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 Cu6Sn5 rods 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 Cu6Sn5 rods 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(-1) for 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 Cu6Sn5 coarsening 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.