A little context: I am trying to restore a Casio PT-7 keyboard, and the biggest issue is that the keyboard has stopped working. As you can see in the photo, the keyboard is a smooth flat surface, there are foil layers inside, and pressure on the keyboard triggers a key press.
Opening the very thin keyboard up, the culprit appears to be that the adhesive holding the connector from the foil keyboard to the PCB has gotten brittle with age, and the ribbon of foil contacts are no longer connected to the PCB surface.
As you can see in the photos, the contacts on the PCB are not bare, but are covered in black, what I am currently assuming was the adhesive. The interesting bit is that if I hold the foil connector back in position with, say, a piece of weatherstripping foam and a clamp to provide even pressure, the keyboard does work again. I suppose I could try to repair the keyboard by making that foam/clamp situation more permanent (and in fact, I know one other person that has repaired their PT-7 keyboard in exactly this way), but I would love to better understand how this connection works, and repair it in a more permanent way.
I have considered stripping the black off of the contacts on the PCB, and soldering the foil connector directly to them, but I fear the plastic/foil connector may not be able to withstand the heat.
Could anyone please enlighten me as to what this black adhesive might have been? Might it possibly be carbon or graphite impregnated in some way? I find it odd that the black extends across the entire pin area, and isn't just on each individual pin, so I expect it can't be that conductive, or it would short all of the pins together. I'm as a bit of a loss as to how this connection works, as it's something I haven't come across before.
Any recommendations on the best method to repair this connection would also be very welcome. Thanks!
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