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Why would more dissipative table mats be better than more conductive mats? Wouldn’t it be better to have the charge go to ground faster? - Anonymous, Marlboro, MA

Answer

The ESD protected working area should provide for charged items to safely bleed off static electricity through the Mat to ground. It also keeps all conductors placed on its’ surface grounded, i.e., at ground potential which should be the same as the operator to minimize the possibility of an Electrostatic Discharge event.

There is a very good reason to use dissipative surfaces for balancing charge imbalances. The rate of charge balance (sometimes referred to as removal) is dictated by the resistance of the material. The faster the energy transfers to ground, the more likely you are to experience an ESD event or a large current pulse with sufficient energy to damage silicon/GaAs semiconductor devices. ESD control has evolved to minimize the risk of ESD Events. By slowing down the rate of charge (current) you can minimize the energy density at any given point in time. A good dissipative mat will discharge a charge imbalance to ground in plenty of time (typically way under 100 ms) relative to the motion of a line worker at the bench.

The more conductive an item is, the greater the energy density in an ESD event. By slowing the charge transfer (ESD event) down with a more resistive material (dissipative), you can minimize the risks associated with conductive ESD events. With a dissipative material, instead of an ESD event, you will have a current ‘bleeding’ or charge balance that is highly controlled.

ESD Protected Workstations ESD-ADV53.1 Electrical Requirements, “Workstation elements shall be connected to, and maintain electrical continuity to, the common point ground as follows: Worksurfaces - Resistance: Between 1 x 10⁶ ohms and 1 x 10⁹ ohms, and 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.”

10⁶ to 10⁸ dissipative range is preferable for worksurfaces as slowing the rate of charge transfer and minimizing energy density reduces the risk of a discharge occurring.