Re: Finding power - gnd shorts

On 23 Jul., 16:31, rickman <gnu...@xxxxxxxxx> wrote:
On Jul 23, 8:33 am, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:



On Tue, 22 Jul 2008 09:30:33 -0700 (PDT), the renowned rickman

<gnu...@xxxxxxxxx> wrote:
I have a small module that is shorted between the +12 volt plane and
ground. I am having a hard time finding where the short is so it can
be fixed.

The bare boards were supposed to be tested, so I don't suspect the
board itself. I have visually inspected everything I can including
looking under the chips as much as I can see and found no sign of a
problem.

My bench supply current limits (foldback actually) and I am seeing
about an Amp into the 12 volt rail. Probing with a volt meter I can
see 10 mV at the point where I connect the power to the board. This
drops to about 1 mV on the other edge of the board. But I can't find
a particular point where the voltage says "here it is"!

Any ideas on how to find and fix this short?

Rick

Do you have a blank board to check? I hope so. Can you see the
clearance where vias etc. pass through the power planes by holding the
board up to a strong light?

As others said you can follow the gradient. If the gradient does not
change then you've found a direction where the plane is not carrying
current. But with two internal planes this won't be all that easy.

Hate to say it, but it kinda sounds like it might be internal to the
board from what you're saying. Like maybe they made two gnd planes or
something like that.

Just a couple of times I've taken almost every single component out of
a board to find the problem with 100% certainty (failure analysis,
just to ensure we didn't get more like that). Whiskery shorts on
boards (sometimes intermittent so it could pass e-test) were the
hardest to find, but you could see them under a microscope. OTOH,
sometimes it's huge and right in front of your face.

When you say you found various shorts, were they design flaws (which
would be repeated on each board) or manufacturing flaws (which would
be individual)? This is one board out of a run of 110. I have found
one other board with a short on the 3 volt rail, but this one is on
the 12 volt rail. So there is no design flaw.

I am currently suspecting a decoupling cap at this point. The
gradient is small and points toward one end of my 4.5 x 0.85" board.
The 12 volt plane is only on about half the board and power has been
applied to the end near the middle of the board. The gradient points
to the opposite end. When I get some more time to work on this, I
will test with power on the "opposite" end and test toward the power
connector. If I see the same gradient, I will start removing
components. If the gradient slopes the other way, I will suspect
multiple shorts which are likely an internal board problem (which I
expect is unlikely).

BTW, thanks for all of the suggestions everyone! Some of the ideas
were obvious to me, others at least made me think a bit. I appreciate
the different viewpoints.

I may try the freeze spray thing. I don't have a supply that will put
out 10 A, but I have an AC transformer that puts out 1 VAC at some
huge current. That with a diode might actually do the job, who
knows?

Rick

use car battery? ;)

-Lasse
.

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