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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!).

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  • shabaz said:
    Could these be useful for perhaps creating membrane-style keyboards?

    Perhaps, but I'd be a bit nervous given the flat structure. Most membranes have a thick mylar sheet with holes or formed "indents" to increase separation. I'm not sure the mask will be enough in the long run, as the copper can elongate under lots of bending.

    shabaz said:
    For instance, are there particular components or connectors that lend themselves to such use, and are some components best avoided?

    Master the use of stiffeners under flexible PCBs - they're basically like glued on bits of substrate that can keep sections of the flex flat and relatively rigid. This is where I would mount my components to - any component mounted above flex that has no stiffener will likely peel off given enough bending, so usually you'd use the flexy-part as an interconnect ribbon (similar to how an inkjet printer uses them to connect the print head to the mainboard). If you design the end right, you could even get it to fit into a flexible flat connector socket for an ordinary rigid PCB, simplifying connectivity, as soldering while possible is often finnicky and risky as overheating is much more damaging to flex.

    shabaz said:
    Has anyone tried soldering flex PCB directly to a FR4 PCB?

    I have, it works as long as you're moderately skilled at soldering ... my students, however, make a mess of this.

    shabaz said:
    bent a lot (or even repeatedly)?

    I can't be considered an expert, but one has to be really careful of the bending radius if they want to repeatedly bend. Folding is generally only good for a few times at the most or the copper will develop fatigue cracks. I tend to make the copper traces as thick as possible to reduce failure risk especially in fatigue, but I've seen very fine traces work okay.

    Just think about the fact that hard disk head stacks are connected to the mainboard using a flexible printed circuit board and they have to last the lifetime of the disk in terms of seek operations (could be billions). I've never seen it fail - in part because there's enough slack. But those differential pairs going to the head are quite thin ...

    - Gough

  • in reply to Gough Lui

    I think you're right, for a keyboard, the gap between the contacts would be way too thin with just solder resist. A layer of "something" more thicker in the sandwich is needed.

    Regarding copper elongation over time in such a keyboard, I was thinking an option might be to use the flex PCB just as a way of electrical contact delivery and not for if's springiness, for instance, by having a circular cut-out around each pad for about 340 degrees or so, and then sticking a firmer thin sheet of plastic on top with the legends, so that the top sheet is being used for its springiness, rather than the flexible PCB. But that could cause its own issues, for instance, tearing at the traces over time since they would flex, maybe buckle a tiny amount each time. Maybe it's only feasible for non-critical stuff with a built-in lifetime.

Reply
  • in reply to Gough Lui

    I think you're right, for a keyboard, the gap between the contacts would be way too thin with just solder resist. A layer of "something" more thicker in the sandwich is needed.

    Regarding copper elongation over time in such a keyboard, I was thinking an option might be to use the flex PCB just as a way of electrical contact delivery and not for if's springiness, for instance, by having a circular cut-out around each pad for about 340 degrees or so, and then sticking a firmer thin sheet of plastic on top with the legends, so that the top sheet is being used for its springiness, rather than the flexible PCB. But that could cause its own issues, for instance, tearing at the traces over time since they would flex, maybe buckle a tiny amount each time. Maybe it's only feasible for non-critical stuff with a built-in lifetime.

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