flex library question

minnowahaw

Hey hi,
I am designing a custom flexible sensor for a project and I followed the excellent tutorial from the Bela blog on how to to this in Kicad (the very first time I use it).
When I run the design checker, I get a message
'trill_flex_connector_long' is missing in the TRILL library, although the 'trill-flex.pretty' folder in included in the Symbols libraries. Any idea on how I could solve this?
Thanks in advance!

max

Hi @minnowahaw thanks for the question.
This is a warning only and not actually a problem. You don't need the footprint in the library for production since all the information needed for production is duplicated in the pcbnew file. You can also open the footprint from pcbnew in the footprint editor and save it in the library, then the warning will go away.

The Symbol library is only for symbols, so that won't apply for footprints.

The video on custom FlexPCBs in KiCad for the Trill Flex sensor is somewhat outdated, as it isn't using the features of the current KiCad version. At some point I would like to make an update, it would be very valuable if you could share your project so I can see what's needed in the community.

minnowahaw

Hi @max , thanks for your quick reply.

For this very first custom flex sensor project I am interested in making a larger flexible slider for implementation inside a wearable instrument. I basically just used the zigzag structure of the flex and tried to turn that into a larger design inside Kicad. Here is a zip file of the project on my server and here is the gerber file. Maybe you could give it a quick glance and give me some feedback?

Two questions that already come to mind: I noticed that OSHpark has as the maximum size for theri PCB's 16x22 inches. My design is a bit larger than that, about 642,1 x 50 mm (25x9 inch). Are there manufacturers out there that you would recommend that produce larger flexible sensor designs on order?
Furthermore: on the PCB design it is mentioned in a user comment just above the connector, where it says 'STIFFENER BOTTOM', it says 'Make sure to export layer "User.1" to Gerber and rename to "Stiffener"'.
Does this mean that the renaming should take place after exporting to Gerber inside the Gerber directory? In my case I find a file in there (along with 12 others) that is called 'es-flexible-sensor-User_1.gbr'. Should this one be renamed to 'es-flexible-sensor-Stiffener.gbr'?
Thanks in advance!
Sjoerd

max

minnowahaw Had a quick glance, the files look OK. You are correct with the stiffener, that's how it's done. For PCB manufacturers, I haven't looked at US based manufacturers, but here is a list for manufacturing in the EU and China

minnowahaw

max Thanks a bunch, @max! That is a super handy list. Wow, the price differences are quite extreme.

minnowahaw

Hi @max . Just a quick other question: I was just contacted by the PCB house about the overall thickness of the end of the sensor / fingers. The tutorial mentions: 'At the end of the sensor you are aiming for overall thickness of 3mm +/-0.5mm to make a good connection with the Flex base board.' The '3mm +/-0.5mm' is slightly confusing for me. Is this a typo and does it mean something in between 0.5 and 3 mm? 3 mm is rather thick, so I communicated 0.5 mm to the PCB house now (they suggested 0.3 mm). Will that work for making a proper connection with the board?
Thanks for all the advice!

max

3mm is definitely not right, that's not a thickness for flex PCBs. 0.3 sounds reasonable. The table below that clears it up: Flex PCB is 0.15 mm thick and Stiffener is 0.2 mm, gives you an overall thickness of 0.35 mm.

minnowahaw

Hi @max, forgive me my many questions: it's the very first time that I design and order a flexible PCB, so it's exciting but also a bit confusing.
In that very same table that you are mentioning it says that the Stiffener Location should be TOP, so this is also what I communicated to the PCB House. The PCB House just contacted me that this would mean that the 'fingers' of the connector would get covered in stiffener, so that it is in their view probably wrong and that the stiffener location should be the bottom side.
Should the stiffener indeed be on the bottom side? Or did I make some mistake while implementing the pre-designed connector in my project?
All the best,
Sjoerd

max

minnowahaw go with what the fab house says. Covered contacts are not good oviously.

minnowahaw

max sure, that was also my line of thought. I was just wondering if I messed something up inside Kicad. Thanks! Fingers crossed now.... 🙂