silicone keypad carbon button contacts

We're doing a custom silicone pushbutton thing, sort of like this one:

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The pcb below the keypad will look something like

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where the center hole lets light through to back-light the key, and the two finger patterns are where the carbon buttons land when the key is pressed. This will be ENIG, gold flash over nickel over PCB copper.

The left and right finger patterns will be electrically in parallel. It's possible for one carbon button to contact, or both, depending on how the button is pushed.

Googling, I find zillions of different and bizarre PCB patterns for the buttons to land on. Crazy shapes.

Has anyone done this? Any opinions on the optimum PCB pattern?

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin
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Any points for doing it wrong? I let the PCB designer do his own thing and ended up with an unreliable switch connection. This was on a marine radio where water sometimes condenses under the buttons. The idea is to get the most contact area under the carbon "pill" without the risk of the traces being too close and getting bridged by condensed water. Some recommendations: I don't recall exactly what we used but a 0.3mm gap sounds about right.

My guess(tm) is that the reason for the weird circular contact patterns is that it eliminates the long parallel trace runs of the interdigital pattern (shown in above URL) which might be affected by leakage between traces. The circular pattern seems to have larger gaps between traces. This is only a guess.

Graphite(+0.25v) and copper(-0.35v) are very much apart on the galvanic series will react. So, you need gold plating.

One thing to watch out for with those switches is the silicone rubber buttons "bleeding" oil onto the gold contacts. I don't know the exact mechanism, but after a few months in the hot sun, an oily liquid (plasticizer?) would leak out of the rubber and create a slimy mess. The oil also seems to attack the carbon making it slowly disappear. Eventually, the switch would fail to make a connection. The problem is common in TV remote controls.

If caught early, some household cleaner (409) would clean up the oil and allow the switch to operate normally. If left too long, the carbon contacts would need to be resurfaced with one of the kits found on eBay. Not every keypad exhibited this problem. I never had the time to investigate beyond finding this and writing a repair procedure.

Anyway, be prepared to make the keypad easy to clean, repair, or replace. The mechanical engineer that did the package saved a few pennies and heat swaged the keypad in place. When it started exhibiting the aforementioned problem, the only option we initially offered was to replace the entire front panel instead of just the keyboard.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

More...

See Pg 25.

See Pg 17.

Seems to be plenty of design guides available. Google for more:

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

Interesting links, thanks.

I don't think I'll worry about water. My pullup resistors will be 10K!

The silicone pad will be difficult to remove, sandwiched between a PCB and an aluminum panel, so I hope it won't need cleaning. I'll ask the vendor about that oozing gunk possibility.

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John Larkin         Highland Technology, Inc 
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John Larkin

Sounds like Amine Blush. It's a waxy oil that leaches out as the Plastic cures. Probably a poor mix on the key matrix.

Cheers

Reply to
Martin Riddle

I've had this happen to several remote controls after a few years. Disassembly and thorough solvent-based cleaning fixes them temporarily, but it recurs more and more quickly. I don't think it's a plasticizer - that would be more noticeable when it was new and would reduce with age. I think the compound is de-polymerising.

It's terminal, just not immediately.

Clifford Heath.

Reply to
Clifford Heath

What, me worry? Here's a simple test to try. Put the keyboard and associated electronics into your environmental chamber or fridge and drop the temperature to well below the dew point. Bring the device out of the chamber or fridge into the warm room. Water will condense on everything but the real question will be will it condense inside the keypad and simulate a keypress.

One of my early jobs was for a company that made communications accessories. In this case, it was a touch tone encoder for 2way radios in skool buses. Everything worked just fine until they were installed in the buses. The buses would sit outside in the cold night. At the crack-o-dawn, the engines would be started and the interiors warmed up in preparation for picking up the students. Water would condense inside the keypad, which would cause the transmitters to key up. Having about 50 transmitters stuck on the air just as the buses were leaving the yard was not a good thing.

Yep, that's the way most of them are built, with the silicon keyboard sandwiched between an expensive decorative front panel, and an expensive PCB full of electronics.

Got any TV remotes, cordless phones, or other devices that use rubberish keypads? If you take them apart and look at how they're built, you'll notice that the back of the front panel has ridges around each button. These ridges prevent water, dust, or debris from getting between the front panel and the silicon keypad. The ridge is under compression from the keypad PCB mounting hardware to the front panel, thus providing a very good seal. However, doing the same thing with an aluminum front panel is not going to happen. There's just no way to coin a ridge into the front panel. It might be possible to glue a frame with similar ridges to the front panel, but that costs money and is labor intensive. I don't know if you're going to need any environmental protection, but I thought it might help to explain why and how it's done.

It's quite real:

Two other potential problems. Women and some men with very sharp fingernails will cut into the soft silicone with their fingernails. That's another reason for an easily replaceable keypad. In the photos, your buttons look small enough that I would not expect a problem but you might want to make sure that the buttons stick out from the front panel far enough that the typical fingernail doesn't scratch the front panel. I also noticed that the silk screened or painted lettering on the buttons uses rather narrow lines. The rubber stretches and moves sufficiently that the lettering might flake off. Try wider line widths for better adhesion.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

Thanks. I'll do some reading but I don't think so. Amine blush is what happens when epoxy paint hardener reacts with water and CO2 and creates a color splotch. I've never seen the effect and have never heard the term used in reference to rubbers or plastics.

"The liquid found in rubber keypads is left over silicone oil from the manufacturing process when the rubber keypad

rubber keypad!" This would also explain why it happens with some keyboards, but not others.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

On Wed, 19 Jul 2017 10:53:27 +1000, Clifford Heath wrote as underneath :

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Recently I painted silver (Ecolit) conductive on the pads (6 Yr old Panasonic TV remote) after careful cleaning with thinners and realising the conductivity of the pads was the problem apart from the goo, this fix has worked extreemly well for the last few months but I have no idea if it will have 'legs' ! C+

Reply to
Charlie+

Do the silicon keyboard design guides (there are several out there) provide any guidance to the PCB patterns?

--sp

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Best regards,  
Spehro Pefhany
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Spehro Pefhany

seems the critical issue for long term reliability is what and how the conductive stuff is attached to the buttons.

m
Reply to
makolber

That's OK, we'll charge a lot too.

The one in that pic was just something we pulled off a dead instrument. Ours will be bigger (1/2 rack instrument) and better. The silicone will be squeezed between the aluminum panel and the PCB, so should seal pretty well. We always specify our temperature range as something like "0-50C non-condensing"

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John Larkin         Highland Technology, Inc 

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John Larkin

Yes, a wide array of sensible and bizarre patterns.

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John Larkin         Highland Technology, Inc 

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John Larkin

The one in that pic was just something we pulled off a dead instrument. Ours will be bigger (1/2 rack instrument) and better. The silicone will be squeezed between the aluminum panel and the PCB, so should seal pretty well. We always specify our temperature range as something like "0-50C non-condensing" =========================================================

Wonder why they don't simply mold ridges into the rubber sheet around each button, on both sides? That would make sort of an oring that would keep stuff from between panel and keypad on the outside, and keep moisture off the button contact on the inside. (Just like GM and others make intake manifold gaskets these days :-).) Would work best if the raised ring didn't cross any traces on the pcb, which would require using vias to connect each switch, but it would probably seal ok even without that.

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Regards, 
Carl Ijames
Reply to
Carl Ijames

The ridge is present, but on the front panel or bezel. That's because there are far more silicone keypads found on cordless phones, TV remotes, radios, toys, test equipment, etc, than on test equipment. All of these devices tend to have a custom molded front panel, while test equipment tends to get a flat aluminum plate. Putting the seal (ridge) on the keypad would work, but then they couldn't use the same silicone keypad on both plastic molded and flat aluminum front panels.

Actually, sealing the entire keypad is also a bad idea unless you can figure out a way to do a hermitic seal. Water will condense out of "thin air" inside the keypad anyway. If one can't have a totally sealed and pressurized keypad, the next best thing is one that drips accumulated water out the bottom. That's why I like to add solder mask on the back PCB under the entire keypad, so that any accumulated water, will not short the traces. However, the current fashion is to punch small holes in the silicone keypad that mate with pins in the front panel or back PCB to align the keypad so that the carbon buttons hit dead center on the contacts. These holes will cause problems if one wants to seal the keypad.

I once suspected that simply pushing the buttons will produced enough positive pressure to pump out some of this accumulated water, but never bothered to test or calculate it.

Incidentally, most of my experience is in marine radio, where such things are a very real problem.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

Our silicone thing will have little cylindrical posty things that drop into holes on the PCB, for alignment. Then the alignment is dominated by how the user pushes the button, dead center or off-center.

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Incidentally, Autocad has a decent online STEP file viewer

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but the measurement tool makes no sense to me.

The keys will be backlit, and there will be a hole in the PCB, in the middle of the contact pattern (see my pic) that will let the light through and ventilate the backside of the key.

Some of the keys are not backlit, so maybe I should add vent holes to those. The instrument will dissipate from roughly 15 to maybe 30 watts, so it will be nice and warm inside.

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John Larkin         Highland Technology, Inc 

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John Larkin

Just out of curiosity why would you use low quality keypad for a high quality instrument?

Cheers

Klaus

Reply to
Klaus Kragelund

We're not. The pic was just an example we have around. Melrose is making us a custom silicone button thing. Tek, Keysight, Fluke use similar buttons.

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John Larkin         Highland Technology, Inc 
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John Larkin

That should work. Where things go awry is when the keypad sandwich is assembled. If over-tightened, the silicone will compress and the flat areas of the keypad will migrate outwards. For the small keypad that you are using, it's probably not going to be a problem. However, better to be safe and put some extra alignment pins near the outside edge of the keypad.

Ugh. You're going to have all kinds of shadows from the center pylon along with variations in brightness caused by different wall thickness. I suggest dark buttons and clear (laser gouged) lettering instead.

I don't think the extra vent holes will be necessary. The heat from the instrument will prevent condensation.

However, you still may have a moisture problem if someone decides to clean the front of the instrument. I had this problem with marine radios that were sold to the USCG (US Coast Guard). The average fisherman cleans his radios exactly once, just before he sells the vessel. However, the USCG is very much into keeping things clean. For example, on the 44ft MLB (motor life boat): the deck and everything on it would be hosed down with fresh water. If the equipment was dirty, someone would clean it with some kind of household ammonia and alcohol cleaner. Both of these were problems. The wash down would drive water past any marginal seals and into the radio. The magic cleaner would dissolve off screening and keypad labels.

I wouldn't expect your customers to hose down your instrument. However, they might decide that clean the front panel. Done vigorously and too often, it might do some damage. I while back, I used some 409 cleaner to wipe the front of some instrument and had all the white paint rub off. Painted lettering is going to be a problem because the paint needs to be flexible to survive on the flexible silicone rubber surface. If it's too hard, it will flake off. If too soft, it will dissolve off with cleaners. I suggest you look into laser scribing the silicone keypad:

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

Compression is controlled by PEMs between the front pabel and the PCB. The silicone thing has features that make it somewhat compressible.

I think that will be OK. The silicone will be milky-white and should diffuse the light pretty well. There is no center pylon; the back side of the key will be flat. We're getting a couple of prototypes to evaluate, so we can iterate.

I could include cleaning guidelines in the manual.

The non-illuminated keys will look better with black lettering.

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John Larkin         Highland Technology, Inc 
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John Larkin

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