How can I troubleshoot an active short circuit?

Question

What are some effective methods of diagnosing an active short circuit? By this I mean a short circuit that presents itself only after a PCB is powered on.

tl;dr

I have a design in the prototype phase. 17 of my 20 boards work great. The other 3 all have a short circuit on a 3.3V rail. This only shows up after the board is powered on. After removing most of the components on the rail, I tracked it down to an Ethernet PHY. If I lift the IC off, my rail is rock-solid at 3.3V. When I put it back on (also tried 2 new ICs), my rail is overloaded again and it drops out.

I have thoroughly visually inspected, and probed the board for shorts, but cannot find one. I have lifted off pretty much everything around the IC (crystal, series resistors, ferrite beads, etc.) but still get the same behaviour. I've also tried holding the chip in reset, but that doesn't help. I lifted individual pins on the IC (VDDIO) and that fixed it, but doesn't offer a real diagnosis. I'm starting to wonder if there's an issue with the PCB fab, but not totally sure how it would cause this. They claim to do 100% E-test. Any advice will be appreciated!

Answer 1

While I wish you best of luck with your thermal camera endeavour, I would actually expect that the camera will show you the IC affected by the short (the Ethernet PHY?), not the short itself. You'd have to be pretty lucky for the actual faulty spot to have higher resistance than the internals of an IC.

If you find nothing with the camera, I would then check continuity to GND / 3.3V rail on any of the IC pins which are not actually connected to GND / 3.3V. Do it with a diode tested, since a forward-biased junction is close enough to a short.

If that comes out negative, I would power the PCB with the IC removed and check the voltage on all pins (ideally, a power-up waveform but it may be tedious for a large number of pins). Any voltage outside of 0..VCC range could potentially cause the IC to latch up, a condition which typically looks like a short.

Finally, I would check if all pads corresponding to the output pins can be actively driven. This can be done by connecting a scope and a signal generator together outputting a 0-3.3V square wave (so that you see the square wave on the scope), then connecting the probe to the pads. A disappearing square wave would mean that something else is trying to drive pad that the IC will want to drive as well. This can be justified for open-drain and bidir pins, but not for pure outputs.

Answer 2

It won't always work, but sometimes you can track down a short with a thermal camera... of course you have to have a thermal camera to do that.

Just power the board up and watch very closely through the thermal camera to see if one area of the board gets really hot, it could help you narrow down the area at least.

Answer 3

Or you can use the "Louis Rossmann Thermal Camera without a thermal camera" technique - cover the board with IPA, run power through the short - the thing that's shorted heats up, and the alcohol will evaporate very fast. It makes it really obvious what's shorted. See demo here: https://youtu.be/gRV0cmIj5Ks?t=236 - yeah, it's kind of a bad example, because he finds the short with the thermal camera in this case, but he usually uses the alcohol method, and it works great.

Answer 4

If you cannot put enough current into the rails to be visible as heat, you may try to use a sensitive voltage meter.

Put the meter in microvolt (µV) range, touch one probe to where the current is coming from, and other probe to where you suspect it might be going. The more current is going through that route, the higher difference the meter will show.

My usual technique is to keep one probe always on power connector or voltage regulator output pin, and move the other probe until I find the highest difference. This point is then closest to the short-circuit.

PCB traces have a resistance of around 1 milliohm per millimeter, depending on width of course (you can check with a calculator). Thus if the short-circuit is drawing e.g. 100mA, you should see 1mV difference per every 10 millimeters.

You can also try to do this from the GND side, which could help if the short-circuit is from an IC output pin to ground. But if the board has a good GND plane, you probably won't see a voltage with small currents.

Answer 5

The good options are already mentioned, but I'd add this. Disclaimer: it's more suitable for shorts on signal traces.

Stuck node tracer

The Art of Electronics, pg. 276, discusses the "stuck node tracer" - a sensitive microvoltmeter, which you use while the circuit is powered, and can point you to where the short is. You keep one of the probes on a fixed location (e.g. the 3V3 rail), the other one you move along the PCB traces. The traces which carry the short exhibit a measurable voltage drop, so the closer you get to the short, the larger the display on the voltmeter will be.

I've never built this, but it looks reasonable (and Tony EE also mentions it). But it's really more useful when the PCB traces in question are thin.

Fixing a short

In the case when the short is within an IC, it's usually a defective IC and I'd just change it. It may have been killed by ESD or some sort of abuse.

If it's on the PCB (and you didn't have electrical test), I enjoy fixing those by placing a large supercap (2kF@2.7V) across the short to vaporize the small whiskers that caused it. Once we had a batch of PCBs littered with problems like this (due to bad conversion to gerbers, which made them noncompliant to the fab's DRC). We fixed a bunch of boards that had whiskers like this, it was a thoroughly fun (albeit smelly) experience :)

If your electronics won't get killed by 2.7V, you can use this method on a populated board as well. Just be careful with polarity, otherwise there'll be smoke everywhere :)