Electronics > Projects, Designs, and Technical Stuff
Flex PCB material selection (high flexibility)
Yansi:
Hello, flex PCB newbie here:
I would like to get rid of connectors and wires in a product, by using a flex pcb. However, there are a lot of materials of different substrate thickness and types and I kind of do not know how to pick a proper one.
What I need is to get the most flexibility, or maximum amount of flex cycles respectively. It is a product with a movable part operated by a user, that needs to be connected by several wires to it.
I understand that to obtain the most cycles of the flexible part, I have to keep maximum bending radius to keep the least stress on the PCB. What I currently do not know and can't find much info about is what type of flex PCB substrate is suitable for such repeated bending.
For example a local PCB fabhouse offers Dupont Pyralux AP 85xx series and AP 91xx series in different thicknesses (1 to 5 mil), with different plating thicknesses. Also some other material ThinFlex A20xx and A40xx series. (2 and 4 mil). That is what I have to go with.
I'd guess that for achieving the most flexibility and flex cycle count I should choose a rather thin material (1mil substrate?) and to also choose a thinner copper plating for minimum stressing of the materials.
Do you have any recommendations on what standard materials and thicknesses are for flexible links, what to be aware of, etc. ?
Thank you for any hints,
Y.
T3sl4co1l:
I don't think it's too onerous. I've seen pretty ordinary looking cable in printers before, either ~1oz and probably around those substrate thicknesses, or the embedded-flat-wire kind (the stuff that's rather substantial feeling, seems like the copper is thicker than the carrier; don't know if you can get PCB made that way, but they're commonly available as plain cables at least).
Some supporting material may help enforce the bending radius constraint. For example, thicker but also more flexible tape, or foam, glued to the cable (one or both sides). Don't forget strain relief, so the cable, with any stiffeners/springiness-ifiers, are anchored to mounts, keeping flex away from the connectors.
What kind of cycle life are you expecting?
I think you could calculate cycle life, based on the moduli of copper and polyimide (or whatever substrate), layer thicknesses, and the fatigue limit or curve of copper.
The copper foil won't be dead-soft, but; well, hm. Raw foil might be annealed, but is more likely supplied half-hard or thereabouts? Plated copper, isn't actually dead-soft (as you might think from the purity?), but contains internal stress due to plating and additives, and, is probably somewhere in the half to full hard range in terms of hardness, but is weaker than a rolled product of the same hardness.
Hah, wow, never thought of that before but it's amusing -- if the stackup is all polyimide and other high temperature resins, I bet the copper could actually be effectively annealed by heating, without compromising the substrate strength too much. Polyimide is rated to crazy temperatures, right around where copper starts to anneal I think.
Anyway, a bit rambly, but should be good for insight and leads on what to look for. Cheers,
Tim
chickenHeadKnob:
I have a roll of 7 mil I can't remember how thick double sided 1 ounce copper on mylar which I purchased from an ebay surplus vendor who claimed it was much tougher than the typical polyimide substrate and was preferred for high flex strength durability in applications like battle-bots. Mylar has significantly lower melting point however so that is the trade off. I don't know where the seller sourced the stuff, it was more than 10 years ago, sorry.
KL27x:
IME, the substrate is not what is going to break from the repeated bending. It's the traces. Yes, I think thinner the traces the better. Dunno how they make FPC jumpers, which you can bend in half no problem. But sadly FPC board doesn't act like that.
I don't know a reason to go crazy looking for a specific brand of polyimide.
Thinner substrate is better for less stress on the traces during bending.
So thin 1 mil substrate and thin half oz copper is what I choose for this, but I could be doing it wrong. Much of the thickness of the final stackup will be in the soldermask/coverlay. So I don't know what I'm doing, but that's how I do it. If you're counting votes.
Yansi:
Hello,
Thanks all for any hints. I spoke with a tech from a local pcb fab, had a bit of talk about the design. We agreed on the substrate thickness to be minimum (1 mil), with also least amount of copper thickness (0.5 oz). Thin coverlays are also available (0.5mil if I've looked correctly).
I was also kind of assured it ain't be cheap, or not at least thaaat cheap. :)
I got also a suggestion to put copper traces on the outside part of the cycled bend in the flexing part, as to not compress the copper, rather the kapton, which has a bit of give, unlike the copper. However I forgot to asked about the hardness of the copper on the substrate, whether it is annealed or what.
Now back to the drawing board and design the final shape. :-/O
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