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solder masking the bottom of a board

I have a PCB layout that will have some hot spots on top, so I have vias down to copper patches on the bottom. Some sorts of heat-conducting insulators will then carry heat down to a cold plate. So I figure there's no reason to solder mask the bottom of the board, which would just add thermal resistance, rough ballpark 0.15 K/W for a

1 square inch patch.

Some of my patches are pretty small. We'll machine four bosses/spacers into the cold plate, which become the PCB mounting points, and those equivalent spacers are pretty small (#2 screws) and might have high thermal power density. I'm expecting 20 or 30 watts dissipated in a board that's about 12 square inches, with no air flow.

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Any reason to not mask the bottom? Maybe the ENIG plating would cost a tiny bit more. All the bottom traces would be exposed, which is not many, but I can't see any harm in that.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin
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I did that a few years ago on a board. Roughly 10W on a 4x5" board, tight box, no air.

The surface-mounted dissipators were thermal-via'd to the back, then mounted metal-to-metal on a cold plate. Only the cold plate contact-points were bared, the rest was solder masked normally.

Cheers, James Arthur

Reply to
dagmargoodboat

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