I'm working on a project to make a wristwatch similar to this one:
Now I have the ability to make flexible PCBs since I accidentally bought some crazy thin copper clad. I obviously want to use SMD LEDs since they're pretty small. What type of \$\mu \$C package should I look for that will be nice and thin but still DIY solderable (the only one I know of is QFP but I'm not sure I could solder one of those)?
Lastly, are there any other common pitfalls when making flexible circuits? I can imagine people bending the PCB all over and accidentally touching traces etc.
EDIT I'm thinking about not making the circuit flexible, but storing components inside each little compartment of the watchband. Then I just need flex cables to go between the parts of the watchband for communication and power. Probably only need a few traces… this would lessen (almost eliminate) the stress on any solder joints and place it only on the material the communication traces are put on. Whether I decide to make that out of polyimide or just my thin FR4 is yet to be determined. Does this seem reasonable?
Thanks
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Best Answer
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.