Hi Don, Each situation should be evaluated on its own merits. I am going to list some general rules of thumb below, but there may be exceptions. I have done lots of ESD current measurements in different configurations and base my opinions on these measurments and personal experience with equipment.
1) Any piece of metal will absorb high frequency components of ESD from a wire that it touches, the bigger the metal, the lower the frequency of components it will remove from the wire. The metal need not be grounded, its free space capacitance is enough. I have done an experiment that shows this hundreds of times in my seminars. So if a cable touches its shield to a metal chassis on the way to a circuit board, the chassis will remove most of the high frequency components of the ESD. 2) I have observed cases where grounding a PWB to the chassis at many points solved ESD problems and removing connections caused ESD problems, but never the other way around although there may be special cases where it could happen. This is likely related to "1)" above. If you have doubt about a particular configuration, don't guess, measure and see what really happens. If you are not sure how to do this, call or email me and I will help you plan an experiment to measure your case over the phone or email. 3) If you ESD to the outside of a chassis, the currents will stay on the outside of the chassis because of skin effect. Holes and seams can let the current in, but the high frequency content will already be reduced by "1)" above. Unless you have gaping holes, multiple connections to the chassis from the circuit board are not usually a problem (remember this can be measured). 4) An interesting thought: even 1 pf of capacitance can easily couple a few amps of ESD current under many conditions so it is almost impossible to isolate the PWB from the chassis anyway! Doug "UMBDENSTOCK, DON" wrote: > > Hello Group, > > We are having a debate concerning the best practice for grounding of a > printed circuit board containing digital logic. These boards are > multi-layer with a ground plane and a power plane. > > One school of thought is to tie the ground plane to chassis ground in many > locations, thus reducing the impedance. > > Another school of thought says to control the point(s) that is (are) tied to > ground or risk upsetting of sensitive circuits with an ESD or other immunity > event. The concept is that an ESD event may be decoupled to chassis at the > I/O ground plane with the use of appropriate circuit elements to control > impedances. Now consider the chassis to be steel, and the digital ground > plane to be copper. If the digital ground plane is stitched to chassis in > several locations, it appears that a lower impedance path (copper vs steel) > will encourage the ESD to travel across the ground plane. If the ESD > travels across the digital ground plane, there appears to be a good chance > of upsetting sensitive circuits. So the thought might be to tie only one > point of digital ground to chassis ground, thereby not providing a path for > any immunity event to flow across this ground plane. > > The rest of the above concept is to use moats to segregate key circuits -- > digital, I/O, analog, switch-mode power supplies. Again, some say to keep > the ground plane in tact to provide the lowest impedance reference possible, > so isolation is provided by carving up the power plane. The alternate > approach is to "carve all the way through", i.e., if you have a moat around > a particular circuit, if you are going to isolate, do it for all planes > (stack, do not overlap). This latter approach, however, carves up the > ground plane which would appear to increase the impedance of the overall > ground reference. The argument is that carving up the ground plane is > justified by eliminating the coupling of "dirty ground" to other circuits in > an overlap situation. > > I would like to hear what you do for pcb grounding and why you do it. > > Don Umbdenstock > Sensormatic > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). -- ----------------------------------------------------------- ___ _ Doug Smith \ / ) P.O. Box 1457 ========= Los Gatos, CA 95031-1457 _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: [email protected] \ _ / ] \ _ / Website: http://www.dsmith.org ----------------------------------------------------------- --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
