The effects of the functionality of an epoxy monomer on the composite properties and reliability of anisotropic conductive films (ACFs) in a flip-chip package were investigated. Three epoxy monomers with different functionalities (f = 2-4) were considered. The ACFs prepared using epoxy monomers with higher functionality resulted in lower molecular weight between crosslinks (M-c). As the M-c decreased, the elastic modulus (E') and coefficient of thermal expansion (CTE) were improved. These results were highly consistent with the rubber elasticity theory. The reliability performance of the flip chip on organic substrate assemblies using ACFs were also investigated as a function of epoxy functionality. The ACFs prepared by using higher functional epoxy monomers showed improved reliability performance.