Cyanate ester resin and its blends with epoxy resin demonstrate a lower viscosity build up when filled with thermally treated silica fillers than with untreated silica fillers. The reduced viscosity build up resulting from the use of the thermally treated materials results in faster capillary flow rates by a factor of 2 to 5 with respect to untreated silica particles. Surface properties of the filler materials are used to interpret the viscosity and flow rate data and guide design of filler surface properties. The flow rate performance improvement contributes to design of new capillary underfill materials for electronic materials applications.