Keyboard ribbon contacts - Page 1
Forum section: electronics / repair

author: mandor
author_rank: Regular Contributor
timestamp: February 21, 2018, 07:34:30 am
content: Hello!I got a used corsair keyboard (old k95 white led) and i knew it had some issues. Like 1/3 LED not working.Now there are two pcbs. On the main one that holds the most common keys i think after some testing that maybe only few leds are actually broken and the rest that don't work don't because of how those bad ones are connected. Will see when i get some LEDs.The issue is with the second PCB that holds the macro keys. Almost no LEDs work and two switches don't work also. I started testing, and i could lite up all the leds. Test the switches and they work. Now i look at solder and some of it is quite broken so I resolder but nothing changed. Then I had a close look at the ribbon that connects the two boards and some contact traces are just broken. Like on the very edge of contact the insulated part that runs to the other contact. It is just like sort of a flex ribbon cable (square sheet rather) that has exposed contacts on either side and those are pushed against the gold plated contacts on pcbs with some silicon rubber.Now I imagine i could just use some thin wire and solder the contacts on two boards together, but then neither of the boards could be taken out again without desoldering them because it runs through a hole. And LEDs do die so I will probably have to replace some more in foreseeable future.What do you suggest i try? Conductive ink pens?

author: ZomBiE80
author_rank: Contributor
timestamp: February 21, 2018, 10:57:13 am
content: Can you show me a photo? Conductive ink might not be a good choice since it could have quite high resistance (enviroment variables) unless you bake it. I've soldered thin wire bridges i got from broken HDMI cables with great success.

author: mandor
author_rank: Regular Contributor
timestamp: February 22, 2018, 06:59:56 am
content: Picture attache

author: Ian.M
author_rank: Super Contributor
timestamp: February 22, 2018, 07:41:56 am
content: Well, that looks like it was designed by Rube Goldberg on shrooms!If its developed cracks where the exposed contact pads emerge from under the top layer, its probably FUBARed. Conductive paint only works on a stable substrate, so will probably break again at the crack, and may give you trouble with it wicking sideways in the crack shorting adjacent traces. A soldered repair (if its even solderable without melting the trace or substrate) with copper foil probably isn't viable because the contacts must remain thin flat and flexible for the silicone pressure pad to be able to effectively press them against the mating PCB.A picture of the inside of the keyboard showing the two PCBs that that flatflex connects to, and whatever obstructions there are that forced that flatflex layout would help.

author: KhronX
author_rank: Frequent Contributor
timestamp: February 22, 2018, 06:42:17 pm
content: I'd be half-tempted to "manually" run wires between the two boards, to entirely replace this thing. And by "wires" i mean really thin ones - thinking of stuff like video cables from laptops, and such...

author: mandor
author_rank: Regular Contributor
timestamp: February 22, 2018, 08:30:07 pm
content: OK, two more pictures here.I thought about thin wires, but i'm saying, i'll probably have to replace more leds in the future and then i'll have to desolder them...The foil is not actually broken and the traces don't seem broken.. they just seem coroded, some of them. Tested with multimeter.

author: KhronX
author_rank: Frequent Contributor
timestamp: February 22, 2018, 09:08:24 pm
content: If they're just corroded, maybe giving them a ginger going-over with some really fine-grit sandpaper might work? Fine-grit being 800 or above.

author: Ian.M
author_rank: Super Contributor
timestamp: February 22, 2018, 09:11:31 pm
content: Using a slightly abrasive pencil eraser, followed by cleaning with IPA to remove any rubber residue would be safer.

author: KhronX
author_rank: Frequent Contributor
timestamp: February 22, 2018, 09:19:10 pm
content: ... Or thatQuote from: Ian.M on February 22, 2018, 09:11:31 pm

author: mandor
author_rank: Regular Contributor
timestamp: February 24, 2018, 08:48:28 am
content: I am not sure but i think that darker spots are where traces are non existant.I sprayed some contact spray on that.I poked with continuity tester and those that have darker spots near the green area do not conduct at all.

author: Ian.M
author_rank: Super Contributor
timestamp: February 24, 2018, 10:15:16 am
content: If its one of those horrible flatflexes made with printed conductive ink on a low melting point plastic substrate, there isn't a whole lot you can do with it once multiple conductors have corroded away. You cant solder them as the conductors and substrate melt before the solder does, and extensive repairs using conductive ink are unlikely to go well even if you are experienced with a calligraphy brush. You'll also have difficulty getting the contacts flat enough.Try cleaning the contacts as I suggested above or with *very* fine grit Wet & Dry paper, folded round the edge of a piece of foam plastic sheet so you cant apply too much pressure easily - 1200 grit would probably be suitable, you'll find it at an auto repair specialist, or with a fiberglass retractable pencil brush, set long so its soft and dragged towards the edge at an angle so the fibre tips don't dig in. Stop cleaning as soon as you get the surface tarnish off as the contacts are very thin. If it goes all the way through, as you suspect, well its FUBARed anyway, so nothing lost.If its FUBARed, the only option (short of designing and ordering a custom flatflex or robbing spares from another unit) would be the thin wires. What's the contact pitch, what are the dimensions of the slot the flatflex contact tail passes through, what's the clearance between the board edge and the case wall with the slot, and how much clearance is there between the track side of the board and the case? We *may* be able to come up with a removable connector solution.

author: mandor
author_rank: Regular Contributor
timestamp: February 25, 2018, 09:12:19 am
content: I think few of them are really fubared. Those contacts and traces carry keypresses and led current and half of that wasn't working.The pitch is 1.6mm. 2x11 contacts each side.One side is 20mm, 3mm gap, 20mm, other side is 40mmThe contacts are pressed with silicon strip 1.5mm thick, sot that would be clearence between pcb contacts and the case.Not sure what you mean by clearance between the board edge and the case wall with the slot.If it's too much trouble, don't bother. I can just solder a piece of flat PATA cable I guess. The 90 degree turn could be difficult, though.

author: Ian.M
author_rank: Super Contributor
timestamp: February 25, 2018, 10:00:15 am
content: I was hoping it would be 2.54mm pitch so you could have used strips of turned pin SIL sockets as mating plugs and sockets, with the socket pins soldered flat to the PCB at the edge, and the wires soldered into the sockets of another strip to make a plug, that would hopefully be thin enough to pass through the slot. However the 1.6mm pitch ruins that idea.If you start with PATA-66 cable, its 0.635mm pitch, so you can approximate 1.6mm pitch by cropping intervening wires so you are left with one per pad as close as possible to the pad center. For the end through the slot - strip the remaining wires long, then sandwich the tips between two layers of Kapton tape, to hold them all straight and prevent the strands unravelling. The gap between the tape and the end of the ribbon insulation should match the width of the silicone pressure pad. That should give you a removable connector compatible with the existing system. For the other end, split the ribbon cable back in strips to make the rightangle turn practical and solder it to the PCB.