Ed,
Desirability may be a moot point. Whether you want to test the
membrane keypad as a contact discharge or not the test equipment may not let
you. The Haefly-Trench ESD gun (PESD 1600?) for one checks continuity
through the tip before it enables a contact discharge. If you pull the
trigger and there is no continuity it displays a message to that effect and
waits to be reset (trigger release).
To the second point I agree with Chris that contact discharge is
only valid for conductive surfaces. The standard prefers contact discharge
WHERE APPLICABLE (emphasis mine) because it is a more repeatable test. It
doesn't have the variables of approach speed and such that air discharge
does. It does need a conductive surface to be valid. Your example of the
membrane being a must test point due to real world considerations is well
taken; but the real world event it sees is an air discharge (which is not a
defining criteria, the defining criteria is if you have a conductive surface
do a contact discharge, even if the real world threat is an air discharge)
and do to its physical characteristics (non-conductive) it is tested with an
air discharge.
This is based on definitions and test procedures for IEC 1000-4-2
and EN 61000-4-2. there may be different requirements in other ESD test
standards. There is at least one commercially available ESD gun that will
only allow you test in the manner I describe.
Paul McCoy
-----Original Message-----
From: Price, Ed [mailto:ed.pr...@cubic.com]
Sent: Monday, April 23, 2001 9:47 AM
To: emc-p...@ieee.org
Subject: RE: ESD generators max Contact discharge level
>-----Original Message-----
>From: Chris Maxwell [mailto:chris.maxw...@gnnettest.com]
>Sent: Monday, April 23, 2001 4:45 AM
>To: 'Dan Kinney (A)'; Douglas C. Smith; Terry Meck
>Cc: emc-p...@ieee.org
>Subject: RE: ESD generators max Contact discharge level
>
>
>
>Hi Dan,
>
>Air discharges are made anywhere on the product where an
>insulator covers
>possibly sensitive features.
>
>Membrane keypads are a great example. The keypads contain traces and
>circuitry that may be sensitive to ESD. However, since the
>outermost layer
>is an insulator, making a contact discharge would not be a
>real world test.
>I believe that it may also damage the contact discharge relay
>in certain ESD
>generators. I can explain that in more detail if you would
>like. In my
>experience, membrane keypads are always tested with air discharge.
>
SNIP
>
>Chris Maxwell
>Design Engineer
>NetTest
>6 Rhoads Drive, Building 4
>Utica,NY 13502
>email: chris.maxw...@gnnettest.com
>phone: 315-266-5128
>fax: 315-797-8024
>
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
Ed Price
ed.pr...@cubic.com
Electromagnetic Compatibility Lab
Cubic Defense Systems
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (Fax)
Military & Avionics EMC Services Is Our Specialty
Shake-Bake-Shock - Metrology - Reliability Analysis
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