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Re: Finding power - gnd shorts
In article <165e19e7-44ef-4419-bfaa-6a398a4c6cf4
@z66g2000hsc.googlegroups.com>, snipped-for-privacy@gmail.com says...
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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


Re: Finding power - gnd shorts
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but you have checked anyway, right ?

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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


Re: Finding power - gnd shorts
In comp.arch.embedded,
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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)

Re: Finding power - gnd shorts

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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.

;)
 
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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,
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Re: Finding power - gnd shorts
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Rick, what ever you do, we want to see it on You Tube ;-)
Looking forward to that,
Best Regards, Dave

Re: Finding power - gnd shorts
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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 /

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Re: Finding power - gnd shorts
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... snip ...
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Have you taken an unstuffed board and tested that assumption?

--
 [mail]: Chuck F (cbfalconer at maineline dot net)
 [page]: <http://cbfalconer.home.att.net
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Re: Finding power - gnd shorts
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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

Re: Finding power - gnd shorts
In article <8ea6b8c3-2bed-40ec-8734-2378efc3e2b0
@l42g2000hsc.googlegroups.com>, snipped-for-privacy@gmail.com says...
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What was the size of your vias?   I've been using 8 mil trace and space
and 21 mil vias for many years without problems.   However, some of
the larger fine-pitch stuff I use now will soon force finer traces
and spacing.
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Mark Borgerson


Re: Finding power - gnd shorts
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I used 6/6 and 10 mil vias in 24 mil pads.  Sunstone originally said
they could make 10 mil vias, but after I got the boards they said they
used a 13 mil drill since that was within their spec of +- 3 mil.  As
long as they got the holes on the pads, I didn't care, but some of the
holes are right at the edge of the pads.  I've never been able to tell
if this is a problem as long as the trace is not cut from the pad.

Rick

Re: Finding power - gnd shorts

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It can be a problem if etchant is able to sneak into the plated hole,
you can guess what happens next!

-jg


Re: Finding power - gnd shorts
In article <165e19e7-44ef-4419-bfaa-
snipped-for-privacy@z66g2000hsc.googlegroups.com>, snipped-for-privacy@gmail.com says...
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Is this multi-layer or 2 layer?

I have seen creases/folds on inner layers (8 layer board) cause this
type of problem.
 
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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.

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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.
 
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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).

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See everybody elses comments as well.

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: Finding power - gnd shorts
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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

Re: Finding power - gnd shorts
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Did they also claim they had been 'bare-board tested' ? ;)

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

-jg


Re: Finding power - gnd shorts
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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.

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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

Re: Finding power - gnd shorts
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It might make sense to rent one, find the short, then take it home and
image your house from the outside. It'll find all the leaky areas where
heat or A/C losses occur. One guy found a really hard signal near a
crawl space vent. Turned out an A/C duct had fallen off.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Finding power - gnd shorts
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Let's see... the board cost $150 to make and a FLIR camera cost maybe
$500 to rent.  My ducts are in the basement where I can see them all
and this 50 year old house is nothing but a draft with windows.

I don't think I'll rent the FLIR camera, but I sleep well at night
knowing I don't have to worry about Radon building up inside...  :^)

On the other hand, I am getting very worried about making enough money
to be able to fill my heating oil tank this year.  After all, I only
make an engineer's pay.  It might not be long before I sell the
furnace as scrap to pay for electric space heaters!!!

Rick

Re: Finding power - gnd shorts
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That's going to cost you in winter.


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Why is everyone concerned about Radon? Does it really occur a lot?


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Careful, in our neck of the woods the utility then really socks it to
you. Get past 130% of a rather paltry baseline usage and it jumps up.
Get past 200% of baseline and electricity becomes hyper-inflationary.
Out here electricity is certainly not the future. So, we got wood
burners. I already know what next winter's heating bill will be: $1200,
for roughly six months or winter (global warming didn't happen out here).

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Finding power - gnd shorts
On Wed, 23 Jul 2008 14:44:20 -0700, Joerg

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Radon can be a problem in houses built on sand that has been recently
(e.g. during the Ice Age) split from some uranium rich stones, such as
granite.

Instead of ventilating the whole house, just make 2-3 vertical holes
under the house and install an air pumps sucking the air and radon out
of these wells and blow it outside the house. The slight
under-pressure in the wells will such the air from the surrounding
sand and thus prevents the radon from entering the house through the
floor.

Paul
 

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