An underfill is used to fill the gap between the integrated circuit chip and substrate to improve the solder joint fatigue life in flip-chip packaging. The influence of aging in an environment with a high temperature and a high humidity on the adhesion performance of an underfill material (epoxy cured with acid anhydride) to the passivation layer in flip-chip packaging is discussed. Adhesion of the underfill to all passivation materials investigated degrades after aging in a high temperature and high humidity environment. The extent of this degradation is dependent on the hydrophilicity of the passivation layer surface. Hydrophilic passivation layer surfaces, such as silicon dioxide (SiO2) and silicon nitride (Si3N4), show much more severe adhesion degradation than hydrophobic passivation layer surfaces, such as benzocyclobutene (BCB) and polyimide (PI). The mobility of both the absorbed water and polymer chains is studied with solid state nuclear magnetic resonance (NMR) spectroscopy. Higher mobility of both the absorbed water and polymer chains in the rubbery state of polymers contributes to faster adhesion degradation during high temperature and high humidity aging. The adhesion stability of hydrophilic passivation layers can be successfully improved by use of a silane coupling agent that introduces a stable chemical bond at the interface. A flow micro-calorimeter is used to study the adsorption of silane coupling agents onto glass surface. The difference in adhesion retention improvement between aminosilane and epoxysilane is discussed.