Connecting to a Model M membrane

jsheradin

06 Nov 2020, 15:49

I'm making a fake-SSK out of a Unicomp 104 key and can't figure out a good way to attach to the membrane. It melts immediately on the lowest temp my soldering station can go (200C). Silver epoxy and carbon glue seem to break off very easily even if I rough up the traces with a glass fiber pen. I had some luck putting copper foil tape on the contacts and soldering to that but it had intermittent connection after a few days.

None of this would be an issue except that the ribbon connector is 2.0mm pitch, not 2.54mm like everything else on earth. The only 2.0mm FPC connector I can find has 2 pins; I need something like 24. I'll be eternally grateful if anybody can find an FPC connector that will fit.

Other options I can think of are:
  • Trim and solder to the controller board, reuse the carbon press fit contacts, clamp it to the membrane somehow
  • Take the contacts out of a 2.54mm FPC housing, cut between the membrane contacts, attach and heatshrink individually
  • Trim the sides off of a bunch of the 2 pin housings, attach them side by side
  • Use a 1.0mm pitch FPC connector and hope that two pins line up for each membrane trace
Any ideas?
Last edited by jsheradin on 08 Nov 2020, 06:09, edited 3 times in total.

pandrew

06 Nov 2020, 21:42

Hey!

What do you mean by FSSK and membrane? The FSSK that has been developed previously on deskthority has a custom super-thin PCB, that is soldered through wire, or through a ribbon cable, and you throw away (or set aside) the original membrane. You don't need a membrane in an FSSK.

If you are only thinking about using model F flippers with everything else Model M, that will likely not work. The Model M flippers are designed to press down in a single point, while the model F flippers aren't.

If all you want is a more F-like feel, you can try just removing the rubber mat. Some people reported that to feel more F-like.

Andrei

gipetto

07 Nov 2020, 21:35

I don't own a model m but i came across part numbers for the membrane socket in the 101 key here. I hope it is of use to you, though it says 16 pin.

https://github.com/qmk/qmk_firmware/blo ... /readme.md

jsheradin

08 Nov 2020, 05:52

pandrew wrote:
06 Nov 2020, 21:42
Hey!

What do you mean by FSSK and membrane?
My bad, didn't realize that wasn't clear. I'm making a fake-ssk out of a Unicomp 104 key. I'm trying to reuse the Unicomp membrane but not the stock controller board which has a different connection method than a normal 101 key model M.
Last edited by jsheradin on 08 Nov 2020, 06:00, edited 1 time in total.

jsheradin

08 Nov 2020, 05:59

gipetto wrote:
07 Nov 2020, 21:35
I don't own a model m but i came across part numbers for the membrane socket in the 101 key here.
That's what was in my OG model M and what I assumed the 104 key would use as well. Unfortunately those are 2.54mm spacing where the Unicomp uses 2.0mm and a single 24 pin connector.

Image

The connection method is not the normal FPC connector like that (so I can't simply harvest the part); they use a carbon contact press fit that is clamped in place with foam.

I've looked everywhere for that style FPC connector in 2.0mm spacing but have run up dry. I ordered some 1.0mm connectors yesterday so hopefully those line up in a 2-1 fashion.

jsheradin

09 Nov 2020, 21:31

Using a couple 1.0mm pitch FPC connectors seems to work! A pin lines up with each trace and there's no shorts. They are all measuring under 1 Ohm with about 0.4 Ohm average.

Digikey #: WM7097CT-ND

Image

pandrew

11 Nov 2020, 08:29

Nice!

Hanslau

12 Nov 2020, 01:01

So how do you attach them? Sorry for my my lack of knowledge in soldering

jsheradin

12 Nov 2020, 14:09

Hanslau wrote:
12 Nov 2020, 01:01
So how do you attach them? Sorry for my my lack of knowledge in soldering
The FPC connectors basically have a little metal clip inside that clamps onto the ribbon cable's contact. That clip extends outside of the plastic housing with a nice big patch that a wire can be soldered to (kind of like an SMD pad). The total connection resistance from membrane trace to wire was around half an ohm; it's not great but it's more than enough for strobing a keyboard.

Once I had all the wires soldered I slathered it in silastic to keep it from moving.

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