Optimization of black oxide coating thickness as adhesion promoter for copper substrate

Black oxide is a conversion coating applied onto copper surface to improve the interface adhesion with polymeric adhesives and moulding compounds. State-of-the-art analytical instruments, including SEM, TEM, XPS, AFM, XRD, Goniometry, TEM, Dynamic SIMS and RBS were employed to characterize the coated surface and interphase with glob top resins. It was found that the copper oxide layer consists of cupric and cuprous oxides with a continuous change of oxide composition from the top surface to the inside without a distinct boundary in between. Crystallinity of the oxides was barely detected directly from the black oxide coated copper. The cupric oxide exist in the form of a long, fibril structure on nanoscopic scale. The interface bond strength between the copper oxide and glob-top resin increased rapidly in the low range of oxide thickness and became almost constant with further increase in oxide thickness. A functionally similar dependence of oxide thickness and interface adhesion on treatment time was also revealed. Fracture occurred mainly within the oxide layer for black oxide coated substrate (i.e. cohesive failure of black oxide), while fracture tended to occur along the coating-resin interface (i.e. adhesive failure) once the coated surface is debleeded by sandblasting.