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Question Hello,
   We had our DSCC audit last week. They did not like that out wire rack shelves (which are lined with ESD mat) were not grounded. These shelves are for placing / storing parts, in aluminum trays, during the manufacturing process. Do you see any problem with this? Could you send me a statement that we could show DSCC? I would really appreciate the help. Thank you, Jeb Evans, Micropac Industries, Garland, Tx.
Answer

The ESD shelving should be grounded to meet the recommended technical range of Table 1 of ANSI/ESD S20.20 for Shelving of < 1 x 10^9 ohms. You cannot get a RTG of < 1 x 10^9 ohms if the ESD shelf matting is not grounded. Per ANSI/ESD S20.20 Fundamental Principle “All conductors in the environment, including personnel, must be bonded or electrically connected and attached to a known ground”.

You could, in your ESD control plan, state that the shelving is not part of the program and not part of the ESD protected area. This would then need you to store the ESD sensitive items there in shielded ESD packaging such as closed Metal-In Bags or Tote Boxes with Lids.

Storing ESD sensitive items in very conductive containers [your aluminum trays] can be done, but is a concern if there is Charged Device Model possible failures. Per ESD Protected Workstations ESD-ADV53.1 Electrical Requirements, “Surfaces of shelves and drawers intended to be used for unprotected ESD sensitive devices - Resistance: Between 1 x 106 ohms and 1 x 109 ohms.” Per ESD Handbook TR20.20 section 5.3.1.7 Electrical Considerations “[RTG is] The most important functional consideration for worksurfaces. This establishes the resistance of the primary path to ground for items, placed on the surface. When worksurface materials are being selected, consideration should be given to possible Charged Device Model (CDM) damage to ESD sensitive products. If CDM damage is a concern then setting a lower resistance limit for the worksurface should be considered. Typically, the lower limit for these types of worksurfaces is 1 X10^6 ohms.”

 
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