Bizarre and Unexplainable High-Impedance "Shorts" Across PCB - Page 1
Forum section: electronics / general

author: tom66
author_rank: Super Contributor
timestamp: September 03, 2021, 02:27:39 pm
content: Has anyone seen this behaviour before?We had a number of high-density boards back from a UK manufacturer (PCB manufactured and assembled by them.) Roughly 10-12 layers, microvias and laser-drilled vias used, density roughly 4/4mil and a few small BGAs/QFNs but nothing beyond 0.5mm.I am getting all sorts of strange logic level signals on my oscilloscope or multimeter. So for instance we have a 3.3V programming signal to our microcontroller but the logic levels are all over the place, highs of 2.5V and lows of 1.5V with the high and the low jumping all over over the course of a few minutes. This then disappears and comes back at arbitrary times. LVDS signals are also affected, but very low impedance power supply rails seem to be mostly okay. When I measure with a meter, I might see some 50-100 ohms between adjacent signals, even when all that is on that board is a connector and traces without any actual logic on that board. Most recently I have a board that just smells a little bad and gets warm and draws more current than expected when 3.3V is applied to it, but a thermal camera and my finger can't locate a specific source - it's almost as if the whole board is just a bit leaky and something is burning somewhere.The boards had connectors fitted to the board by us, so our suspicion was at first that it might be conductive flux causing shorts between adjacent nets, as I've heard this can be an issue after some years but maybe we were getting unlucky. I found the technician was using a standard paste flux (years old and awfully contaminated) that required cleaning and that he might not be cleaning the flux off after soldering, so I made sure the boards were thoroughly cleaned before testing them. This, for the time being, seemed to resolve the issue, however it has begun rearing its head again even after we use no-clean flux AND make sure to properly clean the board with flux-off. We threw away the old contaminated flux altogether. The issues usually appear some time after rework and appear to be located in the area work was done. We are using 60/40 SnPb solder for most of the work from RS.Has anyone seen this issue before and are they aware what causes it? Could we be seeing some kind of internal layer shorting that becomes more prominent after soldering is done in that area, exposing the board to heat? Some kind of 'tin whiskering' effect but only apparent internally in the board? Contaminated FR-4 or prepreg that isn't well-insulating? I'm tearing my hair out!As long as I know how to avoid it I can take steps in that direction, but right now we only have a vague sense of the triggers and not really an understanding of the fundamental effects. I've considered x-raying boards or even doing a SEM scan on solder joints, but the cost of doing that isn't trivial and I'm not sure it'll give us that much.Any thoughts appreciated ...!

author: Tomorokoshi
author_rank: Super Contributor
timestamp: September 03, 2021, 04:49:56 pm
content: 1. Do you have blank boards?2. How much variation is there between the blanks?3. Could metallization from the laser drilling be recondensing somewhere?4. Does it change when flexed?5. Can you cut out an area with isolated copper planes only and measure between them?

author: Marco
author_rank: Super Contributor
timestamp: September 04, 2021, 10:27:43 am
content: How about first doing a cross cut after thermal cycling, polishing it and looking at it under low magnification?If there's voids (crazing/delamination/whatever) there's a lot of other possible problems which become worse. Easily visible and easy to slap the PCB manufacturer over the head with for.

author: jmelson
author_rank: Super Contributor
timestamp: September 04, 2021, 04:22:31 pm
content: Many years ago, I had a computer tape controller that had worked for several years, but then failed. Checking the internal reset/ signal showed it was stuck low (in the reset state.) Checking all ICs that drove this node, and that received it, did not show a failure. I cut the trace, which ran all over the board, to isolate the cause. I finally found a 1 inch strip of trace that was shorted to ground. Peeling that trace from the 2-layer board revealed no visible anomaly, but there was some kind of conductive contamination inside the fiber-epoxy substrate that had become conductive. One of the wierdest cases from my years of fixing stuff.Jon

author: jmelson
author_rank: Super Contributor
timestamp: September 04, 2021, 04:29:31 pm
content: Are the blank boards 100% tested on a flying probe tester? I've had lots of boards that failed after assembly, where I had paid for electrical test, which had inner-layer shorts. The probes leave tiny dimples in the center of the pads that are easily detectable under a microscope. This was the one cardinal sin that made me blacklist a PCB fabricator. If I paid for electrical test, damn it, they need to DO it!The kind of conductivity you describe OUGHT to be picked up on a flying probe tester.Jon

author: jmelson
author_rank: Super Contributor
timestamp: September 04, 2021, 04:36:53 pm
content: OK, reading your OP more closely, it seems to have something to do with hand soldering and rework. Instead of no clean flux, try water-soluble, and then wash the board in running water and scrub the area with a soft toothbrush. The WS flux usually comes off COMPLETELY, leaving a totally clean board and pads.I have some hand soldered connectors that are tested with a bed of nails fixture, and flux residue would keep the fixture from making good contact. The old no-clean flux would absorb moisure, eventually, and become slightly conductive. It is VERY hard to completely remove NC flux residue, even with strong solvents.Jon

author: coppercone2
author_rank: Super Contributor
timestamp: September 04, 2021, 05:53:49 pm
content: ask the manufacturer for impedance/electrical tests, that sounds like a defective board.my usual attempt at cleaning would be to put it in a bag of flux remover and throw it in a ultrasonic for 5 minutes then rinse it with more flux remover in a squirt bottle while you are holding it with thongs over a sink, then do inspection under bight stereo microscope after its dry looking for more sheen from residues. I think you need ultrasonic because maybe you have fucking flux under small parts.. that kind of contamination only comes out with ultrasonic (if you don't believe me, stick grease between two peices of metal and throw it in there, you will see grease plumes pouring out between the surfaces under water, in a way not otherwise possible.You can also try to bake it agressively *(70+C)* to see if you can get any residual humidity out over 1 hourContacts can be a bitch, I thought my car remote broke because I replaced the battery and despite everything looking primo clean, it only worked after I gave the coin cell and the contacts a good going over with isopropyl alcohol on a kim wipe and q tip. there was 0 visible residue but there must have been some film preventing the battery from making contact with the remote, and that battery cavity was real tight too, I had to use a screw driver like a prybar for a 2016 coin cell... I would have naturally thought it would displace any crap on something that looks so clean.

author: Kerlin
author_rank: Regular Contributor
timestamp: September 04, 2021, 11:47:55 pm
content: A high impedance short is the opposite of a low impedance open circuit.

author: tom66
author_rank: Super Contributor
timestamp: September 06, 2021, 05:25:15 pm
content: So, interesting result. I took a 'bad' board which had 30 ohms from one signal to GND and 10 ohms from another signal to GND. Ideal would be Mohm+. I cleaned the board with fluxclene, no difference. Flexed the board in the relevant area: slight increase in resistance (30->42 ohms and 10->12 ohms) but hard to say how significant this is.I then heated the board up in that area at 400C with a hot air gun for 30 seconds. Result: problem gone. And no matter what I do, flex the board, heat it up with an iron, tap it in the area, it doesn't come back.I checked with board manufacturing data, laser drilled vias not used, it's only microdrilling. The boards DON'T show electrical test points on the gold pads but these are only on the blanks I have, I don't know if these were just untested because they weren't sent for assembly. Am following up with manufacturer to confirm.Further testing is required.Thanks for all the input so far.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: September 07, 2021, 04:25:42 pm
content: Quote from: tom66 on September 06, 2021, 05:25:15 pmThat suggests a couple tests:1. If you have a Hi-Pot tester, run it from 0 to maybe 500V with a limit of a few mA.2. Soak that board in distilled water for maybe 5 minutes. Dry with a towel. Measure. Repeat. Something like that. Is the resin absorbing water? If it shorts, try the hit air gun. Does it fix it again?

author: floobydust
author_rank: Super Contributor
timestamp: September 07, 2021, 11:28:45 pm
content: The PC boards are defective- it could be a contaminated bath at the PCB fab.If it's dissimilar metals i.e. copper and tin aqueous, there would be a 1/2 cell galvanic potential seen between traces as well. Dry, a thermal EMF would be seen.I don't think tin whiskers develop fast though.I have seen shorts due to copper specs caused by dust. I just used a power supply at high current to clear them, as proof.

author: station240
author_rank: Unknown
timestamp: September 08, 2021, 01:29:18 am
content: Quote from: tom66 on September 06, 2021, 05:25:15 pmI wonder if this indicates there is water (or a solvent) trapped between the inner layers.Worth asking a few different PCB fabs if this is an issue they have seen, and what the possible cause could be ?