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.

The development of Ag-free Sn solders has attracted significant attention due to the requirement of high-density electronic packaging. In this study, we investigate the Ni element on the microstructures and mechanical properties of Ag-free Sn-Cu-Bi-In solders. This paper details the microstructures and phases of the as-prepared Sn-Cu-Bi-In-Ni solders, as well as its mechanical properties. Specifically, the intermetallic compound (IMC) Cu₆Sn₅ is observed to be distributed in the Sn matrix, forming near-eutectic structures. The incorporation of Ni into Sn-Cu-Bi-In enhances the mechanical properties of the solder joints, including the shear strength and vibrational stability. In the joint obtained using the as-prepared Sn-Cu-Bi-In-Ni solders, a (Cu,Ni)₆Sn₅ IMC layer forms at the interface between Sn ball and Cu pad. The beneficial effects of Ni can be primarily attributed to its ability to adjust the mechanical properties and thermal expansion, enhancing the stability of solder joints. A TEM analysis reveals the closely packed atomic interface of Cu/(Cu,Ni)₆Sn₅ and (Cu,Ni)₆Sn₅/Sn, elucidating the joining mechanism involved.