Next step: custom flex PCB

gcsalzburg

Hi,

I've successfully used the instructions on created a custom FlexPCB to prototype my touch sensor for a pair of RayBans. See photo attached (pairs via BT to PC running WebBluetooth app in Chrome for data readout).

Was thinking about the next step of moving the CY8C20636A-24LTXI and passive parts directly onto this flex board.
Based on the notes on this github issue (https://github.com/BelaPlatform/Trill/issues/1), looks like the options are either a CY3217 programmer (https://www.aliexpress.com/item/32381156184.html) or using the Cypress sample code to create my own programming device, which might be a bit beyond me.
Also - the chips seem to be out of stock everywhere, with no expected leadtime shown on Mouser.

I don't really need any custom firmware on it! Since I only need the chip to move from trill board to my own, and the programmer is expensive anyway, I was thinking of buying a couple more boards and using a small hot plate to just remove the chip and directly move it across to my own custom flexPCB.

Has anyone tried this, or does anyone have any more sensible suggestions?

Thanks!

George

giuliomoro

No pic attached! (if it failed while trying to attach it, try reducing its size before you try again).

gcsalzburg I don't really need any custom firmware on it! Since I only need the chip to move from trill board to my own, and the programmer is expensive anyway, I was thinking of buying a couple more boards and using a small hot plate to just remove the chip and directly move it across to my own custom flexPCB.

That seems like a decent option to solve both issues at once ... can't guarantee it would work, though. During prototyping we did desolder the Cypress chip with a hot air gun, reworked the PCB and put the part back on (solder paste and hot air). We did it twice, I think in one case all pins worked while in the other 2 pins were not connected. Not too bad, may be worth a try. Make sure you order a few spare Flex PCBs!

gcsalzburg

Oops, thanks! Picture attached again, you can also see a test video here: https://1drv.ms/v/s!AtRjkBTu0eqTiP18YyN7QC-pXJmQ2Q?e=QbaXrH

Thanks for the advice. I've never tried reworking a FlexPCB before but hopefully it will be straightforward 🙂.
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giuliomoro

Note that you will most definitely need a stiffener under the QFN on the Flex PCB.

gcsalzburg

Great tip, thank you! OSH Park don't do stiffeners but I think PCBWay do. I will investigate 🙂

bela_robert

This project is brilliant! Looking forward to seeing where this goes. I would suggest using a Rigid-Flex PCB design which combines both the flexible substrate for adhering to surfaces and the more solid FR4 for holding the ICs securely. This techniques allows you to remove the bulky connector and make things as streamlined as possible.

PCBWay offer this service: https://www.pcbway.com/blog/News/What_is_Flex_PCB____An_Overview_of_Flex_and_Rigid_Flex_PCB.html

There are full instructions on how to design your PCB on the page above, and on how to indicate which parts are flexible and which parts are rigid.

This type of PCB can be quite pricey in small batches so if that is a concern then I might recommend getting in touch with PCBWay via instagram and asking if they can sponsor this project. From what I've seen so far this is definitely the kind of project they'd love to feature on their blog so they might offer you discounted manufacturing in return. Worth a try!

We'd also love to feature this on the Bela blog in the future and we can discuss getting cypress chips and best ways of programming them. For now I think the rework option is best. Please let us know when you are ordering the extra Trill Flex at robert(at)bela.io. We can probably arrange a discount, if you don't need the standard flex PCB for example.

gcsalzburg

bela_robert

Wow, thank you Robert!
Yes, PCBWay tried to sponsor the first flex in fact, but even with their sponsorship it was cheaper to order via OSHPark (although very slow!). I will use them for the next version though I'm sure.

Would be very happy for you to use it on your blog too, especially if you can help a little with the chips. You're right, I won't need the standard flex too, just a programmed cypress chip ultimately. I'm going to do a test on a normal FR4 board to see if the circuit is good and the chip-moving process works before I commit to the expense of the rigid-flex. Thanks for your tips, I hadn't seen that blog note before, I'm definitely quite new to rigid-flex as a manufacturing technique.

This project will get nudged along slowly, but I'll message you when I'm back to looking at it again for sure.