Assessing the electrostatic suitability of modern materials: are today's ESD test methods up to the challenges presented by modern materials?(ESD TEST)

The suitability of materials to avoid the risks of and problems from static electricity traditionally has been assessed by measuring resistance. This technique, unfortunately, may not be appropriate for many modern materials.

The Basics

Avoiding ESD-related risks and problems depends on four main features:

Capability of Surfaces to Drain Charge Away From Conductors in Contact

The capability of materials to drain charge from a conductor is relevant to materials used for flooring and footwear to keep the voltages of people at a low level during walking and similar activities. The performance of materials for such applications is appropriately assessed by measuring resistance.

As walking involves times of a few tenths of a second, and a body capacitance typically is around 150 pF, the resistance to ground from the person must be well below [10.sup.9] [ohm]. (1) This is not, however, the same situation as controlling the voltage on a person getting out of a car or up from a chair.

Voltages Arising on Surfaces When Contacted or Rubbed by Other Surfaces

The voltages that arise on surfaces after contacting or rubbing are relevant to the creation of electric fields on items nearby and the induction of charge that may lead to other problems. As a general point, if surface charge on the material that is contacted or rubbed can migrate over the surface in less time than it takes the surfaces to separate, then no significant surface voltages will arise from retained charge, and there will be no influence on items nearby.

Considered in this way and appreciating that many modern materials such as clean-room garment fabrics are not very homogeneous, the definition of the suitability of materials must be in terms of charge decay time--not resistance. As times for separation of surfaces with manual activities typically are at least 0.1 s, this means that a dissipative material is one in which the decay of surface voltage to 10% of its initial value takes no more than a few tenths of a second. …

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