Kinetics of Pb-rich phase particle coarsening in Sn-Pb solder under isothermal annealing-cooling rate dependence

摘要

The coarsening behavior of the Pb-rich phase particles in 63Sn-37Pb (wt percent) solder was investigated following isothermal annealing treatments. Samples were exposed to cooling rates of 0.1, 1.0, 10, and 100 deg C/min. Annealing temperatures were 25, 55, 70, 85, and 100 deg C, and times were 2-100 days. The mean particle diameter decreased from 1.8 X 10~(-3) to 0.8 X 10~(-3) mm with increased cooling rate, indicating two solidification regimes: one for cooling rates <=l deg C/min and the other for cooling rates of >=10 deg C/min. The Pb-rich phase particles coarsened more quickly in samples made at the two fastest cooling rates. There was little Pb-rich phase particle coarsening at 25 and 55 deg C for all annealing times. Coarsening rate kinetics were examined specifically for the 10 and 100 deg C/min data using the expression A_t~n exp[-triangle open H/RT]. The values of n were 0.23 +-0.11 and 0.36 +- 0.13, respectively; n was not sensitive to annealing temperature. The corresponding ln values indicated that the coarsening mechanism changed from a fast diffusion to a bulk diffusion controlled process with a faster cooling rate. The apparent activation energy triangle open H ranged from 16+-8 to 41+- 8 kJ/mol; the values increased with cooling rate from 10 to 100 deg C/min. The triangle open H value was sensitive to annealing temperature only for the faster cooling rate of 100 deg C/min. Together, the n and triangle open H values indicated that an accelerated, fast diffusion mechanism with low activation barriers characterized the Pb-rich phase coarsening in samples exposed to a slower cooling rate, greater annealing, or a combination of the two conditions. That mechanism likely originated from the in situ development of recover/recrystallization microstructures in the Sn-rich phase. At faster cooling rates, those microstructures were not as well developed, so coarsening was controlled more by the higher activation barriers of bulk diffusion processes.