In article ,
John-Del wrote:

On Saturday, November 8, 2014 8:18:22 PM UTC-5, Jeff Wisnia wrote:
John-Del wrote:
So I had this membrane keyboard assy from a Swiss machining center in for
repair as a couple of the switches had failed, and intended to replace
the two with mechanical NO switches as the membrane is NLA from TESA. I
couldn't trace one of the printed switch conductors back to the main
circuit, so I had to peel the membrane apart to see what happened. One
of the printed traces had opened, so I picked up some Bare conductive
paint from Radio Shack to repair the trace.

http://www.amazon.com/Bare-Conductiv.../ref=sr_1_1/18
8-6989773-8983665?ie=UTF8&qid=1400163421&sr=8-1&keywords=bare+conductive+p
aint

This product sold on the premise that it could be used to replace or
fabricate circuit traces or use to attach components without solder. On
the back of the package, they show a painted circuit connecting an LED to
a battery.

Since this is just low voltage/low current key matrix, I figured that
this would work fine. Before painting it on, I drew a two inch trace
(maybe an eighth wide, about the width of the printed traces) on a piece
of plastic and measured the resistance the next morning. The two inch
trace read over 1K. That's right, 500 ohms per inch on a fairly wide
circuit trace. The small blob I would need would read between 50 and 75
ohms. Maybe it would work, maybe not, but I'm not taking a chance.

I can't imagine any use for this stuff whatsoever.

Has anyone used this stuff and measured the resistance of it? :

http://www.amazon.com/CircuitWriterT...ased-grams/dp/
B0002BBVQO/ref=sr_1_7?ie=UTF8&qid=1400163782&sr=8-7&keywords=conductive+pa
int




I've used conductive epoxy with excellent results over the years,
repairing things where soldering was not readily possible.

I just looked around a bit at conductive paints and this one:

http://tinyurl.com/mdv7a7x

Seems to have pretty low resistance. Unless I've forgotten my math the
listed resistivity of Less than .015 ohm/square per 1.0 mill
(25 microns) dry film. That would mean to me that a 50 mil wide trace
made from this paint would have a resistance of only 0.3 ohm per inch.

Someone please correct me if I'm wrong on that.

Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.

I don't know that product, but since this thread popped back up and I was the
OP, I'll update it.

I ended up throwing out the Bare crap and bought a Circuit Works silver paint
pen. The traces I made to replace the missing traces on the plastic membrane
keyboard were about an ohm for an inch IIRC. What I do remember is that I
made several samples traces on a plastic sheet before committing to the
repair, and a couple of them were *much* higher in resistance than they
should be. Circuit Works recommended shaking the pen for a short time, but
if you get one, shake the beejeesus out of it for 10 minutes or more. Short
shake times result in high resistance.

Since the signaling currents in a keyboard scan matrix might very well
be in the microamp range, a few k -- or a few dozen k -- simply won't
make any difference at all. Except maybe for response time. Maybe.

Isaac