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Repairing Flexible PCBs

Discussion in 'Electronic Design' started by Paul Hovnanian P.E., Sep 26, 2004.

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  1. I have one with a broken trace (a very fine crack). Any ideas about
    repairing these things? Do those 'repair pens' with the silver-based
    conductive ink work on these?

    This PCB is the contact grid in a Fluke ScopeMeter, so continuous
    flexing isn't as big a problem as it would be with a flex cable in a
    hinge, for example.

    Where are these pens available on line?

    Thanx in advance.
  2. Robert Baer

    Robert Baer Guest

    As long as the copper is exposed, that might work.
    However, if the crack is near a connector, then it likely will be
    flexed and stressed when re-connected, and possibly breaking the
    conductive coating.
    Does not hurt to try.
    Possible sources: DigiKey, Mouser, Newark, Allied.
  3. Guy Macon

    Guy Macon Guest

    Can you find the signal on whetever is at each end of the Flexible PCB?
    If so, you can runs a wire in parallel with the bad trace.
  4. This is the contact grid for the meter pushbuttons. The buttons are
    connected in an X-Y grid. The break is between one button contact (which
    is a part of the flexible PCB) and the end connector.
    I'd have to solder on the flexible substrate, which won't stand up to
    the heat.
  5. Guy Macon

    Guy Macon Guest

    This is a long shot, but what color is the flexible substrate?
    If it's Kapton (it probably isn't) it *might* be solderable.

    I think that your best chance is to solder a wire on the PWB
    that the end connector goes to, strip an inch or two on the
    other end, lay the bare wire on it on the flexible PCB trace,
    and glue it down with a series of small dots made from the best
    conductive adhesive you can find.
  6. Joerg

    Joerg Guest

    Hi Paul,

    As Guy said, conductive epoxy and soldering are the only techniques that
    have worked for me in flex circuit cases. But both have risks, epoxy in
    that it can dissolve some stuff and soldering in terms of too much local
    heat. Kapton should be able to take that heat though.

    Check the button area. Is the material flexing when a button is pressed?
    If so that might be the problem that caused the crack. Possibly you
    could backfill to avoid that the repaired section fractures again..

    Regards, Joerg
  7. Jim Yanik

    Jim Yanik Guest

    How about a low-melting point solder? Like Chip-Quik(160degF) or similar.

  8. Its clear with a slightly bluish tint.

    I went with the 'PCB repair pen' option (the silver paint stuff). It
    doesn't appear that the PCB moves very much since it lies against a flat
    backing. Its nothing like trying to repair the flex cable at a laptop
    hinge. The crack in the trace is nearly impossible to see. I had to
    diagnose it with an ohmmeter.

    I figure if this paint stuff fails, I can go to plan 'B', some low temp
    solder. There is plenty of real estate on this PCB where I can do a melt
    test on a corner first.

    Paul Hovnanian mailto:p
    note to spammers: a Washington State resident
    It's easier said than done.
    .... and if you don't believe it, try proving that it's easier done than
    said, and you'll see that it's easier said that `it's easier done than
    said' than it is done, which really proves that it's easier said than
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