Microstructural coarsening in Sn-Ag-based solders and its effects on mechanical properties

Solders based on Sn-Ag alloys are susceptible to microstructural coarsening during storage or service, resulting in evolution of joint properties, and hence reliability, over time. Coarsening can occur during static aging, and even faster during thermo-mechanical cycling (TMC). The kinetics of coarsening may also depend on the scale of the joint. These effects lead to evolution of the mechanical properties of the joint over time, as well as spatial variations of property within the joint. Therefore, accurate prediction of joint properties during service or storage requires a quantitative understanding of coarsening under both isothermal and TMC conditions, and incorporating these in constitutive laws. This paper discusses the kinetics of coarsening in Sn-Ag based solders, and presents a rationale for joint-scale dependence of coarsening. The impact of coarsening on creep and fracture properties of joints under drop conditions are also presented.