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Question So is it ok to place a metal plate on the properly grounded dissipative mat in an ESD Protected Area? - Anonymous, Atlanta, GA
Answer ANSI S20.20 states that it is a fundamental ESD control principle that “All conductors in the environment, including personnel, must be bonded or electrically connected and attached to a known ground or contrived ground.” Placing a process essential metal plate on the dissipative mat would ground it and be OK. If the metal plate is not process essential, although it is grounded, it would partially cover the worksurface with a highly conductive surface raising concerns about experiencing Charged Device Model [CDM] ESD events. A device can become CDM charged from motion such as sliding down a defective integrated circuit shipping tube. Another example of CDM charging is by static induction. If an insulator is charged, it will remain charged even if located on grounded mat. If an ESD sensitive device is placed on that grounded mat, electrostatic induction can occur inducing a potential on the ESD sensitive device. The problem is that a charged ESD sensitive device can experience a damaging discharge if touched by a person even if that person is properly grounded via a Wrist Strap or Foot Grounder. So the fact that the conductor [which might be a person] is grounded is not sufficient. Care must be taken that the metal plate not come close to or into contact with a charged ESD sensitive item. If contact is made, grounding will occur via the metal plate to dissipative mat to ground. However, the rate of charge transfer may be too fast, and a damaging discharge may occur.
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