Схемы, платы, компоненты

вот здесь (оказался Intersil) -> стр.2, правая колонка, третий сверху абзац http://www.intersil.com/content/dam/Intersil/documents/tb38/tb389.pdf

Solder masking is also required for thermal vias to prevent solder wicking inside the vias, drawing solder away from the thermal land-to-die pad interface. The solder mask diameter should be about 100μm larger than the via diameter. The vias can be plugged or tented with solder mask, either from the bottom or top surface of the PCB. Tenting from the top is considered a better option as it results in smaller voids under the die pad. Solder masking of vias from the bottom side can result in increased outgassing during reflow, creating bigger voids around vias.

сам так не делал - шибко экзотично имхо :>