Alloying effects in near-eutectic Sn-Ag-Cu solder alloys for improved microstructural stability

This study included a comparison of the baseline Sn-3.5Ag eutectic to one near-eutectic ternary alloy, Sn-3.6Ag-1.0Cu and two quaternary alloys, Sn-3.6Ag-1.0Cu-0.15Co and Sn-3.6Ag-1.0Cu-0.45Co, to increase understanding of the beneficial effects of Co on Sn-Ag-Cu solder joints cooled at 1-3 degreesC/sec, typical of reflow practice. The results indicated that joint microstructure refinement is due to Co-enhanced nucleation of the Cu6Sn5 phase in the solder matrix, as suggested by Auger elemental mapping and calorimetric measurements. The Co also reduced intermetallic interface faceting and improved the ability of the solder joint samples to maintain their shear strength after aging for 72 hr at 150 degreesC. The baseline Sn-3.5Ag joints exhibited significantly reduced strength and coarser microstructures.