Effect of the microstructure of copper oxide on the adhesion behavior of epoxy/copper leadframe joints

The thermal oxidation of copper leadframe was carried out at 175°C and the adhesion behavior of the epoxy/copper leadframe joint was analyzed by investigating the microstructure changes of copper oxide with the thermal oxidation time of copper. The peel strength increased sharply at an early stage of oxidation (~20 min) followed by a slight increase. After further oxidation (120 min), the peel strength showed a slight decrease. The contact angles of water and diiodomethane decreased sharply at an early stage of oxidation with negligible change afterwards. As the oxidation time increased, X-ray photoelectron spectroscopy (XPS) results revealed that the chemical composition of copper oxide had changed (Cu/Cu2O → Cu2O → CuO); this change improved the wettability of the copper surface, which affected the peel strength. Increase of the surface roughness of copper oxide, investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM), causes the epoxy resin and copper oxide to undergo mechanical interlocking, which increases the peel strength. Failure analysis by SEM and XPS indicated that failure was largely in the copper oxide, and the amount of copper oxide on the peeled epoxy increased as the oxidation time increased, due to the weak mechanical strength of the oxide layer. However, a small portion of the epoxy resin was also fractured during the failure process, regardless of the oxidation time. Consequently, fracture proceeded mainly in the copper oxide close to the epoxy resin/copper oxide interface.