Effects of alloying in near-eutectic tin-silver-copper solder joints

Abstract

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 effects of Co on Sn-Ag-Cu solder joints cooled at 1-3°C/s., typical of reflow assembly practice. The results revealed joint microstructure refinement 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 h at 150°C. Some recent additional results with Co and Fe additions are consistent with this catalysis effect, where a reduced total solute level was tested. The baseline Sn-3.5Ag joints exhibited significantly reduced strength retention and coarser microstructures.