@MarcusMüller, thanks that makes sense. I think I will stick with the solder mask to also gain the benefit of it being flatter surface area. Tim Wescott, I’d rather not use epoxy. I want to be able to rework these if necessary, and that is all but impossible with a heatsink permanently attached to each plane.

Thanks, this is the most helpful comment. The surface finish will be HASL, and I hadn’t thought about the advantage of having a more flat surface to work with. The vias are fully tented already on the bottom solder mask. The only heatsinks that I can find which fit perfectly, and are reasonably priced, are un-anodized aluminium. do you think that the thin layer of solder mask and thermal grease will be enough to keep separation over a long time period? The highest voltage potential between any two pads is 27V but potentially I could design it so that it’s only 12V on the heatsink area.

I will still need to have a thermal interface material between the bottom of the PCB and the heatsink anyways, so what is the benefit of keeping the solder mask? I can't just use the solder mask to thermally couple the copper to the heatsink. The top of the IC packages are remarkable cool. When the heatsink get's warm, I can't really feel any heat from the top side components. When the heatsink is HOT then they get warm but, but nothing compared to the heatsink. I don't think the top side of the components radiate heat very well.

Thank you! Raising the voltage to 12V solved my problem. 5V supply wasn't quite enough to turn on the FET. As for stability, this was solved by adding 10N in series with 1k from the output of the op-amp to -In. it works great now!

Yes this was part of the problem. Placing 10N in series with 1K from the output to -In of the op-amp helped a lot with stability. thanks!