Soaring thermal load levels are a huge challenge amid the PCB design revolution. The trends means the inter-relationship between PCB heat sinks and thermal interface materials (TIMs) is becoming increasingly significant. In this paper, Laird first looks at heat transfer solutions applied historically. We then turn to performance characteristics of the TIM-heat sink interaction. We highlight thermal gap filler pads offering low resistance and softness, liquid gap fillers featuring thin bondlines, thermal adhesives enabling low thermal resistance and a thicker bond layer, thermal tape for easier application, and phase change materials which allow the material to fill nooks and crevices at the heat sink interface.