Fundamental adhesion issues for advanced flip chip packaging

One of the key materials used in flip chip packaging is the underfill encapsulant, which enhances the reliability of the flip chip device by more than an order of magnitude. This paper discusses the fundamental adhesion issues of the underfill encapsulant, which are critical to the reliability of the flip chip package. This study found that surface properties strongly depend on the surface preparation conditions. The effects of wet cleaning and dry cleaning, and combination of both were systematically studied. Although wetting is improved after UV/O₃ treatment or O₂ plasma treatment, the adhesion strength of underfill to the passivation or the substrate did not increased significantly. It was demonstrated that high temperature and high humidity aging resulted in adhesion degradation. The adhesion retention after high temperature and humid aging was studied systematically. The results indicated that the adhesion retention strongly depended on the underfill formulations and the surface property of the passivation materials. The degradation rate was controlled by the mobility of the absorbed water and the polymer matrix.