Solder Mask and PCB Heat Sinking

[beaker353]

This maybe more of an academic question than a practical one.  I have a project that I plan on using some area on a PCB to act as a heatsink for a TO-220FP package (an St Micro LM317P).  My calculations tell me my heat dissipation don't require a heatsink, but as I have the extra PCB space, why not add some more margin.  I have just a tad under a square inch on each sides of the PCB that I plan on using, stitching the two sides together with a bunch of vias for heat transfer to the back side (also seen the trick of using 22GA solid copper wire though the vias and soldering on both sides to increase the heat transfer efficiency).  My question is whether or not having solder mask over the copper would have an appreciable effect on the heat dissipated from the copper to the air  I know it's important to keep the mask off where the package touches the PCB, but is there really a difference for the rest of the copper pads to be mask free? Thanks!

- EM

[JuKu]

Do you have any reason not to take off the mask? I know I'm warmer with my sweater on than off...

[marshallh]

Either use a proper heatsink or design in a switching reg and make heat a non-issue.

[SeanB]

More important is if there is air flow over that area. The biggest heatsink in an enclosed box has very little heat transfer.  Make a hole and mount it with a nut and bolt to the board, and use a thermal pad between the device and the board as well, this will make a bigger difference than solder masking. The board does not need the solder mask removed where the device mounts either, as it is an insulated device in any case, and the mask is going to be as good as the board in heat transfer. If you have the space a U shaped heatsink would be a better idea, it does not have to be that big for low power if the bare device alone is capable, it just makes it run cooler and more reliably.

[mrflibble]

Unless you're going to heat up your PCB to several thousand Kelvin, the main chunk of heat transport is going to be convective. So as SeanB pointed out already, air flow is much more important to worry about that soldermask or not.

The main reason for my PCBs to keep the soldermask off is so I can add big solder blobs to the trace. Mainly to increase current carrying capacity and as a bonus you get a bigger thermal mass. This thermal mass helps somewhat ... but you still have to get rid of the heat, so either heatsink or forced air flow (or both ).