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

Abstract

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 Ag3Sn fibrous precipitates form the Ag3Sn/Sn eutectic network surrounding the β-Sn primary grains. Increasing Ag content produces finer precipitates and finer networks. Sn–4.0 mass%Ag has additional large Ag3Sn 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 4.0 mass%Ag slightly. The formation of primary Ag3Sn 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, Cu3Sn adjacent to Cu and Cu6Sn5 adjacent to the solder. The Cu6Sn5 layer is thicker than the Cu3Sn 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.