Let the smoke out

My desk is often a mess. Especially when testing new boards, with wires going everywhere and all boards on the open. I've been fairly lucky, no major mishaps for a long time now, but it was bound to happen soon...

This was taken with my Jolla phone through x10 stereo microscope I use for most of fine work. Photo turned out surprisingly good, although finding the exact location where picture is best(ish) took some fine balancing.

See that dark weird thing circled with red? That's a crater. When I connected power there was pfft sound and a bit of smoke. Shit.

So, I've got two tri-color LEDs on the board for debugging/signalling purposes. I highly recommend you to place some LEDs on your prototype boards too, having nice slow "I'm alive" blink tell you that main() is, well, looping makes debugging way easier when nothing else works.

For now however I had connected some wires directly to pins driving one LED (well, to series resistor, off to left from this view), other end connected to scope. These were for timing tests, I wanted to see how quick power ramps I can generate. Turns out that fairly fast - output transistor isn't best (gotta try NFET instead of NPN I had to spare for first tests) but I still can drive output up very quickly (at 200mA) and even down relatively quickly, at rate of "just" 5v/ms. Perfect for simulating short power outages that are typical in cars (anyone want one? ;)

For a moment I had to do something else, disconnected this board and later connected it back. Pushing things around on table moved the board and one of the test wires moved to meet large-ish  thermal fuse that was directly connected to incoming 12v. And then damage happened. And that nice LED telling me software is running (that being the one LED I hadn't connected wires to) isn't flashing any more.

Ouch.

Taking that out and putting new one sucks. And see those wires at upper part? These are three address pins I stupidly forgot to route in first board version so I needed to manually solder them to processor pins. Processor itself is relatively easy to solder in (despite 0.5mm pin pitch), but those wires are more annoying, being annoyingly large and close together. Markings in wires (1, 2 and 3 dots, number 2 is not visible and third dot on 3 is also out of view) mark the pin position where I need to solder them back in.


Unfortunately I have no choice but to start repairing...