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In order to enhance the reliability of flip chip packages, the initiation and propagation of various interfacial failures and robust interfacial bonds between the underfill and the other components are highly desired. The water molecules inside the plastic material were chemically bonded with polymers by hydrogen bonds in the microholes formed by the polymer molecule chains. The bonding of water molecules and polymers reduced the adhesion strength at the interface between epoxy material and die. In this study, the interfacial bond strengths of commercial underfills with silicon nitride passivation are measured using bottom shear test. The thermal degradation of epoxy-based underfill material has been studied by thermogravimetric analysis (TGA). The results show that adhesion strength is correlated with TGA weight loss curve. Besides, epoxy is sensitive to moisture at high temperature storage. Moisture diffusion characterization at high temperatures in polymeric packaging materials is important since moisture absorption of polymeric packaging materials plays a determining role in “popcorn cracking” of IC packages during the solder reflow process especially for lead-free solder reflow profile. The moisture absorption was measured by using mass transport and diffusion theory.