Effect of BCPs on Temperature-dependent Fracture Behaviors of Underfill: In-situ SEM-DIC Characterization

The growing complexity of advanced packaging architectures, such as fan-out and chiplet integration, requires underfill (UF) materials with high fracture toughness and thermal stability to ensure interconnect reliability. However, the temperature-dependent toughening mechanisms of UF systems remain poorly understood, hindering their optimization for thermal cycling conditions. This study investigates the influence of block copolymer (BCP) content on the fracture performance of micro-silica-reinforced epoxy underfill. A hybrid toughening strategy was employed, combining intrinsic nanoscale (BCP) and extrinsic microscale (SiO₂) mechanisms. The optimized formulation exhibited a 67.1% increase in mode-I critical energy release rate (GIC) compared to the unmodified system while maintaining comparable tensile strength and glass transition temperature (Tg). Fracture tests were conducted over a wide temperature range (25°C–260°C) to simulate real-world operating conditions in advanced packaging. In-situ SEM three-point bending, post-fracture morphology analysis, and digital image correlation (DIC) were used to examine crack propagation and strain localization behavior. The results demonstrate that synergistic toughening remains effective at elevated temperatures, enhancing energy dissipation. Furthermore, thermal cycling warpage tests on UF–Si structures revealed that the 0.5wt% BCP-filled system exhibited the lowest warpage, indicating improved thermomechanical reliability. This study provides fundamental insights into multiscale toughening mechanisms under thermal stress and supports the development of robust UF materials for next-generation heterogeneous integration.