摘要
With the rapid development of automotive electronics, artificial intelligence, and mobile applications, electronic packaging faces increasing demands for miniaturization, performance, and environmental sustainability. Although flip-chip packaging is widely adopted in high-density applications, the coefficient of thermal expansion mismatch (CTE) among packaging materials often induces solder joint fatigue under thermal cycling. Conventional underfill can mitigate thermal stress but suffers from long processing time, void formation, and high energy consumption. Unlike conventional underfill, epoxy flux structure selectively encapsulates regions prone to solder joint failure without fully filling the chip-substrate gap. This study establishes a three-dimensional finite element model (3D FEM) to investigate the thermo-mechanical reliability of wafer-level chip-scale packages (WLCSPs) with epoxy flux structure under thermal cycling from -40 °C to 85 °C. The effects of epoxy flux fill height and coverage area on the equivalent plastic strain of SAC305 solder joints are evaluated. The results demonstrate that epoxy flux structure provides effective mechanical reinforcement and improves solder joint reliability in high-density packaging applications. © 2026 Japan Institute of Electronics Packaging.