The ESD Control Program Handbook. Jeremy M. Smallwood

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.

      1.4.2 Electromagnetic Interference (EMI)

      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.

1.5 Earthing, Grounding, and Equipotential Bonding

      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 <10⁹ Ω 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

1.6 Power and Energy

      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

      

1.7 Resistance, Resistivity, and Conductivity

      

      1.7.1 Resistance

      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.

      

      1.7.2 Resistivity and Conductivity

      1.7.2.1 Surface Resistivity and Surface Resistance

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_s with linear electrodes of width w placed parallel on the surface a distance d apart, the surface resistance R_s measured between the electrodes is

      where d = w, which reduces to ρ_s = R_s.

      The unit of surface resistivity is ohms (Ω). In some industries, it is quoted as ohms per square (Ω/sq). This reflects the property that the value of the surface resistance measured with a square electrode pattern (d = w) is the same, no matter what the dimension of the side of the square is.

      In practice, standards exist for measuring surface resistivity using concentric ring electrodes (IEC 62631‐3‐2 (International Electrotechnical Commission 2015), IEC 16340‐2‐3, ANSI/ESD STM 11.11 (EOS/ESD Association Inc. (2015a)). This is further discussed in Chapter 11.

Figure 1.2 Surface resistivity definition.

Surface resistance is a resistance measured between two electrodes on the surface of a specimen. The electrodes may be of any convenient form. Sometimes this measurement is made using electrodes designed so that conversion from surface resistance to surface resistivity is a simple calculation. In ESD control practice, conversion to surface resistivity is often not needed, and the surface resistance result obtained with defined standard electrodes is used directly.

      1.7.2.2 Volume Resistance, Volume Resistivity, and Conductivity

Volume resistivity is a bulk material property based on the resistance of a cube of material with sides of unit length, with a voltage applied to