On Wednesday, March 8, 2017 at 12:56:22 PM UTC-5, frank wrote:
Hello folks,

did anyone suceeded in repairing traces on that old plastic sheet printed
circuits keyboards? Some traces are interrupted and it's impossible to
solder on that kind of plastic/trace combination. I've had some success
on plastic substrate when the tracks were copper strips of sufficient
thickness, but in this case they looks like black conductive material and
the plastic substrate is so thin that any tip at a useful temperature for
soldering instantly makes a hole.
I've been thinking about conductive glue, but it might not stick or make
a good contact. So if anyone already had experience (positive or negative)
is very welcome.
It's a 30 years old mitsumy custom keyboard, so not replaceable with something
modern.
Thanks
Frank

Get a Circuit Writer pen from Caig labs. I've tried using black conductive paint from several sources and it's not conductive enough.