Effect of Ag addition on the microstructural and mechanical properties of Sn-Cu eutectic solder

The effect of adding Ag up to 1 mass% on the microstructural and mechanical properties of Sn-Cu eutectic solder alloy was examined. Without Ag, primary beta -Sn grains are surrounded by the eutectic network band of Cu6Sn5 needle precipitates/beta -Sn. With increasing Ag content, the primary beta -Sn grain size and the eutectic network size become finer. In the eutectic band fine Ag3Sn particles appear in addition to Cu6Sn5 precipitates. The DSC experiment revealed the presence of four endothermic reactions on heating for Sn-Cu-Ag alloys; the two peaks near 217 degreesC correspond to the Sn-Cu-Ag ternary eutectic melting reaction and those at 223-225 degreesC/224-226 degreesC are for Sn-Cu binary melting. The 0.2% proof stress and tensile strength decrease with the addition of 0.1 mass%Ag and then gradually increase up to 1 mass%Ag. Even with 1% Ag, they are less than the values for a Sn-0.7Cu binary alloy. In contrast, elongation increases with increasing Ag content up to 1%. Thus, the addition of Ag to Sn-0.7Cu alloy can effectively improve its ductility. The strain rate dependence of 0.2% proof stress of Sn-0.7Cu-0.5Ag is similar to that of Sn-Ag eutectic alloy but is different from that of Sn-Cu eutectic alloy. A small amount of added Ag results in a change of the deformation mechanism of Sn-Cu alloy.