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Copper or silver-based should be lower resistance. These conductive paints tend not to be very conductive, the carbon stuff is essentially making a thin-film carbon composition resistor. Good for repairing rear window defroster heating elements, not so great as a 0-ohm trace in a keyboard. For short (<1cm) wires it's usually not too bad, but with the amount of damage I'm not sure you'll be able to repair the thing.
It looks like it might be from a Model M-style keyboard. Unicomp sells those.
Unfortunately the shipping fees would kill me since I'm in Europe, I really wish I could buy a new membrane but it seems like I need to do the repair myself.
Would copper tape suffice as a low resistance trace?
Worth trying. It's already broken, you can't really make it much worse. It'd probably work, and worst case you're back where you started & paying for expensive shipping.
I would suggest soldering on thin wires, even a bit of copper is going to be orders of magnitude more conductive.
Don't think I can solder on plastic, plus it would create new thickness and I don't think that would help some sandwiched membranes, but I'm going for double sided copper tape, that should help I hope.
Do you have a multimeter? Or other way of gauging the resistance of the new traces?
I don't know the specific product you've used, and my experience with conductive paints and glues is almost nonexistent. But what I remember is that it was neither useful as a glue or a conductor. So I suspect that the resistance of the trace is too great to be used for traces going into the 100s of mm.
If my suspicion is correct, then maybe you can fix it by using the paint to attach something with little resistance in parallel to the paint traces. Maybe stripping a multicore wire and using a single strand of copper, lay it down on the trace and paint over it? Or cutting the traces out of tinfoil and gluing them down to the existing traces with some of the paint?
I was thinking about using thin copper tape, it's cheap and it should be better that some graphite ink I hope,
Definitely better conduction than the paint. Just make sure that the tape isn't coated where it has to make a connection to the pads.