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.