Repairing keyboard flexible membrane  - Page 1
Forum section: electronics / repair

author: soldar
author_rank: Super Contributor
timestamp: June 01, 2019, 01:30:49 pm
content: Most of my computers have PS2 keyboards which are getting harder to find and more expensive. Usually I would get them used at Goodwill stores for a dollar or two but even those are hard to find now.I have had a couple keyboards fail and this one is on the way out, I can tell. All are membrane keyboards which I assume everybody knows how they work. A couple flexible printed circuits separated by a plastic film with holes where the keys and contacts are.I assume they can fail in two ways. One is if dirt accumulates between the contacts and another if the contacts wear out from use. I guess dirt could also speed up wear.The flexible printed circuits have the conductors and contacts made with some silvery stuff which I do not know what it is. I wonder if there is some inexpensive product which could be applied like paint, with a small brush. The only requirements I can think of is that it be conductive, flexible and not dissolve the plastic. And inexpensive. Did I mention inexpensive? Yeah, inexpensive.I could try repairing a couple keyboards I have because I am running out of spares. Not urgent but better to be prepared.

author: SeanB
author_rank: Super Contributor
timestamp: June 01, 2019, 04:44:57 pm
content: You might be able to repair them by simply using the PCB from the PS/2 keyboard, and simply mounting it in the USB keyboard housing. just look for the same model keyboards as you have with USB, as the manufacturers redesigned them to use the same membrane internally, the same mouldings and thus the same PCB with a COB on it, and the interface to the keyboard will be the same for the same keyboard layout, and the mountings will be isentical, with the same pitch, same hold down strip and hole spacing.Otherwise you need to find some silver conductive ink used for windscreen demister repair, or go to the art supply stores and look at the silver pens that write conductive traces. If you just get the ink then get some Rotring pens ( large tip types), some solvent for the ink , and hope the silicone bulb in the pens does not dissolve too fast in the solvent.

author: soldar
author_rank: Super Contributor
timestamp: June 01, 2019, 05:25:07 pm
content: ThanksSeanB. Buying another similar USB keyboard does not make much sense to me because the chances of finding the same one are remote and I might as well just buy a PS2 keyboard directly.Searching for -silver conductive ink- lead me tohttps://www.amazon.com/Grams-silver-Conductive-paste-brush/dp/B01GKLIJMIwhich would probably work. But while it might makes sense for expensive repairs or for someone who does these types of repairs every day it does not make sense to buy that because I can buy a keyboard for that price. Still, it is good to know it exists should I ever need it.I realize PC keyboards are not worth repairing and just thrown away when they stop working but I was hoping maybe someone had some home-brew idea that would give me some more time out of the keyboard.I remember many years ago I thought I'd take a chance to see if I could solder to a flexible printed circuit. I thought if I was very quick I could not do much damage but as the tip touched the plastic it instantly melted a huge hole in it.

author: amyk
author_rank: Super Contributor
timestamp: June 01, 2019, 06:05:32 pm
content: Silver conductive ink, yes. I believe that's how they were manufactured in the first place. You can actually find videos on Youtube where people recover real silver from keyboard membranes (warning, might be a little disturbing to watch if you're into repairing things.) A small bottle of it is expensive for that same reason, but the amount in a single keyboard is far less.The contacts are essentially sealed, so that isn't the problem. They usually stop working for other reasons.

author: soldar
author_rank: Super Contributor
timestamp: June 01, 2019, 06:17:44 pm
content: Not sealed in the keyboards I have seen. Just two sheets with the contacts and conducting parts separated by a sheet with holes. I have cleaned a few over the years.If sealed ones exist they are probably much better quality and more expensive.I wonder if it may be practical to use some solvent to recover the ink from a bad keyboard and use it to repair another keyboard.

author: Ian.M
author_rank: Super Contributor
timestamp: June 01, 2019, 06:53:32 pm
content: Nope. Its BER. Silver recovery would be uneconomic unless you had several container loads of just the membranes, + the appropriate licences for handling eWaste and precious metals recovery commercially, + an existing silver recovery line.However this topic might be of interest:https://www.eevblog.com/forum/microcontrollers/converting-usb-hid-devices-(keyboard-and-mouse)-to-ps2/

author: soldar
author_rank: Super Contributor
timestamp: June 01, 2019, 08:42:05 pm
content: I am not talking of recovering Ag to any degree. I am talking of taking a swab of it from one place and depositing in another, just enough to reinforce and repair a weak track.In any case, all this is for fun as I have enough PS2 keyboards to last me for a while, I can buy PS2 keyboards easily for now, whether new or used and, in the future and in the worst of cases the computers can take USB keyboards anyway.

author: amyk
author_rank: Super Contributor
timestamp: June 01, 2019, 09:23:30 pm
content: I said "essentially sealed", there are no holes in the top or bottom sheets so how does dirt get into the switch chambers (which is what the one with holes in the middle forms)? I've cleaned dirt off the top or bottom sheets, but not had to separate the membrane stack.

author: soldar
author_rank: Super Contributor
timestamp: June 01, 2019, 10:15:54 pm
content: Well, yes. The one I am using and which I just cleaned has three plastic sheets which are not sealed, just held in place by pins. The top and bottom sheets of plastic do not have holes so, yes, any dust, to get between the sheets needs to enter from the edges. Not easy but not impossible. After years of use some can get in.Another issue is that every time a key is hit and the contact pads are closed there is some wear on the pads and after thousands of hits they can begin to fail. And maybe some of the material lost from the pads becomes non-conductive dust.I don't know exactly the modes of failure but I have repaired many keyboards and remote controls by opening and cleaning them. A few times no amount of cleaning would fix them.This keyboard I just repaired because the space bar was not working. I disassembled it and cleaned it and now the space works much better but the M doesn't register half the times so I might have to open it again.

author: dacman
author_rank: Frequent Contributor
timestamp: June 01, 2019, 10:35:09 pm
content: I've used CaiKote 44. It's a silver paste made by the same company that makes DeoxIT. One small tube goes a long way.

author: soldar
author_rank: Super Contributor
timestamp: June 01, 2019, 11:33:39 pm
content: Quote from: dacman on June 01, 2019, 10:35:09 pmAt $40 a gram it better go a long way! It must be made with heroin!I guess it is justified in expensive items but not in a cheap PC keyboards. Good to know it exists though.

author: warrjon
author_rank: Newbie
timestamp: June 02, 2019, 01:41:55 am
content: Conductive ink pens are pretty cheap these days, but before they were I repaired numerous amplifier remote controls with kitchen foil.Clean the pad under the key with IPA very well. I use rubber glue to glue the foil to the pad, use the glue sparingly otherwise it goes everywhere. Push the foil onto the pad and let it dry. Once dry use an x-acto knife to trim up the foil being careful not to cut the keypad or tear the foil. And obviously clean the membrane with IPA as it will have all of the conductive material off the key pad stuck to the membrane.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 02, 2019, 01:59:29 am
content: You might try Aquadag. Much lower conductivity, but also much cheaper.

author: GLouie
author_rank: Regular Contributor
timestamp: June 02, 2019, 07:05:12 pm
content: The product I've used for this is the Circuitworks silver pen:https://www.chemtronics.com/circuitworks-conductive-penI haven't done it in years, but I recall the pen dries up within a year or so, so I'd do as many repairs as possible within a short time.Decades ago, GC chemicals sold little bottles of a similar stuff with a brush-cap applicator for this purpose, maybe they still do. I think it's essentially like nail polish, but with silver particles.

author: soldar
author_rank: Super Contributor
timestamp: June 02, 2019, 07:36:49 pm
content: They're too expensive and might make sense if you use them a lot in expensive projects but for repairing one single solitary keyboard it doesn't pay.When you think about it it is amazing that something so simple should be so expensive. If it were rigid you could just use normal wire which costs nothing.

author: amyk
author_rank: Super Contributor
timestamp: June 03, 2019, 02:08:08 am
content: Quote from: soldar on June 02, 2019, 07:36:49 pmThat's because silver is a precious metal. They use only a tiny amount of it in manufacturing, and at those scales it costs less than a PCB and far less than the point-to-point wiring that I think you're suggesting.

author: GregDunn
author_rank: Frequent Contributor
timestamp: June 07, 2019, 07:51:44 pm
content: I have a similar issue - I bought a used Apple A1048 keyboard, but when it arrived, it had several dead keys. From what I can see, they appear to all be on the same branch of the key matrix; I suspected a bad connection on the ribbon cables, but after removing, cleaning and reinserting them, no change. I contacted the seller and he volunteered a refund, so I may have a free, partly working keyboard to experiment with.This keyboard can be disassembled without damaging anything:https://www.ifixit.com/Guide/Apple+Keyboard+disassembly+to+clean+PCB+Membrane/15598So I'm going to try to fix it; hopefully it's just dirt or a bad trace at one end of the run rather than multiple bad pieces. My question is: There seem to be numerous recommendations for what to use to replace the conductive traces. Will any silver/nickel conductive paint pen do the job here, as long as it's properly applied?Edit: looks like coffee spilled and some seeped between the layers near the bottom. This is near the traces of the bad keys, so pretty obvious. I'm assuming IPA will take it away without damaging the traces, or is it better to start with distilled water?

author: amyk
author_rank: Super Contributor
timestamp: June 08, 2019, 01:10:34 am
content: Quote from: GregDunn on June 07, 2019, 07:51:44 pmUse only a small amount of water, and be careful too. I would recommend testing first in a non-critical area.Yes, it's likely to be silver.

author: GregDunn
author_rank: Frequent Contributor
timestamp: June 08, 2019, 05:16:37 am
content: Carefully used distilled water to clean the gunk off, dried it thoroughly, no change.Now begins the search for bad traces or contacts.

author: LateLesley
author_rank: Frequent Contributor
timestamp: June 08, 2019, 06:32:26 am
content: I would also check the button side. There are cases where the small black pads lose their conductivity. The quick n dirty fix is with a pencil, but you can actually buy new conductive pads.https://www.ebay.co.uk/itm/REPLACEMENT-Rubbers-for-Remote-Controllers-Pack-of-100-Conductive-Rubber-Pads/253585418995?hash=item3b0ade6af3:g:QpcAAOSwo4pYgySH&frcectupt=true

author: amyk
author_rank: Super Contributor
timestamp: June 08, 2019, 07:44:00 pm
content: Quote from: LateLesley on June 08, 2019, 06:32:26 amI haven't come across a computer keyboard that has "open" switch chambers, they are almost always either discrete switches or the 3-layer membrane discussed above.

author: LateLesley
author_rank: Frequent Contributor
timestamp: June 09, 2019, 04:41:49 am
content: Quote from: amyk on June 08, 2019, 07:44:00 pmOh, I've come across keyboards with the small rubber domes, with the pads in the underside.I didn't realise it was the 3 layer membrane type.I should read better.

author: GregDunn
author_rank: Frequent Contributor
timestamp: June 11, 2019, 09:20:49 pm
content: This is a 3-layer membrane, correct. I managed to isolate the two traces which were damaged, and they are on separate layers of the membrane, practically on top of each other - which tracks the spill location perfectly.I found a couple of less expensive options for repair; does anyone have experience with either of these?https://www.amazon.com/CaiKot-Conductive-Silver-Coating-surfaces/dp/B003D8G8SY/Or this?https://www.amazon.com/Keypad-Restore-Conductivity-Carbon-Copper/dp/B0026PRMVM/

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 10:25:03 pm
content: Those products look interesting and well worth the price if you have more than one keyboard to repair.The problem with anything that comes in a tube is that you use it once and next tie you need it it has dried up.The space bar was not working on my keyboard so I opened it and cleaned it, Now the space bar works but the M key works like 1/3 or the time. Every post I write I have to go back and insert all the missing M's.I am still using the original MS Intellimouse that I got in 1986 but I believe this is the third keyboard.

author: kizmit99
author_rank: Regular Contributor
timestamp: June 11, 2019, 10:45:33 pm
content: I haven't used either of those products, but they look to me like they are both meant to refresh the carbon pad on the underside of the switch 'bubble', not to repair the traces...I have tried this stuff to repair the traces:https://www.ebay.com/itm/Silver-0-2ML-Glue-Wire-Conductive-Electrically-Paste-Adhesive-Paint-PCB-Repair-/113169201331I doesn't work for crap - I nice thick test trace has something like 600ohms resistance (if I recall correctly) - totally worthless for repairing keyboards.I'm still watching to see if you can find something cheap to make the repair though - I have a keyboard I'd rather repair than junk and the only thing wrong with it are a couple of those membrane traces that have 'evaporated' away...

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 10:57:44 pm
content: I didn't measure the resistance of the tracks on my keyboard but I believe relatively high resistance is normal and does not interfere with the normal functioning of the keyboard.

author: GregDunn
author_rank: Frequent Contributor
timestamp: June 11, 2019, 11:28:15 pm
content: Yes, a typical working trace on this keyboard is between ~3-50 Ω depending on how hard you press and how far apart - so it's not anywhere close to what you'd find on a PC trace. A friend of mine recommended this:https://www.amazon.com/CircuitWriter-Conductive-Ink-silver-based-4g/dp/B00B88B9KI/but as suggested above, I really hate to spend a lot of dough on something that in all likelihood will only get used once. I tried a metallic (decorative) pen on a piece of scrap and found that it was essentially non-conductive. Any circuit pen that I buy will cost more than the actual keyboard, but I suppose I am going to have to try something that isn't junk or else I'll just spend even more.

author: kizmit99
author_rank: Regular Contributor
timestamp: June 12, 2019, 02:13:18 am
content: I dunno - but the traces on my keyboard's membrane buzz out to basically 0 ohms...This is just touching two ends of the trace with meter probes, not measuring across a pressed key...pretty sure this is a very standard, cheap, keyboard...

author: GregDunn
author_rank: Frequent Contributor
timestamp: June 12, 2019, 03:00:53 am
content: My probes are small enough that I didn't want to push very hard for fear of distorting the membrane; but I could generally get the trace to measure as low as I wanted by pushing harder. I very much doubt all membrane keyboards use identical scanning hardware, so some of them probably tolerate higher contact resistance than others. This is the first one I've ever taken apart, though, so I have a history of exactly one to go by.I ordered some silver-based conductive ink so we'll see in a couple of days whether it's OK or not.

author: amyk
author_rank: Super Contributor
timestamp: June 12, 2019, 03:30:55 am
content: Quote from: GregDunn on June 12, 2019, 03:00:53 amIt's almost certainly going to be a CMOS input on a microcontroller, so next to no current is needed.

author: magic
author_rank: Super Contributor
timestamp: December 03, 2021, 02:01:55 pm
content: The silver stuff can now be bought from AliExpress in very small quantities in syringes, costs a few bucks delivered. I bought mine in Poland because I didn't want to wait, but the packaging leaves absolutely no doubt where it originated fromI tried it on a damaged IBM Model M membrane and it workedInitially it has virtually no detectable conductance, I was barely able to get a reading of 100MΩ by bringing the probes within 1mm of each other. But after 24 hours of curing, resistance fell down to <3Ω, which is almost on par with the original traces on the membranes. I applied a bit too much and ended up needing to scratch the gaps between traces.I also performed some testing on the keyboard itself: the original traces are 30~50Ω end-to-end per one sheet, and the controller reacts to a 4.7kΩ resistor applied directly between its PCB contacts (didn't try other values). So it looks like even a much sloppier repair with hundreds or thousands ohms of resistance would still do the job. Cheap graphite ink for TV remote keypads, perhaps?Now gotta bolt mod the damn thing, put it back together and reinstall 102 hammers, springs and keycaps

author: planenut2000
author_rank: Contributor
timestamp: December 05, 2021, 10:42:40 pm
content: I would bet that would work on car key fobs too!

author: magic
author_rank: Super Contributor
timestamp: December 05, 2021, 11:26:45 pm
content: If there is some sort of printed membrane keypad in there that can go wrong then sure.Note that some low power devices like TV remotes or calculators use carbon for the conductive traces and they tend to have more resistance than silver. Those devices can be repaired with conductive graphite ink, which is cheaper. Been there and done that as well.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: December 06, 2021, 01:44:54 pm
content: Good Morning Soldar,Adapters are made to go from PS2 to USB as well as the opposite. Look here:https://www.amazon.com/ps2-usb-adapter/s?k=ps2+to+usb+adapter

author: magic
author_rank: Super Contributor
timestamp: December 06, 2021, 01:57:36 pm
content: Soldar is long gone from the forum and the problem with USB-keyboard to PS/2-computer adapters is that they are usually passive - the keyboard is expected to find out that it is connected to PS/2 rather than USB and deal with it. At some point in time USB keyboards (and mice) had this capability, but current ones probably don't.editJust check out Amazon reviews of the products you linked: