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Question We are currently looking for alternatives to hard wiring our wrist strap sockets and table matting, and were hoping if you could tell us if it is sufficient to do the following and still provide a safe ESD environment for PBA manufacturing? "Attach copper strips to the feet of the production tables, then hard wire the wrist strap socket and table matting to the table frame. (Our floor is static dissipative, on average, to between 10to7 & 10to9 ohms per square.) This will allow our tables to be more flexible, as the table doesn't necessarily have to have power to be earthed." - Clayton Gardner, Australian Electronic Manufacturing Services, Sydney, Australia
Answer I would not recommend wiring your table mats and wrist straps to the ESD floor in luei of a hard ground. Your floor's resistance of 100 Megohms to 1,000 Megohms would not protect you adequately as a "ground substitute" for your wrist strap or table mat. The recommended grounding (for common point grounds, table matting, and wrist straps) is a hard ground that does not exceed 1.0 Ohm according to the ANSI/EOS/ESD-S6.1-1991 under section 4.4.1. One of the biggest concerns to not using a hard ground is the increase in your electronic time constant /RC (discharge time). We have a white paper available on our Web Site that better discusses discharge times. You don't need to have power at the benches to run an "Earth" ground to them. A common ground bus can be used and wired to these benches. An electrician can easily help you with this. You will want to be careful of ground loops for possible electrical noise problems both to sensitive electronic equipment and ESD grounded operators.
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