PHASE COARSENING AND CRACK-GROWTH RATE DURING THERMOMECHANICAL CYCLING OF 63SN37PB SOLDER JOINTS

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 dA(c)/dN were investigated. The phas e coarsening by isothermal annealing was in accord with the mechanism of interphase boundary diffusion; that during TMC was significantly gr eater than by isothermal annealing. The phase coarsening in the range considered (D = 1-5 mu m, N < 150 cycles) had only little, if any, inf luence on dA(c)/dN, which occurred mainly in Stage I of the fatigue cr ack growth rate regime. Results obtained previously, however, indicate that for N > 150 cycles dA(c)/dN is significantly decreased by a redu ction in the as-reflowed phase size. Reasonable agreement occurred bet ween the calculated fatigue lifetime vs initial as-reflowed phase size and that measured.