Microstructure coarsening during static annealing of 60Sn4OPb solder joints: I stereology

The phase size distributions (PSDs) of the Sn, Pb, and the combined Pb and Sn phases, and the shape factor delta were determined for as-reflowed 6OSn4 0Pb solder joints and following their annealing at 50 degreesC to 150 degre esC. The PSDs in all cases had a positive skew which was approximated by a log-normal function. The PSDs were time invariant and were in accord with t he Bitti and Nunzio model for phase coarsening. The shape factor, delta, de creased with the mean of the combined Pb and Sn phase size, and with temper ature. Whether or not delta affects the PSDs is not clear.