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Delamination at polyimide-epoxy interfaces is a major concern in microelectronic packaging (flip-chip assemblies). Typical interfacial fracture toughness values for such interfaces range from 10-60 J/m2 and the failure mode is predominately adhesive. In this study, the adhesive strengths of commercial and model underfill resins are evaluated using interfacial fracture mechanics and the adhesion mechanisms are elucidated using various microscopy and surface analysis techniques. We show that selection of the proper chemistry of the epoxy resin or treating the polyimide surface can produce tough interfaces with interfacial fracture toughness values 100 J/m2 and a failure mode that is predominately cohesive!