Repairing printed carbon trace vias? How are the vias made?

[Jeff_Birt]

Picked up a couple of 'junk' Casio VX-4s from Japan. Both had had battery leakage, the worldwide scourge of electronics. #GTDBO (Get the Darn Batteries Out). Anyhow, one needed some cleaning and a few jumpers on the LCD row driver. The second has a lot of corrosion on the keyboard PCB.

The keyboard PCB is two sided but the bottom is printed carbon only, there is no copper layer. The top is copper with a few printed carbon resistors. The missing traces are no problem to fix but the vias are another issue. I cannot tell how the vias were made. They have an opening in the center as a plated though PCB typically would but as the bottom of the PCB is only printed carbon there would have been no way to plate the holes. I suspect then that the holes are injected with a conductive adhesive but then how is the hole in the center formed? The vias are also all coated with a drop of something translucent from the top side. Perhaps to protect the copper/conductive glue bond?

Not knowing how this type of PCB is manufactured I'm not sure how to attempt to repair the few damaged vias on the PCB. Not that it is a particularly important PCB but I like a challenge. If this PCB had copper on both sides I would just get out the through hole PCB copper via ferrule kit and have it repaired. I could still try and but put a tiny dab of conductive glue under the ferrule on the bottom side. If the carbon is not too brittle such that it would crack when the ferrule was pressed flat this might work.

So, after all my rambling I guess the questions are:

1) How are the vias on these types of printed carbon PCBs made?
2) Any other repair ideas?

Thanks!

[Jeff_Birt]

Decided to scrape off the translucent coating on a good via and found a brass ferrule had be pressed through the PCB. It looks like it protruded just through the board, was flared out to hold it in place and then overprinted with the carbon. I'm not sure how it will work to insert a ferrule after the fact, as one would do with a two copper layer PCB but it is worth a try. It is already broken at this point.

[Jeff_Birt]

I was able to repair this PCB. Using the Pace repair ferrules and trace segments worked great. After pressing the ferrule into place I put a dab of CiaKote on the carbon trace side of the PCB and soldered the ferrule to the trace on the top side. If anyone is curious I did a video on the repair.

https://youtu.be/o0a6JhiZ1Gk