Microstructural evolution in lead-free solder alloys: Part II. Directionally solidified Sn-Ag-Cu, Sn-Cu and Sn-Ag

The tin–silver–copper eutectic is a three-phase eutectic consisting of Ag₃Sn plates and Cu₆Sn₅rods in a (Sn) matrix. It was thought that the two phases would coarsen independently. Directionally solidified ternary eutectic and binary eutectic samples were isothermally annealed. Coarsening of the Cu₆Sn₅rods in the binary and ternary eutectics had activation energies of 73 ± 3 and 82 ± 4 kJmol^-1, respectively. This indicates volume copper diffusion is the rate controlling mechanism in both. The Ag₃Sn plates break down and then coarsen. The activation energies for the plate breakdown process were 35 ± 3 and 38 ± 3 kJmol^-1for the binary and ternary samples respectively. This indicates that tin diffusion along the Ag₃Sn/(Sn) interfaces is the most likely the rate-controlling mechanism. The rate-controlling mechanisms for Cu₆Sn₅coarsening and Ag₃Sn plate breakdown are the same in the ternary and binary systems, indicating that the phases evolve microstructurally independently of one another in the ternary eutectic.