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 n ear-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 th e 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 microstr ucture refinement is due to Co-enhanced nucleation of the Cu6Sn5 phase in t he solder matrix, as suggested by Auger elemental mapping and calorimetric measurements. The Co also reduced intermetallic interface faceting and impr oved the ability of the solder joint samples to maintain their shear streng th after aging for 72 hr at 150 degreesC. The baseline Sn-3.5Ag joints exhi bited significantly reduced strength and coarser microstructures.