Effects of Bi and In Addition on Microstructure and Properties of Sn-0.7Cu Solder

With the widespread application of lead-free solder, solder represented by the SAC series has been widely used. However, with the miniaturization and multifunctionalization of electronic devices, the distance between solder joints is becoming increasingly smaller, and the problem of electrochemical migration caused by Ag in the SAC series is gradually emerging. Therefore, it becomes imperative to develop a solder that has a melting point, mechanical properties, and other properties similar to those of the SAC series but does not contain silver. In this study, 1–3% of Bi and 1–3% of In were added to the Sn-0.7Cu solder to investigate the effects of these elements on the melting characteristics, microstructure, mechanical properties, wettability, and electrical conductivity of the Sn-0.7Cu solder. Due to the addition of Bi and In elements, the melting point of the Sn-0.7Cu solder was lowered, the shear strength was improved, and the solderability was enhanced, but the electrical conductivity was reduced. This article obtained the mechanism of the influence of Bi and In elements on the properties of the Sn-0.7Cu solder through experiments and theoretical analysis, providing a certain degree of theoretical support for the development of silver-free solder.