Recrystallization behavior of lead free and lead containing solder in cycling

The present work addresses the effects of thermomechanical history on the recrystallization behavior of lead free and backward compatible solder joints. 30 mil SAC305 balls were reflowed onto BGA pads using either a SAC305 or a eutectic SnPb paste. Systematic variations of the rate of recrystallization with precipitate coarsening as well as with cycling and annealing parameters were characterized and correlated with observations from thermal cycling experiments on lead free BGA assemblies. The introduction of a few percent of Pb has been seen to not only affect the distribution of secondary precipitates but also add minute inclusions of Pb within the individual Sn dendrites. These also act as barriers to dislocation motion but do not coarsen as rapidly as the precipitates, and we find recrystallization in cycling to be strongly delayed.