The Arduino Mega has an SMD voltage regulator soldered to a copper pad connected with thermal vias to a similar pad on the bottom surface. The datasheet says the (roughly) 10×10 mm copper pad sets RθJA=55 °C/W, more or less; probably a bit less with the double-sided setup.
It’s the chubby black slab snuggled up just to the right of coaxial power input jack. The four vias on each side go to an isolated copper pad under the solder mask on the other side.
The board draws about 75 mA with nothing fancy on the I/O pins, so the regulator dissipates half a watt with a 12 VDC input supply. Figuring an ambient of 30 °C, the junction temperature is ticking along at 50-60 °C.
That’s all well and good, but my rule of thumb for semiconductors is:
- If you can’t hold your thumb on it for any length of time, it’s too damn hot.
That regulator fails my rule of thumb even before I start adding LEDs and other doodads.
A bit of rummaging turned up an old Thermalloy sample box with a DIP heatsink. A dab of quick-setting epoxy and there it is:
Now, I’ll grant you there are a number of things wrong with that approach, but my thumb is much happier. If it gets unhappy, I’ll just crack that puppy off and stick something larger in its place.
The Smell of Molten Projects in the Morning 
If you use the thumb rule on everything, how do you smell molten projects in the morning? :-)
Those creatures that walk by night toast my toys!
Nice fix, I should do this to my STK500 regulators as they get very hot with a 12V supply. This is why boxes of junk in which to rummage are so important.
Although putting the heatsink on the epoxy side of the package is, um, suboptimal, I think it’s more useful there than on the bottom of the board.
I also think that regulator should have an actual heatsink designed into the board; the thermal via thing is probably working as well as you can expect, but …
boxes of junk in which to rummage
Ya gotta have stuff!
Given that you can almost have a switcher in the same amount of board space, I wonder why anyone still uses linear regulators for such circuits (12V down to 5V or 3V3). I do sometimes use linears out of laziness for micros that draw ~30mA (12V to 3V3), but those are fairly large TO252-3 parts coupled to a square inch or more of copper, often on two layers on a 4 layer board. Of course those are designs where you can spare that much board area; here it doesn’t seem to be the case?
They always stay warm to touch, nothing more. I agree with your thumb rule. That way even if the circuit is in a cabinet where the fan has failed and the inside air temperature goes crazy next to a couple 2kW servo drives, it doesn’t miss a beat. I’ve had a cabinet with 10kVA transformer in it and requisite servo drives (to provide load for said transformer) in for certification testing. As luck would have it, the AC fan didn’t quite like operation at 50Hz and it was decided to test without the fan running. It passed anyway.
I think that about covers it… plus you need more parts for a switcher and they take a bit of layout care. I use linear regulators simply to avoid all the fuss for the simple stuff I do, but that’s not enough justification for a real product!