On Jul 22, 12:30 pm, 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

Rick, what ever you do, we want to see it on You Tube ;-)
Looking forward to that,
Best Regards, Dave

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?
> 

First try gradient. Dump in constant current, switch the DVM to the 
200uV range and probe at 1/2" or so distance. IOW the probes "walk" 
behind each other in lockstep. Move the trailing one for greatest 
gradient, then keep going until you see a steep drop or reversal. That 
should lead you to the "sink". Of course this won't work well if there 
is more than one short. I once had a whole series of tested (!) boards 
that had four shorts each.

If this fails try to get a hold of a camera that is somewhat infrared 
sensitive. Snap a pic in the dark, load onto PC and stretch the 
histogram to wazoo. Sometimes that shows a distinct hot spot or possible 
more than one. The temperature at the short is usually a lot higher than 
elsewhere on the plane.

If you are near San Francisco maybe John Larkin lets you put it under 
his FLIR camera. That ought to show it.

-- 
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

rickman wrote:
> 
... snip ...
> 
> 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.

Have you taken an unstuffed board and tested that assumption?

-- 
 [mail]: Chuck F (cbfalconer at maineline dot net) 
 [page]: <http://cbfalconer.home.att.net>;
            Try the download section.

In article <165e19e7-44ef-4419-bfaa-
6...@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.

Is this multi-layer or 2 layer?

I have seen creases/folds on inner layers (8 layer board) cause this
type of problem.
 
> The bare boards were supposed to be tested, so I don't suspect the
> board itself.

If you still have any blank boards I would test them FIRST!

I have seen bare board testing that basically follows the netlist for
continuity, but does NOT check for track to track or track to plane
shorts.

>  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.

First obvious thing is does ANYTHING get noticeably warm/hot.

Without knowing the circuit you could have a combination of
faults (reversed components, interplane short, wrong component on
a power amp can cause some combinational effects).

Let alone a drop of solder from a soldering iron bridging two power 
rails I once did by accident.
 
> 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"!

When you measure 1mV have you just moved one probe. Moving both probes 
gives to nearest and further away points on the two tracks to determine 
voltage gradient. The gradient method and how tone ohms basically work
(the closer the two probes are to the short or one point, the lower
 the impedance, giving usually a higher frequency o/p).

> Any ideas on how to find and fix this short?

See everybody elses comments as well.

-- 
Paul Carpenter          | p...@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/>;    PC Services
<http://www.pcserviceselectronics.co.uk/fonts/>; Timing Diagram Font
<http://www.gnuh8.org.uk/>;  GNU H8 - compiler & Renesas H8/H8S/H8 Tiny
<http://www.badweb.org.uk/>; For those web sites you hate

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.
> 

I had this some years ago with my very first 4 layer PCB. Aaaargh! I 
took a board and sawed it in half: the short should be in either the 
left or the right half. It was in both. So I sawed the two halves in 
half. The short was now in all four pieces....

Eventually I took a belt sander and removed the outer PCB layers, thus 
revealing the fault: the VCC layer has VCC connections, while the ground 
layer had both VCC and ground connections. Armed with this, and the 
Gerbers, I went back to the manufacturer... who was forced to confess 
that some underling had decided that the two layers were meant to be 
merged. So I eventually got my clean PCBs, but it took several weeks all 
told.

JS

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?
> 
> Rick
If it is a lot of board, and they are not too expensive, Try more 
current.  1 amp is a lot.  Something should be getting warm.  If it is 
not the board, that leaves wrong / bad components, backwards parts, and 
a schematic error.

On Tue, 22 Jul 2008 09:30:33 -0700 (PDT), rickman <g...@gmail.com> wrote:


>The bare boards were supposed to be tested, so I don't suspect the
>board itself. 

But tested to prove that they conform to the supplied CAD data.
Some CAD systems will pass ERCs but if there are any manually split planes
problems can be in the Gerbers.
<memories of 0.9mm drills and bits of green wire and tack.>

Geo

A couple of people have mentioned the heating effect but if you only have a
maximum of 10mV and 1A that is only 10mW.
Even with a 10A limited supply, 100mW is not much dissipated over a couple of
copper planes?


Geo

In comp.arch.embedded,
Geo <9...@disposable.spamcon.org> wrote:
> A couple of people have mentioned the heating effect but if you only have a
> maximum of 10mV and 1A that is only 10mW.
> Even with a 10A limited supply, 100mW is not much dissipated over a couple of
> copper planes?

A short is ohmic, so with a current of 10A, the voltage increases to 100mV,
power goes up to 1 whole Watt (P = I^2 * R). Enough to see at least some
heating effects. But as mentioned, with my method of freezing the board a
little, some luck is involved as well, it does not always work.

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

JSprocket wrote:
> 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.
>>
> 
> I had this some years ago with my very first 4 layer PCB. Aaaargh! I 
> took a board and sawed it in half: the short should be in either the 
> left or the right half. It was in both. So I sawed the two halves in 
> half. The short was now in all four pieces....
> 
> Eventually I took a belt sander and removed the outer PCB layers, thus 
> revealing the fault: the VCC layer has VCC connections, while the ground 
> layer had both VCC and ground connections. Armed with this, and the 
> Gerbers, I went back to the manufacturer... who was forced to confess 
> that some underling had decided that the two layers were meant to be 
> merged. So I eventually got my clean PCBs, but it took several weeks all 
> told.

Did they also claim they had been 'bare-board tested' ? ;)

They may well have passed, as many testers learn from a sample!

-jg