Mechanical constitutive properties of a bi-rich high temperature solder alloy

BiAgX is one of the potential lead-free substitutes for lead-rich solders for high temperature interconnections, with a melting temperature above 533K after reflow. This study focuses on the mechanical properties of a particular BiAgX solder alloy. These properties are compared to a selected lead-rich solder for reference. The paste for this solder is in the form of mixed solder powder alloys and the reflow process is done in a vacuum oven to prevent oxidation and to minimize voids. The mechanical constitutive tests conducted in this study include: (i) monotonic elastic-plastic stress-strain response at constant strain rates and constant temperatures; and (ii) isothermal, viscoplastic creep strain response at several constant stress levels and different temperatures. The sensitivities of the material viscoplastic constitutive response to temperature and stress level were systematically studied, so that the mechanical performance of this lead-free BiAgX solder can be compared with those of other solders, at similar test conditions. At temperatures above 373K, BiAgX solder paste is found to strain harden more than the selected lead-rich reference solder in monotonic displacement-controlled tests. In isothermal constant-force creep tests, BiAgX is found to be 1-2 orders of magnitude more creep resistant than the selected lead-rich reference solder. The Ramberg-Osgood model is used to represent the elastic-plastic behavior of this BiAgX solder paste and the corresponding model constants are presented in this paper.