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

Yahoo, that would be it! In 220V countries even better than in the
US... :-)

I have not resorted to that, but have been close - I used a brand-new
12V 7Ah NiMH battery I had just assembled in similar circumstances.
I had a tiny (visible) short on an inner layer and I did need that
board to
work at that moment... It worked and has been working for
many years since.

Didi

------------------------------------------------------
Dimiter Popoff               Transgalactic Instruments

http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/sets/72157600228621276/

Original message: http://groups.google.com/group/comp.arch.embedded/msg/06a618bf575439fa?dmode=source

On Wed, 23 Jul 2008 04:39:39 -0700 (PDT), Didi <d...@tgi-sci.com> wrote:

>larwe wrote:
>> ....
>> Of course a Real Man(sm) would just blow the short out with AC line
>> voltage.
>
>Yahoo, that would be it! In 220V countries even better than in the
>US... :-)

The problem is that in 230 V countries, the mains fuses are only 10-16
A, so if there is a stubborn short on the PCB, the 50-100 A short
circuit current may blow the mains fuse but leave the PCB short
unaffected :-(.

For stubborn shorts, it might be a better idea to use a welding
transformer. 

Even if you do not try to evaporate the short, one idea might be to
let some AC current (a few amps) flow through the PCB and use a
miniature coil connected to the oscilloscope probe and try to locate
in which PCB track branches the AC current flows to the short. By
rotating the coil, you might even be able to determine if the current
is flowing in a track along the long or short axis of the PCB.

Paul

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



Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
s...@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Paul Keinanen schreef:
> On Wed, 23 Jul 2008 04:39:39 -0700 (PDT), Didi <d...@tgi-sci.com> wrote:
> 
>> larwe wrote:
>>> ....
>>> Of course a Real Man(sm) would just blow the short out with AC line
>>> voltage.
>> Yahoo, that would be it! In 220V countries even better than in the
>> US... :-)
> 
> The problem is that in 230 V countries, the mains fuses are only 10-16
> A, so if there is a stubborn short on the PCB, the 50-100 A short
> circuit current may blow the mains fuse but leave the PCB short
> unaffected :-(.

The mains fuse doesn't immediately blow at 16A. I've seen copper 
evaporate from the PCB with a short on the the mains side with the mains 
fuse still left intact.

On Jul 23, 8:45 am, Dombo <do...@disposable.invalid> wrote:
> Paul Keinanen schreef:
>
> > On Wed, 23 Jul 2008 04:39:39 -0700 (PDT), Didi <d...@tgi-sci.com> wrote:
>
> >> larwe wrote:
> >>> ....
> >>> Of course a Real Man(sm) would just blow the short out with AC line
> >>> voltage.
> >> Yahoo, that would be it! In 220V countries even better than in the
> >> US... :-)
>
> > The problem is that in 230 V countries, the mains fuses are only 10-16
> > A, so if there is a stubborn short on the PCB, the 50-100 A short
> > circuit current may blow the mains fuse but leave the PCB short
> > unaffected :-(.
>
> The mains fuse doesn't immediately blow at 16A. I've seen copper
> evaporate from the PCB with a short on the the mains side with the mains
> fuse still left intact.

Yes, this sounds like an excellent idea!  Maybe I can even evaporate
the entire power plane completely eliminating any possibility of a
short!

Rick

On Jul 22, 6:49 pm, langw...@fonz.dk wrote:
> 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

Some 20 years ago I had a 28 slot, 8 layer backplane with this sort of
problem - dust on the artworks during manufacture.  I had to us a 10
amp power supply to vaporise all of the shorts.  Most went at about an
amp but there were a few that took more than 5 Amps before they
opened.  Fortunately no active components on that board.

Ian

In comp.arch.embedded,
rickman <g...@gmail.com> wrote:
> On Jul 23, 8:45 am, Dombo <do...@disposable.invalid> wrote:
>> Paul Keinanen schreef:
>>
>> > On Wed, 23 Jul 2008 04:39:39 -0700 (PDT), Didi <d...@tgi-sci.com> wrote:
>>
>> >> larwe wrote:
>> >>> ....
>> >>> Of course a Real Man(sm) would just blow the short out with AC line
>> >>> voltage.
>> >> Yahoo, that would be it! In 220V countries even better than in the
>> >> US... :-)
>>
>> > The problem is that in 230 V countries, the mains fuses are only 10-16
>> > A, so if there is a stubborn short on the PCB, the 50-100 A short
>> > circuit current may blow the mains fuse but leave the PCB short
>> > unaffected :-(.
>>
>> The mains fuse doesn't immediately blow at 16A. I've seen copper
>> evaporate from the PCB with a short on the the mains side with the mains
>> fuse still left intact.
>
> Yes, this sounds like an excellent idea!  Maybe I can even evaporate
> the entire power plane completely eliminating any possibility of a
> short!

For that purpose I can recommend one of our machines with 40,000uF caps
at 350 Volts, they're ideal for evaporating copper from boards!

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

On Jul 22, 8:02 pm, CBFalconer <cbfalco...@yahoo.com> wrote:
> 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?

No, but I have over a hundred boards that are working just fine.
Actually, I have 103 working boards... unfortunately, I need to
deliver 104, no kidding!  I have three other boards where I have
identified the problems and can be fixed.  One of those has at least
two open vias.  These boards are from the batch I got from Sunstone
where they had a 25% X-out rate due to plating problems in the vias.
I ordered 6 panels.  They made 7 and I still didn't get enough
boards.  They had to make a seventh panel with 16 working boards to
complete the order.

I was very concerned about vias opening in the field.  So I am glad
that I have only found one board with open vias after assembly.  I
just got 56 boards out of a four day bake which I will be retesting.

Rick

On Jul 23, 3:17 am, Neil <NeilKu...@worldnet.att.net> 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.
>
> > 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.

Many posts indicate that I should expect the something to be getting
warm from the 1 Amp of current.  Yes, the power supply is getting
warm.  The voltage on the board is only 5 mV max.  That equates to 5
mW.  I think it would take a very sensitive device to see the
temperature rise from 5 mW of power.  Handing the board wearing gloves
would leave a larger thermal imprint than this.

But using a FLIR camera does sound like fun!

Rick

On Jul 22, 9:30=A0am, rickman <gnu...@gmail.com> wrote:
> I have a small module that is shorted between the +12 volt plane and
> ground. =A0I 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. =A0I 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. =A0Probing with a volt meter I can
> see 10 mV at the point where I connect the power to the board. =A0This
> drops to about 1 mV on the other edge of the board. =A0But 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

Hi Rick
 I've described this methode before. Most people seem to think
that a method of finding the short requires feeding current through
the short. My method uses a different idea.
  Put your supply across the 12v line, from one end to the other,
in current limit. The line then has a small voltage drop across it.
 Place a DVM lead on the ground. The location isn't important
since there is no current in the ground.
 Use the 200mv scale. This works best with a 4 or 5 digit meter.
With the other lead, probe along the 12V line. When the meter
reads closest to zero, you are at the location of the short.
 I have used this method many times when other methods
seem to fail.
 The idea is to consider the 12 v line to be like a resistor with
a tap on it. The ground it the tap. With the second lead as
another tap, when both taps are at the same location along
the resistor, there is 0 volts.
Dwight