30,1992.