Hello, Not sure I like the case to ground layer connection unless the ground layer is very well bonded to mains earth. If it isn't, then you're going to get currents flowing and voltages flying in your ground.
When equipment is housed in a metallic enclosure this itself can be used to guide the ESD current around the internal circuitry, provided that it is properly constructed!. A poorly-built enclosure can actually enhance ESD coupling to the circuit within. Apertures or seams in the enclosure will act as high-impedance barriers to the current and transient fields will occur around them, so they must be minimised. All metalic covers and panels must be bonded together with a low impedance connection (<2.5 milli-ohms at DC) in at least two places; long panel-to-panel "bonding" wires must be avoided since they radiate high fields during an ESD event. I/O cables and internal wiring may provide low-imp. paths for the current, in the same way as they are routes into and out of the equipment for common-mode RF interferance. The best way to eliminate susceptibility of internal harnesses and cables is not to have any, through economical design of the board interconnections. External cables must have their shields WELL decoupled to the ground structure (i.e. 360 degree bonding of cable screens to connector backshell and no pigtails!. Insulated enclosures make the control of ESD currents harder to achieve, and a well designed and low inductance circuit ground is essential. But, if the enclosure can be to designed to have no apertures which provide air gap paths to the interior then no direct discharge will be able to occur, provided the material's dielectric strength is high enough. ESD waveforms are usually in the sub-nanosecond region. 1. Keep all external interfaces physically near each other 2. filter all interfaces to ground at their point of entry 3. if this is not possible, isolate susceptible interfaces with common-mode ferrite choke or opto-couplers 4. use screened cable with the screen connected directly to gnd 5. screen pcbs from exposed metalwork or external discharge points with extra internally grounded plates * breath * !!! For reduced ESD susceptibility, circuit ground needs to remain stable during an ESD event. A low inductance ground network is essential, but this must also be coupled (by caps. or directly) to a master referance ground structure. Kind regards, Darren Logan BSc(Hons) Development Engineer ------------------------------------------------------- Michell Instruments Ltd tel: +44(0)1223 434 854 Nuffield Close fax: +44(0)1223 434 895 Cambridge www.michell-instruments.com CB4 1SS mailto:[EMAIL PROTECTED] ISO9002 registered: Q6284 * Member BCAS UKAS accredited: 0179 * Investor in People ---------------The dew point specialists-------------- This communication contains information which is confidential and may also be privileged. It is for the exclusive use of the intended recipient(s). If you are not the intended recipient(s), please note that any distribution, copying or use of this communication or the information in it is strictly prohibited. If you have received this communication in error, please notify the sender immediately and then destroy any copies of it. -----Original Message----- Sent: 24 July 2003 13:39 To: Multiple recipients of list CHIPDIR-L On Thu, Jul 24, 2003 at 04:10:39AM -0800, Elbert Smit wrote: > > Where abouts are you applying the ESD? (case, window etc.?) > > Metal part of the housing, no earthing available. Are all metallic parts (of the housing and other parts) connected together? If not you will have some sparks at overlapping positions and more problems... >- The metal part is connected to the ground layer of the PCB Is there one connection or more than one? You can have currents through the PCB layer that create high voltages in other wires... >- The PCB contains a transzorb like component (one direction) Is it fast enough? Maybe 10�s are too slow. Matthias -- Author: Matthias Weingart INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Author: Darren Logan INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
