: Knowledgebase

Repairing Rubber Buttons


Since the first phone that was sold, there have been electrical contacts in phones. Originally, the only contact was the hookswitch. It didn't take long to realize that two pieces of metal touching in the same place every time, was not a switch that was going to last a long time. The spark that occurred when the switch was operated left some carbon residue. Eventually, the electricity had a hard time flowing through the built-up carbon, and the switch stopped working.

On low current applications, there is usually no spark, but a coating develops over the two pieces of metal - kind of like the gook that covers your windows before you wash them - and the electricity has a hard time passing through this coating which acts like an insulator.

Later phones began using self-cleaning contacts. This works by designing the two pieces of metal so that instead of touching in the same place each time, one of the contacts slides over the other a little when operated. Designers then found that if they put a piece of knife shaped metal on the contact, facing opposite directions on each side, the scraping action of the "blades" would get the best cleaning action.

ITT was one of the first companies to change over from metal contacts on their telephone dials to conductive rubber buttons that short out two traces on a PC board. This cut down the number of times that the dials would fail - but on ITT's first dials, they used some pins that would slide into sockets to connect the dial electronics to the button PC board. They used a cheap material for the pins and sockets, so oxidation would develop pretty quickly. The fix was to slide the board with the connector up and down a few times on the pins, to clean off the oxidation. This would fix it for a little while, but a new dial was the real fix.


The rubber button type keys are not self-cleaning, but because of the size of the area that makes contact, there are relatively few contact problems except in really dirty environments where the traces or buttons can get covered with non-conductive stuff.

Most telephones use conductive pads that are conductive thru and thru. If these buttons stop working right you can usually clean them with a pink type pencil eraser (very mildly abrasive) on the rubber pads, and on the traces on the circuit board. Be careful with the traces on the circuit board that are not shiny metal, but a raised carbon track. It's easy to take off too much of the conductive material, and the trace will be dead. If you do kill it, use a conductive paint made for PC boards to repair it - DON'T use our rubber button repair paint on the circuit board! You can also get a conductive pen that you can just paint over the metal or carbon trace, but that may not stick to the trace that's constantly being hit by the button?

It's easy to test the conductivity of a rubber pad or PC board trace. Simply use your ohm meter across the face of the rubber pad (not on the sides). On most pads, you will see 100 to 200 ohms. If you test a bad pad, it will often read 500 to 10,000 ohms or more. After using our rubber button repair paint, you will see less than 10 ohms. Metal traces will usually be a direct short, but carbon traces could be less conductive. Comparing working buttons to non-working buttons will usually be your best guide. In general, you shouldn't use any chemicals on the rubber buttons or the traces to clean them... use only a pink pencil eraser.

There are some phone systems that use conductive pads that are NOT conductive thru and thru. On these phones, EVERY phone will end up coming back for a button problem!


In the early eighty's, Japan was producing lots of toys with lots of buttons. A Japanese company designed an ink that would coat non-conductive pads on these toys, making these toys pretty inexpensive to produce. This ink dries out eventually and flakes off the pad. You can often see these little black flakes transferred to the PC board. When the toy stopped just tossed it in the garbage. The aforementioned companies are using this same ink in their business phones. They would be quite happy if you tossed their phone when it stopped working, but they are almost as happy to have you send them the phone for repair, for $50 to $80 or more. This is no accident. These companies consider this an annuity. They sell you the phone knowing that you have to send it back for repair a couple of times through its life or throw it away and buy a new one. Other companies do the exact same thing, but with a different part. The hookswitch is another popular annuity for phone manufacturers.

If you go in and try to clean a pad with an ink coating, you just remove more of the coating...and usually the button won't work at all. If you use a chemical to clean these buttons - they're dead for sure. Different repair companies have been using different stuff to repair these pads to make them conductive. I've seen alarm foil super glued to the pads, graphite coatings and all kinds of carbon, silver, and gold conductive paints. It all flakes off the button after a while, until now!

In October of 1992, I asked a chemical company if they could come up with a solution to this problem. They seemed to think that it would be a pretty easy fix, and they sent me a few prototypes. After some testing, they realized that they all flaked off the pad after a while, mainly because they would dry in a non-flexible state. They continued to work on the problem for many months. They finally sent me a prototype that would definitely work - they had tested the heck out of it and it wouldn't fall off the button. This prototype was adhesive backed copper foil "dots". The adhesive was the secret, which was specially formulated to adhere to the rubber pad and would never dry out. I sent some of these prototypes out to some customers, and everybody felt that it was much too time consuming to stick these "dots" on every button in the phone. The phones that used the ink needed EVERY button done, because the ink would flake off all the pads eventually (and cause lots of comebacks).

The chemical company went back to the drawing board armed with knowledge of what chemicals were needed to adhere to the buttons. In June of 1993, they came up with a two-part mixture that turned into silicone rubber when it cured, and it stuck to the button so well that they needed a razor blade to cut it off the couldn't be pulled off. The two-part mixture was 90% silver colored goo, and 10% clear liquid that got mixed together. There was a 20-minute working time, which meant you had to work fast to paint as many buttons as you could in 20 minutes.

They spent months trying to change the formula so they could put it in two syringes, allowing you to lay down two equal strips of stuff, so it could be mixed up as needed. This stuff is like an epoxy...once it's mixed it starts to cure, and there's no way to stop it. There was no way that anyone could mix only a portion of the 90-10% mixture in the field since you were dealing with small quantities and a lopsided mixture that had one component that was only 10% of the mixture. You'd need laboratory equipment to cut it in half, and if you messed it up the stuff wouldn't adhere to the button, wouldn't be conductive, or it wouldn't have good durability.

Speaking of durability, they tested this stuff on a custom-made machine that pushed the button on an actual NEC or Comdial button assembly, counted the mechanical keystrokes, and counted the electrical impulses on the PC board. They have tested this stuff over to over 500,000 operations, and the two counts were the same, it didn't flake off the button, it stayed just as conductive as when it was put on and was so durable that you couldn't even see the pattern of the PC board traces on the pad after 500,000 pushes!

In October of 1993, they decided that they couldn't make it anything other than the 90-10% mixture, it just would lose one of the properties it needed. They started production of the Rubber Button Repair Kit, and it has worked great! Using the whole bottle at one time may not be perfect in terms of economy, but even if you only fix one phone, it's still a lot cheaper than sending the phone out for repair and waiting for it to come back. Many guys are taking the rubber buttons out of junk phones and painting them in advance all at one sitting. They use these to do repairs, and save the old ones into a box, so they can repair them when they have a chance.

Since then, they've tuned the formula to give you maybe a day or two of working time if you keep the bottle with the mixed epoxy closed when you're not using it.

We've learned a couple of things along the way from our customers:

First, you must clean the button with Acetone (pure Acetone, with no oils or coloring) before applying our stuff. If you leave the old ink on the button, or a previous repair if it's not a virgin phone, our stuff will stick to the ink or the old repair - which will then flake off the button. Don't bother to use this stuff if you don't want to clean the buttons first!

Second, you must wipe off any repair material you get on the flat part of the rubber mat that sits on the PC board. If you don't wipe it off, it may short out the traces that are running all over the PC board, and none of the buttons will work. You can use a little Acetone on a swab to clean it off if it has started to cure, or a knife edge to scrape it off if it's still wet. When painting, a thickness of 3 mils (about 3 pieces of scotch tape thick) works best.

Third, you must mix the clear liquid in the brown bottle (Part B) into the silver goo (Part A) as soon as you open it. It readily picks up moisture from the air, which stops the mixture from working right. The chemical company changed their formula in January of 1994, to help prevent these problems if you do leave it open, but why ask for trouble? The Part B bottle is now also sealed with Nitrogen. You can leave it open for a maximum of fifteen minutes before you will screw up the formula! A good indication that the chemical reaction didn't occur properly is that it will never harden, or the silver particles will rub off on your finger after it does harden. The epoxy has a 2-day working time, it will stay workable for a while so you won't have to rush before the stuff gets hard. The shelf life of this product should be at least 6 months from the date of manufacture if left unopened.

Fourth, don't try to make the chemical company's formula better. These guys are pros. They talk about molecules and stuff like that (I never did well in chemistry in school), and they have the proper knowledge and equipment to design and test this stuff. Adding your own stuff to the mixture will definitely make one of the needed properties worse!

Get a small bottle cap or something to put the Pure Acetone in to dip the cotton swap in, to clean the buttons. You should see black stuff coming off the buttons onto the swab as you clean them. Acetone is a great solvent and dries quickly. Dip the brush in Acetone every couple of minutes and wipe the brush onto a paper towel to make painting easier.

On Comdial phones and some other business systems as well as many cordless and consumer phones, they've used melted over plastic posts to hold the circuit board together instead of screws. Some guys have told me that they use epoxy, hot glue, or try to re-mush over the plastic posts to get them back together. About 20% of the Comdial phones that come in for repair have plastic posts broken off along the bottom anyway because the people have been pounding on the buttons to get them to work. They are used to pounding on these buttons and will continue to pound on them after you fix them correctly. The shock of the pounding will cause the above repair methods to fail. The only dependable repair I have found is to cut the mushed over plastic off the post flush, with a flush cutter or razor blade to get the keyboard open. To close it, drill a hole in the middle of each post, and use a tiny screw and washer (use a plastic washer if there are traces next to the post) to hold the board down.

The most popular phone system made today with ink-coated buttons is the Avaya IP Office. Our Rubber Button Repair Kit will repair the intermittent button operation.