Repairing carbon traces on a Casio WK-110 keyboard PCB?

[Agent24]

I have been looking at a friend's Casio WK-110 keyboard as one of the keys was not working.

The rubber membrane part looks in good condition, but it seems there was some liquid ingress onto the keyswitch PCB which has caused a couple of carbon 'fingers' for the switch contact of the dead key, to come away from the board. Unfortunately Casio cheaped out where they could have used ENIG or such instead, I guess.

Anyway, I have been researching if it's possible to repair these with some kind of conductive paint...

At first I thought silver conductive paint such as https://www.synergyelectronics.co.nz/store/electrolube-scp03b-silver-conductive-paint might  work, but after further research I see there is concern that such paints may tarnish over time and become non-conductive (at least, externally). This would of course make it useless for a switch contact.

Another suggestion was black "Indian ink" from an art supply shop, e.g. https://www.warehousestationery.co.nz/product/W2802098.html Apparently this is conductive due to the carbon content.

But has anyone used this to repair key contacts? Did it work? And what was the longevity?I also see some Indian ink advertised as "high carbon" - does anyone have information whether this is better or worse for such a repair?

Or ... is there some other product which would work? I do like the sound of the Indian ink though, it's pretty cheap 

Additionally... is there something I can safely clean the carbon contacts/PCB with before attempting repair? I only soaked up the liquid that remained with a cotton swab I don't know what it was or if it has left a residue that would hinder adhesion.

[QOTF-Alexi]

I repaired a few carbon contacts in calculators with a relatively soft graphite pencil and they seem to hold fine over the years (although they don't see the same use as a keyboard of course), so I'd assume Indian ink would work just fine, although I'm not 100% sure how well it'll adhere to a smooth surface. As far as I'm aware, graphite does not dissolve in alcohol so you could try that for cleaning.

[Agent24]

I repaired a few carbon contacts in calculators with a relatively soft graphite pencil and they seem to hold fine over the years (although they don't see the same use as a keyboard of course), so I'd assume Indian ink would work just fine, although I'm not 100% sure how well it'll adhere to a smooth surface. As far as I'm aware, graphite does not dissolve in alcohol so you could try that for cleaning.

Interesting. I would have thought just the mark made using a pencil would come off quickly. I'm surprised you say it lasted years!

I wondered about the smooth surface too but forgot to add it. Maybe the PCB surface could be roughened with a fiberglass pen or sandpaper etc. I guess it'd make sense to just get some of the Indian ink and do some tests. It was mentioned that some are not conductive, while most are.

I also see now that some are described as waterproof https://www.whitcoulls.co.nz/fas-waterproof-indian-ink-60ml-black-5544777-black - I wonder if the waterproof version would be less or non-conductive. Does anyone know that?

[2N3055]

https://www.kontaktchemie.com/koc/KOCproductdetail.csp?division=&product=GRAPHIT%2033&ilang=en&plang=en

[Agent24]

https://www.kontaktchemie.com/koc/KOCproductdetail.csp?division=&product=GRAPHIT%2033&ilang=en&plang=en

That looks quite good but doesn't appear to be available in New Zealand, however.

[2N3055]

https://www.kontaktchemie.com/koc/KOCproductdetail.csp?division=&product=GRAPHIT%2033&ilang=en&plang=en

That looks quite good but doesn't appear to be available in New Zealand, however.

Yeah, maybe not, but that is what you need to aim for. Look for conductive graphite paint or spray.

[squadchannel]

If the original trace is gone, I don't think it can be done with the pencil method.

The recommended solution is to use conductive paint. Are conductive "pens" available locally?
In my country they are very common and are sold in bottles like car touch-up pens for the purpose of providing electromagnetic shielding to plastic cases. Spray cans are also available.
The paint is brown in color, I believe copper is used instead of silver.
I don't know about degradation over time, but I think it depends on the conductive material content. I think it can be repainted again when it deteriorates.

Another method: cut and apply copper tape in the shape of an "E" and solder it. There is nothing that cannot be done.



Cleaning is always with isopropanol.

[ozcar]

This stuff maybe?

https://www.jaycar.co.nz/wire-glue-9ml/p/NM2831

P.S. No, I don't know what "not intended to be used at a PCB level" means or why.

[Agent24]

This stuff maybe?

https://www.jaycar.co.nz/wire-glue-9ml/p/NM2831

P.S. No, I don't know what "not intended to be used at a PCB level" means or why.

I did not notice that product before, thanks, it looks interesting. (If not for this, then maybe something else)

Going by the rest of the description I'm guessing it may not adhere to non-metallic surfaces well? But it's hard to tell.
I wish companies would be more upfront with data... why so many vague details? Ugh...

I did find some carbon paint at DigiKey https://www.digikey.co.nz/en/products/detail/mg-chemicals/838AR-55ML/23028605 but it'd have to be part of a bigger order to avoid shipping costs. And while they state it adheres to plastics well, they don't mention anything else. Not an insignificant cost if it didn't actually bond.

At least it's got a better working life than the only conductive stuff Farnell sells - a dedicated membrane key repair compound (!) https://nz.element14.com/chemtronics/cw2605/rubber-keypad-repair-kit/dp/128790 but it's 2-part epoxy style and doesn't last more than 3 days once mixed. And it has the highest cost at just over $50 for something a hobbyist can realistically only use for a single repair...

Does anyone have experience to suggest _against_ the Indian ink idea?

[jzx]

You can try the indian ink on some old board, but I don't thik it is conductive.

There is also "aqudag", the material used in the outer surface of crt, it is conductive, but I have never used, and I do not know how is applied and fixed.

[squadchannel]

Does anyone have experience to suggest _against_ the Indian ink idea?

I think "Indian Ink" probably means the same thing as "India Ink". If you are referring to the black ink used in calligraphy, this is something that is dissolved in water, not something that can be glued on.
Someone in my language experimented with conductivity, who sprayed cloth with a conductive spray, smeared it with the purpose of further reducing the still high resistance, and used it as a flexible conductive cloth.
He said it was about 50-500 ohms.

It might be possible to mix epoxy with carbon or metal powder and make it conductive, but I've seen articles before that say no matter how much you mix it, it will never be conductive.

If you want to experiment with india ink, there is Inkstick in solid form.