Microstructural evolution in lead-free solder alloys: Part I. Cast Sn–Ag–Cu eutectic

Coarsening of the ternary eutectic in cast Sn–Ag–Cu lead-free solder alloys was investigated. The process was found to follow r³ ∝ t kinetics where r is the rod radius of the dispersed phase and t is time. The effective activation energy for the process is 69 ± 5 kJmol^-1. The two types of intermetallic rods, Cu₆Sn₅ and Ag₃Sn, in the eutectic structure coarsen at different rates, with each having a different rate-controlling mechanism. The overall coarsening kinetics for the Sn–Ag–Cu ternary eutectic is significantly slower than that found for the Pb-Sn eutectic, which has implications for long-term reliability of Sn–Ag–Cu solder joints.