Understanding the strength of epoxy-polyimide interfaces for flip-chip packages

Polyimides are commonly used as organic passivation layers for microelectronic devices due to their unique combination of low dielectric constant, high thermal stability, and excellent mechanical properties. Polyimides are well known to have poor adhesion to epoxy resins. Many surface treatment methods have been developed to increase epoxy-polyimide adhesion. These include various ion beam, plasma treatment, or chemical treatment methods. The goal of this research is to understand the strength of epoxy-polyimide interfaces by studying the effect of polyimide chemical structure on epoxy-polyimide adhesion. The four polyimides chosen in this study are commonly used in the microelectronics industry: poly (pyromellitic dianhydride-oxydianiline [PMDA-ODA], poly (3,3´,4,4´-biphenyltetracarboxylic dianhydride-phenylene diamine [BPDA-PDA], poly (hexafluoroisopropylidene-diphthalic anhydride-oxydianiline) [6FDA-ODA], and 5(6) -Amino-1-(4-aminophenyl)-1,3,3, trimethylindanbenzophenonetetacarboxylic dianhydride copolymer [BTDA-DAPI]. The adhesive strengths between an epoxy resin and these four polyimides were characterized using interfacial fracture mechanics and the critical interfacial strain energy release rates ranged from 20 to 179 J/m² depending on the particular polyimide used. The loci of failure for these epoxy-polyimide interfaces were analyzed using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR), and found to be at the interphase region for all four interfaces. It is interesting that the strength of the interfaces appears to be related to the predicted interfacial widths from solubility parameter theory.