Resistance or Resistivity?

Q. What is the difference between resistance and resistivity?

A. Resistance and resistivity are core terms in static control. Often incorrectly used interchangeably, the terms provide us with indications of a material's static-control performance and a way to describe materials based on their resistive properties. They also serve to define the test methods used to evaluate those materials. Although the concept of surface resistance seems favored in this new century, both terms are still widely used.

Volume resistivity is defined as the ratio of the dc voltage drop per unit thickness to the amount of current per unit area passing through the material.¹ A basic material property, volume resistivity indicates how readily a material conducts electricity through the bulk of the material. Volume resistivity is expressed in ohm-centimeters (Ω-cm).

Surface resistivity is a material parameter when the material is a thin film of constant thickness. Surface resistivity is defined as the ratio of the dc voltage drop per unit length to the surface current per unit width for electric current flowing across a surface.¹ In effect, the surface resistivity is the resistance between two opposite sides of a square. It is independent of the size of the square (as long as the size is much greater than the film thickness) or its dimensional units. Surface resistivity is expressed in ohms per square (Ω/sq) and is traditionally used to evaluate insulative materials for electrical applications.

Resistance, on the other hand, describes the opposition of a material to the flow of an electric current based on that material's shape (area and length) and its resistivity. It also indicates the degree of electrical continuity across a surface or from surface to ground. It also may indicate the ability of an object to dissipate a charge. Resistance is expressed in ohms.

Surface resistance is defined as the ratio of dc voltage to the current flowing between two electrodes of a specified configuration that contact the same side of a material. This measurement is also expressed in ohms.² It is applicable to materials regardless of construction.

Volume resistance is defined as the ratio of dc voltage to current passing between two electrodes (of a specified configuration) that contact opposite sides of the material of the object under test. Volume resistance is reported in ohms.³

It should be noted that surface resistance per ANSI/ESD STM 11.11 and volume resistance when measured in accordance with ANSI/ESD STM 11.12 are the preferred methods for evaluating and classifying static-control materials used in electronic applications. These standard test methods are the primary references employed in current ESD control program standards, e.g., ANSI/ESD S20.20.

References

1. ESD-ADV1.0, "Glossary," ESD Association, Rome, NY.

2. ESD STM 11.11-2001, "Surface Resistance Measurement of Static Dissipative Planar Materials," ESD Association, Rome, NY.

3. ANSI ESD STM 11.12, "Volume Resistance Measurement of Static Dissipative Planar Materials," American National Standards Institute, Washington, DC.

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