Elucidation of Adhesive Interaction between the Epoxy Molding Compound and Cu Lead Frames

Clarification of adhesive interactions in semiconductor packages can improve reliability of power electronics. In this study, the adhesion interfaces between the epoxy molding compound and Cu-based lead frames were analyzed using the density functional theory. A resin fragment was prepared based on the polymer framework formed in the curing reaction of epoxy cresol novolac (ECN) and phenol novolac (PN), which are typical molding materials. The resin fragment was optimized on the surfaces of Cu and Cu₂O. We calculated the charge density differences for adhesion structures and discussed the origin of adhesive interactions. The ECN-PN fragment's adhesion to the Cu surface relied mainly on dispersion forces, whereas in the case of Cu₂O, the resin bonded chemically to the surface via (1) σ-bonds formed between the ECN-PN's OH group oxygen and coordinatively unsaturated copper (Cu_CUS) and (2) hydrogen bonds between resin's OH groups and coordinatively unsaturated oxygen (O_CUS) located close to to Cu_CUS, resulting in a stable adhesive structure. The energy required to detach the resin fragment from the optimized structure was determined using the nudged elastic band method in each model of the adhesive interface. Morse potential curve was used to approximate the obtained energy, and the energy differentiation by detachment distance yielded the theoretical adhesive force. The maximum adhesive stress was 1.6 and 2.2 GPa for the Cu and Cu₂O surfaces, respectively. The extent to which the ECN-PN fragment bonded to the Cu₂O surface stabilized was 0.5 eV higher than in the case of the Cu surface.