ESD Systems - Questions And Answers

You may recall receiving a query from us 1-2 months ago regarding matting and RCD's? Many thanks for your reply. For us, it laid the issue to rest.I have no intention of making a habit of this but.....just another little query...lastely. Here is some background first. I am involved with our ESD group, writting up ESD bench testing procedures. The bench matting we are currently using is conductive in nature, its resistance being less than 1M ohm. I have recently orderedand received ESD S4.1 which is a bench testing standard. ESD S4.1 does not detail any methods for testing benches <1M ohm but they don't reallysay why. My questions are: Why do they not provide test methods for <1M ohm resistance matting materials? Does ESD bench matting material need to have a resistance greater than 1M ohm in order to provide the required ESD dissapation rate, or is it an electrical (Mains) safety issue? In other words can we use our cheap conductive bench matting (with RCD's) or do we need to replace it with the much more expensive dissapative matting? -Anonymous

The ESD S4.1 can be used to measure materials with surface resistance less than 1 megohm. The standard states to set the megohmeter to 10 volts unless the surface resistance reading is above 1x10^6 ohms, then change setting to 100 volts. You can use materials that are below 1 megohm, just be concerned for the safety of the operators if the RTG falls much below 100 kilohms. The ESD S4.1 recommends: Due to the wide variety of applications for worksurfaces, specific requirements that could be broadly applied are difficult to determine. However, the following set of guidelines can be used as a starting point for establishing local requirements on the performance of worksurfaces. Resistance-to-groundable point 1x10^6 to 1x10^9 ohms Resistance from point-to-point 1 megohm. These guidelines represent a range of performance that has generally proven to provide protection in the manufacturing environment. They also are loosely based on models for the effects of ESD on devices. It should be noted that worksurfaces with point-to-point resistances below 1 megohm may be a hazard at a hot workstation.

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