Flex-Rigid / Rigid-flex

I’m about to start work on a new project, and it will be a flex rigid PCB.
with a couple of small sensors on a stiffener backed flex PCB, which then goes to a rigid PCB with most of the electronics on.
With the inner two layers being the flex layers.

I’ve noted that a few others have tried this but I’ve not found any concrete answers on this (posts from 2016, 2018 and 2020).

The big problem is that I need to be able to place parts on the flex portion, which are on the inner layers.
The company I work for is a start up, and we can’t afford Altium.

I’m using KiCAD 7.02 (and it’s been great for flex PCBs so far) I’ve been a long time user of KiCAD (~7 years) and I think it’s fab and this seems to be the first time I’ve hit a problem that looks like a show stopper.

Is there anyway to do this? or is it just a simple “no it’s not possible”?

many thanks in advance

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There is no supported way to do this, not primarily because of the flex but because KiCad won’t willingly put components on internal layers. That being said, you can edit footprint definitions (they are just text files) and move parts of the footprint to internal layers. So in theory you could create a little project library of manually edited footprints with landing pads on copper layers 2 and 3, you just have to be careful to only use them in the flex section. That’ll probably be obvious since the flex section will also need keepouts for copper layers 1 and 4, along with solder mask and silkscreen on top and bottom and some sort of fab layer to explain it all to your fabrication house. KiCad’s 3D render-er won’t like it, but the generated Gerbers should be manufacturable.

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Components on the flexi part is fairly unusual (though i have seen it in CD players).
The more common problem are gold flashed fingers at the end of a flexi section to plug into a socket, components mounted on the rigid part.

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Thanks, that’s an interesting approach I hadn’t considered.

I may just end up making two PCBs and joining them somehow.
Sadly due to space constraints and routing (this is a very small medical device) a few of the parts must go on the flex part of the PCB.

Thanks for the advice, I’ll have a think and see how best to approach this.

I decided to stick with a flex PCB and use multiple stiffeners, not ideal, but it should be fine for a prototype.

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