Dissolution and Interface Reactions between Palladium and Tin(Sn)-Based Solders: Part II. 63Sn-37Pb Alloy
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THIS Part II study examines the dissolution of Pd by molten 63Sn-37Pb (wt pct) solder. The results of this study are compared to Pd dissolution data for molten 95.5Sn-3.9Ag-0.6Cu Pb-free solder, which were described in Part I. Briefly, the development of a soldering technology must consider both interconnection processing and reliability issues associated with alternative Pb-free surface finishes.[1,2] A printed circuit board finish that is attracting increased interest has a Ni solderable layer and Pd protective layer.[3–5] A layer of immersion Au has been added on top of the Pd layer to mitigate a slight loss of solderability caused by Pd oxidation. Bader measured the dissolution rate of Pd in molten 60Sn40Pb solder to be about one-half that of Cu.[6] Several studies have examined the effects of Pd on the mechanical properties of Sn-Pb solder joints. Vaynman et al. concluded that 0.5 wt pct Pd had very little effect on the stress strain, creep, and isothermal fatigue of eutectic Sn-Pb solder.[7] However, data from other PAUL T. VIANCO, Distinguished Member, JEROME A. REJENT and GARY L. ZENDER, Technologists, and PAUL F. HLAVA, Contractor, are with the Sandia National Laboratories, Albuquerque, NM 87123. Contact e-mail: [email protected] Manuscript submitted October 23, 2009. Article published online August 10, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
investigations have indicated that Sn-Pb solder joints made to Pd protective finishes can experience a small degree of embrittlement.[8,9] Earlier studies examined the interface microstructure resulting from Pd dissolution by molten Sn-Pb solder. Ghosh performed extensive studies describing the thermodynamics of Pd-Sn and Pd-Pb reactions, identifying many of the phases that are observed in the present work.[10] The experimental results of the present study can be compared to phase analysis presented in Reference 10. In experimental studies performed by Ghosh, the PdSn4 intermetallic compound (IMC) phase was identified after Pd was exposed to molten eutectic Sn-Pb solder at 493 K (220 C) for a 60-second exposure.[11,12] Longer exposures of up to 1800 seconds caused a PdSn3 layer to form between the PdSn4 layer and Pd substrate. However, the study examined only two temperaturetime combinations, which would not support a rigorous calculation of the interfacial layer growth rate kinetics. In the second work, the authors readily dissolved the thin Pd layer so that the study addressed primarily the Ni-Sn reaction. Kobe and McIntyre exposed Pd foil to eutectic Sn-Pb solder at 493 K (220 C) for 100 to 400 seconds and observed the formation of only the PdSn4 at the interface.[13] Wang and Tu examined the reaction between Sn-Pb solder and Pd foil at 523 K (250 C) and 2- and 5-minute exposure times.[14] They observed a very rapid reaction between the solder and
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VOLUME 41A, DECEMBER 2010—3053
Pd, generating thick IMC layers of PdSn4 and PdSn3, the latter having formed between the Pd substrate and PdSn4 layer. This Par
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