Study on mold flow during compression molding for embedded wafer level package (EMWLP) with multiple chips

Abstract

The rapidly increasing demand of embedded wafer level package (EMWLP) due to its advantages, smaller form factor and flexibility in system level integration leads to the development of reconstructed wafer level encapsulation. The reconstructed wafers are encapsulated with epoxy molding compound using compression molding. Due to EMWLP advanced applications, there is a need to use multi chips with different layout in a single package. The overall reconstructed wafer design then became complex that eventually leads to asymmetrical chips layout within the wafer. One major challenge in molding of reconstructed wafer with multi-chip layout was the incomplete filling due to imbalance mold compound flow during compression molding. This study was conducted to determine the actual mold compound flow during compression molding of EMWLP with multi chips layout. Mold flow studies has been carried out on different multi-chip layouts using ANSYS Poly flow/Fluent software and results revealed that asymmetrical chips layout had imbalance mold flow response. The result of the mold flow simulation was then compared to the actual mold compound flow during compression molding by performing intentional short shots at different mold filling stages. It was confirmed that actual molding with asymmetrical chips layout also resulted to unbalance mold filling. The flow of the molding compound in areas with wider gaps was faster compared to areas with narrow gaps. This suggests that the chips layout determines the actual mold compound flow during compression molding. Balanced mold compound flow was achieved by re-arranging the chips into a symmetrical layout. In addition, this paper also shows that by changing the dispensing pattern to oval shape, the actual mold compound flow on asymmetrical chips layout became balanced. The mold flow simulation results with different chips layout were validated with experimental mold compound flow tests. The simulation and experimental results revealed that the chips layout and mold compound materials dispensing pattern are critical to achieve excellent molding quality results.