While the electronics industry appears to be converging on Sn-Ag-Cu as the alloy of choice for lead free electronics assembly, the exact composition varies by geographic region, supplier and user. Add to that dissolved copper and silver from the printed circuit board traces and surface finish, and there can be significant variation in the final solder joint composition. This paper presents the results of a systematic study of the mechanical and microstructural properties of Sn-Ag-Cu alloys with Ag varying from 2wt% to 4wt% and Cu varying from 0.5wt% to 1.5wt%. Different sample preparation techniques (water quenched, oil quenched and water quenched followed by reflow) were explored and the resulting microstructure was compared to that of a typical reflowed lead free chip scale package (CSP) solder joint. The microstructure obtained by oil quenching produced a microstructure similar to that of the CSP solder joint. The water-quenched sample had a finer grain structure, while the water quenched and reflowed sample had a coarser grain structure compared to the CSP solder joint. The sample preparation technique and comparative microstructures are presented. The tensile strength, 0.2% yield strength and the ultimate tensile strength have been measured for the range of alloys at conditions of as cast, aged for 100h at 125°C and aged for 250h at 125°C. The creep test results and the effect of high temperature aging on mechanical properties are also presented for the oil-quenched samples. The results of this work provide necessary data for the modeling of solder joint reliability for a range of Sn-Ag-Cu compositions and a baseline for evaluating the effects of subsequent quaternary additions.
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