Thermodynamics-Aided Alloy Design and Evaluation of Pb-free Solders for High-Temperature Applications

Abstract

High temperature solders that will not be affected in the subsequent thermal treatment are required in the step soldering process of multi-chip module (MCM) packaging. High-Pb solder alloys such as 95Pb–5Sn (numbers are all in mass% unless specified otherwise) are currently being used for this purpose. However, the development of the Pb-free solder alloy for high temperature applications is needed due to environmental issues. The solder alloys of Bi–Ag, Sn–Sb and Au–Sb–Sn systems are considered as candidates in this study. Aided by thermodynamic calculations, several specific compositions have been chosen and they were investigated in terms of melting behavior, electrical resistivity, wetting angle and hardness. The Bi–Ag alloy exhibited poor electro-conductivity while the Sn–Sb system had low melting temperatures. The ternary Au–Sn–Sb solder alloy shows prospects for high temperature applications in spite of poor wetting properties.