Surface property of passivation and solder mask for flip chip packaging

Adhesion of underfill to passivation and solder mask is critical to the reliability of an underfilled flip chip package. In this study, the surface properties of solder mask and four passivation materials: benzocyclobutene (BCB), polyimide (PI), silicon oxide (SiO₂), and silicon nitride (SiN), along with their respective preparation procedures were investigated. A combination of both wet and dry cleaning processes was very effective to remove contaminants from the surface. The oxygen atom, introduced during O₂ plasma treatment or UV/O₃ treatment, led to the increase of the base component of surface tension. X-ray photoelectron spectroscopy (XPS) experiments confirmed the increase of oxygen concentration at the surface after UV/O ₃ treatment. Wetting of underfill on passivation and solder mask was slightly improved at higher temperatures. Although UV/O₃ cleaning and O₂ plasma treatment significantly improved the wetting of underfill on passivation materials, they did not improve adhesion strength of epoxy underfill to passivation. Therefore, the wetting was not the controlling factor in adhesion of the system studied