Overvoltage failure modes/PCB repair

Question

I have a PCB that has >10 ICs intended to be powered by a 3.3V supply. Unfortunately, 12V was applied to the circuit and the board now exhibits a short circuit. The offending power supply is current limited so that now when it is attached to the damaged PCB, the supply limits itself to < 100mV.

My question is this: In situations like this where the supply limits itself after a short develops, removing the overvoltage condition, is it typical for only a single IC to have failed (i.e., the weakest link) at which point the failed IC protects the other ICs on the board, or should I be looking for multiple device failures?

Answer 1

If 3.3V was the intended supply voltage, I'm guessing that you didn't have an in-line regulator to many of these ICs. I try not to design circuits without some sort of regulator for exactly this reason. Also, you can design in Zener diodes with an inline fuse, or other forms of protection to try to prevent this.

However, it happened, and for finding damaged components, nothing beats a thermal camera. In this situation especially, you've got components that are trying to pull too much current. They will be hot. You can pull them off one at a time until your current consumption stabilizes at what you would expect.

What kind of thermal camera? I really like my Fluke TiS ($2k from Newark), but you can also get models that are intended to be used for your phone for less than $350.

Shorts from supply rails to ground are more difficult. You can find these typically by pushing 1A through the supply rail, then probing different vias to ground on the board and looking at the voltage differences around the board. This will help you find where on the ground plane the current is entering.

Good luck!

Answer 2

Mike Barber gave some excellent suggestions. However, you may not have a thermal imaging camera available.

In that case, the decidedly-low tech technique simply uses Isopropyl alcohol. Spread a thin layer of alcohol over the entire board surface. Set your power supply to the rated supply voltage (3.3 Vdc) and set the current limit to 500 mA or so. Connect the power supply to the circuit and observe all of the devices.

Increase the current if needed until you see the alcohol evaporating from one device more than the others. Add more alcohol and ensure that the devices is indeed running warm.

Remove that device that is running hotter than the rest and repeat the test. You will eventually be able to find all of the components that can't handle your 3.3V power supply rail.

This technique worked well for several decades for me, until I finally purchased my first Flir thermal imager. I still use this technique if I am in the field and don't have access to a thermal imager.