Microstructure coarsening during thermo-mechanical fatigue of Pb-Sn solder joints
Phase coarsening in as-cast solder joints produced by isothermal annealing and by thermo-mechanical cycling (TMC) (-30 to 130°C) was determined and its influence on the plastic deformation kinetics of 63Sn37Pb solder joints was investigated by stress relaxation (SR) tests. The static, isothermal coarsening kinetics were in accord with Senkov-Myshlyaev’s theory. The rate of coarsening by TMC was appreciably greater than that by isothermal annealing. The SR curves following TMC exhibited two regions with a stress exponent n > 5 at high stresses (Region A) and n ≈ 2-3 at low stresses (Region B). In both regions, the stress exponent varied in an irregular manner with the mean phase size. In Region A the plastic strain rate at a constant value of τn (τ = shear stress) was relatively independent of the average phase size, while in Region B the strain rate increased with increase in phase size. The activation energy in both regions appeared to be that for lattice diffusion. Possible reasons for the anomalous effects of phase size on the plastic deformation kinetics are offered.