Hi Chris,
I agree with your whole heartedly. If it weren't for people there
wouldn't be an ESD problem. We are wither walking around charging others
stuff like furniture or worse yet we charge ourselves and the go around
touching stuff. For the most part I know of nobody that is able to hold a
charge until they contact a surface and then dump it to ground at will. Even
though somewhat less repeatable I think the test should attempt to model the
discharge mode - Through air as people approach the equipment.
I also have a hunch, and I have no data nor have I studied real
hard, but I believe that if the guns are designed be more and more alike, a
great deal of the variability goes out of the test. The test operator comes
next, but they can be trained for reasonable repeatability when the are
taught some of the issues.
Even some of that gets leveled out if the test is run carefully and
uses multiple test levels, say every 2k or even at 1K increments.
Gary
-----Original Message-----
From: Chris Maxwell [mailto:chris.maxw...@gnnettest.com]
Sent: Monday, April 23, 2001 11:36 AM
To: 'EMC-PSTC Internet Forum'
Subject: FW: ESD generators max Contact discharge level
> Hi Ed,
>
> I fully agree that membrane keypads are one of the most common places
> where a finger could discharge to the instrument.
>
> However, I disagree that contact discharge is the appropriate test method
> for these surfaces. Current state of the art in test instruments and
> methods dictates air discharge testing of such surfaces. Just for
> clarity's sake, when I say air discharge testing, I mean approaching the
> device under test with a blunt, charged ESD gun tip. If the device under
> test has a weak insulator, an air discharge will occur.
>
> The IEC standards use contact discharge testing as the "preferred" method
> because it is more repeatable than air discharge testing. The IEC
> standards recognize air discharge testing on surfaces that won't allow a
> contact discharge.
>
> I could write a book explaining why air discharge testing is the closest
> simulation to real life (unless you're in a vacuum) and another book about
> why contact discharge is used as a repeatable model for air discharges
> but I'll spare everyone the details. I could explain more fully if anyone
> is interested.
>
> The manual for my ESD gun (Keytek MiniZap, which is a compliance grade
> instrument) recommends against contact discharges to insulated surfaces as
> it has the potential to damage the high voltage relay in the product. I
> can't speak for other guns.
> .
> For insulated membrane switches, the IEC standards and the equipment that
> I have dictates that I use air discharge testing.
>
> Until a better test method comes along, I stand by my original answer to
> Dan's question.
>
> Best regards,
>
> Chris
>
> P.S. Being forever curious, If anyone knows of an ESD simulator or test
> method that does perform contact discharges to insulated surfaces, I'd
> love to hear about it. I'm sure that it would be of interest to the group
> as well.
>
>
> ************************ Ed's email attached *****************************
>
>
> Chris:
>
> I don't understand why a membrane switchpad would not be tested for
> contact
> discharge. It think that this is the absolutely most likely place where a
> finger, attached to a charged human body, might be applied to the EUT. It
> seems to me that you would want to be testing the dielectric strength of
> the
> insulation over the keypad conductive traces.
>
> The issue of whether a triggering of the testing gun, without a completed
> discharge current, would damage the gun, isn't relevant to the need for
> the
> test. Dumping 15 or 20 kV to a probe tip, while not exactly trivial, still
> shouldn't be critically sensitive to load conditions.
>
> Regards,
>
> Ed
>
>
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