Repairing rubber membrane keyboards (carbon dot elastomer buttons + a coated PCB trace)

[ac...@provide.net]

Has anyone come up with a *reliable*, *permanent* fix,
for both the carbon dots on the elastomer, and the
black textured traces on the PCB?

I have an electronic piano with dual height (velocity sensing)
elasomer/carbon button pairs on the inside, which I need
to repair. (The button dot PAIR senses velocity via the
"1.. 2.." timing of a press.)

I have a lot of dots to fix. The mfg does sells new strips
in long strip modules with molded mounting pins, but this
makes an OEM repair well over $100 in parts alone.

I'm hoping there's an easy/better/cheaper/reconditioning
fix for this, such as finding the original chem mfgr of
"carbon dot elasomer ink", etc. and refurbishing the dots.
(Hey, they OEMs it once, get THAT product!)

The net shows a number of techniques and suggestions.
Most boil down to a combo of cleaners, adhesives and some
sort of conductive paint (eg carbon dust & super glue,
silver slurry ink pens, rear window defogger repair kits,
et al).

BUT... others' feedback on each shows than MANY of the
suggestions have serious long term problems, and fail quickly.
Cited failure modes include:
.. cleaning (and/or inks) actually making things WORSE,
.. conductive adhesives and paints quickly flake off flexing rubber,
.. graphite/ink dust shedding, shorting out OTHER things,
.. true silver tarnishes fast, so silver ink traces quickly fail,
.. "carbon dot sticker" adhesives fail, letting them come loose,
.. velocity sense fails due to a differential height shift
from the attempted "repair".

I'm hoping that SOMEONE here has actually cracked
this common repair, and can advise on which system(s)
DO work (or conversely, which to AVOID!)

Finding a source for the "original conductive paints" the
OEMS use on both the silicone and the PCB sides (eg carbon
dust with a good glue) would be a bonus!

Any advice, on fixing both the elastomer side, AND PCB
refurbishing?

Thanks!
- Keith Mc.

[Nathaniel Bezanson]

On Saturday, June 20, 2015 at 7:16:25 PM UTC-4, KeithMc wrote:

Has anyone come up with a *reliable*, *permanent* fix,
for both the carbon dots on the elastomer, and the
black textured traces on the PCB?

Is it necessary for the PCB side to be coated? I see lots of membrane keypads with just bare ENIG-finish traces on the PCB, and they seem to work fine. I've seen the coated ones too, I'm just not clear on what benefit it offers, or what the effect would be if you left the traces bare. (Theory: Textured surface may reduce stiction when you release the key, and allow it to return silently / faster, by creating channels for air to work between the PCB and the key dot?) This coating must cost money so there's clearly a reason for it in some applications.

 

 

.. conductive adhesives and paints quickly flake off flexing rubber,
.. graphite/ink dust shedding, shorting out OTHER things,
.. true silver tarnishes fast, so silver ink traces quickly fail,
.. "carbon dot sticker" adhesives fail, letting them come loose,

Seems to me that what you want is another piece of actual carbon elastomer, in a form-factor that you can glue on. My first impulse is to look at "zebra strip" elastomeric connectors -- can you peel them apart into layers, and just use a little slice as your new dot? Bond it with something flexible like rubber cement, but I'm sure there are better options. Certain flex-in-place caulks might be worth a look, try a tube of Geocel from the roofing aisle. 

.. velocity sense fails due to a differential height shift 
from the attempted "repair". 

This is gonna be the kicker, as whatever you do to one dot, you'll have to do identically to the other. I think that should be made easier by using the zebra strip described above, as all the layers should be the same thickness. 

...

While we're talking conductive goop and adhesives, I had another thought recently -- "smartphone gloves" fetch a premium, because normal gloves don't work with capacitive touchscreens. Could you simply dip the fingertip of a regular glove in conductive goop, eg trace repair compound, and bring this capability to cheap gloves for pennies a pair? I plan to find out come winter! 

 

-Nate B-

[Michael Rudas]

In cases where the tarnishing of silver print paint causes problems, I use nickel print instead. Its conductivity is slightly lower than silver or copper, but it doesn't oxidize as badly as either.

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