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.
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!