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geekhack Community => Off Topic => Topic started by: dotancohen on Tue, 16 August 2011, 17:06:47

Title: How to solder onto membrane pads
Post by: dotancohen on Tue, 16 August 2011, 17:06:47

I'm trying to solder wires onto the pads where a nylon membrane would otherwise sit on a PCB. When I heat the board to tin it, some waxy substance sweats out and the solder cannot reach the pads. I tried scraping it off but more sweats out. So I tried drilling a hole with the intention of filling it with solder that just might make contact with the pad. That worked out just as well as you might expect.

How does one attach leads to a keyboard controller? Conductive glue?

[ATTACH=CONFIG]24507[/ATTACH]

Title: How to solder onto membrane pads
Post by: ricercar on Tue, 16 August 2011, 17:21:18

It looks like the metal has some sort of coating on it. Gently scrape at the pads with a knife and see if you can scrape off a coating.

Title: How to solder onto membrane pads
Post by: RickyJ on Tue, 16 August 2011, 21:51:54

Those are carbon-based pads, and it's possible that the controller needs the resistance of the pads to keep from shorting out (I've seen that in other devices).  You can gently scrape the solder-mask off of the copper traces and solder straight to those.

Or you could mix up some brush-on electrical tape mixed with graphite powder and use it to glue the wires to the carbon pads, but it's really freaking messy.

Title: How to solder onto membrane pads
Post by: Pylon on Wed, 17 August 2011, 07:20:29

You can also use some very fine sandpaper and rub it against the carbon pads a little bit and then solder. I've found that that helps copper wires and the solder adhere much better.

Title: How to solder onto membrane pads
Post by: bluecar5556 on Thu, 18 August 2011, 15:57:36

[ATTACH=CONFIG]24631[/ATTACH]

As RickJ suggested, soldering to the traces leading up to the carbon pads should be your best bet as illustrated above.  The best way i've found to remove the solder mask is to take a razor blade and hold it over a 45 degree angle and pull the sharp end so it is trailing, not leading.  This will make the blade near impossible to cut or gouge anything.

Carbon has resistance but it being a factor into the design of a switch has my doubts but I may be wrong.  As always, I hold no responsibility for any misfortunes.


Resistivity of Carbon, Graphite (http://hypertextbook.com/facts/2004/AfricaBelgrave.shtml)

Title: How to solder onto membrane pads
Post by: bluecar5556 on Thu, 18 August 2011, 16:02:03

[ATTACH=CONFIG]24631[/ATTACH]

Carbon has resistance but it being a factor into the design of a switch has my doubts.  As always, I hold no responsibility for any misfortunes.

Resistivity of Carbon, Graphite (http://hypertextbook.com/facts/2004/AfricaBelgrave.shtml)

Title: How to solder onto membrane pads
Post by: The Solutor on Thu, 18 August 2011, 17:10:18

Remove the carbon with a piece of sandpaper and then solder normally.

Title: How to solder onto membrane pads
Post by: bluecar5556 on Thu, 18 August 2011, 17:13:42

Quote from: The Solutor;401226

Remove the carbon with a piece of sandpaper and then solder normally.

Swell idea, that way the traces can be soldered to alternatively if one of the pads lifts. What Solutor said...

Use any grit that is not excessively aggressive such as 400.