Chris: Sorry, my ignorance is showing! I was assuming that an "air discharge" was just that; i.e., a test electrode to test electrode discharge in air some specific distance from the EUT. From your elaboration, I now see that "contact discharge" is a deliberate conductive connection before application of the discharge. And an "air discharge" is actually a "touching" discharge to some surface, like paint or the non-conductive membrane over a keypad, which may then penetrate the dielectric and discharge onto the conductive paths below.
Gary; don't forget that many industrial processes create ESD events, from chip handlers to conveyor belts to fuel transferring. Regards, Ed Ed Price [email protected] 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 >-----Original Message----- >From: Gary McInturff [mailto:[email protected]] >Sent: Monday, April 23, 2001 3:39 PM >To: 'Chris Maxwell'; 'EMC-PSTC Internet Forum' >Subject: RE: ESD generators max Contact discharge level > > > >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:[email protected]] >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 >> >> > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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