Understanding the Resistivity of Materials

Q: What are the resistance ranges for conductive, static dissipative, electrostatic shielding, insulative, and antistatic materials?

A: For ESD purposes, many materials are classified by their resistance or resistivity characteristics. The following ranges and definitions are found in ESD Association or EIA standards publications.

Conductive materials are defined as those having a surface resistivity less than 1 x 10⁵ Ω/sq or a volume resistivity less than 1 x 10⁴ Ω-cm.¹ With a low electrical resistance, electrons flow easily across the surface or through the bulk of these materials. Charges go to ground or to another conductive object that the material contacts or comes close to.

Dissipative materials have a surface resistivity equal to or greater than 1 x 10⁵ Ω/sq but less than 1 x 10¹² Ω/sq or a volume resistivity equal to or greater than 1 x 10⁴ Ω-cm but less than 1 x 10¹¹ Ω-cm.² For these materials, the charges flow to ground more slowly and in a somewhat more controlled manner than with conductive materials.

Electrostatic shielding materials have a conductive layer with a surface resistivity of less than 1 x 10⁴ Ω/sq or a volume resistivity of less than 1 x 10³ Ω-cm per millimeter of thickness.² For ESD-sensitive devices, these materials provide Faraday cage protection from energy transfer.

Insulative materials are defined as those having a surface resistivity of at least 1 x 10¹² Ω/sq or a volume resistivity of at least 1 x 10¹¹ Ω-cm.¹ Insulative materials prevent or limit the flow of electrons across their surface or through their volume. Insulative materials have a high electrical resistance and are difficult to ground. Static charges remain in place on these materials for a very long time.

Antistatic materials are not defined by resistance or resistivity. Antistatic refers to the property of a material that inhibits triboelectric charging. A material's antistatic characteristic is not necessarily correlated with its resistivity or resistance.²

Material Type	Surface Resistivity (Ω/sq)	Volume Resistivity (Ω-cm)
Conductive	<1 x 105	<1 x 104
Dissipative	³1 x 105, <1 x 1012	³1 x 104, <1 x 1011
Electrostatic shielding	<1 x 104	<1 x 103
Insulative	³1 x 1012	³=1 x 1011
Antistatic	Not correlated	Not correlated

Table I. Resistance ranges for materials.

References

1. ESD-ADV 1.0-1994, "Glossary of Terms," ESD Association, Rome, NY.

2. EIA-541, "Packaging of Electronic Products for Shipment," Electronic Industries Alliance, Arlington, VA.

To submit your questions to the ESD Help Desk or to browse the archives of past questions and answers, go on-line at http://www.ce-mag/esdhelp.html. Information on the ESD Association may be found at http://www.esda.org.

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