Every engineer has that “worst case scenario,” story that they survived and kept designing after. During the worst week of my professional life, we had a shipment of overdue PCBs come in. These PCBs were supposed to have been installed in hardware and deployed at customer sites more than a month earlier. We were beyond a little mortified.
Needless to say, there was a slight rush to ship these new boards. So we powered them up to test and things did not look good. You could smell ozone off the boards and one of the most expensive components heated up so hot that it actually burned a couple people doing a “touch test.” We didn’t have the time, we thought, to get a test batch of boards, and just ordered the fully populated PCBs.
Obvious frustrations aside, explaining major hardware failures to your boss while sucking on burned fingers is one of the worst meetings you can have. The best thing you can do is offer a plan forward. If you’re ever in that situation, here’s a starting point for tracking down that short circuit.
The first thing you want to do is take a really careful look at the entire surface of the PCB. Use a magnifying glass or low magnification microscope if you’ve got one. Look for tin whiskers between pads or solder joints. Any cracks or blobs of solder should get careful attention. Check all your vias. If you specified unplated vias, make sure that’s the case on the board. Plated vias can short layers together and leave you with everything tied to ground or VCC, or both.
If the short is really bad, you’ll actually see burned spots on the PCB. They can be quite small but will be dramatically discolored brown instead of the normal green solder mask. If you have multiple boards, a burned PCB can help you narrow down the specific location without powering up another board to sacrifice in the search. Unfortunately, our board didn’t have any burns on the PCB itself, just the unlucky fingers that checked for overheating ICs.
Burns can definitely help you locate a short but at a very smoky cost.
Potential Locations of Short Circuits on Your PCBs
Outside of the first step in using your trusty eyes to check the board, there are a few other spots where you can check to find the potential cause of short circuits on your boards.
Some components have a tendency to go bad, like electrolytic capacitors. If you have suspect components, check those first. If you aren’t certain, you can usually do a quick google search for components that you suspect “failed” to find out if it’s a common issue.
If you’re not at a startup that just blew its hardware budget, you might be lucky enough to have access to infrared imaging. Using an infrared camera can help you to locate a short circuit, even if the short is between internal layers. The short is usually higher resistance than a normal trace or solder joint since it didn’t have the benefit of being optimized in your design (unless you’re really bad about ignoring rule checks). That resistance means the short circuit will heat up, and announce itself in an infrared view. Start with the lowest current you can. Ideally, you’ll see the short before it does any more damage.
Digital Multimeter Testing
Since a visual inspection didn’t reveal any clues as to the location or cause of the short circuit, we pulled out a multimeter and tried to track down the physical location on a bare PCB. The multimeter approach gets mixed reviews in most electronics forums, but tracing your test points can help you figure out what isn’t the problem. You’ll need a very good multimeter with milliohm sensitivity, and it’s easiest if it has a buzz function to alert you when you’re probing a short.
Multimeter probing can help you to track down a short, but they aren’t always sensitive enough to find it.
The Last Straw: Destructive Testing
After all of our testing, which did not include infrared because we were that broke startup I alluded to, all we could figure out was that the short was in one half of the board. So, we cut it in half, then in half again. Destructive testing is obviously a measure of last resort, but that’s definitely where we were.
Going back to the multimeter confirmed that most sections did not have Vcc and ground tied together. But that single quarter of the board was a little black hole of mystery, and we never got any closer to the short. We did change manufacturers and get test boards the next time which both seemed to help.
If you want to avoid the soul-crushing anxiety of searching out short circuits, make sure you have robust rule checking, for errors, design issues, and manufacturer tolerances. A solid design software like Altium’s CircuitStudio can do most of that for you, as well as providing the unified design environment you need to accomplish your designs with as little headache and singed fingers as possible.