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Underfill (UF) is an important process in flip-chip packaging because of significant impact on the reliability of the IC's package. For three-dimensional integrated circuit (3DIC) demand, fin e pit ch an d fine gap are the market trend in the future due to t he requirements of functionality and performance in electronic device. In this study, a two die stacking, with Cu pillar bumps area of multiple pitches, joint by thermal-compress ion bon ding without flux has been demonstrated in chip on chip fashion. The maximum standoff height after micro bump joint is less than 25 um. Because of different pitch and fine gap structure, underfill dispensing becomes a challenge process for 3DIC stacking. Two different types of underfill were chosen to study in this paper. UF-A has higher viscosity and better stress simulation than UF-B. Different dispensing design were also studied in this paper, and the mechanism of underfill flowing property was determined by scanning acoustic tomograph (SAT) for comparing the effect of dispensing parameter and material.