Static Dissipative Materials

Static dissipative (or simply dissipative as used in this document) materials have electrical resistance between insulative and conductive materials (1.0 x 104 – 1.0 x 1011 ohms for surface and volume resistance). Electrons will flow across or through the dissipative material, but the rate is controlled by the surface or volume resistivity of the material. This can be very useful in protecting ESD sensitive devices since limiting the current flow during a discharge event reduces the threat of damage. Since the defined dissipative range involves seven orders of magnitude, there is a very large difference in electron flow from or through materials on the low end of the range compared to materials on the high end of the range. When choosing dissipative materials for any application, the end user is cautioned to determine the suitability of any given material since there is such a large range of performance. If a material is chosen for an application that is too conductive, there exists an increased threat of harmful discharge if an ESD sensitive part has built up a charge. Conversely, if the material is too resistive, the risk is that not all the charge will be drained from the part while it is in contact with the dissipative surface.

As with the other two types of materials, charge can be generated triboelectrically on a dissipative material. However, like the conductive material, the dissipative material will allow the transfer of charge to ground or to other conductive objects. The transfer of charge from a dissipative material will generally take longer than from a conductive material of equivalent size. Charge transfers from dissipative materials are significantly faster than from insulators, and slower than from conductors.

The most obvious difference between insulative, conductive, and dissipative materials is their electrical resistance. As you will see in later sections, the measurement and evaluation of resistance characteristics of items are important in determining the appropriateness of an item in a given application. Measurement of electrostatic fields and charge decay times are also basic elements of ESD control and analysis.