In this article, the filler distribution in an epoxy molding compound (EMC), used in IC packaging, is studied across a wafer using high-resolution X-ray computed tomography (HR-XCT). It is widely assumed that the fillers are uniformly distributed across the wafer. However, it is demonstrated that the distribution of the filler deviates across the wafer and that filler sizes affect the distribution on edge and center of wafer too. Quantitative HR-XCT of the filler distribution provides accurate data for simulation. Based on this approach, the differences between the coefficient of thermal expansion (CTE) and Young's modulus values of the filler material for the center and edge of the wafer are explained. Furthermore, compression simulations are conducted by applying an XCT-based finite element method (FEM) model to understand the role of EMC for the mechanical behavior of advanced IC packages. These findings are validated by in situ and stand-alone compression experiments. The accurate simulation results demonstrate that the use of an XCT-based FEM model provides insight into the mechanical behavior of the EMC itself and, furthermore, of the whole IC package.