TQFN footprint pad via design guide for proper thermals from Diodes Incorporated. Link here (PDF)
TQFN packages have exposed pads to provide excellent electrical grounding paths to the PCB and transfer the device heat through thermal vias on the PCB thermal landing to the internal copper planes. In order to maximize the removal of heat from the package, the number of vias, the size of the vias, and the construction of the vias must be considered for the thermal landing pattern. The exposed pad must be soldered down to ensure adequate heat conduction from the package.