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Question Currently, we put our PCB's in a static dissipative cassette holder and then transport them on a mobile cart.  We may go to using a conveyor / flow rack system which means pushing the cassettes over rollers using a ball bearing system (the cassettes would move about 10 feet).  Are there ESD issues to be concerned with, and if so, do we need to use a conductive roller / rack system that is groundable?  Derrick S.

Table 3 of ANSI/ESD S20.20-2007 has the required limit for Mobile Equipment (working surfaces), such as on carts, to be RTG (resistance to ground) less than 1 x 10E9 ohms. So cart should be grounded while being moved via a drag chain or conductive casters to an ESD Floor.

Compliance Verification ESD TR53 for carts should be measured for RTG using one 5# electrode. The objective of this periodic test procedure is to verify the resistance of the Cart grounding system (cart through grounded floor). A well-designed Cart compliance verification program not only tests the Cart, but also indicates the quality of the conductive path through the cart and floor grounding system.

Regarding your conveyor / flow rack system, the roller / rack system should be grounded. Per ANSI/ESD S20.20-1999 section Automated Handlers “All conductive or static dissipative components of automated handling equipment should provide a continuous conductive path to ground, whether stationary or in motion. The equipment should minimize charge generation of the ESDS [ESD sensitive] items that are handled. Where insulative materials are necessary in the device path, they should be designed to minimize electric fields and the charge imparted to devices being handled.”

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