Figure 6. Transmission electron micrographs of the exfoliated graphite platelets for the production of the thermal interface material.
The effect of pressure on the bond line thickness of the thermal interface material for smooth surfaces is illustrated in Figure 7 for graphite platelets and carbon black. The thermal interface material structures include protruding surface features to reduce thermal interface material migration. The thermal interface material structures incorporate surface features onto a particular mating surface, such that the features protrude into selected areas of an interface separating the particular mating surface from another mating surface. In some cases, the protruding features may be incorporated onto a mating surface of a heat spreader that surrounds an electronic component and distributes heat away from the electronic component [81, 82]. In other cases, the protruding features may be incorporated onto a mating surface of a heat sink that overlies the module lid and is separated from the module lid by the interface [83, 84]. During thermal cycling, a coefficient of thermal expansion mismatch between the module lid and the heat sink may cause relative motion between the module lid and the heat sink [85, 86]. By incorporating surface features that protrude from a mating surface into selected areas of the interface, the potential relative movement of the mating surfaces in the selected areas may be limited [87, 88]. Limiting the relative movement may reduce strain on the thermal interface material in areas of the interface proximate to the protruding features. Reducing the strain on the thermal interface material may reduce the potential for thermal interface material pump-out and the associated increase in thermal resistance due to loss of material from the interface. A heat source dissipates heat using a heat sink that is joined to the heat source by a thermal interface material, such as a thermal grease or a thermal putty. Compressing the thermal interface material between the heat source and the heat sink may fill an interface gap between a mating surface of the heat source and a mating surface of the heat sink in order to form an interface for efficient removal of heat from the heat source via the heat sink. The heat source and the heat sink may correspond to different materials that have different coefficient values of thermal expansion. Due to the coefficient of thermal expansion mismatch between the heat source and the heat sink, thermal changes associated with thermal cycling cause relative movement of the heat source and the heat sink. To illustrate, during thermal cycling, the heat source may bow upward into a central area of the interface, and the heat sink may bow downward into the central area of the interface, resulting in a significant reduction of interface thickness between the heat source and the heat sink in the central area of the interface. The resulting strain may cause the thermal interface material to migrate away from the central area of the interface over time. Pump-out of the thermal interface material results in increased thermal resistance due to loss of material from the interface. By contrast, the thermal interface material structures incorporate surface features onto a mating surface of a particular heat transfer component of an electronic component cooling assembly that includes two heat transfer components separated by an interface that includes a thermal interface material. The heat spreader surrounds an electronic component and distributes heat away from the electronic component. In some cases, the surface features may be incorporated onto a mating surface of the heat spreader. In other cases, the surface features may be incorporated onto a mating surface of the heat sink.