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Flexible PCBs; Any interest, and any tips? (NOT a discussion about specific PCB manufacturers, that will get reported, so no spam please : )

shabaz

The cost of prototype flex PCBs seems to have come down a _lot_ with some (not all) prototype PCB manufacturers.

Here's an example quote from a popular (3-letter name) prototype PCB manufacturer, for a 2-layer 160x100mm board with a lot of holes (I randomly chose Gerber files for a normal FR4 project, to generate this quote for now), and ENIG finish by default. $33 for 5 boards, so $6.60 per board basically,. This might actually be cheaper than an FR4 ENIG board!

image

These are the options (default settings); the board thickness is 0.11mm:

image

Another popular PCB manufacturer (one that is famous for a lot of spam) is currently quoting $158 for five boards of similar size, so that's quite a difference!

I had a variety of general questions since I've never done a flex PCB design.

Could these be useful for perhaps creating membrane-style keyboards? I'm thinking that perhaps the solder mask might be thick enough to create separation between the flex PCB and (say) an FR4 base, to create a PCB maybe. Alternatively, a laser-cut sheet could be placed in between for added separation. Has anyone tried such a thing? I'm considering it for a keypad with many dozens of keys for a variant of this project:  Triggered Sound FX: On-the-Fly Sound Mixing with Pi Pico  

Are there any tips/tricks for flex PCBs? For instance, are there particular components or connectors that lend themselves to such use, and are some components best avoided? Any checklist you use for flex PCB designs?

Would you rarely apply components to a flex PCB (using it as an interconnect), or are you comfortable with components on them? Both sides or one side? : )

Has anyone tried soldering flex PCB directly to a FR4 PCB? I have done this with the flex on some LCD displays, and it is great; they have tiny holes for allowing the flux or excess solder to pop out of, and it works very well. I was wondering if the prototype flex PCB would behave the same as the flex on typical LCD screens.

Any problems with warpage, etc., i.e., if the flex PCB outline is intricate, will it curl up while it is being manufactured, potentially getting damaged?

Are any particular copper features or trace thicknesses best avoided for portions of flex that may be bent a lot (or even repeatedly)?

What applications do you have for flex PCBs? Are you considering them for any prototype or product, perhaps where you would typically not consider them, now that they are lower cost?

I apologize for all the questions, but I figure someone must ask the basic questions!

All comments are welcome! (apart from spam!).

Parents

  • One recommendation I would offer is to use MLCCs with "flexible terminations." Different vendors name it differently, but it is generally a layer of silver epoxy that allows the terminations (solder points) to flex slightly. On rigid PCBs, this pliable layer helps resist cracking when the board flexes. (The dreaded "board flex" failure.)

    Also, if you have a specific bend point in mind, I would recommend keeping MLCCs as far from it as possible. If you have to get close, keep the terminations in parallel with the radius so that both terminals (hopefully) see the same stresses.

    Or you can do what automotive customers do: put two caps in series but turn one of them 90 degrees.

  • in reply to baldengineer

    Good point. I've hit this issue with normal FR4 and hot air tools too : ( To me 0402 decoupling capacitors (100nF or higher) seemed especially delicate compared to say 0603, and I've hit situations where the hot air didn't heat the component evenly when reflowing, and presumably micro-flexed the component, and ended up with shorted 0402 capacitors : ( After that I've also tried to avoid capacitors on regions of a board that may flex slightly.

    Neat idea with the orientated dual capacitors for some protection!!

  • in reply to shabaz
    This reply was deleted.
  • in reply to anniel747

    You're right. I rarely do that (that board happened to be an RF project with a QFN that was being reflowed (the nearby 0402 capacitors were the casualties). I also probably didn't heat very evenly with the tool at the time. 

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