Kick-starting and LED ??

)

w that orders are coming in at a fast enough pace to justify the expense of a "real" keypad.

en at 8 mA on a 5-volt supply. The other 96% were fine, and would reliabl y illuminate even at 3 to 4 mA. (The keypad also has lots of red LED's, bu t no problems with any of them.)

o illuminate. Whereupon, removing power and reconfiguring for 5 mA, ALL wo rked fine, and no matter what we did, we could not get them to fail. (We'd ruled out anything mechanical prior.)

and mechanical crimp-style connector" on the plastic ribbon tail. That som ehow the one-time jump to 10 mA was enough to permanent "weld" a reliably c onductive trace of lower resistance, and now those keypads are "fixed".

ormal?

The "welding" sounds a bit sketchy to me. First, if there was no connectio n, how was it passing current? If there was a connection, but poor, why di dn't the LED light when 8 mA was being passed? But I'm unclear on what the circuit is exactly.

Is this a simple row/column arrangement of contacts that have no electronic s other than the LED? Is there an alternate path for the current other tha n through the LED? How did you pass more or less current given a constant

5 volts on the supply? Did you run the voltage above 5 volts?

If there is no alternate path around the LED and you were able to pass curr ent, the LED should light unless it is defective. I would not trust a defe ctive part that "healed" itself, but how critical are these devices? I'd a t least burn them in for a few days just to prevent a nuisance return even if the devices are not critical.

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  Rick C. 

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Maybe a bit of stray carbon ink that burned off at 10 mA?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

ion, how was it passing current? If there was a connection, but poor, why didn't the LED light when 8 mA was being passed? But I'm unclear on what t he circuit is exactly.

I'll try to describe the circuit:

5-Volt rail (regulated, never changes) This goes to a resistor on the PCB. Then to a header connector (.100 pins) Then to the ribbon on the keypad tail. Then, to whatever is inside the keypad (just traces and the LED) Then, a shared trace on the return back to ground (shared with the red LED' s)

We adjusted the current by swapping out the resistors on the PCB.

I should also note that upon further investigation, some in the 96% pass pi le would actually illuminate down to 3 mA, but we don't drive them that low .

It does seem that the Chinese Green LED spec is all over the place. But what is curious is that once you get them lit (via more current) they s tart to behave normally. And by "normally", I mean here that a 300-ohm res istor on a 5-volt rail seems to be a happy safe medium for them.

We calculated the current as 5V minus the drop across the dedicated resisto r, divided by the resistor value. (The board was originally designed with

470-ohm resistor, which does work, 96% of the time.)

About now I should state: We were forced to select the dropping resistor v alue "in the dark", so to speak, as the Chinese vendor doesn't give the spe c for the green LED's they're using. (It's possible they're not even all t he same LED's??!)

I'm not so much worried about the forward current through the LED, so long as it is in the range of what's might be considered reasonable (i.e., < 10 mA), but why the "kickstart" thing. That's a mystery right now....

And - we can't really generate a table of R-values vs. illumination because we only had a handful fail and by the time we really got prepped for a dee p dive on the problem, all of them were working! I can say that before the higher current was applied, the green LED's WERE CONDUCTING (evidenced by the R-drop), but there was no visible indication whatsoever of the LED bein g "on". Under normal lighting at that point, of course. But I don't think they were even barely on. It's almost like a hysteresis behavior. But in an LED?

I had them affected units quarantined, and they are burning in now. Three days so far. No problems.

I got a feeling these membrane keypad LED's are assembled like a sandwich. Maybe we burned some "intra-layer glue off", or something??

I should also state my Senior Engineer on this project is dead set against membrane keypads with that "silver printed ink" stuff.

He's probably right. (He usually is.)

But margins are thin on this one, and unless ordered in high-enough quantities, can't make the BOM work. Although now, we're getting past that point and I'm likely to start shopping around for better keypads.

And I don't want to knock the JRPANEL stuff. For what it is, (and overall, it ain't bad!), it does fulfill a price-point.

We did put one of the failing parts under the microscope. Didn't see anything obvious (of course, it's all translucent white, so a bi t hard to tell). Also, that visual inspection test is possibly compromised because we ordered these keypads with the 3M300LSE adhesive, and it was di fficult to remove it from the enclosure door.

This JRPANEL outfit is a little squirrely with the file requirements. Basically, they want a life-size PDF.

That's right: PDF

Example at link below. And having just looked at it, the web page is MUCH BETTER than it was back when we placed the order!

Link:

f

)

w that orders are coming in at a fast enough pace to justify the expense of a "real" keypad.

en at 8 mA on a 5-volt supply. The other 96% were fine, and would reliabl y illuminate even at 3 to 4 mA. (The keypad also has lots of red LED's, bu t no problems with any of them.)

o illuminate. Whereupon, removing power and reconfiguring for 5 mA, ALL wo rked fine, and no matter what we did, we could not get them to fail. (We'd ruled out anything mechanical prior.)

and mechanical crimp-style connector" on the plastic ribbon tail. That som ehow the one-time jump to 10 mA was enough to permanent "weld" a reliably c onductive trace of lower resistance, and now those keypads are "fixed".

ormal?

Hmm well as a crazy idea maybe tin whiskers are shorting the green led's? And 10 mA is enough to blow it out? This would be easy enough to check. Measure voltage across failed led.

George h.

bit hard to tell). Also, that visual inspection test is possibly compromis ed because we ordered these keypads with the 3M300LSE adhesive, and it was difficult to remove it from the enclosure door.

k when we placed the order!

pdf

Ok, so each LED has a separate lead and wired to a common. You still don't explain where you measure the 5 volts and what you are doing to change the current. I would assume you use a resistor from a 5 volt supply and chang e the resistor to get different currents?

If that is the case, what is the voltage on the LED when it didn't work? T hat will tell you what is wrong.

There may well be a short around the LED. It could be some of the ink maki ng the traces if they are printed. You won't know if you don't have both t he voltage and the current.

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  Rick C. 

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y.)

now that orders are coming in at a fast enough pace to justify the expense of a "real" keypad.

even at 8 mA on a 5-volt supply. The other 96% were fine, and would relia bly illuminate even at 3 to 4 mA. (The keypad also has lots of red LED's, but no problems with any of them.)

to illuminate. Whereupon, removing power and reconfiguring for 5 mA, ALL worked fine, and no matter what we did, we could not get them to fail. (We 'd ruled out anything mechanical prior.)

k and mechanical crimp-style connector" on the plastic ribbon tail. That s omehow the one-time jump to 10 mA was enough to permanent "weld" a reliably conductive trace of lower resistance, and now those keypads are "fixed".

normal?

My understanding is tin whiskers won't even take 1 mA to blow out. They ar e so tiny you can't see them without a microscope, so they blow out on very low currents. Not likely here though as they usually take time to form.

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  Rick C. 

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Rick C wrote in news: snipped-for-privacy@googlegroups.com:

"blow out" is ambiguous. Now, you have TWO whiskers, one on each node, pointing toward each other. I am pretty sure that can lead to some signal issues on some circuits... enough so that NASA and the gov boys wanted no part of having to characterize that realm. We remain exempt for that reason. No COTS in space rated for external excursions.

63/37 should STILL be the rule. Metallic form lead does NOT glean into the environment.

If it did, the lead reading at water tables near gun shooting ranges would show higher, and they do not!

RoHS was the most stupid thing the euro boys ever did. It cost the entire industry billions and it is ongoing.

They should have gone after the mercury and Cadmium and such, but NOT metallic form lead.

it sounds more like some sort of whisker (or other fine concentrated current path) that was destoyed by the 10mA current

How did the voltage drop of the bad LEDs look at 1mA?

--
  Jasen.

it's a mature technology. my first two computers had that for their keyboard, they worked ok but eventually the plastic failed with stress cracks, then several PCs with mechanical switches, then several with that silver ink stuff deep inside. but I'm back with mechanical switches now.

--
  Jasen.

Oh they have far more stupid things, both done & planned. This continent doesn't know what it's doing half the time.

I had a Microsoft keyboard with what appeared to be the silver printed ink. One of the traces corroded and it quit working. lol It took some four years or so, but still, the trace corroded???

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  Rick C. 

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On Wednesday, January 15, 2020 at 10:30:16 PM UTC-8, Rick C wrote: now.

Or, maybe an impurity in the plastic film didn't wet properly, and the trace wasn't quite intact to begin with... but yes, any metal with current can corrode, if you have a moisture film a trickle of current will rust stainless steel. Silver or nickel aren't immune either (aIthough if you WANT to etch silver, a little chlorate ion helps).

I don't know if we had a chance to measure that. I've been out sick the past two days with a cold, and my tomorrow is out of the office. So, it'll be Monday before I can post the readings we did tak e.

From memory, the currents looked about right, but no illumination. This would have been in the neighborhood of 3.5 mA upwards to 6 mA. But, after hitting them with higher current, they illuminated and would the reafter work at 4 to 5 mA with no problems.

My money is on something strange going on with the physical construction of the layers in the membrane keypad. The traces themselves are far enough a part I don't see how that could be an issue. And as you may know, keypads are layed out like single-side PCB's, with no options for jumpers. (At lea st, not through this vendor.)

of the office. So, it'll be Monday before I can post the readings we did t ake.

hereafter work at 4 to 5 mA with no problems.

of the layers in the membrane keypad. The traces themselves are far enough apart I don't see how that could be an issue. And as you may know, keypad s are layed out like single-side PCB's, with no options for jumpers. (At l east, not through this vendor.)

This reminds me of a batch of boards at Microdynre. The product has a ten y ear old design that had been rock stable. All of a sudden the Intersil volt meter ICs weren't working properly. Someone had scratched one of the films to make the blank boards You couldn't see the copper whisker without a micr oscope. The was an even smaller gap in the whisker which didn't show up wit h an ohm meter.VCC was 5VDC, and it would arc over the gap, starting around 3.65VDC One of the traces was the input pin to the meter. The RF caused it to display random numbers. Purchasing wanted to scrap the whole run and wa it for new blanks, but a quick pass with a new Exacto blade would remove th e entire whisker and allowed us to ship the radios on time. A new set of fi lms for the board house was the permanent fix.

of the office. So, it'll be Monday before I can post the readings we did t ake.

hereafter work at 4 to 5 mA with no problems.

of the layers in the membrane keypad. The traces themselves are far enough apart I don't see how that could be an issue. And as you may know, keypad s are layed out like single-side PCB's, with no options for jumpers. (At l east, not through this vendor.)

Well that does sound like a short somewhere. I'm still guessing some sort of metalic whisker.

George H.

t of the office. So, it'll be Monday before I can post the readings we did take.

thereafter work at 4 to 5 mA with no problems.

n of the layers in the membrane keypad. The traces themselves are far enou gh apart I don't see how that could be an issue. And as you may know, keyp ads are layed out like single-side PCB's, with no options for jumpers. (At least, not through this vendor.)

If this is a printed ink membrane my money is on stray ink. Not sure how a metallic whisker would be formed. Tin whiskers grow and take time to form .

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  Rick C. 

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Yep, all it takes is a bit of fiber dangling from the (tip/brush/nozzle). I'm guessing that it didn't show up in visual inspection because it was very thin. Had that happen on printed circuits, too (etched not-quite-all of the way through).

Wet items are complex at all scales (from wet chemistry up to hillsides about to avalanche).