Crack evolution and rapid life assessment for lead free solder joints

This paper reports on the crack behavior and microstructure evolution of lead free solder joints during thermal and isothermal fatigue testing. Daisy chained Ball Grid Array (BGA) assemblies were subjected to either accelerated thermal cycling or cyclic bending at room temperature. In the case of thermal cycling effects of strain range were assessed by comparing assemblies with different distances to neutral point. Crack areas were measured by dye and pry and recrystallization detected by optical microscopy with crossed polarizers. The stochastic nature of the fatigue crack growth associated with variations in Sn grain orientation was addressed by using pattern recognition with Artificial Neural Network (ANN). Different statistics were calculated for the different crack curves at specific life percentages to address the change in crack growth distributions at these different live intervals. Results suggested that the number of cycles to failure could be predicted by the measurement of crack lengths after less than 10% of a typical thermal cycling test. Preliminary results suggest that this may also be true for isothermal cycling, but trends appear to be less easily generalized.