Finding shorts on a populated PCB. - Page 1
Forum section: electronics / beginners

author: Cramer
author_rank: Newbie
timestamp: November 09, 2022, 04:08:06 pm
content: We use a circuit that takes 3 volts, DC, and converts it to 6 and 9 volts, DC. In many of my populated PCB's I have a high current drain with 3 volts applied, and I can trace it to the, shall we call it, the converting section of the circuit. I have seen posts on inputting a square wave to locate a short, my question is, what would that look like? I have access to good equipment, just need to be pointed in the right direction. As much as I like a good troubleshooting challenge, this is killing me!TIA.

author: tszaboo
author_rank: Super Contributor
timestamp: November 09, 2022, 04:12:38 pm
content: You can connect it to a power supply, and place there a large current limit. The short will heat up. Often times it's enough to even evaporate the invading hair-short. Or you can see it with a thermal camera or Mark 1 Fingertips.

author: Cramer
author_rank: Newbie
timestamp: November 09, 2022, 04:19:17 pm
content: Thanks, I will try this method. Can't hurt, unless I burn myselfQuote from: Cramer on November 09, 2022, 04:08:06 pm

author: kripton2035
author_rank: Super Contributor
timestamp: November 09, 2022, 04:37:07 pm
content: I use my shorty-with-diplay for that...http://kripton2035.free.fr/Projects/shorty-display.html

author: Bud
author_rank: Super Contributor
timestamp: November 09, 2022, 04:46:09 pm
content: Quote from: tszaboo on November 09, 2022, 04:12:38 pmDo not do large current, the shorted component may explode. 100mA is sufficient for thermal detection.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 11, 2022, 02:09:45 am
content: I think I saw Dave do this with a multimeter and measuring 6-7 digits. The short was where the resistance was lowest.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 11, 2022, 08:03:20 pm
content: Stick a 100mA through the path and work along the shorted lines with your DVM. Start at the powered end.As you get closer to the short, the mV reading across the lines will decrease and there's the location. You need a decent DVM that will resolve sub mV voltages.An amp betwen the pcb and the DVM will help. Use a single supply precison opamp (sub 1mV offset) to increase your sensitivity by say 100xZero drift opamps are very affordable these days. eg MCP6V66 don't connect your current source ground to the amps ground. Power the amp with its own battery.In practice this will probably be fairly time consuming unless you have the PCB artwork to show the routing.If you have buried layers and tracks your're stuffed.Dont try and vapourise it away with a lot of amps. Bad idea.

author: Cramer
author_rank: Newbie
timestamp: November 17, 2022, 04:23:38 pm
content: Thanks for the input and suggestions. The PCB is a double sided, with no hidden layers. My problem seems to be component related, not trace or layer related. I have a method to look for shorted vias and have ruled this out on many of the bad PCB's. When I do have this issue it's masking related so I scrape the area clear and it generally works, wish they were all that easy.....I can just throw darts at the board and replace components that appear to be blocking the voltage, but I would much rather find a scientific method of troubleshooting.Onward thru the fog!

author: exe
author_rank: Unknown
timestamp: November 17, 2022, 09:20:07 pm
content: I once got a solder bridge on a proto board. It was happily passing several amps without manifesting on thermal camera. I had to remove components one by one to find the problem (fortunately, many parts were in socket, so it didn't take much time).

author: Siwastaja
author_rank: Super Contributor
timestamp: November 18, 2022, 04:34:16 pm
content: Apply power. Let your lab supply limit current. 1-2A is enough. Take a multimeter with most sensitive voltage range (0.1mV is usually good enough but it does not hurt to have more resolution). Follow your power traces with the multimeter: wherever flows current, there is voltage drop. Branches where no current is flowing have constant voltage over them.Way better than the "try what heats up" method, although that's sometimes very simple, too.I'm assuming no power planes, but power planes are crap anyway.

author: jmelson
author_rank: Super Contributor
timestamp: November 18, 2022, 05:12:22 pm
content: Quote from: Terry Bites on November 11, 2022, 08:03:20 pmWell, no. I had 4-layer boards with power and ground on inner layers, and didn't think to test for shorts, as I paid the fabricator to do that testing. They cheaped out and didn't test. Knowing which power net was shorted, I put a strong light behind the board, and using a loupe-type magnfier, I searched for anything in the vias and through-hole clearances that didn't look right. I was then able to drill through the speck of copper that didn't etch away and break the shorted path. So, it IS possible to fix a board that has an inner-layer short, at least in some cases. Actually, this may have been a case not of copper that should have etched away, but of a tiny void in the lamination where the through-hole plating went sideways into the void. And, this would naturally happen close to a through-hole.Jon

author: Bud
author_rank: Super Contributor
timestamp: November 18, 2022, 05:13:34 pm
content: Quote from: exe on November 17, 2022, 09:20:07 pmSorry mate, no believe, with several amps PCB traces would just glow red on a thermal camera.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 18, 2022, 09:14:55 pm
content: It depends on the pcb traces and the short resistance.If the short is 20mOhm, you could throw 5 amps at it and only dissipate 0,5W.This works great in large boards, you have hundreds of ceramic caps, one is shorted, are you going to measure millivolts in every single cap? Will take a whole day.For example laptops usually take 3-5A, these boards have heavy power layout with extremely low impedances.So you begin with 100mA, give it few seconds while searching for hot spots, repeat with 200mA, 500mA, 1A... until you notice something starts heating up.If you started with 5A and 1/2 board blew away, well, that's your fault!

author: xavier60
author_rank: Super Contributor
timestamp: November 18, 2022, 09:58:12 pm
content: For boards like laptop mainboards, I used to use 500mA limited to 500mV into the shorted rail then home in on the shorted component by measuring ground plane voltage drops looking for the highest reading.Best done with a DMM with 1uV resolution. The black probe should be connected to the ground plane near to where the power supply negative is connected, not at the same point.To confirm a kill, more direct measurements can be taken. A shorted MLCC's usually have a very small voltage drop between the base and the top of the endcaps. If it's a shorted low side MOSFET, a voltage drop can be detected along its Source pin or along a short distance of copper near the Source connection for leadless parts.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 19, 2022, 10:19:18 am
content: Quote from: Bud on November 18, 2022, 05:13:34 pmPower traces are usually wide, so a few amps is something like a few degrees of temperature rise. If you have a multi-layer board, the ground fill at distance of 100-200µm sucks all that heat away. With a very low-noise and accurate thermal camera you may be able to see it. Increasing the current to tens of A probably helps.The problem here is a short (as adjective, opposite to "long") short does not become a hotspot; then basically the whole power path heats up. This can still help finding paths where current doesnotflow, ruling some parts of the circuit out.I prefer the multimeter method because it is more accurate, but fundamentally these two methods are similar and sometimes you can't access the power tracks with probes (inner layers, few vias).Power planes are difficult.