Solder mask withstands heat pretty well, but it is easily scratched. Although the FET is screwed in place, it is still liable to move around (especially with vibrations during transport, or during its lifetime), and thus I think the FET's drain tab is likely to scratch through the solder mask in some units. I think it's a clever idea for reducing radiated EMI (they avoid having a large heat-sinking conductor which is attached to a switching node), but for reliability purposes I would avoid it.
There are commercially available insulating sheets, and I think it's a better idea to use these as an interface between the FET and the PCB. These insulating sheets, which are called Mica insulators, are specifically engineered for use as a heat sink interface material and are thermally and mechanically robust, and have good thermal conductivity.
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Solder mask withstands heat pretty well, but it is easily scratched. Although the FET is screwed in place, it is still liable to move around (especially with vibrations during transport, or during its lifetime), and thus I think the FET's drain tab is likely to scratch through the solder mask in some units. I think it's a clever idea for reducing radiated EMI (they avoid having a large heat-sinking conductor which is attached to a switching node), but for reliability purposes I would avoid it.
There are commercially available insulating sheets, and I think it's a better idea to use these as an interface between the FET and the PCB. These insulating sheets, which are called Mica insulators, are specifically engineered for use as a heat sink interface material and are thermally and mechanically robust, and have good thermal conductivity.