Understanding adhesion issues at underfill/passivation interfaces

The factors affecting the adhesion of underfill/polyimide interfaces relevant to flip-chip assemblies were investigated in this study. The study consisted of three distinct parts: an examination of the chemical and mechanical contributions to practical adhesion, a comparison of the initiation and propagation phenomena, and a comparison of the relationship between sub-critical and critical interfacial failure.; The first part involved the use of fracture toughness testing to characterize the interfacial strength of several underfill interfaces. Molecular interactions studies using flow microcalorimetry and identification of crack tip shielding mechanisms were used to explain the observed adhesion. It was shown that the interfacial fracture toughness exhibited a strong dependence on the strength of the molecular interactions, with the basicity of the underfill curing agent playing a strong role. The amount and type of the crack tip shielding mechanism had a significant effect on the interfacial toughness, with the extrinsic shielding mechanisms in the filled epoxy underfills increasing practical adhesion if allowed to operate at the interface. The results served to highlight the importance and interplay of the two contributing factors of interfacial toughness.; The initiation of underfill interfacial failure was studied by characterizing failure at a free-edge with a critical stress intensity factor. In comparison with the interfacial fracture toughness testing, it was shown that a good correlation exists between the initiation and propagation of interfacial failures. Such a correlation justifies the continuing use of fracture mechanics to predict the reliability of flip-chip packages.; The final aspect of the research involved fatigue testing of tensile butt joint specimens to determine lifetimes at sub-critical load levels. The results display an interfacial strength ranking similar to that observed during monotonic testing. Additionally, flip-chip four point bend geometries tested under both thermal and mechanical fatigue also exhibited a ranking of interfacial strengths in the same order as other adhesion tests. The fatigue results indicate that monotonic fracture mechanics testing may be an adequate screening tool to help predict cyclic underfill failure; however lifetime data is required to predict reliability.