Program Organizers: M. McCormack, AT&T Bell Laboratories, Rm lA-123, 600 Mountain Ave., Murray Hill, NJ 07974; S. K. Kang, IBM, T. J. Watson Research Center, P. O. Box 218, Yorktown Heights, NY 10598; M. R. Notis, Department of Materials Science and Engineering, Lehigh University, 5 East Packer Ave., Whitaker Laboratory, Bethlehem, PA 18015-3195; K. N. Tu, Department of Materials Science and Engineering, UCLA, Los Angeles, CA 90095-1595
Tuesday, AM Room: Grand K
February 6, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: M. McCormack, AT & T Bell Laboratories, Rm 1A-123, 600 Mountain Ave., Murray Hill, NJ 07974
8:30 am Invited
INTERFACIAL REACTIONS DURING SOLDERING WITH LEAD- TIN EUTECTIC AND LEAD (Pb)- FREE, TIN- RICH SOLDERS: S.K. Kang, R. Rai, S. Purushothaman, IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598
Lead (Pb)- containing solders used for the interconnections of microelectronic subsystem assembly and packaging have been a subject of the environmental issues recently. Extensive research and development activities for replacing Pbcontaining solders with Pb- free solders are in progress in electronic industries, universities, and national laboratories. One key technical issue recognized with the Pb- free, Sn- rich solders is a need to develop a good barrier metallurgy to control the interfacial reactions, namely, dissolution of the base metal(s) and concurrent formation of intermetallics at the soldering interfaces. In this study, the interfacial reaction of Cu and Ni metalization with several Pb- free and Pbcontaining solders are investigated. The dissolution kinetics of the base metal(s) as well as the growth kinetics of the intermetallic are discussed.
8:55 am lnvited
PHASE EQUILIBRIA, COMPOUND FORMATION AND INTERFACIAL INSTABILITY IN THE Au- Cu- Sn SYSTEM: M. R. Notis, Lehigh University, Bethlehem, PA 18015- 3195
Phase equilibria in the Au- Cu- Sn system is important in determining which phases and compounds form in solder connection in several electronic device applications such as TAB and BGA joining. Ternary Au- Cu- Sn compounds were formed by reaction diffusion in Cu3Au, CuAu, or CuAu3 vs. Sn solid- state diffusion couples annealed at 170C. These compounds are identified as (at 25Au- 55Cu- 20Sn, atomic %) and X (at 33.3Au- 33.3Cu- 33.3Sn). Significant interfacial breakdown occurred in the CuAu3 vs. Sn diffusion couple. Diffusion paths for the various CuAu vs. Sn couples were determined by EPMA and were plotted onto the 170deg.C isothermal section of the phase diagram. The location of the ternary phases in relation to other known phases is displayed, and compared with the results of other studies in the literature. The present study also included measurement of the liquidus temperature of a number of ternary alloy compositions. These are also displayed together with other literature data and used to discuss the applicability of this ternary system to joining technologies.
9:20 am Invited
MICROSTRUCTURAL EVOLUTION OF Ag- Pd- Pt THICK FILM CONDUCTOR DURING STEP SOLDERING: D. Millin, G. M. Crosbie, T.- Y. Pan, H. D. Blair, R. P. Cooper, J. M. Nicholson, Ford Research Laboratory, Ford Motor Company, Dearborn, MI 48121- 2053
Thin film packages with ceramic substrate have been widely used in automotive and military applications. The material usually involved in thick film are the A12O3 substrate, a Ag- based conductor pad, one layer of the low- Sn, high- Pb solder (solder 1), and one layer of the eutectic or near- eutectic Sn- Pb solder (solder 2). Both solder layers are reflowed sequentially, with the process referred to as "step- soldering." In this study, microstructural changes in a AgPd- Pt thick film conductor bonded to an A12O3 substrate were observed during both stages of the step soldering. Samples were reflowed using a solder paste (solder 1) having a composition of 88Pb- lOSn- 2Ag. Initial microstructural observations exhibited minimal reaction between solder 1 and the conductor. Samples were then dipped into a bath of eutectic Sn- Pb solder (solder 2) at varying times and temperatures. Solder 1 was found to rapidly dissolve at all temperatures tested. At lower temperatures, the rates of dissolution were found to follow a classic diffusion based dissolution equation. This equation was used in a processing model that provided an optimum processing window. It was used to predict the soldering lifetime of the conductor based upon alloy composition, time, and temperature. The model does not account for convection effects, that are likely to play an important role at higher temperatures and during the dynamic soldering processes. Catastrophic damage to the conductor, in the form of intermetallic formation, occurred once the conductor contacted molten solder 2.
9:45 am
A COMPARISON STUDY OF THE WETTING REACTIONS OF EUTECTIC SnPb ON Cu, Pd, AND Ni FOILS: Jessica P. Almaraz, Carmen Fernandez, Shanay R. Trunnell, Y. Yang, H.K. Kim, and K.N. Tu, Department of Materials Science and Engineering, UCLA, Los Angeles, CA 90095- 1595
We performed a comparison study of the morphology and reaction rate of molten eutectic Sn- Pb solder on Cu, Pd, and Ni foils. Overall, the reactions on Cu and Ni are quite similar except that the wetting angle on Cu is smaller and reaction rate is faster. However, no stable wetting angle can be observed on Pd and the solder spreads out until the reaction is completed. The reaction rate of intermetallic compound formation on Pd is much faster than those on Cu and Ni. Only Sn forms compounds with them and no Pb compounds can be detected. The morphology of the Sn compounds has been examined by selectively etching the Pb away from the cross- section of the samples. We observed a strong dependence of the reaction rate on the morphology of the compounds.
10:10 am BREAK
10:25 am
XPS STUDY OF THE OXIDATION BEHAVIOR OF Cu- Sn INTERMETALLIC COMPOUNDS AT LOW TEMPERATURE: D. Wang, A. C. Miller, M. R. Notis, Lehigh University, Bethlehem, PA 18015- 3195
The oxidation of Cu- Sn intermetallic compounds is of significant technological interest in relation to the wetting behavior of Sn- base solder alloys in contact with Cu and Cu alloys. In this present study, oxide films formed in- situ in pure O2 on a number of different Cu- Sn intermetallic compounds held at temperatures <350deg.C have been analyzed using XPS. Single- phase Cu- Sn intermetallic compounds used for this study were made by hot isostatic pressing of rapidly solidified powders and were provided by NIST. In order to characterize the nature of the oxide layers formed on the intermetallic compound surfaces, spectra obtained from the Cu- Sn intermetallics are compared with spectra for pure copper and pure tin oxidized at the same conditions. The role of copper and tin on oxidation behavior in Cu- Sn intermetallics will be discussed in terms of available phase equilibria diagrams.
10:50 am
MORPHOLOGY OF THE WETTING REACTIONS OF EUTECTIC SnPb, EUTECTIC SnBi AND PURE Sn ON Ni: Shanay R. Trunnell, Carmen Fernandez, Jessica P. Almaraz, H.K. Kim, K.N. Tu, Department of Materials Science and Engineering, UCLA, Los Angeles, CA 90095- 1595
The soldering reaction on Ni is of interest because of the use of Ni as a cushion layer on Cu to reduce the consumption of Cu by the solder. We have conducted a systematic study of the soldering reactions of eutectic Sn- Pb, eutectic Sn- Bi and pure Sn with Ni. The wetting angles and wetting interfaces have been studied by using SEM and EDX, The wetting angles do not change much with time, yet a reaction band has been observed to grow in front of the wetting tip Or the Sn- Pb solder. The cross- sectional views showed the formation of Ni- Sn intermetallic compounds. The morphology of the compound interfaces is rather smooth.
11:15 am
WETTING BEHAVIOR OF EUTECTIC Sn- Ag ALLOY ON Cu SURFACE: H.K Kim, K.N. Tu, Department of Materials Science and Engineering, UCLA, Los Angeles, CA 90095- 1595
The Pb- bearing solder used in microelectronic industry is becoming an environmental issue. Since the eutecffc Sn- Ag alloy is one of the Pb- free solders which have received much attention, we studied its wetting and aging behavior on Cu. The Cu- Sn and Ag- Sn intermetallic compound formation has been studied. At the wetting tip, Cu- Sn compounds form and grow into the solder side, and a scalloplike Cu6Sn5 compound structure forms at the interface between the solder and Cu substrate. Large Ag3Sn grains have been observed to grow in the matrix of the solder and the amount increases with aging time. Upon solidification, the solder surface showed crack formation.
11:40 am
INTERFACIAL REACTIONS OF MOLTEN Sn AND In WITH THE Cu SUBSTRATE: L. Su, S.Chen, Department of Chemical Engineering, National Tsing- Hua University, Hsin- Chu, Taiwan 33043, China
The interfacial reactions of molten Sn and In with the Cu substrate were
determined by studying the reacffon diffusion couples. Cu foil was encapsulated
with Sn or In shots in a quartz tube. The sample capsule was then annealed at a
predetermined temperature. The annealing temperatures were 300deg.C and
450deg.C. These temperatures were higher than the melting points of Sn and In,
whilst than lower than that of Cu. At these annealing temperatures, the Sn and
In shots became molten and reacted with the Cu substrate. The phases formed at
the interface were examined by using OM, SEM, and EPMA. The thickness of the
reaction layers was measured by using an image analyzer. Only Cul11In9 phase
was observed at the interface of the Cu/In couples annealed at both
temperatures. For Cu/Sn couples, two phases, and were found. In comparison
with the results available in the literature for couples of solid Cu/solid Sn
or In, the phase formation sequences were similar, however, the interfacial
morphology and the reaction rates were quite different. For the solid/liquid
couples, the reaction rate was much faster and interface was nonplanar.
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