страница 23 DS: "The thermal pad is directly connected to the substrate of the IC, which for the TPS704xx series is a secondary
electrical connection to device ground. The heat-sink surface that is added to the PWP can be a ground plane or
left electrically isolated."
И далее, на странице "Mounting Information": The primary requirement is to complete the thermal contact between the thermal pad and the PWB metal. The
thermal pad is a solderable surface and is fully intended to be soldered at the time the component is mounted.
Although voiding in the thermal-pad solder-connection is not desirable, up to 50% voiding is acceptable.