Why do I keep blowing up Nanos when I solder to them?

I have blown up 2 Arduino Nanos and I'm trying to figure out what I'm doing wrong.

In both cases, I was powering the boards with a 16V supply (I know this is high, but the regulator datasheet says it is good to 25V, and the regulator was not damaged). I unplug the supply and USB cable and then start soldering to one of the pins (A6). When I plug the board back in, the L LED does not flash, indicating the bootloader has been erased. The POW light is on and I still measure 5V, indicating the regulator is working. When I connect the board to my PC, the serial port pops up just fine. But when I try to upload a sketch, the IDE times out and says it cannot communicate with the board.

I attempted to reprogram the bootloader using a raspberry pi but avrdude also says it cannot find the device either. It looks like the microcontroller is blown.

My iron tip is grounded, so my best guess is some residual power in the supply travels through the microcontroller to my iron and shorts something out. But this seems unlikely seeing as A6 is an input and grounding it should be just fine even when the supply is fully on.

Any ideas what I could be doing wrong? FYI, I am cross-posting this on Arduino Stack Exchange.

Hi,

Are you soldering with the Nano attached to power / PC, etc?

Are you holding the soldering tip on the pin for too long and putting too much heat into the Nano?

Too much solder shorting to the next pin?

I've soldered a couple of Nanos recently, the last one every digital pin and three of the analogue pins and not had a problem.

Peter

I do not have power connected (PC or supply) when soldering, and no I didn’t cause any shorts. I have successfully soldered to pins as well. It seems to be an intermittent thing.