Environmental influence on adhesion of underfill with passivation materials

The environmental influence on adhesion of underfill to passivation materials for flip chip packaging is discussed in this paper. Thermal cycling between -55°C and 125°C does not significantly affect the adhesion strength. Adhesion strength of underfill material decreases with the increase of test temperature above room temperature due to the decrease of modulus of the underfill. The sharp decrease in adhesion strength occurs at a temperature approaching and below the glass transition temperature of underfill material. The adhesion degradation after aging in a high temperature and high humidity environment is dependent on the hydrophilicity of the passivation material. Hydrophilic passivation, such as SiO₂ and Si₃ N₄, shows much more severe adhesion degradation than hydrophobic passivation, such as benzocyclobutene and polyimide. It is demonstrated that the adhesion stability for hydrophilic passivation can be successfully improved by use of a coupling agent such as silane that introduces stable chemical bonding at the interface. The adhesion degradation process during aging in an 85°C 85% chamber is slower than moisture diffusion