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4

My device uses a silicone keypad to detect a key press rather than a physical button.

After setting up, it works smoothly, even without applying much pressure to the keypad.

However, after a while ( say 2 months), you will need to apply much pressure on the key pad before a key is detected. It continues like this for a while, and then no keys can be detected again.

So we open, clean the PCB keypad traces with "methylated Spirit". And it works back as new. Sometime, we see black residue on the keypad PCB traces, which appears to come off the conductor of the silicone keypad. We wipe this off and everything is back to normal.

My question is how to avoid this problem.

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  • 14
    Your contacts really should be gold-plated. Tinned with solder or silver-plated will corrode. – brhans Jun 10 '17 at 16:30
  • I have seen other that are not gold plated, and looks silver....and it's does not act this way. – Paul A. Jun 10 '17 at 16:40
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    We used gold plating on office equipment keyboards for worldwide export, so in all sorts of humidities, and had no keyboard problems. You can weigh up the extra cost of the gold plating against the extra cost of replacing or repairing the device. It's hard to imagine that the former cost's anywhere near the latter. Spot on in first going for root cause analysis, though. – TonyM Jun 10 '17 at 17:09
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    I've seen very poor quality conductive silicon pads which readily leave parts of themselves on the contact surface and where all I had to do was to buy some good quality conductive pads and the problem completely disappeared. You have a lot of good advice focused on an improved board surface. But all of that would not have helped in the case I remember. It was the pads, themselves, and replacement with better quality cured the problem. – jonk Jun 10 '17 at 17:48
up vote 22 down vote accepted

Electricity and water is the issue. The tin in the solder plating will grow a crystalline structure and form an oxide that doesn't conduct very well. Spent many months in the 1980s solving this problem and the bottom line is use gold plate. Don't be cheap on this. The company I worked for at the time sued the supplier a lot of money for their incompetence and they were big in the industry at that time.

If you can't seal it (and clearly you can't because you can clean the contacts) then water will get in. It's inevitable.

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  • I see. First, we have some that we are sure water did not get to and it still acts bad. I am really open to go gold plated. But I need to sure that there is no other thing that can cause this problem. Like I said, we have some that are not gold plated and it worked without this issue. – Paul A. Jun 10 '17 at 16:53
  • Let me add that we did not use solder paste on the key pad PCB contacts. – Paul A. Jun 10 '17 at 16:58
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    The keypad tech we used back in the 80s was a glued and sealed plastic membrane but water got in because the keypads were made in a non humidity controlled environment. If you think that a rubber mat is going to stop moisture then you are deluding yourself. Each time you open it to clean it water gets in by way of air humidity. – Andy aka Jun 10 '17 at 18:20
  • Most silicones are also quite open for water vapor, so even though you think you sealed everything, the keypad will let water vapor through which might cause trouble later on (condensation, too high humidity etc.). – Arsenal Jun 12 '17 at 9:13
  • Comments regarding only use real gold are old news. Like I said, Process Controls have vastly improved uyemura.com/pcb-finishes_ENIG.htm and vendor selection is key. uyemura.com/pcb-finishes_ENIG.htm Note 2 versions are highly corrosion resistant.... GoBright® TWX-40 & KAT SP ENIG – Tony EE rocketscientist Jun 12 '17 at 18:22
up vote 13 down vote

If you can't afford decently thick* gold plating, I suggest you specify the PCB with printed carbon conductive ink. HASL or tin plate will not be reliable over the long term and what you have experienced is to be expected. Nickel would be better, but still not great.

You will find that conductive ink is standard in most consumer remote controls, and something like that is what is on the other side of the contact (a carbon-loaded elastomer pill).

If you can't find PCB makers willing to do that in your quantities, get the gold (it should always be supplied plated over a nickel barrier layer) and be done with it.

*ENIG (electroless nickel immersion gold) is not really to be strongly recommended for keypad use- too thin, only a few microns thick.

Hard (electrolytic) gold over nickel is literally the gold standard for contact surfaces. Unfortunately, the gold has a negative effect on solder connections (there are typically too few gold atoms in ENIG to seriously embrittle the joints for most applications) so it should be limited to areas that are not soldered or it must be removed later (as is detailed in the IPC J-STD) for high reliability applications.

J STD-001 Revision “F” now states: (note that the new wording/changes are highlighted below) 4.5.1 Gold Removal

Gold removal is performed to reduce the risk of failure associated with embrittled solder. Gold embrittlement is not a visually inspectable anomaly. In cases where analysis has determined there is a gold embrittlement condition, the gold embrittlement shall be considered a defect, see IPC-HDBK-001 or IPC-AJ-820 handbook for guidance. Except as noted above, gold shall be removed:

   a. From at least 95% of the surfaces to be soldered of the through-hole component leads with >2.54 μm [100 μin] gold thickness and all through-hole leads that will be hand soldered regardless of gold thickness.
   b. From 95% of all surfaces to be soldered of surface mount components regardless of gold thickness.
   c. From the surfaces to be soldered of solder terminals plated with >2.54 μm [100 μin] gold thickness and from all solder cup terminals, regardless of gold thickness.
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  • Obviously, I would have to go gold plated going forward as I can relate with most problems explained here. However, I have about 100 PCB boards with me with tin plated. Can I get the carbon conductive ink and do it myself? – Paul A. Jun 10 '17 at 17:30
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    (I fell silly for forgetting carbon ink in my comment above, good call SP and @TonyStewartEEsince75) We used carbon ink on a later product to that in comments above, again for worldwide export into offices and humidities. We shipped at least 20,000 machines. We had no problems with the keyboard at all and subjected it to wear tests, using solenoids to hit the keys over a million times as the spec'd life. Our test department was pretty gung-ho and this test rig was clicking its way through keyboards for what felt like months. – TonyM Jun 10 '17 at 18:45
  • If you have unpopulated boards I don't see any reason why you couldn't print the ink over the keypad areas, but if you have not done silk screening before and don't have the facilities to cure the ink (probably just air cure at some specified temperature/time) it might be more trouble than it is worth. – Spehro Pefhany Jun 10 '17 at 19:14
  • @SpehroPefhany obviously, I need to print the ink over the keypad areas ASAP and I am familiar with screen printing, as we use that for silk printing in our prototyping PCB lab. But are there any specifics to look out for, such as; ink thickness, air cure temperature etc. I looked over at digikey to buy some carbon ink, but I see digikey.com/product-detail/en/chemtronics/CW2000/CW2000-ND/… could this do it? – Paul A. Jun 12 '17 at 11:59
  • I would try to get something more like this and check withe the supplier first regarding your application. – Spehro Pefhany Jun 12 '17 at 13:17
up vote 2 down vote

Late Edition

Other new process are now available.

  • Nanofics technology uses a low pressure, dry plasma system to deposit fluoropolymer nanocoatings that provide permanent hydrophobicity and/or oleophobicity. The system is inherently “green,” and does not generate chemical waste; coatings are PFOA and PFOS-free. ( Used to make cellphones waterproof unlike every Asus and iOS product ( That I have owned and seen fail from moisture) that rust with moisture from lead-free corrosive no clean flux)

New Chemistry Deposits 4-8 µin

Uyemura has introduced a reduction-assisted immersion bath for board customers who demand an immersion gold deposit above the standard 1-2 µin on ENEPIG. Called TWX-40, this is a mixed reaction bath – an elite hybrid – that delivers both immersion and autocatalytic (electroless) modes of deposition.

TWX-40 is a proven alternative to other attempts to achieve heavier gold deposits on ENEPIG, (electrless Ni then electroless Palladium then immersion Gold) Cu > Ni > Pa > Au

It may come down to who has the better process controls.

Carbon Ink with short shelf life or ENIG from porosity and even distribution of paladium catalyst or C sense with finger noise .

*One of the biggest Process Improvements was a saturation gold density limit with monopolarity pulse electroless plating. They now use a specific burst profile with periods of reverse polarity pulses, repeated in profile for desired characteristics and is faster and cheaper than traditional ENIG or EP

This also provides finer gold deposits and higher density for less porosity.

In any case CPk or 3 sigma analysis on SNR ratio with accelerated humidity heat cycles are suggested. Min SNR of 10 worst case without skew meaning threshold under all conditions. Robotic fingers however do not replicate human touch with side swipe. e.g. Bosch have terrible low sensitivity touch sensors in some appliances.(due to finger capacitance variations and internal settings too high)

Another is a thin plastic film between metallized conductors on urethane membrane and the electronics only measure the capacitance change.

Anecdote

( reminds me of an auto car wash with digits for wash code in Toronto and the membrane buttons were gouged out with keys and pens because frantic users wanted to get their car washed without delays.)

Carbon ink works but softer and reliability depends on user excess abrasive pressure which can also be a reliability problem. My old keyfob and old car garage door opener used this and it is wearing out now.

Details

Electroless Nickel Immersion Gold (ENIG) is a type of surface plating used for PCB with a thin layer of immersion gold, which protects the nickel from oxidation using a Palladium catalyst while the Nickel is electroless plating over copper.

ENIG has several advantages over more conventional (and cheaper) surface platings such as HASL (solder), including excellent surface planarity , good oxidation resistance, and good usability for untreated contact surfaces such as membrane switches and contact points.

IPC Standard IPC-4552 covers the quality and other aspects of ENIG finish on printed circuit boards. IPC-7095 covers some "black pad" related features such as so called mud crack appearance and nickel protrusion spike

Ref

Other

The other requirement of membrane switches is to have at least 15mm air clearance to human finger or else a plastic insulation layer with 15kV breakdown protection against ESD..

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  • Capacitive sensing is a good idea. I haven’t thought this through but a microcontroller with a comparator with a capacitive sense mode (e.g. EFM32G) should be able to do it without any changes to the keypad, shouldn’t it? – Michael Jun 10 '17 at 19:34
  • Depends how your keys are addressed . Cortex uses 8 parallel Cap to ground inputs to tuned RC oscillators to detect switching. THat wont work if you use row column muxing. But if you Row-Column mux a 1MHz signal then you can measure voltage drop with a diode peak and decay or active row clamp and suitable hysteresis (1/3Vss tpy) in a Schmitt gate. ENIG plating is 1-2u" flash Au plating 2~3u" and good Au plating 20~30u" for gold. I think adhesion depends on surface roughness. <1u" too smooth may be a problem. – Tony EE rocketscientist Jun 10 '17 at 19:51
  • 2
    Uh.. Is it just me and my visual preferences, or is that big blob of old at the beginning unnecessary? – quetzalcoatl Jun 12 '17 at 7:59

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