Study on influence of environment on adhesion performance of underfill for flip chip application

This paper systematically discusses environmental effects (temperature and humidity) on the adhesion performance of underfill materials. The adhesion strengths of underfill with passivation materials were measured by die shear tests for various conditions. The adhesion strength between underfill and passivation is not affected significantly after thermal cycling tests for 1000 cycles. Die shear tests at elevated temperatures showed that underfill adhesion strength decreases with increase in test temperature above room temperature. The decreased adhesion strength versus temperature is probably due to the decrease of underfill modulus with increase in temperature. The sharp decrease occurs at a temperature below the glass transition temperature of the underfill. Underfill adhesion strength with different passivation materials decreases after aging in a high temperature/humidity environment. The adhesion degradation after high temperature/humidity aging is dependent on the hydrophilicity of the passivation. Hydrophilic passivation, e.g. SiO₂ and SiN, shows much more severe adhesion degradation than hydrophobic passivation such as benzocyclobutene and polyimide. It is shown that adhesion stability for hydrophilic passivation can be successfully improved by use of coupling agents such as silane that introduce stable chemical bonds at the interface. Moisture diffusion kinetics into the 2 mm×2 mm die shear sample were studied. Although the adhesion strength degradation is due to moisture diffusion into the underfill, the adhesion degradation rate of die shear samples during aging at 85°C/85% RH is slower than moisture diffusion