Non-conductive paste (NCP) formulation for chip-on-board (CoB) flip chip packages should contain a large amount of inorganic filler to achieve the low CTE (coefficient of thermal expansion) of cured joint and resultantly the reliability of overall packages. As the interconnection pitch decreases, the filler particle size should also be decreased because the particles trapped between bump and pad at the joint may deteriorate the joint reliability. In this study, effects of submicron-sized silica filler additions on the uncured and cured properties of NCP formulation and the void formation behavior at the flip chip joint were investigated. Two kinds of spherical fused silica fillers having particle size of 1 ㎛ and 100 nm were used. The abrupt increase point of viscosity of NCP formulation was shorten in the finer silica addition due to the increased probability of agglomeration and collision of particles. It resulted in the maximum filler content of 60 wt% and 30 wt% for 1 um and 100 nm silica, respectively. The addition of 100 nm silica showed lower effect on the CTE reduction and increased void formation at the flip chip joint compared to 1 ㎛ silica. It was considered that the defects at the interface between epoxy resin and silica filler originated from the surface contaminants, such as moisture and adsorbed gases, of silica particles was mainly responsible for the bad effects of the finer silica particles on CTE reduction and void formation behaviors.