Effect of indium addition on the microstructural formation and soldered interfaces of Sn-2.5Bi-1Zn-0.3Ag lead-free solder

The microstructural formation and properties of Sn-2.5Bi-xIn-1Zn-0.3Ag (in wt%) alloys and the evolution of soldered interfaces on a Cu substrate were investigated. Apart from the relatively low melting point (about 195 C), which is close to that of conventional eutectic Sn-Pb solder, the investigated solder presents superior wettability, solderability, and ductility. The refined equiaxial grains enhance the mechanical properties, and the embedded bulk intermetallic compounds (IMCs) (Cu6Sn5 and Cu5Zn8) and granular Bi particles improve the joint reliability. The addition of In reduces the solubility of Zn in the beta-Sn matrix and strongly influences the separation and growth behaviors of the IMCs. The soldered interface of Sn-2.5Bi-xIn-1Zn-0.3Ag/Cu consists of Cu-Zn and Cu-Sn IMC layers.