Fatigue Life Prediction and Analysis of Wafer Level Packages with SnAgCu Solder Balls

Currently, various mainly tin-based lead free solders are in use or under evaluation. Many solder fatigue models have been developed to predict the fatigue life of solder joints under low-cycle fatigue conditions, however, because of the diversity of solders, more work is needed in this respect. The theoretical fatigue life prediction of solder joints, based upon non-linear finite element (FE-) calculation results, requires both a solder constitutive model and a Coffin-Manson type failure criterion, if an engineering type of evaluation is chosen. The material investigated was near-eutectic SnAgCu solder with lower silver content, i.e Sn98.5Ag1Cu0.5, which is one choice currently discussed. Properties like the lower creep resistance but a better brittle failure performance combined with lower cost can make the SnAg1Cu0. 5 solder a candidate material for a broader application. The study deals with the low-cycle fatigue performance of this material. A special wafer level test package (WLP) was used for the evaluation. The derived fatigue model is based on experimentally observed low-cycle fatigue data compared to FE results. In FEA effects of input quantities scatter, of different element types and mesh densities, and different analysis codes are studied and a general averaging strategy, to be used in Coffin-Manson type failure criterions for ball interconnects, is proposed