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The nanosilica filled composite is a promising material for the no-flow underfill in flip-chip application. However, as the filler size decreases into the nano length scale, the rheological, mechanical, and thermal mechanical properties of the composite change significantly. The filler-filler and filler-polymer interactions have a profound impact on the material properties. The purpose of this paper is to achieve an in-depth understanding of the effect of the filler size and surface treatment on material properties and therefore to design a nanocomposite formulation with desirable material properties for no-flow underfill applications. Mono-dispersed nanosilica filler of 100nm in size were used in this study. An epoxy/anhydride mixture was used as the base resin formulation. The nanosilica fillers were incorporated into the resin mixture to different filler loadings from 5wt% to 40wt% with or without silane coupling agents as the surface treatment. UV-Visible spectroscopy showed that the underfills with nano-size filler were transparent in the visible region even at high filler loading. The curing behavior and the Tg of the nanocomposite were studied using a modulated differential scanning calorimerter. It was found that the presence of the nanosilica could hinder the curing reaction, especially at the late stage of cure. The Tgs of the nanocomposites with untreated silica were found to decrease with the increasing filler loading. The measurement of the dynamic moduli from dynamic mechanical analyzer indicated that there was a secondary relaxation related to the filler-polymer interface. The coefficient of thermal expansion of the nanocomposite was measured using a thermal mechanical analyzer. The rheology of the nanocomposite was studied using a stress rheometer. It was found that the filler treatment could significantly reduce the viscosity of the nanocomposite and improve the processing capability of the underfill. Density measurements and moisture absorption ex- eriments both indicated that the addition of nanosilica could increase the free volume of materials. The dispersion of the nanosilica in the cured composite materials was observed using scanning electron microscopy. Control samples with micron-size silica fillers were formulated and characterized for comparison.