flexiblepcb
Having never worked with flexible PCB's before I was wondering how much and how often it can be bend? I'm thinking of using a flex PCB inside a pet collar where it would have to fold in order to adjust collar length.
Would a flex PCB be able to cope with this much mechanical stress?
Thanks for sharing your thoughts!
To start with your last question: this really looks like the weakest link in your project. Flexible PCBs are not made to work under mechanical stress.
They're often in a fixed position and the parts which can become under mechanical stress (like user controls) are always fixed, either with a screw, or with a molten piece of plastic. In particular the parts of the flex-PCB which have components soldered on them should not flex, as creepage may cause solder connections to fail in time.
This is more of a mechanical than electronic problem, but I would make sure that there's always another mechanical part which will take the stress when flexing. It could also shield the PCB from touching.
As for the package, it's true that QFN is becoming more popular, but most microcontrollers are still available in LQFP/TQFP (Low Profile Quad Flat Package/Thin Quad Flat Package), with pitches from 0.5mm over 0.65mm to 0.8mm (wider pitch usually on devices with less pins).
The last one should be rather easy to solder, and even 0.5mm is doable with a fine tip soldering iron and a steady hand (ask your dentist! :-)).
It's hard to select a microcontroller purely by package, so maybe you could give us some details, like number of I/Os.
The question does not specify the expected number of flexing cycles across the device lifetime. For applications where the FPC will be flexed just once at installation, such as connections between LCD panel and attached backpack controller board, pretty much anything goes. At worst, infantile death of the FPC if it happens, will be detected in a test run.
Assuming multiple flexing cycles:
Best Answer
As with most engineering questions, the answer is "it depends".
You can purchase flex circuits optimized for frequent bending (dynamic) applications, and you should minimize the copper thickness, and ideally use just a single layer, to make the PCB as robust as possible. Tight radius bending is bad, but how bad depends on the flex construction (number of layers, substrate), and thickness and type of copper you specify from the maker. If you need multiple layers, consider using more than one single or two layer flex.
Repeated bending with a tight radius (a small multiple of the substrate thickess) should be avoided.