Influence of temperature and humidity on adhesion of underfills for flip chip packaging

This paper systematically discusses the influence of temperature and humidity on the adhesion performance of underfill material (epoxy cured with acid anhydride), which was evaluated by die shear test after exposure to various conditions. The inherent adhesion strength between the underfill and passivation is not affected significantly by thermal cycling between -55°C and 125°C for 1000 cycles. The adhesion strength of underfill material decreases with the increase of test temperature in the investigated range, due to the decrease of modulus of the underfill with the increase of temperature. A sharp decrease in adhesion strength occurs as temperature increases toward the glass transition temperature of the underfill material. Adhesion strength of underfill with different passivation materials decreases after aging in a high temperature and high humidity environment. The extent of the decrease depends on underfill formulation and the hydrophilicity of the passivation material. Hydrophilic passivation such silicon oxide (SiO₂) and silicon nitride (Si₃N₄) shows much more severe adhesion degradation than hydrophobic passivation such as benzocyclobutene (BCB) and polyimide (PI). Adhesion degradation is slower than moisture diffusion. The adhesion stability for hydrophilic passivation can be successfully improved by use of a coupling agent such as silane that introduces stable chemical bonds at interface.