Effect of Ag Content on Properties of Sn-Ag Binary Alloy Solder

Sn-Ag binary alloys, with Ag content in the range between 0 mass% and 4.0 mass%, were examined in order to understand the effect of Ag addition on microstructural and mechanical properties of the solders. Fine Ag 3 Sn fibrous precipitates form the Ag 3 Sn/Sn cutectic network surrounding the β-Sn primary grains. Increasing Ag content produces finer precipitates and finer networks. Sn-4.0 mass%Ag has additional large Ag 3 Sn primary particles. Thermal expansion coefficient of the alloy decreases with increasing Ag content. The 0.2% proof stress of Sn-Ag alloy increases with increasing Ag content up to 4.0 mass%Ag, and is higher than that of Sn-37 mass%Pb solder above 2.0 mass%Ag. In contrast, tensile strength increases up to 3.5 mass%Ag but decreases at 4.0mass%Ag slightly. The formation of primary Ag 3 Sn is attributed to the degradation at 4.0 mass%Ag. The wettability of the Sn-Ag alloys on Cu is slightly improved by the Ag addition but is worse than Sn-37 mass%Pb solder. Two intermetallic layers are formed at the interface, Cu 3 Sn adjacent to Cu and Cu 6 Sn 5 adjacent to the solder. The Cu 6 Sn 5 layer is thicker than the Cu 3 Sn layer and grows into the solder forming scallop shape. The thickness of the reaction layers slightly increases with increasing Ag content. The composition of Sn-(2-3.5 mass%)Ag is the best selection for obtaining high joint strength. Sn-Ag alloy is superior to Sn-37 mass%Pb solder for establishing a rigid interface.