Effect of In on microstructure, thermodynamic characteristic and mechanical properties of Sn–Bi based lead-free solder

• A moderate amount of In element will increase the ductility of Sn–Bi solder.
• BiIn intermetallic compounds will form in the Sn matrix after adding 4% In.
• In element is found to be distributed in the BiIn phase and in the Sn matrix.
• The peak temperature will decrease with In element addition.
• In element participated in the interfacial reactions forming Cu–Sn–In IMCs.

The influences of In element on microstructure and properties of Sn–Bi based lead-free solder were investigated in this study. With In element addition, the amount of primary Sn-rich phase increased in the Sn–Bi alloys. When the In content increased to 4%, granular BiIn phase formed distributing in the Sn matrix and along with the Bi phase. The addition of In lowered the peak temperature and increased the reaction temperature. Tensile test results showed that the tensile strength changed slightly with increasing In addition, while the elongation increased remarkably first and then decreased after adding 2.5% In. In element was confirmed to be participated in the interfacial reactions forming Cu–Sn–In IMCs and affect the wettability of Sn–Bi solder on Cu substrate.