An apparatus for heat spreading and low parasitic transistor mounting in high power microstrip circuit applications. An electrically and thermally conductive strip is conductively affixed to the ground plane of a substrate containing the circuit. The strip is shaped to fit substantially within a slot in a circuit carrier block (heat sink) such that the strip makes intimate contact between the mounting surface of the transistor mounted in the slot and the carrier block. The strip provides the electrical connection between the ground plane of the substrate and the carrier block in addition to increasing the surface area for more efficient heat transfer, and thus, allows for the ground plane of the substrate to be attached to the block either by a non-metallic adhesive or by solder.