Tuesday, December 13, 2011

Devil's in the details

And I just got burned. I inadvertently designed a short circuit in a pcb!

I create circuit board artwork using DesignSpark PCB and when printing it out on transparency film I always have the board outline printed out as well. The outline allows for precise alignment of the mask (the film) and the presensitized board prior to exposure. In this particular design of a simple 5-volt power supply I had a wide strip of copper for both the +5V and ground. And I extended those tracks all the way to the edge of the board (see the left side of the artwork image below).

Yep, disaster in the waiting. I printed the artwork, exposed the presensitized board, etched it, drilled the holes, soldered the components, and all the while I was absolutely oblivious to the short I had created. I powered up the board while probing the output with a multimeter. Reading? 23mV. I cut off power immediately. Tried it again. Still the same zero voltage. I was so clueless of the booboo that I even desoldered one of the filter caps suspecting it might be defective. Only while poring through board with a magnifying lens did I finally spot the fine 5-mil trace at the very edge of the board bridging the power rails. Facepalm moment indeed. Actually if I had moved the mask by some half millimeter that outline would've been off the physical board and the short wouldn't have appeared.

It's worth noting that even a 5-mil track is faithfully reproduced--implying there's insufficient undercutting during etching to remove the track. This is good of course--if we want such fine copper traces. Luckily it's easy to cut and gouge out the offending copper track. After making sure the bridge had been completely obliterated I soldered the cap back on and the circuit worked as designed. No electronic component was harmed in the making of this sophomoric design mistake.

Hopefully the stress, anxiety, and panic I was subjected to has burned the lesson in my head--since I'll still continue the practice of printing the board outline, I must remember to leave some 20 mils around the perimeter of the board copper-free. I better burn that minimum gap requirement into DesignSpark's design rules. And more importantly, I better start paying attention to the error messages which the design rule check routine spits out--because I don't usually take a look at the track-to-track, pad-to-track, shape-to-track, shape-to-pad, etc. spacing errors. I have a knack for breaking rules. Unfortunately, this time around I got bitten.

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