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Is Charge Decay important for ESD Mats? Gene, Chino, CA
Worksurfaces should charge to less than 200 volts per ANSI/ESD-S20.20 which references ESD-STM4.2, ESD Protective Worksurfaces, Charge Dissipation Characteristics 1.1 Purpose
The purpose of this standard is to aid in determining the ability of ESD protective worksurfaces to dissipate charge from a conductive test object placed on them. This ability may not be revealed through standard resistance measurements as outlined in S18.104.22.168 Scope. This standard provides a test method that measures the charge dissipation characteristics of worksurfaces. To accomplish this, a conductive test object is charged, placed on the worksurface under test, then removed. The resultant charge on the test object is an indicator of the ability of the tested worksurface to dissipate charge from the test object placed on it. This is only applicable however for the test object specified within this standard. The standard is designed for use in a laboratory environment for qualification, evaluation or acceptance of worksurfaces and not for periodic testing. Note:The FTMS 101c method 4046 is a test method and doesn’t give a constraint for charge decay time for mats. (at least I couldn’t find any) Also, It can be said that the method 4046 test does not replicate real world events that is, it demonstrates only a material’s propensity to dissipate an induced charge when grounded in accordance with the specified test conditions. The correlation of material resistivity (volume or surface) to other parameters such as static decay time may be inconsistent for materials of complex construction. For packaging, EIA-541 states that material acceptance is for average decay times less than 2.0 seconds.
For what it is worth, FTMS 101c method 4046 is not an appropriate test for any material that has an exposed or buried conductive layer. It was designed for homogeneous materials only and specifically sheet films - like pink poly bags! This is why there is no FTMS 101c - 4046 required for static shielding films or bags. If you look at Mil 81705, you will see the FTMS 101c method 4046 required for dissipative / anti-static / 6 mil pink stuff and not for any of the other conductive or shielding materials. It ain’t appropriate.By the way, want to know where the 2.0 seconds came from? Take two pieces of plain plastic film (make it a good insulator), sandwich a conductive material between them and run charge decay. It will be very fast. All it measures in this instance is the most conductive layer.
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