Comparative Analysis of Warpage Behavior of CTBN-Modified Epoxy-Based Underfill for Flip-Chip packaging

Warpage in advanced semiconductor packages is mainly caused by the coefficient of thermal expansion (CTE) mismatch between silicon chips and PCB substrates, leading to delamination, bonding failure, and reliability degradation. To mitigate these issues, underfill materials with improved stress-relaxation capability are required. In this study, silica-filled epoxy underfill materials were formulated using epoxy resin with different reactive diluent compositions to investigate the influence of Carboxyl-Terminated Butadiene Acrylonitrile (CTBN)-modified epoxy on package warpage behavior. The underfills consisted of an epoxy resin, reactive diluent, silane-treated silica fillers, and 2-Ethyl-4-methylimidazole (2E4MZ) as the curing agent. CTBN-modified epoxy systems with different low molecular weight contents and a commercial underfill formulation were investigated. The prepared underfills were applied between silicon chips and PCB substrates and thermally cured. Warpage behavior was characterized using the Shadow Moiré technique during thermal cycling from 25 °C to 260 °C and back to 25 °C. All specimens exhibited a transition from crying to smile warpage during heating and returned to crying after cooling, reflecting the CTE mismatch between the silicon chip and PCB substrate. Differences in warpage behavior were observed depending on the reactive diluent composition, and the effects of low molecular weight content and epoxy functionality on package deformation were compared. These findings provide insight into the relationship between CTBN-modified epoxy composition and package warpage behavior, demonstrating the potential of CTBN-modified epoxy for developing low-stress underfill materials for advanced semiconductor packaging.
- This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. RS-2025-25396489).