The LMG341xR070 is a lateral transistor grown on a Si substrate. The thermal pad is connected to the Source node. The LMG341xR070 may be used in applications with significant power dissipation, for example, hard-switched power converters. In these converters, cooling using just the PCB may not be sufficient to keep the part at a reasonable temperature. To improve the thermal dissipation of the part, TI recommends a heatsink is connected to the back of the PCB to extract additional heat. Using power planes and numerous thermal vias, the heat dissipated in the LMG341xR070(s) 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 adhesive thermal interface material (TIM). The soldermask from the back of the board underneath the heatsink can be removed for more effective heat removal.
Please refer to the High Voltage Half Bridge Design Guide for LMG341x Smart GaN FET application note for more recommendations and performance data on thermal layouts.