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

> 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.

Faced with the same problem I had no choice but to use a dremel tool
to do a binary search to isolate the problem to a particular half,
then quadrant, then octant, then ....

Of course a Real Man(sm) would just blow the short out with AC line
voltage.

Un bel giorno rickman digitò:

> 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?

Increase the current until you see something heat up or melt. :)

-- 
emboliaschizoide.splinder.com

On 22 Jul., 18:30, rickman <gnu...@gmail.com> 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

I've heard a story of some big expensive board that had similar
problems, I believe dust or something on the films for the pcb made
some tiny shorts.

some brave guy just took the biggest PSU he could find and puff they
all worked

-Lasse

On Jul 22, 5:30=EF=BF=BDpm, rickman <gnu...@gmail.com> wrote:
> I have a small module that is shorted between the +12 volt plane and
> ground. =EF=BF=BDI 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. =EF=BF=BDI have visually inspected everything I can includi=
ng
> 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. =EF=BF=BDProbing with a volt meter I =
can
> see 10 mV at the point where I connect the power to the board. =EF=BF=BDT=
his
> drops to about 1 mV on the other edge of the board. =EF=BF=BDBut 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

A small compass

rickman 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?

If you were a really old fart like me, you'd
have a dusty old Toneohm on one of the back
shelves of the lab.  Ask around.

In article <165e19e7-44ef-4419-bfaa-6a398a4c6cf4
@z66g2000hsc.googlegroups.com>, g...@gmail.com says...
> 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?
> 
If this is a 4-layer board,  take a careful look at any mounting
holes.  I forgot to do that on a recent design and the inner layers
came right up to the plated-through mounting holes for one module.

I had to drill out the mounting holes and warn the end user to
mount the module with nylon screws until the next board revision.

Mark Borgerson

rickman 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. 

but you have checked anyway, right ?

> 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?

Normally the 4 wire 'microvolt gradient' method is enough to
narrow down the location.

That, and a highlighted view of the PCB layers, showing just the two 
offending nets, can give you a smaller set of 'candidiate locations' for 
a short.

Find the lowest voltage diff point, and then vary the current 2:1
and get a milliohm value from the dV - that also gives a clue of what
can be the cause, and possible solutions.

Copper or solder whisker faults can be cleared with a fusing current, 
applied in the right place :)

-jg

In comp.arch.embedded,
rickman <g...@gmail.com> 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?

Ground is a plane as well? If so, most of the power will be dissipated
in/around the short.

Spray the board (or section by section) with cold spray until you get
a thin layer of ice (just a white haze). Then run a current big enough
to get some power in the short. If you're lucky, you will see a spot
defrosting more rapidly then the rest of the board. Try to spray
evenly, uneven spraying can defrost in different times too.

The fancy option would be a heat camera, but I suspect you don't
have one sitting on a shelf. ;-)

Maybe filming the defrosting board and playing it back slowly will
help?

-- 
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

On 2008-07-22, rickman <g...@gmail.com> 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.

Finding the short is easy:

 1) Turn up the PS current limit until until the voltage jumps
    and the current drops to 0.

 2) Examine board for charred spot.

;) 
 
> 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?

We used to use a current probe to find shorts on blank PC boards.

-- 
Grant Edwards                   grante             Yow! In 1962, you could buy
                                  at               a pair of SHARKSKIN SLACKS,
                               visi.com            with a "Continental Belt,"
                                                   for $10.99!!