Surface Resistivity & Surface Resistance

Do you know the difference between surface resistivity and surface resistance? Although there has been a lot of discussion centered around these parameters, they are probably some of the least understood in the ESD industry. ESD practitioners need to have a clear understanding of what the differences are to make informed material selections in their work environments.

Let’s start with the basics. Surface resistivity in ohms/square is used to evaluate insulative materials where high resistance characteristics are desirable. Surface resistance in ohms is a measurement to evaluate static-dissipative packaging materials where lower resistance characteristics are required. Now let’s explore the standards and tests that address these measurements.

For many years, surface-resistivity measurements have been used to classify ESD-controlled packaging materials. The primary reference for this measurement has been ASTM D-257Standard Test Methods for D-C Resistance or Conductance of Insulating Materials. ASTM D-257 measures the resistive or conductive properties of insulative materials rather than the dissipative characteristics of ESD-control materials. Regardless of its title, ASTM D-257 has been used throughout the military and commercial world for classifying the performance of static-control packaging materials. People always define the surface resistance measurement procedure for a planar (flat) static-dissipative material.