Influence of microstructure size on the plastic deformation kinetics, fatigue crack growth rate, and low-cycle fatigue of solder joints

The influence of microstructure size on the plastic deformation kinetics, f atigue crack growth rate and low-cycle fatigue of eutectic Sn-Pb solder joi nts is reviewed. The principal microstructure feature considered is the ave rage eutectic phase size d = (d(Pb) + d(Sn))/2. The effect of an increase i n reflow cooling rate (which gave a decrease in d) on the flow stress and o n fatigue life was irregular at 300K, depending on the stress or strain lev el and cooling rate. In contrast, a consistent increase in fatigue life wit h decrease in d occurred for thermomechanical cycling between -30 degrees a nd 130 degrees C. Constitutive equations for plastic deformation and fatigu e crack growth rate are presented which include the microstructure size. It appears that the rate-controlling deformation mechanism is the intersectio n of forest dislocations in the Sn phase. The mechanism for both static and dynamic phase coarsening appears to be grain boundary diffusion with a t(1 /4) time law. Some success has been achieved in predicting the cyclic stres s-strain hysteresis loops and fatigue life, including the influence of the as-reflowed microstructure size and its coarsening. Additional definitive s tudies are however needed before we can accurately predict the fatigue life of solder joints over the wide temperature range and conditions experience d by electronic packages.