Worksurface Requirement

What is the concensus of checking worksurfaces other than ESD benches and such? It is difficult to relay the information to a group when the worksurface is not an ESD bench per say when it comes to fixtures, standoffs, hotplates to help with thermal shock components, glass beds for inspection microscopes, and other types of “worksurfaces” that are not the typical set-up of an ESD workstation.

How would you “qualify” these non-standard surfaces since the STM does not apply? Can it be “explained away”?

Also, where can I find information as to why 1E9 ohms was chosen as the maximum RTG allowed instead of <1E11 ohms or even <1E10 ohms? Also, what is the additional risk we can see once we start increasing these limits?

ANSI/ESD S20.20-2021 shows an example of this worksurface requirement being “tailored” out of an ESD Control Plan, but it is hard to see how to excuse this change in the limit without seeing the technical rationale for why it was chosen in the first place.

mancho87, those are quite the questions but I can give you my perspective.

We treat any flat open surface as a potential work surface, as sooner or later someone will lay something on it. As such if we don’t want the surface to be used for product we label it as Non-ESD to indicate product or parts cannot be placed there. All actual work surfaces are tested for compliance as identified in S20.20-2021.

Start your qualification process with this question “Is this an ESD Control Item”? Items such as a microscope would not be. You of course don’t want it to introduce a static charge. But it would not be used to manage the build up of a charge. Footwear, flooring, wrist straps, smocks… these are the types of items you would introduce to manage that and thus must be qualified. Those non-control items would then need to have verification testing as per TR53.

The test limits you chose to use should correlate to the sensitivity of your product and components. What class of protection are trying to achieve? S20.20 has limits to comply with 100v HBM for example and components more sensitive than that would need additional control. So the risk, is all about your specific sensitivity needs… depending on that maybe there is no risk at all raising your limits. But this would be up to you to determine through your own testing.

Tailoring statements are basically a deviation from the standard. The how and why are up to you to define and the example(s) are not meant to give you details you can duplicate. But rather give you an example of what you should consider. Explaining the what and the why, along with any supporting data provides an auditor all the detail as to why you chose not to comply to the standard. In the workstation example it might have been that the station did not meet the expected work station standard, but the only parts used at the station were shown to not need any additional level of ESD protection. Tailoring statements are a great benefit… but you must consider what you are allowing and what is the risk of doing so. In the event of a workstation… how do you control the “what” is used there? Is the risk worth the cost it would be to modify the station to comply?

Hope this helps you out.

C. Smith