Finding a short on a prototype board

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Hi All,
Hoping ya'll will have some brilliant suggestion to help me out...
again!

Just received a batch of prototype boards.  Added the through hole
parts to two, and tried to power up.  No power!

After a bit of testing, determined that my regulated VCC is shorted to
ground!  :-(  So, I started looking for the obvious - solder bridges,
vias shorted to an adjacent pad, going over my design files in detail
for maybe an error on my part, etc.  No joy, can't see where the error
is.

So, my problem, how can I locate this elusive short. Any ideas?  It is
a populated board, so I can't just crank up current till it indicates
the problem 'visually' (and aromatically!)  ;-)

Charlie

Re: Finding a short on a prototype board
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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

Re: Finding a short on a prototype board

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



Re: Finding a short on a prototype board

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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.


Re: Finding a short on a prototype board
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".

Re: Finding a short on a prototype board

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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?


--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: Finding a short on a prototype board
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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.

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Great question.

Re: Finding a short on a prototype board
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[...]

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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.

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*That* is cool.

--
Cheers,
James Arthur

Re: Finding a short on a prototype board
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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.

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Could be handy.

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

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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


Re: Finding a short on a prototype board

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That might be an application for the fluxgate current probe discussed here
a while ago.

The question (still unanswered) about inner layers remains.

--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
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Re: Finding a short on a prototype board

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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.

--
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.

Re: Finding a short on a prototype board
On Wed, 06 Jun 2012 14:51:52 -0400, Ecnerwal

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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

Re: Finding a short on a prototype board

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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:
<http://www.eevblog.com/2011/12/31/eevblog-230-arducopter-ardupilot-troubleshooting/

Always good stuff over there (down there?).

--
Rich Webb     Norfolk, VA

Re: Finding a short on a prototype board

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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-(


Re: Finding a short on a prototype board
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   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.



Re: Finding a short on a prototype board
On Wed, 06 Jun 2012 12:55:43 -0800, Robert Baer

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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

Re: Finding a short on a prototype board

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


--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: Finding a short on a prototype board
On Wed, 06 Jun 2012 14:34:29 -0700, John Larkin

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

Charlie

Re: Finding a short on a prototype board

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Ok, found the problem.

I had to add the power pins to my symbol for the MCU.  When adding the
pins, got one VDD-VSS pair reversed!  As the boss says, Wrong Way
Charlie strikes again!

Now I have to see what I can do to fix this on these prototype.  Can't
waste $1200 to do a re-spin... :-(

Charlie

Re: Finding a short on a prototype board

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Blue wire ?:-)
        
                                        ...Jim Thompson
--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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