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enter image description here

Figure 1. (1) Lenovo keyboard controller (upside down), (2) keyboard membrane lower layer and (3) upper layer. The spacer layer is just about visible around the contact dots.

I'm building a footswitch PC controller with four momentary pushbuttons, up, down, PageUp and PageDn. I've found an ideal controller in a Lenovo USB keyboard with some missing keycaps and figured out the matrix. Pins 1 - 18 are the "columns" and 19 - 26 are the rows. http://keyboardtester.com has a simple online app which shows the "pressed" key as I short out the rows and columns. I will dump the flexible membranes and connect my footswitches directly to the green PCB.

The FPC\$^1\$ keyboard membranes are clamped against the PCB edge connector which appears to be coated with a dark grey material that looks like graphite. A quick test with the soldering iron on the inner edge of pin 26 failed to wet the pin.

Does anyone know what the correct term is for this edge connector coating material and can anyone suggest a means of attaching wires to the green PCB pins?


\$^1\$ FPC = flexible printed circuit.

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    \$\begingroup\$ Why not just scrape the coating away with some sandpaper or a knife? \$\endgroup\$ – user25093 Apr 24 '16 at 16:21
  • \$\begingroup\$ Because I've got to do it 26 times and only get one shot. If I damage one that I need I'll have to start again on another keyboard. This one is almost ideal. I could also look for an edge connector. Pins are 2 mm spacing with the centre screw taking up three positions. I'd need 29 ways to catch all 26 pins plus the three blanks. I only want to jump to a perf strip board so I can solder in some pin headers to make the unit reconfigurable, if required. \$\endgroup\$ – Transistor Apr 24 '16 at 16:44
  • \$\begingroup\$ It is possible the contacts are Chromium (it resists tarnishing) and a conductive epoxy would be the best bet in that case. \$\endgroup\$ – Peter Smith Apr 24 '16 at 16:56
  • \$\begingroup\$ Update my previous comment: CrO2 looks just like graphite and has excellent electrical characteristics, but is not solderable. \$\endgroup\$ – Peter Smith Apr 24 '16 at 17:10
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    \$\begingroup\$ You will only use four keys. You should be able to test in unused contacts some techniques to scrap the black coating using a knife or sandpaper like someone suggested. Anyway, if something goes wrong with them, you can scrap the green soldermask (carefully) and solder wires direct to then, using some hot glue or other material to fix the wires to the board and act as a stress relief for the connections. Do not rely on the soldering itself to hold the wires. They are not strong and may break the copper tracks to a unrecoverable state. \$\endgroup\$ – ricardomenzer Apr 25 '16 at 13:02
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I would recommend that you use a stiff piece of non-conducting board (of the correct dimensions) and 3 screws and nuts to clamp the FPC to the PCB, and forget the soldering.

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This might be a bit late, but for anyone browsing this later for solutions, just use that a pencil eraser to rub out the graphite, revealing the tinned pads below, which you can solder directly on to.

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  • \$\begingroup\$ Thanks. It wasn't that easy. You've reminded me that I didn't post the solution. +1 anyway. \$\endgroup\$ – Transistor Jun 3 '19 at 11:26
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In the end I used a screwdriver to carefully scratch the coating off the PCB edge connector. I was then able to solder onto the PCB.

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