Measurement of interfacial fracture toughness under combined mechanical and thermal stresses

The interfacial fracture resistance of a polyimide passivation/underfill interface was measured using an asymmetric double cantilever beam technique. A thin layer of polyimide was coated on a standard epoxy as one of the beams and an underfill was flowed over the polyimide film and cured to form the second beam. The relative contributions of the thermal and mechanical stress were systematically varied by varying the thickness ratio of the two beams. The results indicate that the effect of thermal residual stress must be taken into account in the calculation of critical strain energy release rate to obtain the interfacial fracture energy.