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December 2006

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Subject:
From:
Carl VanWormer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Carl VanWormer <[log in to unmask]>
Date:
Wed, 13 Dec 2006 10:59:41 -0800
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I have two membrane keypads that have shown intermittent "short circuit" failures.  I am looking for suggestions to identify and eliminate the problem.  Both units were recently returned to our manufacturing facility with complaints of non-human initiated activity, typically during the night.  Analysis of one product showed that a key was being pressed "all the time" but when the keypad was removed, the contact measured "open circuit".  Further analysis showed a "leakage path" to an unrelated (LED drive) circuit on the keypad.   I created a test connector that shorted all the LED terminals together and shorted all the keypad terminals together. The resultant two terminal connection should never show conduction between the two terminals, but can detect any short between the two (isolated) sections of the keypad array.  I found a 1.5k Ohm "short", and figured I had the problem identified.  The next morning, the reading was about 15 Meg Ohms.  After wiggling, banging, and hooking the keypad up to the original circuit, the problem returned.  I found that the problem could come and go with applied voltage bias.  

I set up a scope to monitor the system with a 5V supply and a 20k series resistor.  After a while, a "short" would cause a voltage to drop across the normally open nodes giving less than 0.5V. When the 20k resistor was shunted with 2k, the "short" disappeared and the measured voltage returned to 5V.  When the 2k was removed, the circuit would stay "open" for 1 to 10 seconds, then short again. I think this behavior may be typical of dendrite growth. We tested a second keypad that had reports of similar problems but could not repeat the problem in our initial tests. I moved this second keypad into a large plastic bag, placed a cup of water inside the bag, then closed the bag around the test leads. The next day, this second unit was failing in a similar manner as the first, indicating a short between the normally open traces. 

Examination of the membrane circuit layout shows the two sections of my system are closest at a crossover trace, insulated by a dielectric layer.  I suspect either a localized contamination in the area of the crossover trace, or possibly contamination in the actual dielectric substance used as the insulator at the crossover point. When there is enough moisture in the system, maybe dendrites can grow through the dielectric and short out the conductors at the crossover point.  I think the conductors are a screened on silver base material.

Is there an easy way to find the root cause of this problem?

Your thoughts and comments (and war stories)  would be appreciated.

Thanks,
Carl


Carl Van Wormer
Cipher Systems
1800 NW 169th Place, Suite B-100
Beaverton, OR  97006
Phone (503)-617-7447 extension 20    




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