The New Hotness

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 R_θJA=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.