> 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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