Effects of microstructure evolution on damage accumulation in lead-free solder joints

The wear out of lead-free solder joints under realistic loading conditions has been shown to deviate strongly from predictions based on current damage accumulation models. We argue that the deviation must be due to the simultaneous evolution of solder properties and damage. In general, solder properties and fatigue behaviors are determined by microstructure and damage accumulation mechanisms. Literature has reported on effects of precipitate coarsening and recrystallization of SnAgCu solders. However, we show these cannot account for critical trends in isothermal cycling such as repeated drops, bending and vibration. The present paper addresses an additional microstructure evolution path. Thermal aging and room temperature shear fatigue test on SnAgCu solder joints both demonstrated continuous hardness decrease. But precipitate coarsening was not observed in the shear fatigue test. Specially designed sample sectioning allowed the observation of slip bands formation and correlation with cyclic softening in shear fatigue test. In addition, the pattern of slip band formation was shown to be load-dependent, indicating the difference in damage accumulation. The consequences for the prediction of fatigue life under combined loading are discussed.