Characterization methodologies for copper/polymer interfacial strength

Abstract

Copper/polymer structure is a prominent part in wafer level package (WLP) for electrical and thermal connectivity of package-to-system. Due to higher density of Cu redistribution layer (RDL) in advanced wafer level packages, the robustness of Cu/polymer structure such as adhesion is becoming more critical than ever to ensure long term reliability. In this paper, the characterization methodologies of adhesion in Cu/polymer interface are investigated. Compared to stud-pull test which is widely used in industry, fracture toughness characterized by four-point bending (4PB) test is not only effectively to identify interfacial strength but the critical strain energy release rate (Gc) also can be determined. Furthermore, a practical design of Cu/polymer structure is simulated based on the fracture modeling methodology where the VCCT (Virtual Crack Closure Technique) is applied to estimate strain energy release rate (G) at a specific Cu/polymer interface. Results show a predictive modeling approach has been successfully developed to evaluate the risk of delamination compared to critical strain energy release rate. More importantly, the material selection strategy of polymer and Cu structure design optimization for adhesion enhancement and also ensure product reliability.