Abstract-An alternative test technique to the wire bond pull test is presented for wire bond interconnects. The new test tech- nique, based on electromagnetic ...
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IEEE TRANSACTIONS ON COMPONENTS. PACKAGING. AND MANUFACTURING TECHNOISIGY-PART
A, VOL. 17, NO 4, DECEMBER 1994
Development of an Alternative Wire Bond Test Technique Michael Pecht, Fellow,, IEEE, Donald Barker, and Pradeep Lall, Member, lEEE
Abstract-An alternative test technique to the wire bond pull test is presented for wire bond interconnects. The new test technique, based on electromagnetic resonance, has the potential for on-line use as a quality assurance and operational life evaluation method. The new technique greatly reduces the test time in comparison with the existing MIL-STD-883 pull test and internal visual inspect ion. This new test technique more closely simulates the operational stress than the wire bond pull test and has also shown a sensitivity to defects that would otherwise escape visual inspection.
I. INTRODUCTION
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RESENTLY, the nondestructive bond pull test. specified by MIL-STD-883C, is used to precipitate nonacceptable wire bonds. The test uses a hook-shaped member to hook onto and pull each bonded wire with a predetermined force. The use of the pull test as a quality ;assurance and process control procedure raises some serious concerns. The pull test is useable on ! S I and MSI chips, but it is difficult to use on VLSI chips. Bond wire spacing on modem VLST chips can be less than 0.004 in. In ULSI chips with even larger I/O counts there is a trend toward even finer bond wire spacing. Placing the testing apparatus hook around a single bond wire without damaging adjacent wires is difficult, tedious, and time consuming. As bond pad spacing continues to decrease, the pull test may become impossible to employ. In the bond pull test, a hook applies a unidirectional force to the wire, which does not simulate the operational load on the wire. Operational life failures are not tensile overstress failures but fatigue failures resulting from wire flexure in temperature cycling or in vibration during operational life or ultrasonic cleaning. In addition to the difficulty i n employing the bond pull test with modem ULSI dies and the fact that the test does not simulate an operational load, there is a problem in interpreting the meaning of the numerical value of bond pull strength. From Fig. I and simple statics, it can be shown that the bond pull strength is a function of the location of the hook on the bond wire. The observed bond pull strength increases as the hook position gets closer to either bond. However, if the bonds have a tendency to peel, hook slippage on the wire close to Manuscript received December I , 1993; revised March 22, 1994. M. Pecht and D. Barker are with CALCE Electronic Packaging Research Center, University of Maryland, College Park, MI) 20742 USA. P. La11 was with CALCE Electronic F'ackaging Research Center, University of Maryland, College Park. MD 20742 USA. He is now with Motorola Radio Products. lEEE Log Number 9404658.
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Hook Position, 11 (b) Fig. I . (a) Force equilibrium during bond pull test. (b) Bond pull strength as J function of location of hook o n bond wire.
the wedge or stitch bond can result in a peel mode of failure, giving a lower bond pull strength value [ 2 ] . The bond pull strength value also depends on the bond types used for the first and second bonds. The ball-stitch combination, often used in microelectronic packages, consists of a wire that rises vertically above the ball bond and gently slopes towards the stitch bond. During the pull test, the hook often slips to the top portion of the loop above the ball bond. Even a weak ball bond due to its larger cross-sectional area is stronger than the wire, and therefore remains untested. Harman has also shown that the elongation of the wire also effects the bond pull test results. With all other factors being equal, wires with a larger percentage elongation have a larger bond pull strength (see Fig. 2) 131. The increase in the bond pull strength is attributed to an increase in the final loop
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A N ALTERNATIVE WIRE BOND TEST TECHNIQUE
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Fig. 2 . The variation in bond pull Yrrength and wire breaking strength versus wire elongation [ 3 I.
height at wire breakage due to the increase in wire elongation. A variation in bond pull strength with increase in final loop height is shown in Fig. 3 . The bond pull strength value, in addition to being dependent on elongation, is a function of geometry of the wire loop. As the difference in the height of the first and second bonds decreases, the observed bond pull strength value can decrease by a factor of up to 5 for the same hook position (see Fig. 4). Internal visual inspection is used for quality assurance and process control based on the assumption that if an interconnect does not conform IO a specified geometry, it may not possess the structural integrity to last the prespecified mission life. Visual inspection is subjective in nature. The subjectivity arises from the definition of defects, which within certain variabilities are a function of the operator skills. In addition, certain internal defects may not be detected by visual examination. Internal visual inspection is time consuming for large MCM packages with hundreds of 110, and special equipment is needed for finepitch devices. With the miniaturization of microelectronics, visual inspection may not be a viable te.chnique for future applications. This paper presents an alternative to the bond pull test and internal visual inspection. The test technique does require the use of a special test coupon and circuitry that allows an alternating current to be passed through all the wire bonds. As such, the technique cannot be used as a screen but is designed to be used as a process control test to evaluate bonding parameters. The experimental setup and some selected test results are described in the following sections. The impact of this test procedure on present approaches to wire bond test and evaluation are then discussed.
11. ELECTROMAGNETIC RESONANCE TEST TECHNIQUE
The electromagnetic resonance test technique is based on a test method proposed earlier by Tustaniwskyj et al. [l]. The test technique derives its origin from the physics of a currentcarrying conductor in a magnetic field. A conductor of length 1, carrying a current of illength, in a magnetic field of B
The current authors found that a single excitation frequency was ineffective in exciting more than one bond wire simultaneously. Small nonuniformities in bond geometries caused the natural frequencies of individual wire bonds to be separated far enough to prevent simultaneous excitation. The present approach uses a narrow-band random signal with a bandwidth wide enough to account for the range of natural frequencies instead Of a sing1e frequency input. The random signal enables the excitation of gangs of wire bonds. The amplitude of vibration of each wire determines the time-to-failure value. The amplitude is directly proportional to the voltage power spectral density of the narrow band random signal (as can be seen from (I)). The magnitude of the power spectral density is measured using the digital signal analyzer. Since the wire bonds on the test coupon are daisy chained, failures are reflected as open circuits. Further, the failure of the test coupon is the time to first failure of the wire, and thus only the worst case defects are precipitated. The test technique assumes that all the wires are stressed equally. To evaluate the effects of local joule heating, the current through the wires was assessed based on the power spectral density of narrow-band random signal. For the highest power dB Vrms, the voltage across spectral density, -0.125 x the wirebonds (wirebonds refers to the chain of wirebonds being excited) was calculated as
P S D = lolog,,, -
0.125 x
(2)’
dB Vrms = 1Olog,,,
~
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where VI is the normalizing voltage (1 mV) and V, is the voltage applied across the wire bonds, calculated to be 0.99 mV (from (2)). The resistance of a wirebond chain is in the neighborhood of 2.2 R. The current through the wirebonds was calculated to be 0.45 mA, based on Ohm’s law. The 12R ohmic heating for the highest power spectral density used in the test technique was 4 . 4 5 10 ~ -7 W, which is negligible. Ohmic heating thus does not effect results. 111. EXPERIMENTAL SETUP
The magnetic field of about 6000 G is provided by a permanent magnet, with a pole-to-pole spacing of 0.5 in. The random signal is produced by a programmable function generator and passed through the test coupon, which is placed in the magnetic field. The test coupons were thick film gold metallization substrates (1.75 in x 0.75 in), with gold wires bonded to the gold metallization. A total of 5 12 wires are daisy chained together, aligned both parallel and perpendicular to the
IEEE TRANSACTIONS 'ON COMPONENTS. PACKAGING, AND MANUFACTURING TECHNOLOGY--PART A, VOL. 17, NO. 4, DECEMBER 1994
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Difference in heigth of first and second bond, h2 (inch, Fig. 4. Variation in normalized bond pull strength versus difference in height of first and second bond.
length of the substrate as shown in Fig. 6. The random current passing through the bond wirer, interacts with the magnetic field to produce a force on the wires causing them to vibrate at their natural frequency. ,' t=O
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I v . SELECTED 1rEST RESULTS The test coupons with 512 wire bonds daisy chained together have been successfully excited simultaneously. Failures have been precipitated within 5-s to 56-h exposure to various levels of random excitation. On a log-log plot of time to failure versus applied voltage power spectral density, the failure times lie in a very tight band. Failure data to date has demonstrated the potential of the technique to precipitate bond pad lifts, wire flexure fatigue failures, and nonconductive microcracking failures. A selected group of defective bonds with balls off the bond pad (BoBP-see Fig. 7). small ball bond (SBB-see Fig. 8), and shallow stamp on stitch bond (S4-see Fig. 9), were tested. The defect magnitudes for each category were varied from small to a large value. Each defect type was then tested at various power spectral densities. Fig. 10 shows that the times to failure for these defective bonds followed a trend other than that for the nominal good bonds. Once a characteristic life curve for nominal good bonds has been generated, it is easy to ascertain the relative quality of sample bond. An operational life value below the characteristic
(b)
Fig. 5 . Principle of electromagnetic resonance test technique. (a) The test coupon is placed in a magnetic field and a random varying signal is passed through the wire bond interconnects. (b) The randomly varying current signal interacts with the magnetic field, causing the wire bond interconnects to vibrate.
curve indicates bonds with marginal life. while life values above the curve indicate better bonds. The technique appears to be more sensitive to bonding defects as the voltage power spectral density is lowered and the time to failure increases. Thus, it is a compromise to determine adequate sensitivity within a minimum period of time. BoBP defect magnitude was varied from 25% bonded to 85% bonded. Fig. 10 shows the time to failure for 25% bonded is much lower than 85% bonded wire bonds (as shown in Fig. 10 by 0, at -4.5 dBV2 rms and -9.5 dBV2 rms). Further, the 85% bonded wirebonds have a time to failure almost equal to nominal good wirebonds (as shown in Fig. I O by 0, at -0.125 pdBV2 rms). This is consistent with MIL-STD-883, Method 2010, which allows a 75% bonded wire bond to pass internal visual inspection.
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Fig. 6. A total of S I 2 wires are daisy chained on the test substrate, both parallel and perpendicular to the length of the substrate.
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SBB defect magnitude was varied from 1D ball diameter to 5 D ball diameter (where D is the wire diameter). The time to failure is consistent with the defect magnitude. While small ball diameters result in lower time to failure (as shown in Fig. I O by 0, at -9.5 dBV2 rms power spectral density), the larger ball diameters have a time to failure almost equal to nominal good bonds (as shown in Fig. 10 by U, at -0.125 ,udBV2 rms and -5 dBV2 rms power spectral density). 5’3 defect magnitude was varied from just the capillary impression to a no-impression stitch bond. S4 is depicted by A in Fig. IO. The defect magnitude is consistent with the time to failure. While the capillary impression results in a life almost equal to the nominal good bonds (as can be seen in Fig. IO, for power spectral densities of -0.125 /,,dBV2 rms and -5 dBV2 rms), the time to failure for no impression is lower than a nominal good bond (as shown in Fig. 1 0 by A, for -9.5 dBV2 rms and - 12.5 dBV2 rms power spectral density). Often, the existence of a defect may not be known and the indication of relative order of goodness of a wire bond may be indicated by a different failure mode and a time to failure in the electromagnetic test technique. The effectiveness of the electromagnetic test technique in identifying different failure modes-including wire break above the ball due to embrittlement (Fig. I I ) , ball bond lifts (Fig. 12), and stitch bond lifts (Fig. 13)-was examined. The time to failure for different failure modes differs from nominal good bonds, as shown in Fig. IO.
___ Fig. 9.
V. CONCLUSION The feasibility of the electromagnetic resonance test technique to precipitate defects has been demonstrated. The potential of the technique to precipitate defects that otherwise escape internal visual inspection has also been demonstrated. The technique uses a special test vehicle to evaluate the wire bonds versus known good bonds. The technique does not measure life, but it does use a stress more closely simulating field stress. The test technique is fast and facilitates conformance to quality assurance requirements of one failure in 10000 bonds on a regular basis. in a cost-effective manner. This test technique provides an altemative technique for quality assurance for wire bond interconnects. The electromagnetic resonance can be used for rapid evaluation of ULSI devices. The research effort was successful in exciting and testing 512 wire bonds simultaneously. This is extremely useful, since bond pull tests, presently used for quality evaluation of wirebonded interconnections in microelectronic devices for SSI and MSI chips, may prove time consuming for VLSI chips with a large number of bond wires (e.g., over 100 wires).
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The desired component failure rates for high-reliability application require one failure in I O9 h. For 100000 h per mission, this requirement translates to one failure in 10000 wires. A sample inspection of 10000 wires using the electromagnetic resonance test technique will take 20 tests on 5 I2 wire bond test substrates. Each test lasts a few seconds, compared to hours using the bond pull test. The electromagnetic resonance test technique, like the bond pull test, is a function of bond geometry. However, the plot of voltage power spectral density versus time to failure can be characterized for a particular geometry. This curve is then used for evaluations of similar geometries. Further, the noncontact nature of the electromagnetic resonance technique circumvents the difficult process of hooking each wire. The electromagnetic test technique can be used to aid the internal visual inspection to assess consistency of bond loop for high-frequency applications. Consistency of bond loop is of extreme importance in high-frequency applications. If a
broad band frequency is so chosen that the extremes of the frequency range correspond to the range of allowable bond geometries, bond loops that are out of range of the allowable bond geometry will not vibrate. Thus the test procedure can be used as a check on the loop uniformity with an accuracy greater than can be attained by intemal visual inspection. The test technique has been used for test coupons as well as for multilayer substrates with silicon chips. The effects of magnetic field and currents on the test device have not yet been evaluated but will be published in a later paper. For the present, the test technique is recommended as an on-line process control and can be used on a sample population to evaluate bonding parameters.
nominal good bonds.
ACKNOWLEDGMENT The authors thank W. Pearson and C. Chevez for their cooperation in obtaining the samples, and R. .I. Usell and S. A. Smiley for loaning their experimental apparatus (originally
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Donald Barker has been working in the general area of experimental mechanics, fracture mechanics, fatigue, and dynamic material response since joining the University of Maryland, College Park, in 1976. He is the Co-Director of the University of Maryland Computer Aided Life Cycle Engineering (CALCE) Electronics Packaging Research Center, which deals with the reliability issues in the design of electronic equipment. The CALCE Center is a National Science Foundation-sponsored state industryhniversity cooperative research center. His research involves the experimental and numerical modeling of mechanical failures in electronic components and assemblies and the integration of design tools into a concurrent engineering environment. This research is leading to practical approaches for microelectronic package design and the implementation of reliability assessment methodologies, based on physics-of-failure concepts.
Fig. 13.
used for their 1987 paper). The work was performed in cooperation with Westinghouse Electric Corp. and Unisys Corporation.
REFERENCES J . I. Tustaniwskyj. R . J. Usell, and S. A. Smiley. “Progress towards a cost-effective 100’7,wirc bond quality screen,” in Proc. 37th Electronic Comporient.s Conj., pp. 557-565, 1987. G . G. Harman and C. A. Cannon, “The microelectronic wire bond pull test, how to use it, how to abusc it,” IEEE 7i.un.s. Corp. Hybrids, Munu$ Tkhnol.. vol. CHMT-I, pp. 203-210, Sept. 1978. G. G. Harman, “Reliability and yield problems of wire bonding in mi~roelectronics,”ISHM, Technical Monograph, 1989.
Michael Pecht (S’82, M‘83, SM’90, F’92) received the B.S. in acoustics, a M.S. in electrical engineering, and the M . S . and Ph.D. in engineering mechanics from the University of Wisconsin. He is a Tenured Faculty with a joint appointment in Systems Research and Mechanical Engineering. He is also the Director of the National Sciencc Foundation-supported CALC Electronic Packaging Research Center al the University of Maryland. Dr. Pecht is a Profcssional Engineer. He serves on the boards of advisors for various companies and was a Westinghowe Professor. He is thc chief editor of the IEEE TRANSACTIONS ON REIJABILITY. a section editor for the Society of Automotive Engineering, and on the advisory board of IEEE Specfrum. He has edited five b o o k on electronics design, rcliability asressment, and qualification.
Pradeep La11 (S’90, M’93) received the B.S. degree from the University of Delhi, and the M.S. and Ph.D. degrees from the University of Maryland. College Park, all in mechanical engineering. He is currently an Engineer at Motorola Radio Products. He has published extensively in the area of temperature effects on microelectronics, reliability assessment and development of failure modeling strategies for microelectronic packages. His research involves finite element modeling of multilayer ceramic surface mount capacitor reliability during wave soldering; development of modeling suategies to address thermal, mechanical, thermomechanical, and electrical effects on advanced electronic technologies; physics-of-failure-based software tools for design, reliability assessment. testing and screening of microelectronics; high-temperature reliability of microelectronics; guidelines for thermal derating; and development of alternative test techniques for first-level interconnects.
