Study on surface tension and adhesion for flip chip packaging

In a flip chip package with underfill, adhesion of underfill to passivation and solder mask is critical to the reliability of the assembly. In this study, the three-liquid-probe method was used to investigate the surface properties of the solder mask and four different passivation materials, benzocyclobutene (BCB), polyimide (PI), silicon oxide (SiO₂), and silicon nitride (Si₃N₄), after different preparation procedures. A combination of both wet and dry clean processes was very effective for removal of contaminants from the surface. The oxygen atom, introduced during O₂ plasma treatment or UV/O₃ treatment, led to an increase of the base component of surface tension. X-ray photoelectron spectroscopy (XPS) experiments confirmed the increase of surface oxygen concentration after UV/O₃ 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 show improvement in adhesion strength. As such, the wetting was not the controlling factor in adhesion of the system studied