Jeremy M. Smallwood - The ESD Control Program Handbook

Polymers often have relative permittivity in the range 2–3 and many other materials in the range 2–10. Materials such as ceramics can have far higher permittivity.

Moving charges form electrical currents. One coulomb of charge has passed if 1 ampere has flowed for one second.

or for a varying current

and so

IEC 61340‐1:2012 defines an electrostatic discharge as “transfer of charge by direct contact or by breakdown from a material or object at a different electrical potential to its immediate surroundings.” IEC 61340‐5‐1:2016a gives a slightly different definition of “Rapid transfer of charge between bodies that are at different electrostatic potentials.”

There are various types of electrostatic discharge that are important in different fields. In ESD in electronics handling, the main types of concern are

Spark discharges between conducting objects or materials

Brush discharges between a conducting object and an insulating material

Corona discharges from sharp conducting objects and materials

Electrostatic discharges are discussed further in Chapter 2.

ESD from different sources produces very different discharge current waveforms. These can be modeled and simulated by simple electronic circuits. Three standard ESD source circuit models, human‐body model (HBM), machine model (MM), and charged device model (CDM), have been developed and standardized for testing ESD susceptibility of electronic components. This is discussed further in Chapter 3.

An ESD event can produce very large and fast‐changing currents and voltages. These produce fast‐changing electromagnetic fields with strong and fast‐changing magnetic and electric field components and a broad frequency spectrum, sometimes extending to over GHz frequencies. This can be radiated and conducted to be picked up by nearby electronic circuits and can cause temporary malfunction. This is known as electromagnetic interference .

Electrostatic discharges occur because of voltage differences between the objects between which the discharge occurs. If there were no voltage difference, then no ESD could occur.

So, one way to prevent ESD from occurring is to eliminate voltage differences between objects. If the two objects are conductors, connecting them electrically ensures that they are eventually at the same voltage. This must be so, as if any voltage difference were to arise, charge (current) would flow due to the voltage difference, until there is no voltage difference. The practice of connecting conductors together to eliminate voltage differences is known as equipotential bonding .

If two conductors at two different voltages are brought into contact, an electrostatic discharge will occur as part of the voltage equalization process. If one of the conductors is susceptible to ESD damage, it could risk being damaged as a result. So, ESD‐susceptible parts must only make contact with other conductors, including grounded conductors, in circumstances designed to protect against damage.

In many practical cases, one of the conductors concerned may already be electrically connected to an electrical earth or can be conveniently connected to earth. The earth is often defined as our 0 V reference point in electricity power distribution, electrostatics, and ESD control. So, it is often convenient and is common practice to electrically connect all conductors to earth. Earth is also known as ground, and earthing is also known as grounding.

The terms earthing and grounding can have different meanings and requirements in different contexts or industries. An electrical engineer may require an earth resistance less than an ohm. A plant engineer may earth bond two items of plant, requiring a resistance less than 10 Ω. An electromagnetic compatibility (EMC) engineer may require an extremely low impedance to be maintained from direct current (DC) to hundreds of MHz or even GHz. To an ESD control practitioner, a resistance to ground <10 9Ω at dc may be sufficient.

In practice in ESD control, there are various types of ground that can be used. In the ESD standards IEC 61340‐5‐1:2016a and ANSI/ESD S20.20‐2014, the term grounding is used to mean any of the following:

Connection to electrical earth (the safety earth wire of a mains electrical system)

Connection to a functional earth (e.g. an earth rod driven into the ground)

Connection to an equipotential bonding system

Energy is the ability to do work. Physics recognizes many types of energy – heat, light, gravitational, mechanical, and of course electrical.

Mechanical energy expended is the product of force and the distance moved. If a force qE is applied to move a charge q over a distance s between points A and B, the work done, W AB, is

Energy (work) expended, W , is also the product of power P and the time duration t that the power is applied.

The electrical power expended is the product of voltage V and current flowing I .

So, the electrical energy expended is

Electrical resistance is the ratio between the dc voltage applied to a circuit or material and the current flowing through it, given by Ohm's law.

Surface resistivity is defined as a material surface property. It is based on the theoretical resistance of a square of material surface with sides of unit length, with a voltage applied to two opposing sides of the square ( Figure 1.2). In theory, the current flows across the surface of the square. For a material of surface resistivity ρ swith linear electrodes of width w placed parallel on the surface a distance d apart, the surface resistance R smeasured between the electrodes is

where d = w , which reduces to ρ s= R s.