We are looking to adopt the S20.20.2021 standard. Currently, we measure shelf/cart surfaces to have a point to ground (RTG) resistance between 10^6 to 10^11 ohms. This falls in the dissipative range. Whereas S20.20.2021 table 4 requires RTG < 10^9 ohms, which overlaps the conductive and partially dissipative regions.
Some of our shelves and conductive trays use antistatic bubble wrap as a liner. This passes our limits but fail the < 10^9 ohms limit in table 4.
Typical situations for these shelves include placement and removal of ESDS items by grounded personnel. The trays containing ESDS items may also be transported by a cart from one EPA to another. The cart would travel in an ESD cart covering and be grounded once in an EPA.
What factors, information, measurements, etc., should I be considering to assess whether or not our current test requirement is suitable for us?
The dissipative range you mention applies to packaging, not worksurfaces, shelves or carts. I recommend you follow the ANSI/ESD S20.20 upper limit of 10^9 ohms as it gives you a faster discharge time than 10^11 ohms. It is also recommended in footnote 9 that “If there is a concern for CDM failures, then a lower limit of 1.0 x 10^6 ohms for point to point and point to groundable point should be considered.”
I would also recommend you not use antistatic bubble wrap as a shelf cover as it usually is an insulative material with a coated antistatic and dissipative layer that wears off over time (typically a few months). If you are putting you ESDS items directly on the shelves, I would recommend you use a more inherently dissipative surface like an ESD mat. They may be more expensive, but you will not need to replace them that often.
If you only put your ESDS items on a tray or into a tote before putting them on the shelf, then you might consider putting the tray or tote directly on the shelves without any additional liner
You question on using an ESD cart with the ESDS items in trays going from one EPA to another is probably okay as long as there is no chance of someone touching the ESDS items in transit. You would need to make sure the cart is grounded in both EPAs before handing the ESDS items either by contact with an ESD floor or by grounding the cart with a grounding cord
Thank you Andy, we are in the process of eliminating the antistatic bubble wrap for this purpose.
I do have another that came to mind when evaluating different materials with different resistivity:
During an ESD event of an ESDS item, I understand that a slower rate of discharge reduces the likelihood of component damage. In the case of an item on an ESD mat worksurface, the mat’s relatively high resistance would provide a slow discharge path to ground. The S20.20 limit of <1.0x10^9Ω suggests you could have a surface with near zero resistance to ground. Is that advisable? Is there such a thing as too low of a resistance for such surfaces (benches, carts, shelves, etc.)?
You ask a very good question. If you look at footnote 9 of ANSI/ESD S20.20, it states “Due to a wide variety of applications for worksurfaces, specific requirements that could be broadly applied are difficult to determine. If there is a concern for CDM failures, then a lower limit of 1.0 x 106 ohms for point to point and point to groundable point should be considered.”
There are some instances like a cleanroom environment where using an ESD mat is not allowed due to cleanliness requirements. ANSI/ESD S20.20 could not make a lower limit mandatory because of cases like these. However it does recommend you consider the 10^6 lower limit if CDM failures are a concern