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FPC / Ribbon Cable for surface mount PCB

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7件のコメント

  • Brad Levy

    Hi Jennifer,

    I actually do this on one product, but the flex circuit only connects two traces, which is easy enough to do manually.

    If you have quite a few contacts to make, the usual technique for direct solder of flex to PCB is known as hot bar soldering.

    Here is one page about it:

    https://www.amadamiyachi.eu/knowlegde-base/hot-bar-reflow-soldering

    You can google hot bar soldering flex circuit to find more articles and info.

    It should be pretty straightforward as far as the PCB design and specification goes. One area to pay special attention to would be the paste mask. Hot bar soldering assumes the PCB and flex circuit are both pre-tinned. You may want to talk with your assembly house for recommendations on paste stencil openings.

    -Brad

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  • Jennifer Smith

    Thanks Brad.  Had a look at the link.  I'd not heard of that before.  I'll ask them about hot bar soldering.  

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  • Jennifer Smith

    I am unsure as to whether several 2 way flex or longer flex is the best option.  The flex cable has to go through a small hole so 2 way or 4 way would be easier but these cables seem less common.

    What connector did you use at the opposite end of your flex cable?  Does it need to be a standard flex connector?

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  • Brad Levy

    The flex circuit we use (which is small enough I frequently refer to it as a flex tab) would technically be known as a FPC (Flex PC board), since it is (a little) more than just straight through wiring from one end to the other, and uses an etched conductor pattern.

    It sounds like you are looking for just a straight through connection (multi-way) between two points. For that, FFC (flat flex cable) is the more common variety. In FFC, copper strips are placed between insulating layers, rather than being etched from one wide plane of copper. The strips are frequently a little (not much) thicker than in FPC cables. One method of termination (connection) of FFC is using pins that are installed by clinching them the conductors, in an area at the end that is clear of insulation on one side of the jumper. The pins could then be soldered to thru-holes on a pcb, or maybe mate with a socket (not sure on that). There are also free-hanging connectors that can be clinched to the conductors on an end, that look relatively sturdy:


    The other common method of termination of FFCs is for a stiffener to be added on one side of the end, with the insulation removed from the opposite side of that end for a short distance, exposing the parallel conductors (backed by insulation and stiffener). This end then mates with zero insertion force (ZIF) or low insertion force (LIF) sockets mounted to a PCB. These can be quite low profile, but these connectors are so small they feel a bit fragile - better for inside a product than outside.

    FFCs are available off-the-shelf in a number of standard lengths, lead pitches, circuit counts, and termination methods. Pitches vary from 1/10 inch to 1 mm. Below 1mm, FPC cables are usually used instead.

    There is also another category that Molex offers, called Premo-Flex™ Round Flat Cable (RFC) Jumpers. These have round conductors, but are still relatively flat and thin. The conductors are exposed at the ends and can be soldered directly to PCBs.
    http://www.literature.molex.com/SQLImages/kelmscott/Molex/PDF_Images/987651-2713.PDF

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    Here is an image of the little flex tab I created for our application. Its purpose is to position the LEDs in a place where there would otherwise be no way to mount them. Since it is just to hold the LEDs, it only connects to one PCB - it doesn't serve as a bridge to another board. The low quantity (compared to mass-market products) and simplicity and loose tolerance on this flex circuit made fabbing it myself a reasonable option. Three 6x6" sheets of Pyralux were enough to make a thousand of them. More online board houses are now adding flex capability, too, and can offer capabilities beyond the DIY approach.
    The flex board is only 0.05 to 0.07 mm thick, and combined with the thin LEDs we use, the total thickness is still less than 1/2 mm. And very flexible.
    We do the soldering of the tabs to the PCB, but don't have a hot bar soldering setup. In this case, with just two wide traces with good space between them, we do the following:
    The pads on the PCB and flex both pre-tinned, and a tiny bit of solder paste applied to the pads on the PCB. We fold the flex tab where I have indicated, then position the tab over the pads on the PCB using a little jig, Then apply heat to the traces of the flex right where it meets the PCB. The copper traces convey the heat to the pads and flow the solder.
    (Applying heat through the Pyralux rather than to the copper is possible but more difficult because of the low thermal conductivity of the Pyralux substrate. Commercial flex circuits frequently have an opening in the substrate of the flex circuit at the point where the soldering will be done, making it easy. But I don't know of an easy DIY method of making those openings.)

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  • Jennifer Smith

    Thanks Brad.  I'll investigate.  I might just solder wire to SM pads as it looks like it might be an easier option, as I'm looking to get it all done by the manufacturer in this case but thank you so much for the information as that will be really useful going forward.  

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  • Boss .

    Very interesting Brad, I haven't used this approach and will certainly consider this for future designs, it opens up some very neat solutions.

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  • Jennifer Smith

    As a slightly side question but relevant to PCB connections for very thin wires, does anyone know of connectors, like Molex or JST that work for magnet wire/enamel copper wire? 

    I have some 28awg magnet wire with a very fine plastic coating and wondered if it's feasible to add connectors to it. 

    Molex picoblade go down very small 28-32 awg but whether the wire would break and whether they are suitable for magnet wire I'm not sure?

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