Designing a PCB with cutouts for better airflow (to avoid overheating)

I'm making a custom PCB to plug onto the top of an Uno R3. Of course, this tends to "enclose" all the Uno components, and it's in an environment which can get warm. (Trunk of a car, during summer.) My added components on the new PCB are to the side of the Uno, which means I can add cutouts above the Uno components which tend to be vulnerable to heat -- To promote better airflow.

A previous version of the project naively assumed the Uno wouldn't be vulnerable to overheating, so it was in a nearly-sealed enclosure. Bad idea, methinks. My sketches locked up after a while. And YES, I'm pretty sure it wasn't memory. code, or power issues. (I'm aware that a car power supply needs good voltage regulation. It has that.)

Nothing was cooked, but the sketches locked up sometimes. Resetting gets the sketches running again.

I don't want "holes everywhere" because the PCB would become flimsy and it would be more vulnerable stray objects creating shorts.

So, I need to know:

  • Please confirm that heat can cause such issues without necessarily destroying the Uno, right? If I'm wrong, then I'll triple-check code and memory management, but those seem solid and my power regulation is good too.

  • What components on an Uno R3 are in greatest need of a good flow of air in a warm environment? (At this point it doesn't seem necessary to add a fan.) Is it the main chip? The caps?

The Uno is really hampered by its voltage regulator. You give that 12V input and 400mA output (like a backlight for an LCD) then it will cook very quickly. You never get to its rated 1000mA capacity. The regulator is supposed to shut down when it overheats although it is possible to kill it if you try.

The rest of the components on the board will run quite happily when blazing hot. I haven't checked all the components but I believe most of them have ratings above 80C. That's too hot to put your finger on the chip for more than 1 second. It will hurt your skin.

Just adding holes "above" a hot component won't cool it down. There must be an airflow. Air must come from somewhere and go to somewhere else. If you really think that it's getting hot, arrange the airflow to flow across the Uno, through the tunnel formed by the headers on each side.

If you can, give the Uno a 7V input. That will make the onboard regulator run much cooler.

Thank you, it sounds wise (and fairly easy) to use 7 volts to power it. I'm just not gonna add a fan and I'm pretty sure it wouldn't be needed... See, the "previous version" of the project pretty much worked most days. But the Uno, DC-DC converter and other parts were almost completely sealed in a cigarette-pack size plastic box, and I had even foolishly packed it in foam, which I'm sure was like thermal insulation.

This new "enclosure" (which is more of a protective cage) will be much more open, and even though it's in a car trunk, I'm pretty confident just the natural air currents will be quite different from the previous version, which "almost worked". (And that version had the Uno powered at 9 volts so your 7 volt advice could help greatly in itself.)

But I'd really like confirmation to my first question in my original post. That is, I have searched and found no issues with my code and I'm confident my power supply was delivering clean power, so does my hypothosis that overheating was causing my sketches to lock up sound correct, given what I've described about the previous enclosure?

so does my hypothosis that overheating was causing my sketches to lock up sound correct

I think, MorganS already gave you the answer:

The regulator is supposed to shut down when it overheats

When it is shut down, the sketch will be interrupted/stopped and you need to re-start the application.

If you can't use active cooling, follow MorganS' advice to run your configuration at the lowest possible power supply. If your application draws 200mA and you are powering it with 12V, the regulator has to transform: (12V-5V) x 0,2A = 1400mW into heat.

Running the same configuration at 9V: energy dissipation = 800mW, 7V: 400mW. More details about powering Arduinos: http://www.open-electronics.org/the-power-of-arduino-this-unknown/

Maybe you can use a DC-DC converter to reduce 12V to 7V and then power your Uno with that voltage. Your power supply in a running car can even go up to 14 "plus something" volts!

rpt007: When it (the regulator) is shut down, the sketch will be interrupted/stopped and you need to re-start the application.

Actually that's the part that makes me uncertain. In the "locked" state, the Uno appears to still be "on", but unresponsive. This is, its onboard LED still glows and its 5v pin is still powering a small OLED display, (which is frozen but glowing with its original graphics.) That's not the behavior I would have expected if the voltage regulator is shut down... Am I wrong?

If I'm reading you correctly, this would still occur with voltage regulator shut down from heat? Maybe it just shuts down the 3.3 volts, but not the 5 volts?

.. at least the shut down of the regulator has stopped the continuous execution of the sketch and what you see is the frozen state of the last command, I suppose.

With the 5V voltage regulator shut down, everything should be off. There would be no LEDs driven by the Arduino.

It is possible to overload the 3.3V regulator. That's only rated for 50mA. It theoretically also has a thermal shutdown. But that won't stop the main Arduino processor, which only requires 5V to run. Has it stopped because it's waiting for a 3.3V sensor to reply?

But the Uno, DC-DC converter and other parts were almost completely sealed in a cigarette-pack size plastic box, and I had even foolishly packed it in foam, which I'm sure was like thermal insulation.

That's new information. Just drill a few holes in the top and bottom of the box. Put some foam or mesh over the holes to keep insects out. I would try to keep the whole Arduino wrapped in foam but allow the air to rise through the tunnel between the headers.

pburto: Actually that's the part that makes me uncertain. In the "locked" state, the Uno appears to still be "on", but unresponsive.

Have you checked the voltage in this situation? The ATmega has a brown out detector that will keep it on reset until the voltage his above his high thresold, may could be your case.

Ciao, Ale.

If the unit is in the trunk of a car and the trunk is getting very hot, no amount of air-circulation in the Uno can improve things much unless you cut holes in the trunk!!

Here's what you should do:

Run the unit out of the car for 2 hours and then measure the internal temperature compared to ambient - that's the temperature rise caused by poor heat-flow in the box.

If that's 5 degrees or less, there's no point improving it, the car is the problem.

If its 10 or more degrees (C) then it is probably worth improving - its still mainly the environment you put the box in that is the problem, but you have some leeway.

Incidentally I've had hot-melt glue melt inside a car on a normal spring day when it was sunny, a vehicle is basically a greenhouse without ventilation.