Epoxy-based underfill encapsulant materials are used in advanced microelectronic packaging to reduce thermal stresses on the solder joints and interconnections in integrated circuits. Therefore, their thermal properties directly affect package performance and reliability. In this study, the thermal properties of an epoxy-based underfill material developed for Intel's flip chip packaging were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermomechanical analysis (TMA), dynamic mechanical thermal analysis (DMTA) and DMA techniques. Experimental results showed that this epoxy can be cured rapidly with low core weight loss. Near room temperature, the cured epoxy has a low coefficient of thermal expansion (CTE) and a moderate storage modulus, resulting in a low stress index. The glass transition temperature and thermomechanical properties as a function of epoxy curing conditions will he discussed.