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Finding power - gnd shorts

Discussion in 'Embedded' started by rickman, Jul 22, 2008.

  1. rickman

    rickman Guest

    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
     
    rickman, Jul 22, 2008
    #1
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  2. rickman

    larwe Guest

    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.
     
    larwe, Jul 22, 2008
    #2
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  3. rickman

    dalai lamah Guest

    Un bel giorno rickman digitò:
    Increase the current until you see something heat up or melt. :)
     
    dalai lamah, Jul 22, 2008
    #3
  4. rickman

    langwadt Guest

    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
     
    langwadt, Jul 22, 2008
    #4
  5. rickman

    cbarn24050 Guest

    A small compass
     
    cbarn24050, Jul 22, 2008
    #5
  6. rickman

    Stef Guest

    In comp.arch.embedded,
    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, Jul 22, 2008
    #6
  7. rickman

    drn Guest

    Rick, what ever you do, we want to see it on You Tube ;-)
    Looking forward to that,
    Best Regards, Dave
     
    drn, Jul 22, 2008
    #7
  8. rickman

    Joerg Guest

    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.
     
    Joerg, Jul 23, 2008
    #8
  9. rickman

    CBFalconer Guest

    Have you taken an unstuffed board and tested that assumption?
     
    CBFalconer, Jul 23, 2008
    #9
  10. Is this multi-layer or 2 layer?

    I have seen creases/folds on inner layers (8 layer board) cause this
    type of problem.
    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.
    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.
    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).
    See everybody elses comments as well.

    --
    Paul Carpenter |
    <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
     
    Paul Carpenter, Jul 23, 2008
    #10
  11. rickman

    JSprocket Guest

    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
     
    JSprocket, Jul 23, 2008
    #11
  12. rickman

    Neil Guest

    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.
     
    Neil, Jul 23, 2008
    #12
  13. rickman

    Geo Guest

    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
     
    Geo, Jul 23, 2008
    #13
  14. rickman

    Geo Guest

    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
     
    Geo, Jul 23, 2008
    #14
  15. rickman

    Stef Guest

    In comp.arch.embedded,
    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, Jul 23, 2008
    #15
  16. rickman

    Didi Guest

    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
     
    Didi, Jul 23, 2008
    #16
  17. 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
     
    Paul Keinanen, Jul 23, 2008
    #17
  18. rickman

    Dombo Guest

    Paul Keinanen schreef:
    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.
     
    Dombo, Jul 23, 2008
    #18
  19. rickman

    rickman Guest

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

    Rick
     
    rickman, Jul 23, 2008
    #19
  20. rickman

    ian.okey Guest

    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
     
    ian.okey, Jul 23, 2008
    #20
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