Thermal Warpage Control of Epoxy Underfill Materials

Warpage in advanced semiconductor packages is mainly caused by the coefficient of thermal expansion mismatch between silicon chips and PCB substrates, leading to delamination, bonding failure, and reliability degradation. In this study, silica-filled epoxy underfill materials were formulated using KDS8805HP epoxy resin with different reactive diluent compositions, and their warpage behavior and reliability were evaluated. The underfills consisted of KDS8805HP epoxy resin, reactive diluent, silane-treated 500 nm silica fillers, and 2E4MZ curing agent. Five reactive diluent systems were examined: CTBN 0.3%, CTBN 0.6%, CTBN 3.0%, CTBN commercial reference, and LD224HP. The underfills were applied between silicon chips and PCB substrates, followed by thermal curing. Warpage was analyzed using Shadow Moiré during heating from 25 °C to 260 °C and cooling back to 25 °C. All specimens showed crying behavior at room temperature, changed to smile behavior during heating, and returned to crying behavior after cooling, due to the larger thermal expansion of the PCB. Among the tested formulations, LD224HP exhibited the best overall warpage behavior. In the CTBN-based systems, CTBN 0.6% showed warpage suppression comparable to the commercial reference, with a total warpage index of 180 μm, close to 178 μm for the reference. In contrast, CTBN 3.0% showed the largest warpage variation and crack formation after reliability screening, indicating that excessive low-molecular-weight components can reduce reliability. After aging and repeated reflow tests, discoloration became more pronounced, especially in the CTBN commercial reference, possibly due to higher chlorine content. Overall, CTBN 0.6% provided effective warpage control and improved material stability, suggesting its potential as a reactive diluent for epoxy-based underfill materials in advanced semiconductor packaging.

Keywords: Epoxy, Underfill, CTBN, Warpage, Shadow Moiré, Reliability