Study on Surface Tension and Adhesion for Flip Chip Packaging

摘要

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 property of solder mask and different passivation materials: benzocyclobutene (BCB), polyimide (PI), silicon oxide (SiO_2), and silicon nitride (Si_3N_4) after different preparation procedures. Combination of both wet and dry clean processes was very effective to remove the contaminants from the surface. The oxygen atom, introduced during O_2 plasma treatment or UV/O_3 treatment, led to the increase of the base component of surface tension. X-ray photo spectroscopy (XPS) experiments confirmed the increase of oxygen concentration at surface after UV/O_3 treatment. Wetting of underfill on passivation and solder mask was slightly improved at higher temperatures. Although UV/O_3 cleaning and O_2 plasma treatment significantly improved the wetting of underfill on passivation materials, they did not show improvement on adhesion strength. As such, the wetting was not the controlling factor in adhesion of the system studied.