Underfill materials had been widely employed in the flipchip packaging to fill the gaps of solder bumps connecting IC chip and organic substrate in order to prevent failure of the solder joints. For radio-frequency (RF) device applications, underfill materials should possess low dielectric constant to alleviate power loss at high-frequency, in addition to good thermal and mechanical properties. In this study, a novel approach of incorporating porosity through porous silica filler was attempted to develop low-k underfill materials. An inorganic, sacrificial material, hexamethylcyclotrisiloxane (D3), was used to temporarily seal the interconnected pores in the porous silica at temperature < 95°C by thermal and solvent pretreatment methods, and was later removed thermally at 125-165°C during the crosslinking reaction of underfill materials. For underfill materials with 15% filler content, a 7.8% reduction in dielectric constant has been successfully demonstrated and achieved by pore sealing of porous silica (60% porosity) using solvent pretreatment, while maintaining the mechanical strength of porous silica, 2.6 GPa. Moreover, the adhesion between epoxy and porous silica in the underfull materials was found to critical in preserving its mechanical strength when pore sealing pretreatment was applied.