Viscoelastic Modeling for Heterogeneous Fan-out Wafer Molding Process

Abstract

Fan-out wafer level package (FOWLP) is a disruptive technology in the semiconductor packaging industry. Demand for higher system performance has caused an increase in both package size and the complexity of heterogeneous integration. Large warpage, which arises from significant volume changes in molded underfill (MUF) during the curing and subsequent assembly processes, is a top process and reliability issue.The selection of molding material is critical importance in FOWLP, as the material must meet multiple manufacturing requirements. In this study, an integrated modeling approach is used to predict fan out wafer form warpage. This approach considers both chemical shrinkage and cure dependency of molded underfill. Viscoelastic relaxation behavior over the course of compression molding cured (namely CMC) and post molding cured (namely PMC) has been carefully modeled. Measurements of material properties used in our models were characterized through differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The result of our integrated modeling approach was validated by comparing actual warpage data of various temperature loading conditions. Predicted warpage values are in good agreement with in-line experimental data.Furthermore, we apply this methodology to study the wafer fan-out ratio and Cu density effect. It is shown that Cu density effect is not sensitive, and higher wafer fan-out ratio induces larger warpage due to more molding volume.