Evaluation of Polymer Core Solderballs for Wafer Level Reliability Improvements

Abstract

Nearly half a century ago the first die bumping was developed by IBM that would later enable what we call Wafer Level Packaging. It took nearly 40 years for Wafer Level Chip Scale Packaging (WLCSP), with all of the “packaging” done while still in wafer form to come into volume production. It began with very small packages having solderball counts of 2–6 I/Os. Over the years, the I/O count has grown, but much of the industry perception has remained that WLCSPs are limited to low I/O count, low power applications. But within the last few years, there have been growing demands for WLCSP packages to expand into applications with higher levels of complexity. With the ever increasing density and performance requirements for components in mobile electronic systems, the need has developed for an expansion of applicability for Wafer Level Package (WLP) technology. Wafer Level packaging has demonstrated a higher level of component density and functionality than has been traditionally available using standard packaging. This has led to the development of WLCSPs with larger die and increasing solderball connectivity counts. Development activity has been ongoing for improved materials and structures to achieve the required reliability performance for these larger die. For this study, we have evaluated several different metallic structures used for polymer core solderballs with two different WLCSP structures. The WLCSP structures which were evaluated included a standard 4-mask design with redistribution layer (RDL), using a Polymer 1, Metal RDL, Polymer2, and Under Bump Metallization (UBM); as well as a 3-mask design with RDL, using a Polymer 1, Metal RDL, and Polymer 2. In the first case, the solderballs are bonded to the UBM, while in the second case the balls are bonded to the RDL, using the Polymer 2 layer as the solder wettable defining layer. All of the combinations are tested using the standard JEDEC Temperature Cycling on Board (TCOB) and Drop Test (DT) methodologies. The two different metallurgies of the polymer core solderballs appear to react differently to the two different WLCSP structures. This suggests that the polymer core solderball compositions may perform best when optimized for the specific WLCSP structures that are manufactured. We will review the results of the impact of the different polymer core metallurgies on the TCOB and DT reliability performance of the WLCSPs, showing the interactions of these materials with the two WLCSP structures.