The LMG342xR030 is a lateral device grown on a Si substrate. The thermal pad is connected to the source of device. The LMG342xR030 can be used in applications with significant power dissipation, for example, hard-switched power converters. In these converters, cooling using just the PCB can not be sufficient to keep the part at a reasonable temperature. To improve the thermal dissipation of the part, TI recommends a heat sink is connected to the back of the PCB to extract additional heat. Using power planes and numerous thermal vias, the heat dissipated in the LMG342xR030 can be spread out in the PCB and effectively passed to the other side of the PCB. A heat sink can be applied to bare areas on the back of the PCB using an thermal interface material (TIM). The solder mask from the back of the board underneath the heat sink can be removed for more effective heat removal.
Refer to the High Voltage Half Bridge Design Guide for LMG3410 Smart GaN FET application note for more recommendations and performance data on thermal layouts.