Warpage measurement and design of wBGA package under thermal loading

Window ball grid array (wBGA) package has become one of the most popular alternatives for DRAM packaging due to its high I/O density, high heat dissipation, low profile and high electrical performance. The reliability issue with excessive warpage of the wBGA packages, which results in solder joint failure during or after solder reflow process, or during thermal cycling, is needed to be resolved. To enhance the reliability of the wBGA, the goal of this study is to identify the key material and geometric parameters of affecting the coplanarity of wBGA and propose the solutions to it. First of all, the residual strain of epoxy molding compound (EMC), induced by its chemical shrinkage after post-mold-cure process, is characterized with the warpage measurement of bi-material using shadow moiré method associated with the finite element method (FEM) and theory. Then, the out-of-plane displacements (or warpage) of the wBGA under thermal cycling (from 25°C to 260°C) are measured by the shadow moiré method and simulated by the finite element method. The bi-material result indicated that the residual strain of the EMC with 72% volume fraction of fillers after post-mold curing was found to be 0.15%. The consistency of the experimental and simulation results also showed that chip thickness can significantly affect the warpage of wBGA with involvement of the residual strain of the EMC. And the wBGA package with 225μm-thick chip and 155μm-thick core is recommended to minimize the warpage (within 15μm ) during the thermal cycling.