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> Finding A Short Circuit In A Pcb, Locating it with Eagle?
zonks
Posted: August 22, 2015 04:52 am
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I got two PCBs from a cheap fab and there's a short between +5V and ground on one of them, which I had noticed when using a multimeter to do a continuity test. It's a large two-layer board so I've been visually inspecting it but that's painstaking work and I haven't found the problem yet.
In Eagle's board viewer I've ripped up all nets except for +5V and ground and hid all layers but the top and bottom copper, which at least lets me focus on the areas I need to examine. This helps a little.

I'm assuming the short happens at some point where the +5V and ground rails run close to each other and there's some bridging under the solder mask, or where a off-center drill hit shorted them together after the hole was plated. So I was wondering if there's some way (perhaps via scripting) to have Eagle calculate locations on the board where two nets are the closest together so I could narrow down the areas to examine. Or is that outside of the type of functionality Eagle can provide?

Are there any other strategies for finding a short? The board is mostly through-hole with just a few surface mount parts and I've checked the those parts for bridging on the pads.

The board passed the Eagle DRC/ERC checks and the fab's DFM check.
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Village Idiot
Posted: August 22, 2015 06:40 am
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If everything else fails, connect a high current power supply between the +5 and ground, and see what lights up. Do it in a dark room and shoot a video while you do it, so that you can record where the problem is, just in case your eyes aren't quick enough.

A side benefit of this is that when you're finished the test, the short will be gone.
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MacFromOK
Posted: August 22, 2015 06:43 am
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QUOTE (Village Idiot @ August 22, 2015 12:40 am)
A side benefit of this is that when you're finished the test, the short will be gone.

As may be the circuit board... biggrin.gif


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Village Idiot
Posted: August 22, 2015 07:03 am
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Well, I did say, "If everything else fails..."

I'm only partly kidding. A very experienced electronics tech friend of mine actually has used this method. If he can't find the source of the short in a reasonable amount of time he puts the power on it. He figures that even if you have to repair a trace, it's still the fastest way to find the short, and for him, time is money.
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Nothing40
Posted: August 22, 2015 07:19 am
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If you have a current limited supply,and a thermal camera.. Run just enough current through there to make the short heat up..it's not necessary to pump a bunch of current in,and vaporize it,but just enough so that it heats up,and you can see the hot-spot with an IR camera,FLIR,whatever.

Short of that,give 'er some juice,and look for the curl of smoke. Should clear the short,anyway. smile.gif


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draget
Posted: August 22, 2015 08:30 am
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Capacitor ESR or other super low ohms meter should be able to help you find the short.
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Sch3mat1c
Posted: August 22, 2015 09:44 am
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Beyond DRC, I don't think EDA will help you much. Such a mistake could happen anywhere. There seem to be two kinds of PCB fab errors: standard deviation in the actual imaging and print quality (trace edges are within so-and-so resolution capability of the tools used to print the layer), and random event errors, such as blobs in the film (positive or negative), bad drill hits and etc.

Clearance rules are designed to avoid the first, because if the resolution is, say, 1 mil variance, then 7 mil minimum clearance means 7 sigma of unlikely-to-short-out. Basically impossible (~parts per billion). That's the purpose of those rules.

Blobby errors aren't practical to treat by clearance, because they are randomly distributed over the panel, fairly large (~mm), and infrequent events (Poisson statistics instead of Gaussian normal). This is why board houses typically make 10-13 copies of a board, and promise to deliver at least 8-10 of them. You're likely to get more than the minimum, because they add just enough extra to allow for that expected error rate. The minimum is set such that, in the rare event that they have too many errors, they only have to re-spin the board (adding delay to your production, and doubling their cost, which they have to swallow) once in a fairly long time, perhaps less than 1% of the time (so the added cost averages pretty small).

Unfortunately, if your board does not include electrical test (or their electrical testing is unable to spot these correctly; unintentional shorts around intentional net bridges would be one example), the cost for failed boards must be borne by you, the end user.

Tim


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kl27x
Posted: August 22, 2015 11:54 am
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QUOTE
So I was wondering if there's some way (perhaps via scripting) to have Eagle calculate locations on the board where two nets are the closest together so I could narrow down the areas to examine.

Go into DRC menu. Examine parameters for any min clearances that are on the small side. Make the minimum clearances incrementally larger until you start getting errors when you run a DRC check.

After you do this and still fail to visually locate the short:

Examine your + rail in CAD to a find strategic place to cut it on the physical board. Make cut, continuity test to determine on which side the short persists, examine the bus in CAD and find the best (middle-est) point to make the next cut.
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zonks
Posted: August 22, 2015 03:55 pm
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I found it, there's a few 7343 tantalum caps on the board and the pad for one of them had these very fine 'fingers' of copper coming out of the corner connecting it to an adjacent via under the soldermask. It almost looks like dust and is pretty hard to see.

I had been checking with a flashlight underneath the board expecting to find a solid dark area where the traces were touching and didn't expect this type of structure. Scraping it away fixed the short.

Thanks for the input everyone, there were a lot of interesting solutions given. In fact I wonder if the high current approach would have vaporized these thin connections and fixed the problem. smile.gif
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JedNaus
Posted: November 13, 2015 02:11 pm
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As per my experience if you have a current limited supply and a thermal camera then Run just enough current through there to make the short heat up.
But it is not necessary to pump a bunch of current in and vaporize it but just enough so that it heats up.
And you can see the hot-spot with an IR camera.

surface mount assembly

This post has been edited by JedNaus on May 18, 2016 11:08 pm
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