Surface reactivity of polyimide and its effect on adhesion to epoxy

Polyimides are commonly used as organic passivation layers for microelectronic devices due to their unique combination of properties such as low dielectric constant, high thermal stability, excellent mechanical properties and superior solvent resistance. Unfortunately, polyimides are well known to be difficult to bond to other materials, especially to epoxy resins. Many surface treatments have been developed to increase epoxy–polyimide adhesion. These treatments include exposure to ion beams, plasmas and chemical solutions. The goal of our research was to relate surface reactivity of epoxy and polyimide resins to the strength of epoxy–polyimide interfaces. The surface reactivity of four polyimides was studied and quantified using contact angle measurements, flow microcalorimetry (FMC), Fourier transform infrared (FT-IR) spectroscopy (using an attenuated total reflection (ATR) accessory) and X-ray photoelectron spectroscopy (XPS). Several ways of analyzing contact angles were tried and only a weak correlation between the polar component or the acid–base components of the surface free energy with the critical interfacial strain energy release rate (i.e., the interfacial fracture strength) was observed. FMC results suggest that the strength of epoxy–polyimide interfaces is related to the molecular interactions between the curing agent and polyimide. The molecular interactions between the curing agent and polyimide surfaces were found to be either greater than epoxy and polyimide interactions or more irreversible. Therefore, the curing agent (2,4-EMI) is thought to play a critical role in controlling adhesion strength.