Thermal and mechanical behaviors of underfills for flip-chip packaging

Recently, research and industrial interest in underfill materials in electronics packaging has increased, especially in flip chip in package (FCIP) and wafer-level packaging (WLP). Underfill materials, typically comprising an epoxy resin matrix with silica particles, are required to improve packaging reliability in chip-level devices by reducing stress due to differences in the coefficient of thermal expansion (CTE) between interfacial materials such as metal bumps and the substrates. The advent of flip-chip packaging has necessitated higher thermal and mechanical performance from underfill encapsulates. In this paper, several underfill materials from different suppliers are characterized in terms of their curing behaviors, kinetics, thermal stability, and thermo-oxidative stability using differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA) techniques. Mechanical properties were also characterized using the shear test method. The morphologies of fracture surfaces of the test specimens were also analyzed and failure modes were investigated with scanning electron microscopy (SEM). The voiding issues of underfill materials at high temperatures were also evaluated using C-mode scanning acoustic microscopy (C-SAM). The composition of underfills after thermal curing was also investigated using energy-dispersive X-ray (EDX) techniques. This study of the characteristics of underfill materials is essential in the application of flip-chip packaging. The results show that the selected underfill materials have potential for flip-chip packaging applications due to their comprehensive properties