FEA study of the evolution of wafer warpage during reflow process in WLP

In this paper, the evolution of wafer warpage during reflow process in wafer level packaging (WLP) is investigated by finite element analysis (FEA). The investigation focuses on three different fan-in WLP technologies: ball on polymer WLP without under bump metallurgy (UBM), ball on polymer WLP with thick UBM layer and encapsulated copper post WLP. Both wafer-level model and ball-level model are built and the results indicate that wafer warpage derived from ball-level model is compatible with wafer-level model. Ball on polymer WLP with thick UBM layer has the maximum warpage after reflow process. Reflow profiles with different cooling rate are also simulated.