Keyboard flexi PCB

[soldar]

This could be titled "Fools rush in where angels fear to tread". ( https://en.wikipedia.org/wiki/An_Essay_on_Criticism )

I have a favorite keyboard which got wet and some keys stopped working. I discovered some discontinuity in a few traces of the flex pcb.

On eBay I ordered some conductive ink and I received it recently except that it looked like thermal paste and not like conductive ink. No matter I thought, I will just try it.

Well, not only it does not conduct but it dissolved the trace so now the problem is made worse.

And now I think "why did I not test it first in some other place or even just test it for conductivity?"

So here I am, having damaged the keyboard even further and feeling silly for having done it.

Now i will have to search for some real conductive ink. I seem to remember I already asked about this in the past.

[squadchannel]

I have repaired a trace on a IBM Model M keyboard with aluminum foil.
It is not unusable, but sometimes I cannot type. It is cured by tapping it, though.
The pressure with which the aluminum foil and conductive trace are pressed together is important.

Where I live, it is relatively easy to purchase conductive paint. It is sold for the purpose of applying conductive shielding inside plastic cases.
I have used this paint to repair bonding wires on ICs.
I am sure there are similar products available.

[soldar]

I bought this as conductive paint but it is more like paste and it does not conduct when applied. Still, I am doing some experiments because I think there is a chance it might conduct (enough) after it dries but it takes many hours, days even, to dry. We shall see. Maybe it will work well enough even if I have to apply several layers. I will see.

[tooki]

Honest question: why are so many people reluctant to buy stuff from reputable distributors (like DigiKey, mouser, Farnell, RS, Reichelt, Conrad, and many others), instead choosing who-knows-what from eBay? It just doesn’t make sense to me.

[soldar]

Well, I did some experimenting and it does seem to work for now. As the paint/paste dries it does conduct enough that the keyboard works. It is tricky to apply because the traces are so thin but I did manage to do it. After it dries it shrinks down to very flat on the PCB.

I suppose I should not expect it to stop working or have problems but only time will tell. For now it is working.

[HwAoRrDk]

Are you sure that stuff is not intended to have some kind of thinner added to it before application?

[soldar]

Are you sure that stuff is not intended to have some kind of thinner added to it before application?

I have no idea but it comes in a syringe with a very fine needle.  It looks to me like it might be the same as thermal paste and I wonder if thermal paste is electrically conductive and if it might work for this type of application.

[tooki]

I have no idea but it comes in a syringe with a very fine needle.  It looks to me like it might be the same as thermal paste and I wonder if thermal paste is electrically conductive and if it might work for this type of application.

LOL what?!?

No, normal thermal paste is not electrically conductive, and it’s a grease, meaning it does not dry out (at least not at any useful speed).

[HwAoRrDk]

Well, there were some thermal pastes for PC CPUs, etc. that were electrically conductive, and you had to be careful with any overspill, but I don't know if those are still around these days. And of course these days you have liquid metal, etc.

The thought occurred to me that perhaps OP bought silver conductive ink intended for screen printing flexible circuits, etc. and perhaps is supposed to have thinners added to mix it to the correct viscosity for printing or whatever. Conductive ink for what OP wants to do is usually sold in pen form, and indeed if you search eBay for "conductive ink", these pens are mostly what you find.

Are we also sure OP didn't just buy solder paste? That typically comes in syringes for small quantities. And of course that is ultimately conductive (because it's solder!), but I would imagine it might not be with all the flux mixed in.

[soldar]

This is what I got: https://www.ebay.com/itm/393931446178

As they say, all's well that ends well.

[tooki]

Well, there were some thermal pastes for PC CPUs, etc. that were electrically conductive, and you had to be careful with any overspill, but I don't know if those are still around these days. And of course these days you have liquid metal, etc.

That’s why I specified “normal” thermal paste. There are exotic exceptions, but the normal stuff is non-conductive.

[jzx]

I dont know the brand, it was long time ago, but I used a silver paint for repair a spectrum keyboard. It was very thin, like water, and had a smell like some organic solvent (not alcohol or acetone).
I  had to do a mask with adhesive cellophane to apply, but it dried fast (perhaps 1-2 hours) and the resistance was low. Perhaps somebody can recognize this paint.

[soldar]

I did not think of using masking tape which is so obvious... I hope I will remember for next time.

I could have used Post-it notes which will come off easily.

[Haenk]

I dont know the brand, it was long time ago, but I used a silver paint for repair a spectrum keyboard. It was very thin, like water, and had a smell like some organic solvent (not alcohol or acetone).
I  had to do a mask with adhesive cellophane to apply, but it dried fast (perhaps 1-2 hours) and the resistance was low. Perhaps somebody can recognize this paint.

Yes, I had this stuff too, like 30+ years ago. It claimed to contain silver (Was it called "liquid silver"?), and was really expensive. It was solvent based and dried up to look like some crappy silver paint. However it did work for me. Unfortunately, the next time I wanted to use it, the little bottle had dried up.

[soldar]

Yes, I had this stuff too, like 30+ years ago. It claimed to contain silver (Was it called "liquid silver"?), and was really expensive. It was solvent based and dried up to look like some crappy silver paint. However it did work for me. Unfortunately, the next time I wanted to use it, the little bottle had dried up.

This is a common problem with anything with solvents and infrequent use. If I know what solvent will work I will add some and also I try to seal the product as best I can.

At my desk I always have a glue stick of the common kind used for paper. I seldom use it so they used to dry up and need to be replaced. Until someone told me the solvent is plain water so now, every time I use it, I put a few drops of water on it before I put it away. Works great.