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Repairing flexible pcb connector track?

Discussion in 'Electronic Repair' started by Ian P, Jul 29, 2010.

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  1. Ian P

    Ian P Guest

    In the course of doing some modifications to a very expensive Sony 3 chip HD
    camcorder I have accidentally cut through 3 tracks on the edge of a mylar?
    flexible cable. On the good side the damage is very accessible but the bad
    bit is the tracks are only 0.1mm wide on a 0.2mm pitch! Ideally one would
    just replace the flex pcb but in practice that would involve dismantling the
    camera and lens to get to all the places the pcb branches out to, and in
    some places it actually goes into the lens mechanism itself.

    I am considering abrading off the top mylar coating to expose the copper
    then bridging the breaks with some very narrow pitch zebra strip. My other
    idea is to make my own miniature insulation displacement/piercing connector
    using a stack of razor blade bits sandwiched with insulation layers.

    The last option would be to use wire links and solder but I think this could
    only be done with some sort of mechanical micropositioning rig in view of
    the small sizes involved.

    I would be interested to hear any thoughts or experiences if anyone has any.

    Cheers



    Ian
     
    hotrodjohn71 likes this.
  2. This is "not unlike" fixing a PCB trace. How would you do that?

    What about globbing some eutectic solder over the traces (even though it
    shorts them), then using "something" to break the solder into individual
    "strands" while it's still liquid?

    I'm wondering whether zebra strip would be conductive enough? It might not
    matter over such a short (ar, ar) distance.

    I wish you success. And if this happened this morning, please put it aside
    and relax. Work on something else, something easy, and try to forget about
    this problem for the time being.
     
    hotrodjohn71 likes this.
  3. Meat Plow

    Meat Plow Guest

    Flexible pcb connector as in a ribbon connector? I've had some limited
    success repairing these with conductive paint designed to repair
    automotive window heaters.
     
    hotrodjohn71 likes this.
  4. N_Cook

    N_Cook Guest


    Assuming you're not in a stripline-type situation with ground plane/rf
    considerations, try plaiting some magnet wire and soldering to the 3 pcb
    solder points at either end of the plaited section of "ribbon", you may
    have to make a hole through the pcb though.
     
    hotrodjohn71 likes this.
  5. Arfa Daily

    Arfa Daily Guest

    Based on my experience of trying to repair accidental damage to flexiprints
    (yes, it happens to us all no matter how long we've been in the game and how
    experienced we are !) I think that you are going to struggle to get a fix on
    one of that tiny pitch. I have had plenty of success with abrading and
    soldering ones of a slightly greater pitch, doing it with a tiny
    needle-point soldering tip and under a microscope, and using a single strand
    from superflex instrument cable, such as is used for better quality meter
    leads, for instance.

    I don't know whether this is a commercial job, or one for a friend, or maybe
    even for yourself, but before you commit to any strategy that's going to
    waste a lot of time, and end up with no fix at the end of it anyway, the job
    would probably stand being left for a day or two, before revisiting it with
    a clear head, to look at how hard replacing the flexiprint would *really*
    be. I've often found that things that look as though they are going to be a
    copper-bottomed-gold-plated bitch to do, are actually not so bad, when
    looked at again after you've calmed down, and forced a degree of 'san fairy
    ann' into your head over it. If you have a copy of the service manual or can
    obtain one, a look at the parts list will tell you if replacement is even an
    option (if you're really unlucky it might be an integral part of the lens
    assembly) and if it is available, a look at the exploded view diagram, would
    be helpful to determine how many 'hidden' branches the flexiprint has, and
    where they go.

    I really feel for you on this one. I've been there many times over the years
    ....

    Good luck with it, and please post back, and let us know how you get on with
    it :-\

    Arfa
     
  6. Ian P

    Ian P Guest

    William

    I too wondered about how conductive zebra strip is but considered it for
    this because the tracks are so narrow they are probably only carrying logic
    level signals whereas some of the other tracks in this particular flex are
    over 1.5mm wide.

    It 'happened' several days ago so have got over the initial shock and
    annoyance with myself. I am not rushing in until feel right.

    Ian
     
  7. Ian P

    Ian P Guest

    Its a paper thin flexible pcb, golden brown in colour which I think is made
    from Mylar. Because of the narrowness of the tracks and their spacing I
    doubt it could be done with paint.

    Ian
     
    hotrodjohn71 likes this.
  8. Ian P

    Ian P Guest

    Its just low level analogue signals but the break is close to the end of the
    ribbon where the tracks are gold plated to fit in the board connector. The
    copper conductors are encapsulated in the Mylar and drilling through the
    track which is only 0.004" wide would not be easy!

    Ian
     
  9. It happened several days ago so have got over the initial shock
    Good. That means you'll probably come up with the "least bad" solution.
     
  10. Ian P

    Ian P Guest

    Arfa

    Thanks for your advice. As you postulated this pcb is integral with the lens
    and although I have the full service manual for the camera it give no
    information at all on the lens which is a bought in item (not removable
    though). The job is not really commercial, for a friend or myself but it a
    sort of combination of all three. I caused the damage though and I need to
    repair it because there is not really any alternative.

    I have in the past repaired and modified boards and components and used thin
    wire just as you described, this cable though has track and spacing widths
    that are really challenging, and I don't have any microscope.

    I have lots of old bits of similar flexible boards and cables so I am going
    to experiment with them.

    Ian
     
  11. Thanks for your advice. As you postulated this pcb is integral with the
    I'm starting to get the feeling that maybe you should "bite the bullet" and
    have Sony do the repair. It's going to cost a lot of money, but it might be
    worth it, simply in the grief saved.
     
  12. Ian P

    Ian P Guest

    If I can repair the broken tracks I will locally stiffen the ribbon so that
    it will not be subject to flexing anyway. Your idea of staggering the links
    (which I might do with wire and soldering) means that I have a bit more room
    to work with.

    Ian
     
  13. Grant

    Grant Guest

    I feel for you, it's a serious oops moment to recover from.

    Contact Sony for an idea of cost of fix, so you can place importance
    of fixing yourself in your mind? If you can afford the fix at least
    there's a way out.

    You've got stuff to practice with, but I can't see how you're going to
    reliably connect to that fine pitch. Are there accessible places where
    the cut tracks get wider? Do the cut tracks terminate to accessible
    places where you can create another, separate cable to bridge the
    breaks?

    IOW, sidestep trying to repair the damage by creating an alternate
    circuit.

    Seems to me it's a cable replacement, but that could be sub-assembly
    replacement if the flex also gets glued in place to some smaller parts.

    Take it easy, it is not a rush job.

    Grant.
     
  14. Ian P

    Ian P Guest

    I am not sure that Sony would want to repair the camera now because I was in
    the process of carrying out electromechanical modifications so its a bit non
    standard.

    Ian
     
  15. Ian P

    Ian P Guest

    The subassembly in question is the lens. Parts of the cable are wrapped
    round and glued to the lens body, and the only way to access it is to
    completely dismantle the bulk of the camera, more or less reversing the
    original manufacturers assembly procedure.

    Ian
     
  16. Arfa Daily

    Arfa Daily Guest

    Another possibility, if you get to the point of 'kill or cure', and
    depending on how much space you've got to play with. I have had considerable
    success on remaking the ends of flexiprints, that have failed from bending
    at the stiffening film at the connection point. You can carefully knife off
    the last few mm of cable, then re-expose the print 'fingers' by abrading the
    plastic. I actually use a blunt curved scalpel blade. Once the fingers have
    been thus exposed, the cable can be reinserted in the connector, and then
    the original stiffener pushed back in behind to give a good tension on the
    connector again. Suppose now that you could cut right across where the
    damage is, and re-expose connector fingers at each cut end. If you could
    then obtain connectors, and solder them back to back, you could then use
    this as a joint to remake your cable. I realise that with such a fine pitch,
    the soldering would not be easy, but at least you would be doing it on the
    bench, in the open, and with good light. A strong magnifier would be enough
    to be able to see what you are doing, and some liquid flux, and desoldering
    braid, should make the job do-able.

    Along similar lines, another possibility might be to again cut right across
    the cable, and expose the conductors on the upper surface of one end, and
    the lower of the other. Then treat it as a surface mount soldering job. If
    you use liquid flux, solder paste, and hot air, and do a good job of lining
    up the tracks and preventing movement before you start, there's a good
    chance of success at making a satisfactory join. Capillary action will pull
    the solder onto the tracks, and providing you've been sparing with it, there
    shouldn't be any shorts between tracks.

    Arfa
     
  17. N_Cook

    N_Cook Guest


    No, making hole/s through the pcb, to take the plait, so you can make proper
    solder joins to pre-existing solder joints on the ribbon sockets
     
  18. N_Cook

    N_Cook Guest


    If heat resistant then probably kapton tape. If signal levels and you have a
    fairy godmother then anisotropic tape is another possibility , if you can
    bare back to the underlying condusctors .
     
  19. Ian P

    Ian P Guest


    Arfa

    You have made some very good suggestions there and I can tell you've had
    some experience in repairing things others would not even consider! I can
    see no reason why the lapped joint idea wouldn't work, however the backside
    of the flex is probably the substrate that the copper was plated onto and
    scraping that off might be very tricky.

    I'm leaning towards fine wire soldered jumpers across the breaks.

    Ian
     
  20. Smitty Two

    Smitty Two Guest

    I wouldn't hesitate to do this, but I'd not attempt it whatsoever
    without a microscope. Can you borrow one?
     
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