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  1. #1
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    Default Lap Joining 2 PCB's

    I am looking into a project to make a CNC keyboard using a emulator from a keyboard.
    A guy on, from the UK, U Tube has made a 'expander' pcb to make more room to solder the wiring to it.
    He has over lapped the PCB's and used 'liquid wire' to make the electrical connection between PCB's.
    I google 'liquid wire' and various other names, but didn't find anything.

    He has had mixed results and it doesn't seem like a good way to go, to me......


    I was thinking of tinning both PCB's, laying them over each other and using a hot air iron to sweat the solder to join the pads.

    Has anyone done this (either method )?

    Did it work?

    Thanks

    https://www.youtube.com/watch?v=9fXhuWGbo5s&t=237s

    Go to 1:50 mark to see both boards.

    Two PCB's.JPG

    Steve

  2. #2
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    Default

    Hi Steve,

    The "Liquid Wire" is a silver loaded paint that provides an electrical connection ! Same stuff that is used to repair car window heater strips by painting over breaks in the element. I've used it a couple of times, its quite expensive and once the container is opened dries out over a few days even in a sealed container.

    EDIT:
    Having seen the video, you might be better off using a "Zebra" strip between the two circuit boards !
    The stuff he's using is in a much larger container ! The stuff I'm referring to, comes in a very small glass tube about 3/8" diameter and 1.5" long with a small plastic plug stopper.
    Best Regards:
    Baron J.

  3. #3
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    Default

    Thanks John,
    It doesn't sound like liquid wire is the way to go.

    Have seen Zebra strips in devices I've pulled part, but not bought any.
    A quick google didn't find any in Oz and the ones I did find had a pitch of 0.1"
    I need a pitch of 2mm.

    PS: I found some in Switzerland...lol.

    You don't think I could re-flow solder to make the connections?

    Steve

  4. #4
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    Default

    Hi Steve,

    The Zebra strip spacing shouldn't make a lot of difference as long as the gap between conductors is greater than the Zebra strip spacing ! So if you have say a 2 mm conductor to conductor spacing then a 0.1 Zebra strip cannot jumper the gap between two conductors.

    As far as re-flow soldering yes it will work ! I would use solder paste if you have some. It contains flux so once the solder melts and flows you are almost guaranteed a good joint. Yes you can just tin the conductors but you won't be able to ensure that the solder would flow enough to make a sound joint.

    The other possibility is solder balls ! They are flux coated and commonly used to secure and make electrical connections to high pin count IC's. The issue there is ensuring that the heat is evenly distributed across the whole area to be soldered.
    Best Regards:
    Baron J.

  5. #5
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    Default

    I wonder whether the liquid wire is this stuff -
    https://www.jaycar.com.au/wire-glue-9ml/p/NM2831

    Michael

  6. #6
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    Default

    Hi Michael,

    Obviously not the stuff I was talking about ! About the same sort of price if you change $ to £.

    I bought my conductive silver paint from a car spares department at a Ford, or might have been Vauxhall garage. Its used and sold for repairing heated rear windows. I've also used it for repairing PCB tracks that have fused from over current.

    Trouble with it is that once you have opened the phial it starts to set and dries in a couple of hours and then within a few days has set almost solid to a thick unusable goo and a couple of weeks later it is solid. I broke the glass phial and it was quite hard.

    Clearly intended for a single application and use !
    Best Regards:
    Baron J.

  7. #7
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    Default

    Geez.. I was convinced it was the right stuff.......
    I don't like the idea of it going off after a single use.


    Sent from my SM-G970F using Tapatalk

  8. #8
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    Quote Originally Posted by Sterob View Post
    Geez.. I was convinced it was the right stuff.......
    I don't like the idea of it going off after a single use.


    Sent from my SM-G970F using Tapatalk
    Hi Steve,

    No ! It surprised me with the speed that it went off ! I suspect that it was something like a super glue, moisture in the air causing it to set.
    Best Regards:
    Baron J.

  9. #9
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Michael G View Post
    I wonder whether the liquid wire is this stuff -
    https://www.jaycar.com.au/wire-glue-9ml/p/NM2831

    Michael

    Thats a carbon (graphite) based conductive paint and contains some sort of acetate based suspension solution which evaporates away leaving a conductive layer. I've seen it used to paint cavities inside guitar bodies to act was an electrical shield. It doesn't go off when it's opened.

  10. #10
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    Hi Bob, Guys,

    A bit like "Dag" the stuff they used on cathode ray tubes to form a capacitor between the inside and outside of the glass, using the glass as the dielectric of the capacitor. At one time you could buy it in bottles for repairing the Dag coating when it got damaged.

    I haven't seen any of that stuff for many many years. Farnell, now Element 14 used to sell it !
    Best Regards:
    Baron J.

  11. #11
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by BaronJ View Post
    Hi Bob, Guys,

    A bit like "Dag" the stuff they used on cathode ray tubes to form a capacitor between the inside and outside of the glass, using the glass as the dielectric of the capacitor. At one time you could buy it in bottles for repairing the Dag coating when it got damaged.

    I haven't seen any of that stuff for many many years. Farnell, now Element 14 used to sell it !
    We made our own dag at work by mixing graphite with meths and a drop of "gum Arabic" but it was still relatively resistive and had to be painted on quite thick to get a conductive path.

    It's possible to make semi-conductive paint using a mix of acrylic pint and graphite but it's not very conductive because the Oxides in typical acrylic paint (TiO) are insulators and should be replaced with graphite - if I were doing it I would find a can of settled acrylic paint and scoop some of the clear binder liquid off the top and mix in the graphite.

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