Development of Sn-Bi-In-Ga quaternary low-temperature solders

With the miniaturization and high density of electronic devices, the development of electronic packaging technologies has grown significantly in recent years. In order to eliminate thermal damages of the CTE (Coefficient of thermal expansion) mismatches of materials in packaging modules, low-temperature lead (Pb)-free solders with low cost and high reliability are in demand in the electronic industry. Eutectic Sn-58Bi with high tensile strength and low melting temperature at 139 °C has caught great concerns in the industry. However, the brittle nature of the Bi-rich phase is a significant issue in employing the Sn-58Bi solder. Therefore, the goal is to design proper alloying elements for improving the elongation of the Sn-58Bi solder, while keeping their low melting temperatures. In the literature, it has been found that minor indium (In)-doping can substantially improve the elongation of Sn-58Bi solder; however, the excess amount of In-doping would lead to the formation of brittle BiIn intermetallic compound (IMC). In addition, according to our previous study, minor gallium (Ga) doping into the Sn-Bi solder can effectively suppress the interfacial IMC growth1. In this study, CALPHAD-type thermodynamic calculations using the PANDAT software and corresponding key experiments were performed to design the Sn-Bi-In-Ga (SBIG) quaternary low-temperature solders. Calculated solidification paths based on the lever rule and the Scheil model were employed to design the desired compositional range of the Sn-Bi-In ternary constituents, without the formation of brittle IMC during reflow and solidification processes. The designed Sn-Bi-In-Ga (SBIG) quaternary low-temperature solder is composed of the primary (Sn) phase, the (Sn)+(Bi) eutectic structure and little amount of unreacted excess Ga and the steps of solidification were furtherly verified in the step quenching experiment. Moreover, the different cooling rate made a great influence on the IMC appearing. Because the primary (Sn) phase was solidified more completely with the lower cooling rate such as furnace cooling, BiIn-rich liquid was formed, and the BiIn IMC was easily observed. As for the mechanical properties of the SBIG solder after air cooling, high yield strength, high ultimate tensile strength, and a much better elongation than the conventional Sn-58Bi solder were obtained. Dimple-like morphology was observed in the fracture surface, indicating a ductile fracture. A new low-temperature Pb-free Sn-Bi-In-Ga quaternary solder with good mechanical properties is proposed based on computational thermodynamics and validated in experiments.