Electronics > Projects, Designs, and Technical Stuff
Flex PCB material selection (high flexibility)
KL27x:
--- Quote ---I was also kind of assured it ain't be cheap, or not at least thaaat cheap. :)
--- End quote ---
Dunno where you live, but you will get bent over for FPC by most "local companies" unless you live in Shenzen, lol.
Unless you are under a time deadline, you are almost assuredly going to save a boatload of money by making your own mistakes and ordering from China, directly. After discussing my first FPC design with a lot of local companies, I found the curious coincidence that they all take roughly 3 weeks, but can't say exactly. 3 weeks is how long it takes for someone to order the thing from China. So I would say 99.99% of the companies I contacted take your Gerber and order it from China.
The "big" legit "FPC company" I found in my country (because they advertise FPC expertise) don't make FPC, either. They have experience and expertise in the design and in fabrication of FPC/rigid composite boards, and the like. I got the distinct impression they make money from big medical/government contracts.
Yansi:
This should be a serious manufacturer, that indeed should have the technology to make them. They can do a lot of specialties (including Rogers, wild stack-ups, extreme copper weight - once they made a 2layer 0.5mm thick buried copper PCBs for a high power motor controller, etc)
But the are EXPENSIVE. Hence why I don't use them and avoid them for personal use. This time, company pays and time is ticking (they can't set up sensible deadlines, because mechanical guys are in command of the electronics design dept... ...) so money are currently not the issue. Meeting deadlines with crazy ideas out of reality is.
jbb:
I’m sure you’re on top of this already, but: use a single copper layer.
If you have stubborn EMI problems, there are special EMC layers which are very thin conductive films (2um vs 18um for half ounce copper).
In my limited experience, a major piece of the puzzle is managing the location of the stress. You need to look at your connectors, fasteners and any guides, slots etc. I had one design that failed after 10,000 cycles because the FPC was flexing against a cut edge of a rigid PCB. Adding a little bit of tape to control the bend location took that up to > 500,000 cycles. Try to engage with a mechanical engineer on this task because it’s actually a mechanical issue. Some clamps or tapes or (shudder) glue may be required.
If you desperately need a component or connector soldered onto the FPC, you don’t need to go full on rigid flex. You can get the FPC manufacturer to fit a stiffener on the back side (polyimide or FR4 is available).
Finally, you’ll need to work out how to manufacture it without putting a kink in the critical bending region. Kinks usually concentrate the stress and speed up the fatiguing.
Yansi:
Yes, only single layer on the outside of the bend, that was also what I was told. If desperately needing ground on the other side, I was suggested to use just a fine mesh, not a solid pour.
Only FPC, no flex-rigid - I think I haven't even mentioned that option. The main PCB in the device is just 2 layer job, not worth the trouble investing into space tech. The FPC alone is expensive enough by itself.
So yes, I will also need to figure out that end with the FPC connection. (Probably an 8pin 1mm pitch job there).
wraper:
--- Quote from: KL27x on September 24, 2019, 04:41:28 am ---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.
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Actually it's usually opposite, at least when inspected under microscope it's rare to see no damage to substrate on faulty flex. Either substrate cracks starting from sides or gets some sharp deformations and only then traces give up. I would say that the most important is considering shearing forces applied to the cable as they are the most dangerous. When folding phones were popular, they had way more flex cable failures than sliders due to that. Bending with small radius is also extremely bad. I marked with red places most subjected to damage by shearing with folding type. IMHO if you make something where cable will go through a hinge, you should use thicker material as it should be less susceptible to shear. If you make sort of sliding stuff or cable will be straight, not going through a hinge, use thinner substrate as it can be bent safely with smaller radius.
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