Non-destructive analysis of molecular behavior and deterioration at the buried epoxy/silver interfaces

As power electronic devices for electric vehicles are advancing in performance and miniaturization, reliability of the bonding interface with sintered silver, which is highly conductive and resistant to heat, and epoxy resin for device protection has become critical. Understanding the adhesion and degradation-induced delamination between epoxy and silver at high temperatures is crucial; however, conventional analytical techniques cannot evaluate the molecular structure at buried organic/metal interfaces in situ. In this study, sum-frequency generation (SFG) spectroscopy was utilized with the transfer matrix method to extract and reconstruct the molecular behavior at the buried epoxy thin-film/silver interfaces, considering optical interference in multilayer systems. We examined the interfacial changes in epoxy/silver samples subjected to continuous heating at 150 °C. The results showed that prolonged heating formed an oxide layer at the silver interface, significantly decreasing the SFG signal intensity and disrupting epoxy molecular orientation. The growth of the silver oxide layer, estimated by a multilayer interference simulation using the transfer matrix method, was validated by cross-sectional transmission electron microscopy and energy-dispersive x-ray spectroscopy. The reconstructed spectral response at the buried interface revealed the deterioration and disruption of the interfacial epoxy molecular ordering owing to silver oxide formation. These findings elucidate the chemical changes occurring at nanoscale interfaces and the macroscopic adhesion reliability of buried organic/metal interfaces, highlighting their connection with adhesive performance.