Finding a short on a prototype board

A on-destructive way to find a short is with a micro-ohm meter.

Google for "micro ohm meter", lots of hits to DIY or purchase.

This is a long and tedious process. (unless its an obvious short easily cleared up)

If you are using thru hold parts, cut off the VCC leg on each chip one at a time. If its a bad chip, you'll find it and repair the cut leads.

If its a PCB problem, go the uOHM meter route.

hamilton

Remove any tantalum caps first, then check for chips the wrong way round... protection diode the wrong way round?

Is it a dead short, i.e. you put 1 volt on it and get just a couple of mV across the power rails, or is it a diode drop? If a diode drop, it is probably a component the wrong way round.

Have you used a type of regulator or transistor you've not used before? Those 3 pin footprints are always the easiest ones to get the pins swapped.

About 20 years ago when multilayer boards were still considered high tech, I got my first 4 layer PCB back... due to some fabrication error, ALL vias were shorted to the 0V plane. Hah!

I have occasionally seen PCB's under-etched, leaving whiskers of copper between tracks. This is why one pays to have boards "electronically tested".

Get a bench supply and set it for, say, 1 volt out with a 1 amp current limit, and connect it to the shorted regulator output. Then get a good DVM, ideally one with uV resolution, and start measuring voltages and drops along the power net. It's not hard to see where the current is going. Sometimes moving the power supply entry point will help localize the short.

If you can get access to a thermal imager, shorts are often visible. If the voltage is limited, you can really crank up the current and see the heat pattern.

I really want an imaging magnetic field mapper for situations like this. I have applied an AC current to a short, and probed with a magnetic pickup, like a small drum core connected to an amp/speaker. You can *hear* the short.

We have all our PCBs bare-board tested, and never see plane shorts any more. Do you see the same short on a bare board?

Great idea.

I was going to suggest thermal imaging to find where the current is going. Reminds me that for very subtle leakages on ICs, we used to use a KLA emission microscope.

Great question.

You say you have populated two. What about using one of the unpopulated (you have more than two made?) and the brute force method? Or are all the other boards populated with SMT parts but not with through-hole parts? In any case, if they are hosed, the parts may be the last thing to worry about (or the problem.)

Before visual and aromatic cues, warmth will indicate where to look as you crank up the amperage. If you don't have a fancy thermal imager (or a friend in the fire department who'd like an opportunity to play with theirs for a good cause,) use your fingertips.

Yes, they all have SMT parts populated. I have asked my board shop for a copy of the unpopulated board, and they are sending me one. Looks like I get to try and make a micro-ohmmeter... ;-)

Charlie

This isn't a direct answer (probably!) but here's a wonderful video from David L. captured live trying to find a short on a populated board:

Always good stuff over there (down there?).

You've gotten some good suggestions.. when you're looking for stuff (after you've absolved the bare boards), look at chips with pads under them, parts with vias or traces under them, mechanical stuff.

Is it possible that a through-hole part is completing a circuit between power nets?

I normally go searching with a bit of current and follow voltage gradients to the fault, but it's not always easy. On one I found a part that the manufacturer had moved from one side of the board to the other without re-routing the board (extra tooling charge) but the legs of the through-hole part were sitting on a trace connected to the opposite supply, separated only by the solder resist. About 499 times out of 500 that was okay. 8-(

Thanks for all the great suggestions. Looks like I will need to finally break down and get me a bench supply! A commercial micro-ohmmeter is definitely not in the budget right now... ;-)

Charlie

Injecting an a.c. current and readout with a DVM on a.c. volts is a poor man's method too. That takes care of thermal emfs and whatnot.

I usually use d.c. current and just chop it manually, by switching the probes back and forth.

-- Cheers, James Arthur

I've used that method, and it works. It's a bit tedious.

As others have pointed out, using alternating current and synchronous detection lets you work with lower currents and lower voltages.

Could be handy.

As with the thermal imager, you can "see" the current from a distance, and move towards it

Cambridge Instruments started paying for bare-board testing late in the 1980's and it cut the number of faults per board by about 65%. Shorts between printed circuit tracks on the bare boards had been about 2/3s of the faults that production had had to find, and finding them automatically save a lot of hours of skilled labour - if it got tricky, final test were perfectly capable of calling on the engineers to help, so I was a small part of the skilled labour that got freed up.

-- Bill Sloman, Nijmegen

Use your standard power connector and wires, a variable supply and resistor, and start with supply set to required operational voltage level and adjust resistor for (say) 10mA. Use a DVM (say) with low side on PCB at ground pad and poke on traces with high side traveling toward supply trace while noting voltage. Follow each physical path. In some area there should be a much smaller change/increase - indicating likely source of problem.

Use a D-cell battery and a 1 ohm resistor to force current, and a good DVM to measure voltage drops in traces.

Of course, these are the power nodes, so I have copper pours on both sides of the board... :-(

Charlie

Also look for a hint from the I/V curve. Low resistive => short. Diode =>

backwards chip or protection diode. Odd diode voltage => backwards capacitor. Etc.

Chill the board in the refrigerator or freezer. take it out and apply power look for the dry spot. The warmth of the short will dry out any condensation making the short easy to find.

regards, al

Chill the board in the refrigerator or freezer. take it out and apply power look for the dry spot. The warmth of the short will dry out any condensation making the short easy to find.

regards, al

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That works. First put the board in the freezer for say 20 minutes. Remove the board to the bench and apply the limited current. The frost will melt along the trace and you can take a quick picture for review.

tm

If applied improperly, that could zap some semiconductor devices on board.