Beneficial Alloy Effects in Tin-Silver-Copper-X Solder Joints for High Temperature Applications

TitleBeneficial Alloy Effects in Tin-Silver-Copper-X Solder Joints for High Temperature Applications
Publication TypeJournal Article
Year of Publication2006
AuthorsAnderson IE, Harringa JL
Journal TitleBrazing and Soldering, Proceedings
Date Published4
Accession NumberISI:000280337000004
KeywordsEUTECTIC SOLDER, microstructure, SN-AG-CU

Modification of a near-eutectic Sn-Ag-Cu (SAC) solder alloy, Sn-3.7Ag-0.9Cu (wt.%), with selected alloy additions to form SAC+X alloys (X = Co, Fe, Si, Ti, Cr, Mn, Ni, Ge, and Zn) that are designed to substitute for Cu has proven successful for retention of solder joints (using Cu substrates) with consistent strength and ductility after thermal aging at 150C for up to 1000h. Such improvements to SAC solders are critical for Pb-free assembly of portable electronics and electrical devices with potential for drop impact failure of interconnects. Although Izod impact strength of most as-soldered SAC+X joints (except SAC+Cr and SAC+Ti) was similar and equivalent to the selected SAC alloy joints, the impact strength retention of the thermally aged SAC+Zn and SAC+Ni joints was superior, especially the SAC+Zn. The current report also added more understanding of key results from previous asymmetric four-point bend (AFPB) testing, especially for SAC+Ni and SAC+Zn solder joints. Microstructural analysis of post-AFPB test specimens of these two alloy joints (aged 1000h at 150C) established that, while all SAC+Zn joints displayed ductile failure, one SAC+Ni joint sample clearly exhibited brittle failure. Moreover, the SAC+Zn solder appeared remarkable in its ability to suppress formation of a continuous Cu3Sn layer adjacent to the Cu substrate, even after aging for 2000h at 150C. Electron microprobe results confirmed that pronounced intermetallic layer enrichment occurs in SAC+Zn joints after thermal aging, similar to most other SAC+X joints in the study, and indicated that some Sn-Cu-Zn intermetallics form in the solder matrix and, perhaps, at the Cu interface on prolonged thermal aging.

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