Thermomigration behavior of Sn–Bi joints under different substrate

In this work, the atomic migration behavior in Sn-58Bi solder joints was investigated under the condition of a temperature gradient. The effect of different substrates including Cu, Ni and Co on microstructural evolution in the solder was also compared. It was found that the main migrating element in Sn-58Bi solder was Bi in the direction of temperature gradient to produce certain Bi-rich phases at the cold end. With the change of hot end from Cu to Ni or Co, the thermal migration rate of Bi phase and the growth rate of interfacial Intermetallic compounds (IMC) were significantly depressed, which can be attributed to the decrease of the temperature gradient in the solder joint. In particular, there is no obvious asymmetric growth during the growth of IMC at both ends of the interface, which is different from other Sn-based Pb-free solders. Finally, finite element simulation analysis also verified the excellent thermal migration resistance of Co and Ni substrates.