This work encompasses the development of low-viscosity cyclic oligomer underfill formulations that cure without heat evolution. Boron nitride, silica-coated aluminum nitride, and alumina ceramic powders were used as fillers in cyclic butylene terephthalate oligomer melts. The melts were heated with a suitable catalyst to induce polymerization. The effects of the filler type and composition on the thermal and mechanical properties of the poly(butylene terephthalate)/filler composites were examined with differential scanning calorimetry, temperature-modulated differential scanning calorimetry, thermogravimetric analysis, thermomechanical analysis, and dynamic mechanical analysis. Scanning electron microscopy was employed to elucidate the morphology of these composites. (c) 2005 Wiley Periodicals, Inc.