The thermally enhanced package provided with the DRV8432 is designed to interface directly to heat sink using a thermal interface compound, (for example, Ceramique from Arctic Silver, TIMTronics 413, and so forth). The heat sink then absorbs heat from the ICs and couples it to the local air. It is also a good practice to connect the heatsink to system ground on the PCB board to reduce the ground noise.
RθJA is a system thermal resistance from junction to ambient air. As such, it is a system parameter with the following components:
The thermal grease thermal resistance can be calculated from the exposed power pad or heat slug area and the thermal grease manufacturer's area thermal resistance (expressed in °C-in2/W or °C-mm2/W). The approximate exposed heat slug size is as follows:
The thermal resistance of thermal pads is considered higher than a thin thermal grease layer and is not recommended. Thermal tape has an even higher thermal resistance and should not be used at all. Heat sink thermal resistance is predicted by the heat sink vendor, modeled using a continuous flow dynamics (CFD) model, or measured.
Thus the system RθJA = RθJC + thermal grease resistance + heat sink resistance.
See the TI application report, IC Package Thermal Metrics (SPRA953), for more thermal information.