The effects of phase coarsening in 63Sn37Pb solder joints produced by isothermal annealing and during thermo-mechanical cycling (TMC) on the fatigue cracked area growth rate dAc/dN were investigated. The phase coarsening by isothermal annealing was in accord with the mechanism of interphase boundary diffusion; that during TMC was significantly greater than by isothermal annealing. The phase coarsening in the range considered (D=1–5 µm, N<150 cycles) had only little, if any, influence on dAc/dN, which occurred mainly in Stage I of the fatigue crack growth rate regime. Results obtained previously, however, indicate that for N>150 cycles dAc/dN is significantly decreased by a reduction in the as-reflowed phase size. Reasonable agreement occurred between the calculated fatigue lifetime vs initial as-reflowed phase size and that measured.
