Effect of Au addition on Microstructural and Mechanical Properties of Sn–Cu Eutectic Solder

The effect of Au addition on the microstructural and the tensile properties of Sn–0.7 mass%Cu alloy was examined. Tensile strength and 0.2% proof stress remarkably increase up to 0.3 mass%Au primarily due to solid solution hardening. Beyond 0.3 mass%Au, due to precipitation of large intermetallic compounds, elongation decreases while tensile strength and 0.2% proof stress increase slightly. With Au addition to Sn– Cu binary alloy, the eutectic endothermic peak in DSC, i.e., the melting reaction at 227◦C, becomes broader and shifts to the lower temperature range. With Au beyond 2 mass%, the broad peak becomes smaller splitting into two peaks and moving towards lower temperature while a new peak appears at about 212◦C. These thermal reactions can be well explained by the formation of β-Sn, Cu6Sn5 and AuSn4 with Au more than 1 mass%. EPMA observation revealed that much amount of Au and Cu dissolve into Cu6Sn5 and AuSn4, respectively.