evaluation of contact resis

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IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART B, VOL. 18, NO. 2, MAY 1995 299

Evaluation of Contact Resistance forIsotropic Electrically Conductive Adhesives

Michael A. Gaynes, Russell H. Lewis, Ravi F. Sad , and Judith M. Roldan

Abstract- Electrically conductive adhesives are discussed andstudied with ever-increasing interest as an alternative to solderinterconnection in microelectronics circuit packaging. A similarlevel of scrutiny that is used to evaluate contact resistanceperformance for interconnections made with solder and separableconnectors is necessary for electrically conductive adhesives.Experience with solder interconnection and separable connectorsshows low initial contact resistance of less than 10mR when bulkconductor material is minimized in the measurement scheme.Stability is typically determined to be ess than a 5-10 mf2 changeas a function of stress. The main intent of this study is to char-acterize the electrical contact resistance performance of jointsmade with isotropic electrically conductive adhesives. A coppercomb pattern test vehicle was designed and fabricated using 0.25-mm thick lead frame material. The plating finishes that wereapplied to the copper substrate included a palladium alloy, gold,tin, and nickel. Test samples were made with several electricallyconductive adhesives. Samples consisted of two comb patternsbonded to each other making a gang of 40 ap joints. Variablesfrom circuit packaging such as coefficient of thermal expansionmismatches are purposely avoided in this study. Contact resis-tance measurements were made initially and asa function of timeduring environmental tests. Stresses included thermal cycling,thermal aging, and temperature and humidity conditioning. Thestability of electrical contact resistance is shown to be influencedby both plating metallurgy and the conductive adhesive itself.Contact resistance equivalent to solder is possible with someelectrically conductive adhesives on appropriate metallurgicalfinishes. Mechanically, adhesive oints are less robust than solderjoints, and therefore care must be aken to eliminateor m i n i the effects of mechanical loading.

I. INTRODUCTIONOR OVER 20 years, isotropic electrically conductiveF dhesives (ECAs) have been used in the electronic pack-aging industry, primarily as die attach materials [13, [2]. Morerecently, ECAs have been proposed as an alternative to solderin surface mount technology (SMT) applications. The benefitsand drawbacks of ECAs compared to solder have beenthoroughly discussed in the literature. Briefly, claimed ad-vantages of electrically conductive adhesives include reducedenvironmental impact, improved processing characteristics,

Manuscript received November 1994; revised January 13, 1995.This paperwas presented at the First International Conference on Adhesive JoiningTechnology in Electronics Manufacturing, Berlin, Germany, November 2 4 ,1994.M. A. Gaynes and R. H. Lewis are with the Assembly Process DesignGroup, IBM Corporation, Endicott, NY 13760 USA.R. F. Saraf and J. M. Roldan are with T. J. Watson Research Center, IBMCorporation, Yorktown Heights, NY 10598 USA.IEEE Log Number 9410557.

and increased resistance to thermal fatigue [3]-[9]. Con-versely, disadvantages of conductive adhesives include limitedelectrical contact resistance data, unknown conductor surfacemetallurgical requirements, and weaker bond strength [11, [41,[ 6 ] , [7], [lo], [ll]. Initial experience of the authors in usingECA to attach SMT components was not encouraging. Thinsmall outline packages (TSOPs) were bonded to PCMCIA(Personal Computer Memory Card International Association)cards. Only mild card flexing or a two foot drop were neededto debond devices. Admittedly, the combination of TSOPson a thin card is a worst case SMT application because of theshort stiff component leads and the thin, flexible card. A lowerstrain application would be to attach discretes to a thick (1.3mm ) epoxy laminate printed circuit board.Silver is typically used in ECAs becauseof its low volumeresistivity of 1.6 x 52-cm. Electrical data reported in theliterature compare the volume resistivity of isotropic ECAs tosolder. The best commercially available conductive adhesivesat 5 x 52cm [61, 191.However, the electrical performance of a joining material is acombination of the bulk material resistivity and the interfaceresistance to the mating conductors. Contact resistances of16-22 m52 have been reported on a gold surface with anECA joint 110 pm in diameter and 50 pm high [111. Duringenvironmental stress testing a change in resistance from 5 to 50mR (from 25% to 300%) was observed. In another work [121,a series of 11 joints was measured and after subtracting bulkcircuitry effects, individual joints were estimated to average30-60 mR. In the actual measurement, the contribution of the11 oints would be between 330-660 m52. If it is assumed thatthe 25% stability criterion employed in this study is appliedonly to the joint resistance contribution of the measurement,changes as much as 82-165 mR are deemed to be acceptablefor the series. In this scheme, it is unclear how individualjoints are performing. In another study [13], 34 electricallyconductive adhesives were evaluated with resistivity beingone of the responses. Only one of the adhesives exhibiteda reasonably stable resistivity. A thick film specimen about12 mm long with two interconnection contact resistances tothick film metallization probes was measured as a function ofthermal cycling. Stabili ty appeared to be between one and four52. A recent study [14], claims that there is one adhesive on themarket that is stable (defined as not exceeding a 20% changein contact resistance). Specifying stability as a percent changecan be misleading depending on the level of bulk resistanceand how many joints are in the measurement.

52an approach solder at 2 x

1070-9894/95$04.00 0 1995 IEEE


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300 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING,AND MANUFA CTURING TECHNOLOGY-PART B, VOL. 18, NO. 2, MAY 1995

Fig. 1 . Copper comb pattem test vehicle.

Experience with solder interconnection and separable con-nectors shows low initial contact resistance of less than 10mR when bulk conductor material is minimized in the mea-surement scheme. Stability is typically determined to be lessthan a 5-10 mR change as a function of stress. Contact areaand bulk resistance contributions must be considered whendoing any comparative evaluation of contact resistance withan interconnect technology. A resistance change on the orderof 5-10 mR is usually descriptive of catastrophic changesin a solder joint (a completed crack) or separable connectorinterface (severe corrosion). Both solder and separable connec-tors have been researched and modeled from the beginning ofthe microelectronics industry. Contact resistance and stabilityare two key responses that have been measured and studiedexhaustively. Failure rates for reliable separable connectorsare on the order of magnitude of 10s of parts per billion percontact per lo00 power-on-hours. Solder, being a metallurgicalbond, is even better by at least an order of magnitude. BecauseECAs are frequently considered as an alternative to solder,it is prudent that a similar level of scrutiny be given tounderstanding contact resistance performance and stability.The purpose of this paper is to measure contact resistancefor bonded joints using various ECAs. The electrical contactresistance was measured initially and as a function of thermaland humidity environmental stress. The challenge is to achievea low joint resistance which is stable over time at stress.

11. EXPERIMENTALPPROACHThe electrical contact resistance of several isotropic con-ductive adhesives is evaluated using a copper comb patterntest vehicle [15]. This 0.25-mm thick lead frame material had

40 photolithographically defined fingers, each 0.25 mm wideand 9.5 mm long on a 0.64 mm pitch (Fig. 1). These cop-per substrates were electroplated with the following separateplating combinations: Palladium alloy over nickel, gold overnickel, nickel, and tin. All plating thicknesses were 1.5 pmexcept Pd alloy and hard Au, which were 1.1-pm thick. Muchof the previously published work has focused on tin-lead(Pb) joining surface finish [16], [17]. Alternate metal surfacefinishes are evaluated for the following two reasons: 1) Since

Fig. 2. Dispensed adhesive dots.

Fig. 3. Comb pattem bonding f ixture.

electrically conductive adhesives provide a path to Pb elimina-tion, non-Pb containing surface finishes must be included, and2) solder coated surface finishes cause junction instability dueto oxidation [3], [9], [14]. With these thoughts in mind, thestudy of alternate metal surface finishes as presented here andelsewhere represent possible future directions [181, [191. Threeadhesives were selected by reviewing commercially availableadhesives from several manufacturers. One additional adhesivethat is being developed was included. Other recent electricallyconductive adhesive reliability evaluations have identified onlyone or two promising candidates [13], [20]. The materialsselected for this study include three thermosetting adhesives(referred to as A, G, and D) nd one thermoplastic adhesive(referred to as B). All adhesives were filled with silver.Using the plated copper substrates, lap joints were madeusing the four different isotropic electrically conductive ad-hesives. First, the adhesive was dispensed on the tips of thefingers (Fig. 2) with automated syringe dispense equipmentusing needles with a 0.2-mm diameter orifice. A fixtureallowed a second comb pattern to be aligned to the first ina lap fashion (Fig. 3). A clamping bar applied a mating forceto the lap joints that controlled bond thickness to 0.08 mm andheld the bonds in place during the temperature driven bondingprocess. An angled view of the lap joints is displayed in Fig. 4.The bond area was typically 0.13 mm. Bonding was ac-complished according to the manufacturers recommendations.After curing, the common bar of one coupon was removed to


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GAYNES et al.: EVALUATION OF CONTACT RESISTANCE FOR ISOTROPIC ELECTRICALLY CONDUCTIVE ADH ESIVES 301

Fig. 4. Comb pattern lap joints.

allow contact resistance measurement of the individual lapjoints. Four coupons with 40 joints each were made on bothgold and palladium alloy surface finishes with four adhesives(32 coupons total). Additionally, adhesive A which providedthe most robust bonds (both mechanically and electrically) wasused to make four coupons each that were tin and nickel plated.One coupon of every surface finish was held as a control, whilethe other three were subjected to one of the following testconditions: O-lOOC accelerated thermal cycling (ATC) for2000 cycles, 12OOC thermal aging for 2000 h, and 85OC/80%RH temperature and humidity (TM) conditioning for lo00h. Electrical contact resistance was measured initially and asa function of time at stress. These tests and conditions aretypically used to assess reliability of technologies and designsunder development. In this case, effects and variables due tocircuit packaging, processing, and materials compatibility havebeen intentionally avoided. The main focus is to understandthe joint resistance behavior as a function of adhesive andsubstrate finish.

111. RESULTS AN D DISCUSSIONContact resistance distributions are presented in graphs thatappear in Figs. 5-7. The z-axis represents contact resistance(in m a) and the y-axis represents a normal probability scale.Data that fit a straight line describe a normal distribution.The slope of the straight line fit is the standard deviation.Contact resistance distributions are plotted initially and at thecompletion of ATC, thermal aging, and temperaturehumidity(T/H) environmental stress testing. During the course of thistesting, the sample size decreased as a function of stress. Asthe samples of bonded joints are placed into and removedfrom the test fixture for contact resistance measurement, bondsbreak due to handling damage and contact force from thespring-loaded electrical probes. In this regard, the experi-mental method requires improvement to eliminate mechanicaldamage. Nonetheless, electrically conductive adhesive jointsare mechanically weaker than solder joints and proper design

consideration must be given when using adhesives. A rule-of-thumb in circuit packaging design is to not allow, or minimize,load carrying by interconnections. Recent mechanical testingof commercially available isotropic conductive adhesives, re-ported a wide range of pull strengths with the best, only onethird of the tensile fracture strength of a typical solder joint[=I.

.-

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2 4 6 8 10RESlSTANCE IN MILUOHMS

Fig. 5. EC A comparison with solder.

Fig. 5 compares the contact resistance of the four selectedadhesives on a palladium alloy surface to a soldered versionof the copper test vehicle at the initial readout. Adhesive Aand solder fit a normal distribution. Adhesive A comparesvery favorably to solder with a mean of 4.4 ersus 4.0 mRfor solder. (The standard deviations for approximately 100measurements are 0.2 mR for Adhesive A and 0.1 mR forsolder.) Adhesive C does not compare well with solder witha mean of 8.2 mR and a larger standard deviation of 2.0.Adhesives B and D appear to have a bimodal distribution.The first mode of the distribution for adhesive B comparesvery well to solder with a parallel slope (similar standarddeviation). The median for adhesive B falls between sol-der and adhesive A. The second mode of adhesive B isrepresented by eight data points that have a high standarddeviation compared to the first mode. These joints are the eighthighest resistances for adhesive B and comprise the secondmode of this distribution. The eight joints were observedto mechanically break at the next electrical readout. Themost likely cause for the higher resistance second mode ishandling damage as the sample is placed into or removedfrom the test fixture. Some lap joints would not align inthe channels of the fixture upon entry or would catch alongthe side walls of the channels when removed. Typically, theforces imparted were sufficient to break the fragile bonds thatwould fracture in the range of 1-5 N. Mechanical damagethat did not result in complete fracture is indicated by anoutlying higher resistance from the main distribution. Thesemechanically compromised joints only showed an increase inresistance between one and four mR. This observation supportsthe 5-10 mR rule-of-thumb for electrical interconnect stability:Only a small increase in resistance indicates incipient failure.Joints with this higher resistance would break before the nextmeasurement.

Adhesive D is also bimodal. By inspection of Fig. 5 ,Adhesive D has a median of approximately 5 mR. However,the slope of the first mode shows a greater standard deviationthan solder. The second mode can also be related to mechanicalhandling damage. The bonds with the five highest resistancesbroke during removal from the test fixture. It appears that thehigher mode of a bimodal contact resistance distribution is anindicator of mechanically disturbed bonds.


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302 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACIUIUNG TECHNOLOGY-PART B,VOL. 18, NO. 2, MAY I995

PALLADIUM ALLOY PLATINGI I I I I i I I

............. ..................................... """.-"-."..1""-.""1......... .............. . ....................... . "-

........... .- .- .-.-I-.-.......................................... --...- ...I__".

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Fig. 6. Adhesive A on Pd alloy and hard Au surfaces.

TIN PLATING1 I 1 I I

NICKEL PLATING................ """"" .

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.......................... "_"

. .--...-----.I."1 !. ! ? . . - 2 l o o - ~ A Totoo?2 1---b 900 HOURS, RH- en HpuRs. , 1 2 0 c,AcE

a'r l 3IDo m = QDD 4 I 1 i a m' 'RESISTANCE IN MllLlOHMS

Fig. 7. Adhesive A on Ni and Sn surface.

Comparing the four chosen conductive adhesives, AdhesiveA had the best contact resistance performance and mechanicalintegrity initially and as a function of environmental stress.The performance of Adhesive A on four different metalelectroplating finishes (Pd alloy, Au, Ni, and Sn) is shownin Figs. 6and 7. The palladium alloy plated surface providedthe best electrical stability during thermal cycling, thermalaging, and temperaturehumidity conditioning. During the lifeof the tests, the change in contact resistance was less than0.2 mR for all but one of the 83 total joints subjected to thethree environmental stresses. The final measurement of thisone joint, after 2200 h of 12OOC thermal aging, was 9.4 mR,which is a catastrophic degradation. The initial measurementwas 4.9 mR. Six subsequent measurements, made up to andincluding 1925 h, were all 4.8 mR. When the sample wasremoved from the test fixture after the 9.4 mR measurement,the bond was broken.

The other electroplating surface finishes did not performas well as the palladium alloy. Both the mean and standarddeviation are higher for all adhesives on gold compared topalladium alloy, which is surprising. Most notable for goldis the high contact resistance increases as a function ofhigh temperature exposure (Fig. 6). Both ATC and thermalaging exposure increase contact resistance dramatically at the

completion of the tests. Starting at a meadstandard deviationof 4.6/0.5 ma, ATC increases to 623.3 mR, and thermalaging increases to 15.4A3.8 mR. One possible explanationfor the increasing electrical resistance of the gold platingfinish is oxidation of the nickel that is used to harden gold.Another explanation is from detrimental effects of organicfilms. Carbon was observed on the hard gold surface byAuger analysis [22]. A contaminant film of carbon couldweaken the bonding between the adhesive and metal surface.This weak interface provides a fast diffusion path for oxygenduring exposure to high temperatures. The oxygen and 120Ctemperature oxidize the carbon film and contact resistanceincreases.Adhesive A is very stable during T/H exposure for bothpalladium alloy and gold. Contact resistance data for Adhe-sive A on nickel and tin surfaces is also shown in Fig. 7.The performance on nickel and tin is significantly inferiorto palladium alloy and gold. The nickel surface yields aninterconnection hat is highly resistive-initial average of 1678mR compared to 4.4 mR on palladium alloy. Interestingly, thestandard deviation of joint resistance improved from an initialvalue of lo00 mR to less than 500 mR for all stresses onnickel. For the ATC and T/H tin cells, most of the bondsbroke during handling. The temperature aging cell was stable


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GAYNES e?al.: EVALUATION OF CONTACT RESISTANCE FOR ISOTROPIC ELECTRICALLY CONDUCTIVE ADHESIVES 303

at 2200 h (8.6 mR initially versus 9.2 mR at end-of-test). Thestandard deviation remained the same at 2.8 ma.

IV. CONCLUSIONSContact resistances equivalent to solder oints are achievable

with electrically conductive adhesives. Plating finish is a veryimportant consideration for adhesive applications. A palladiumalloy surface provided an electrically superior joint comparedto gold, tin, or nickel. The joints made with palladium alloyare stable during environmental stress. Further work is neededto study and understand the instabilities observed duringstress with gold. Contact resistance differences were alsoobserved among the four chosen adhesives under the sameplating finish and environmental stress conditions. AdhesiveA generally performed better than the other three adhesivesin both initial electrical measurements and stability duringenvironmental stressing. Contact resistance increases of lessthan 5 mR resulted from mechanically compromised joints.This observation supports the 5-10 mR rule-of-thumb forelectrical interconnect stability. Only a small increase predictsimminent failure of the bond interface. Mechanically, adhesivejoints are less robust than solder joints and therefore care mustbe taken to eliminate or minimize the effects of mechanicalloading. Encapsulation may be a likely way to provide thismechanical protection.

V. SUMMARYElectrically conductive adhesives are frequently discussedas alternatives to solder interconnection. Advantages includereduced environmental impact, improved processing charac-teristics, and increased resistance to thermal fatigue. Littledata exist in the literature that support the use of electri-cally conductive adhesives in low impedance, active deviceinterconnection. Data that are available, indicate high contactresistances that are unstable and are typically reported as per-cent changes or include large bulk additions to the resistancemeasurement. To achieve a reliable interconnection on theorder of a separable connector or solder joint, much morethorough study of contact resistance is needed for electricallyconductive adhesive joints. This study represents a modestbeginning to understand contact resistance performance. Toobtain an dea of the work that may be necessary to introduceconductive adhesives in electronic packages in a significantmanner, consider the years of research and development thathave supported solder connections and separable connectors.Only when a collective industry focus is given to understand-ing contact resistance and stability for electrically conductive

adhesives, will successful and pervasive implementation be-come a reality.

ACKNOWLEDGMENTThe authors gratefully acknowledge the contributions ofDave Cokely and Randy Hoffman.

REFERENCES[I ] J. Bolger and S. Morano, Conductive adhesives: How and where. theywork, Adhesives Age, pp. 17-20, June 1984.[2] J. Greaves, Jr., Evaluation of solder alternatives for surface mounttechnology, in Proc. Nepcon West Tech. Program , 1993, pp. 1479-1488.[3] G. Nguyen, J. Williams, F. Gibson, and T. Winster, Electrical reliabilityof conductive adhesive for surface mount applications, in Int. Electron.Packaging Con$, 1993, pp. 479486.[4] G. Nguyen, J. Williams, and F. Gibson, Conductive adhesives: Reliableand economical alternatives to solder paste for electrical applications,ISHM Proc., 1992, pp, 510-517.[5] J. Bolger, J. Sylva, and J. McGovern, Conductive epoxy adhesives toreplace solder, Surfoce Mount Technol., pp. 66-70, Feb. 1992.[6] B. Alpert and A. Schoenberg, Conductive adhesives as a solderingaltemative, Electron. Packaging Production, pp. 13&132, Nov. 1991.[7] K. Gilleo, Polymer bonding systems offer alternatives to soldering,Electron. Packaging Production, pp. 52-55, Dec. 1992.[8] A. Burkhart, H. Yoshigahara,Y. agami, and T. Yamazaki, Conductivepolymeric adhesives solve SMD assembly problems, Adhesives Age,pp. 36-39, Oct. 1990.[9] K. Gilleo, Evaluating polymer solders for lead free assembly, part I,Circuits Assembly, pp. 52-56, Jan. 1994.[lo] H. Kim, L. Li, C. Lizzul, I. Sacolick, and J. Moms, Electrical, tructuraland processing properties of electrical ly conductive adhesives, IEEETrans. Comp., Hybrids, Manufact. Technol., vol. 16, no. 8, pp. 843-851,Dec. 1993.[ I l l R. Estes, F. Kulesza, D. Buczek, and G. Riley, Environmental andreliability testing of conductive polymer flip chip assemblies, in Int.Electron. Packaging Cont. 1993, pp. 328-342.[12] J. Bolger and K. Gilleo, Area bonding conductive epoxy adhesivepreforms for grid array and MCM substrate attach, in Proc. IEEEMCM Con$, 1994, pp. 77-82.1131 R. Keusseyan and J. Dilday, Electric contact phenomena in conductiveadhesive connections, in Proc. Su$ace Moun t Int. Con$ Expo ., 1993,

[I41 K. Gilleo, Evaluating polymer solders for lead free assembly, part 11,Circuits Assembly, pp. 50-53, Feb. 1994.1151 M. Gaynes and R. Lewis, Evaluation of contact resistance for isotropicelectrically conductive adhesives, in Proc. 7th Int. SAMPE, 1994, pp.[16] F. Liotine, Jr., Surface mount solderless attachment using electricallyconductive polymer adhesive technology, in Proc. Su rface Mount In?.Con$ Expo., 1993, pp. 572-583.1171 H.Rubin, Altemative interconnect methods using conductive adhe-sives, in Proc. Surfnce Mount Int. Con$ Expo., 1993, pp. 748-752.1181 I. Kadija, J. Abys, E. Kudrak, and I. Maisano, Solderability andwirebonding characterist icsof palladium, palladium nickel and soft goldelectrodeposits,Proc. AESF Annu. Tech . Con$, 1993, pp. 363-377.[19] D. Abbott, R. Brook, N. McLelland, and J. Wiley, Palladium as alead finish for surface mount integrated circuit packages, IEEE Trans.Comp., Hybrids, Manufact. Technol., vol. 14, no. 3, pp. 567-572, Sept.1993.[20] H. Hvims, Solder replacement, IEEE CHMT Int. Electron. Manufact.Technol. Symp., 1993, pp. 128-135.[21] P. Arrowsmith, K. McCafferty, and R. Emery, Early results withisotropic conductive adhesives, presented at the SMTA Symp., RTP,NC, Oct. 1994.[22] E. Sancaktar, Y. Wei, and M. A. Gaynes, Conduction efficiency andstrength of electronically conductive adhesives, presented at the 17thAnnu. Meeting of the Adhesion Society, Feb. 20-23, 1994.

pp. 567-571.

69-78.

Michael A. Gaynes received the B.S. degree in chemical engineering fromBrigham Young University.He joined IBM in 1979. He has held technical leadership positions thatcover a wide spectrum of electronic packaging in manufacturing and devel-opment. These include ceramic chip carrier circuitization, failure analysis,reliability test, and model development. During the past four years, he hasdirected materials and process development effortsfor applications hat requirethermally and electricallyconductive adhesives, die attach adhesives,and flip-chip encapsulants. He has authored 15 technical publications and has sixpatents filed.


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304 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART B,VOL. 8, NO. 2, MAY 1995

Russell 3 Lewis received the B.S. and M.S. degrees in metallurgicalengineering and materials science from Camegie Mellon University.He joined IBMs T.J. Watson Research Center, Yorktown Heights, NY,in 1983, where he is a staff engineer in the Assembly Process Designgroup performing SMT process development. His assignments have includedceramic and plastic BGA rework development, and electrically conductiveadhesive interconnection. Prior to his current assignment, he was the leadtechnical engineer in an IBM Materials Analysis Lab, where he authorednumerous construction and failure analysis reports of electronic compounds.

JudithM.Roldm received the B.A. degree from Manhattanville College andthe M.S. degree from Long Island University, both in chemistry, in 1981 and1985, respectively.She is a senior associate engineer at the IBM T.J. Watson Research Center,Yorktown Heights, NY, where she is engaged in research on interconnectionmaterials.Ms. Roldan is a member of the American Chemical Society.

Ravi F. Saraf received the B.Tech. degree from the Indian Inst itute ofTechnology, Kanpur, India, and the Ph.D. degree from the University ofMassachusetts, Amherst.He is a research staff member in the Silicon Science and TechnologyDepartment at the IBM T.J. Watson Research Center, Yorktown Heights, NY,where he has worked on the physical structure and properties of polymers.He has authored four patents and over 30 papers, and is the co-editor ofa book on characterization of polymer surfaces published by Butterworth-Heinemann. He has received three invention achievement awards in 1991,1992, and 1994, respectively.

evaluation of contact resis

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