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If you want the best thermal exchange with the air, is it preferable to leave a thermal/ground plane raw or to cover it with solder mask? And how important is the difference?

(Dave from EEVblog is pretty self-confident saying it's more efficient without solder mask: https://youtu.be/q3RhWuXFixU?t=782)

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The short answer is no it won’t make a significant difference. The long answer is that conduction will be slightly worse with solder mask but radiation will be better, assuming that the bare metal is in good condition - the emissivity of a shiny metal surface is generally low, around 0.1 while that of a coated surface is 0.9 or more. Thus bare metal may paradoxically be a worse heat sink at higher temperatures. This assumes that the bare metal doesn’t oxidise or corrode, unlikely if it’s copper but possible if it’s tinned or gold plated.

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The solder mask coating is a very thin thermal and electrical insulator. However, "preferable" is a relative term. Exposed copper or solder plate will oxidize / corrode / whatever, and as above is uninsulated from accidental contact with a non-ground-potential conductor of some kind.

So, yes, there are several differences; but no, I've never seen a situation where the thermal difference is significant.

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good thing to avoid solder mask, make sure copper is not exposed and surface finishes are applied. This exposed metal can be touched with a heat sink if required

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