The thermal characteristics of thermally conductive underfill in flip-chip package was studied. To enhance the thermal conductivity of underfill, the epoxy was mixed with thermally conductive fillers, such as silica (1.5 W/mK), alumina (36 W/mK) or diamond (2000 W/mK). Coefficient of thermal expansion (CTE) was changed by filler and its content and CTE was 28 ppm for 60 wt% silica, 39 ppm for 60 wt% alumina and 24 ppm for 60 wt% diamond. Thermal conductivity was calculated from the measurement of thermal diffusivity, density and specific heat capacity under the various temperature conditions with the various fillers. To investigate thermal characteristics of different underfill, diode temperature sensor array (DTSA) was fabricated, which consisted of 32 x 32 array of diodes (1024 diodes) for temperature measurement and eight heaters for heat source on an 8 mm x 8 mm of silicon surface. The DTSA was packaged by flip-chip packaging method and applied with the same power (0.84 W) for different underfilled packages. Finally, the thermal simulations with ICEPAK matched very well with the measurement. (C) 2006 Elsevier B.V. All rights reserved.