Make:

I think the main limit is caused by surface corrosion or contamination. I'd predict that nickel paint would work fine at first, until the resistance at the contact surfaces started getting high.

The keyboard membranes I've seen all seem to use silver ink. Nickel is much darker color. (Compare bottles of "Nickel print" with bottles of silver conductive ink.) But also I've seen membrane keypads that use small silver dots connected together with black carbon-ink traces.

Very interesting... I think you'd have better luck silkscreening one of the liquid inks then you would trying to spray it though. One way or the other, if you try it, let us know how it works.

Have you tried a conductive pen? They have silver conductive ink and you can draw thin lines with them on whatever surface you want. You could also slice the pen body open and get the ink out for stenciling/ screenprinting. RS sells them for $13, item 64-4339. Seems easier and cheaper than a stencil, esp for a one-off minor modification to an existing keyboard plastic.

Although I'm wondering if a stencil and spray shield paint might be a fun way to create PCBs out of unconventional materials (besides brick walls, as shown). I'll have to ponder the possibilities....

FWIW, when I was ringing out the $10kraft pad connections, the black traces on the flexible plastic mat were around 1500 ohms when making contact. The traces were pretty long, but I was surprised that the resistance was that high. But it's not like it's a high current circuit--about 3ma gets through 1500Ohms at 5VDC--it just has to determine the difference between open circuit and not.

Try using a meter to see what the resistance is on the keyboard plastic over the distances as a point of comparison. As an experiment, try putting some various resistors inline with a button and see how much additional resistance is you can get away with before the controller stops functioning. You might not need to worry about having super conductivity.

If corrosion is a concern, you could also spray a protective coating over the traces (masking off your contact points, of course). Multiple layers of the paint / pen conductive ink might help corrosion and wear issues too.

Now, if they had silver ink in a plotter pen ..... :-)

John

The HP #11 inkjet printheads are fed externaly. We have a 3d printer at work that feeds four of them from refillable tanks. Since inkjet printers are so cheap these days, perhaps a hack is in order? Also, does anyone know of an inkjet printer that allows a flat paper path? I would like to be able to print directly
to PCB blanks or sheet brass for etching.I miss the old flat bed pen plotters.

Some epson printers (and probably others) support printing on CDs, so pcb's should probably be possible.

"I miss the old flat bed pen plotters."

They're on eBay all the time, sometimes pretty cheap.