Adhesion and subcritical debonding at interfaces between a silica-filled epoxy underfill and a silicon die passivated by silicon nitride and benzocyclobutene (BCB) layers were investigated. Adhesion was measured in terms of a critical value of the applied strain energy release rate, G (J/m2 ). Subcritical debond-growth rates in the range of 10−9 to 10−3 m/s were characterized as a function of applied G. Adhesion and subcritical debonding were affected by changes in interfacial chemistry and environment. The surprisingly large effect of adjacent layer elastic properties on interfacial adhesion was demonstrated with simulations of interfacial fracture using a mechanics of materials approach. Interfacial chemistry was modified by using different adhesion promoters, by varying the BCB cure state, and by using different epoxy underfill resins. The effects of environmental variables were studied with temperature- and humidity-controlled environments in order to determine the separate roles of moisture activity and temperature.