Electronic – Black carbon like contacts on circuit boards, eg remotes, what are they made of

pcb

Have been given a 30+ year old electric piano that wasn't working. Looking inside, it looks like it's been taken apart 1000 times.

Anyway, I replaced a load of tact switches, replaced one capacitor and it all sprang into life.

All except one note.

I've taken off the keys, cleaned with alcohol but the note that doesn't work has been really butchered, in that the black carbon like stuff doesn't exist (see right hand contact on pic).

Pic of contacts

Someone advised me to try repairing it with high silver content conductive paint. That may well work, but the keyboard isn't really worth spending much money on, and in the UK, the cheap silver conductive adhesives don't get very good reviews (and one said it had to be lacquered over to prevent oxidization).

I wondered what they use originally on all these things such as remote controls etc, I suspect it's something baked on hence not available to the home user?

There's a ton of info about repairing the carbon contacts on the rubber buttons such as on a remote but I've found zero info about repairing the circuit board side of things?

Any info or suggestions greatly appreciated

Many thanks

Best Answer

Carbon Conductive ink is used. Screen printed on the contacts, or using a suitable printer. Industrial of course. Something like Peter's Carbon-conductive ink SD 2843 HAL:

The carbon-conductive ink SD 2843 HAL is a 1-pack screen printing ink that, on account of selected special carbons and high-quality types of graphite, displays an excellent conductivity even after soldering. The achieved conductivities are to a large extent dependent upon the selected curing conditions (see Item "Resistance in relation to curing conditions”). The carbon-conductive ink SD 2843 HAL is suitable for touch-key contacts and enables the substitution of gold on contacts, the production of cross-over conductors (cross-over technology) as well as the creation of printed resistances. Further applications include migration protection for silver-conductive ink, electro-mechanical keyboards, foil keyboards for computers, switch contacts, low-voltage circuits, shield areas and heating elements.

Typically, these do NOT have silver in them, that's a different category of ink. Used in a similar way, but often not needed. Carbon Conductive Ink is used as a protection layer for silver ink, to seal it from the environment.

You can get small tubes of carbon conductive ink just like you do silver conductive ink pens. While these don't have the quality of an oven baked screen printed carbon conductive ink, they will do better than the silver ink pens due to what you are using them for, repairing push button contacts. Carbon conductive ink doesn't tarnish and prevent contact from just being exposed to air and moisture.

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