Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 36

There is no fundamental definition of insulators and conductors in electrostatics. In reality, there is a continuum of material resistivity from highly conducting (low resistance) to highly insulating (very high resistance). Different industry areas may have differing views on the resistance level at which a material is considered to have insulating properties.

For our purposes, a conductor is a material that allows charge to move around on the surface or in the bulk of the material and can thereby be used to transport charge from one place to another. An insulator (nonconductor) is a material that does not allow the charge to move in this way.

One problem in practice is that a material that is considered “insulating” in one application may be considered significantly conducting in electrostatics. So, for some years I have offered the following pragmatic definitions for use in practical electrostatics and ESD control:

 A conductor is a material that allows charge to move away quickly enough to avoid significant electrostatic charge build up.

 An insulator is any material that is not a conductor, in other words, a material that does not allow charge to move quickly enough to avoid charge build up.

Conductors are easily maintained at a low voltage by connecting them to earth (ground). However, an insulator in electrostatic terms cannot be maintained at a low voltage by installing a ground connection. The charge on the material simply does not move to the ground connection quickly enough to be conducted away in the desired timescale.

Materials or equipment are often defined as conductors or insulators based on either their measured resistance or a charge decay time. This is discussed further in Chapter 2.

Table 1.2 shows how the terms insulating, dissipative, conductive, and antistatic are widely used in ESD control. Take care when using these terms, because they may be defined differently in different contexts and may mean different things to different people. When defined in the standards, the precise definition can change as the standards evolve into new editions.

The situation becomes worse if usage of these terms in other industries and for specific products is considered (Table 1.3). In general, these words should be considered unreliable in meaning unless specified by standards as part of an ESD control system.

Table 1.2 Example of how meanings of conductive, static dissipative, insulative, and antistatic can vary with context in ESD control in electronic manufacturing.

Term	Application	General use	Meaning under 61340‐5‐1:2016a	Meaning under S20.20‐2014
Conductive	General	Resistance <106 Ω	Not defined	Not defined
ESD control footwear		Not defined	Not defined
ESD control flooring	<106 Ω	Not defined	Not defined
ESD protective packaging		Surface resistance <104 Ω	Surface and volume resistance <104 Ω
Static dissipative	General	Resistance between 106 and 1011 Ω	Not defined	Not defined
ESD control footwear		Not defined	Not defined
ESD control flooring	≥106 Ω	Not defined	Not defined
ESD protective packaging		Surface resistance ≥104 and ≤1011 Ω	Surface and volume resistance ≥104 and <1011 Ω
Insulative	General	Resistance over 1011 Ω	Not defined	Not defined
ESD control footwear		Not defined, but by implication >108 Ω	Not defined, but by implication resistance >109 Ω
ESD control flooring		Not defined	Not defined
ESD protective packaging		Surface resistance ≥1011 Ω	Surface and volume resistance ≥1011 Ω
Antistatic	General	Widely used to described materials used in static control; can mean almost anything	Not defined	The property of a material that inhibits triboelectric charging (ESD ADV1.0‐2009)
ESD control footwear	Note: Has defined meaning under ISO 20345 in process industry hazard work	Not defined	Not defined
ESD control flooring		Not defined	Not defined
ESD protective packaging		Not defined	Materials that have reduced amount of charge accumulation as compared with standard packaging materials

Table 1.3 Example of how meanings of conductive, dissipative, and insulative can vary with context in static control in other industries (IEC 60079‐32‐1:2013).

Object	Measurement	Conductive	Dissipative	Insulative
Material	Volume resistivity (Ωm)	<105	≥105 to 109	≥109
Clothes	Surface resistance (Ω)		<2.5 × 1010	≥2.5 × 1010
Footwear	Leakage resistance (Ω)	<105	≥105 to <108	≥108
Gloves	Leakage resistance (Ω)	<105	≥105 to –<108	≥108
Floor	Leakage resistance (Ω)	<105	≥105 to <108	≥108