Molded Underfill (MUF) process, used in Stacked-Up Package Technology, suffers from void formation that significantly impacts yield and compromises reliability at the product level. However, a concrete methodology, to resolve such issues, remains unclear due to the complexity of package structure and convolution of viscous material flow during MUF process. In this paper, we focus on the ... [Show full abstract] formulation of void nucleation mechanism and solution to optimize the MUF process. Based on empirical assessments, Computational Fluid Dynamics (CFD) was employed to simulate Epoxy Molding Compound (EMC) flow through bump array in a metal mold frame, where air and liquid EMC coexist as a two-phase flow. When EMC enters the solder bumps, attached to silicon IC in a metal mold, a vortex flow tends to develop due to unbalanced EMC flow associated with bump distribution and pressure gradient during the MUF process. In particular, the vortex flow occurred near the vent zone, in which there is a relatively lower molding pressure compared with the molding entrance. This hardly prevents void nucleation, from air trapping into viscous EMC, which significantly affects micro void formation and leads to a short fail during reflow and solder mount. In fact, location and size of voids depend on bump design such as bump density and structure. Intrinsic key parameters for void formations from MUF process include mold flow direction, cavity pressure, and bump array. Finally, a practical guideline to suppress voids and optimize MUF package process is delivered to ensure a mature and reliable production.