We have proposed Direct Bonded Heterogeneous Integration (DBHi) as a new packaging technology. In a DBHi package, a bridge chip is bonded to and in between two adjacent processor chips, this multi-chip with a bridge is then joined on an organic laminate with a cavity that houses the bridge chip. Because a DBHi package has a coefficient of thermal expansion mismatch between organic and silicon materials, it is important that the space between an organic laminate and silicon chips is filled with underfill (UF) to provide mechanical reinforcement. However, with the particular DBHi package geometry it is difficult to fill a gap between the bridge chip and the cavity, which often leads to void entrapment. Therefore, the objective of this study is to evaluate the effects of dimensional parameters of the package on capillary underfill (CUF) flow and void entrapment propensity. For the evaluation, a transparent glass mock-up of the DBHi package was prepared, and UF flow behavior was observed directly. Eventually, the experimental results indicate that a gap between the sidewalls of the bridge chip and its cavity should be equal to or smaller than the other gaps where UF flows such that the force dragging UF into the cavity prevails. Moreover, it is suggested that when the structure is designed for UF flow control, not only the absolute values but also the relative values of the gaps should be considered because the dragging forces generated by capillary pressure are stronger in narrower gaps.