Electromigration Lifetime Improvement of WLCSP by Using Different Structure of UBM

Electromigration (EM) lifetime needs to be continually improved to satisfy the non-stop demand for the high power and high I/O counts design of commercial product. In this study, the Wafer Level Chip Scale Package (WLCSP) were mounted with printed circuit boards (PCB) followed an EM test with 1.4 A at 160°C. Firstly, the additional Ni layer was added into the under bump metallization (UBM) and the corresponding EM lifetime approached to twice than the one of UBM without Ni layer. Moreover, the very early failure, i.e. infant mortality, can be further avoided when the increased thickness of redistribution layer (RDL) and the pre-solder of UBM were included into the test vehicles. The failure mode analysis of different samples was also included in this study. The composition and distribution of intermetallic compound (IMC) were found quite different in the samples with different UBM structures. Consequently, the content of Ag in Sn-Ag-Cu solder, Ni, Cu, and pre-solder in UBM can be all optimized to approach the better performance of EM lifetime and also to create a competitiveness production process.