Analysis of Underfill-Polymer Interfacial Adhesive Strength by Combined Experimental and Modeling Approaches

Underfill (UF) polymer (PM) structures are becoming increasingly popular in fan-out packages because they exhibit several advantages, such as protection from mechanical stress and package- to-system electrical and thermal conductivity. The laminate integrity of UF-PM interfaces plays a crucial role in determining fan-out package performance and long-term reliability. In this study, we presented a fracture-mechanics-based experimental method and finite element method to quantitatively determine the interfacial adhesive performance of UF PMs. The adhesion in the UF-PM interface was experimentally characterized using the four-point bending (4PB) test. Unlike the stud-pull test, which is widely used in industry, fracture toughness can be characterized using the (4PB) test, which is effective in determining not only the interfacial strength but also the critical strain energy release rate (Gc). Furthermore, an underfill polymer structure design was simulated based on the fracture modeling methodology in which the virtual crack closure technique (also known as virtual crack closure integral) was applied to estimate strain energy release rate (G) at a specific UF-PM interface. This result revealed that the proposed predictive modeling approach could successfully evaluate the risk of delamination. Crucially, these techniques can be applied to rapidly evaluate new UF and polymer material selection strategies and to examine the effect of process modification on adhesive performance in various environments.