This is a copy of the slides presented at the meeting but not formally written up for the volume.

Description: Thermal pastes are a class of soft composite materials of great importance to the microelectronics industry. The function of these pastes is two-fold: (i) to transport heat away from the chip; and (ii) to accommodate mechanical stresses in the package arising from the mismatch in the thermal expansion between the chip and the heat spreader or sink. Due to the former requirement, thermal pastes are among some of the most highly-filled composite systems in practice (solids volume fraction > 70%). These materials are expected to withstand the significant normal forces, lateral forces, and temperature variations associated with chip operations and power off/on transitions. The structural changes and degradation of various thermal pastes during power cycling have been characterized using optical microscopy and IR thermography. Correlations between the evolving structures and variables such as thermal paste inhomogeneity and binder-particle dispersability have been successfully made and will be presented.