The ESD Control Program Handbook. Jeremy M. Smallwood

capacitances between objects are in series (Figure 1.7), they are combined as

Figure 1.6 Capacitors in parallel.

Figure 1.7 Capacitors in series.

1.9 Shielding

The term shielding is used in ESD control in a different way to other disciplines, especially EMC and radio frequency work. Shielding definitions and tests used in ESD control are often highly specific to the standards used. Typically, the term is used to describe the attenuation of electrostatic fields or electrostatic discharge energy applied to the outside of a protective package, measured at the inside of the package. This is discussed further in Chapter 8.

      

1.10 Dielectric Breakdown Strength

      If a low voltage is applied across an insulating material, very little current will flow due to the high resistivity of the material. If, however, the voltage is increased, a level may eventually be reached where the current suddenly increases to a high value. Typically, this current flow may lead to formation and thermal heating of a small electrically conducting channel through the material. For a solid material, melting or damage of a small channel through the material may occur. This is dielectric breakdown of the material.

      Typically, very high electrostatic field strengths are required for dielectric breakdown to occur. The breakdown strength of air is, for planar parallel electrodes, around 3 MV m⁻¹ or about 3 kV mm⁻¹. For curved or sharp electrodes, it is much lower. The breakdown strength of most insulating solids is much higher than air. For polyethylene, it is about 20 MV m⁻¹ (IEC 61340‐1 (International Electrotechnical Commission 2012)).

      

1.11 Relative Humidity and Dew Point

      The relative humidity (rh) or dew point of the atmosphere has a large influence on electrostatic phenomena (see Section 1.2.6). At any temperature, moisture‐saturated air in equilibrium contains a maximum amount of moisture determined by the saturated vapor pressure of water at that temperature (Lawrence 2005). The saturated vapor pressure of water and hence the amount of moisture in saturated air increase strongly with increasing temperature. This saturated state is defined as 100% r.h. The relative humidity of air with lower than the saturated amount of water vapor present is given by

      As the saturated vapor pressure increases strongly with temperature, if the amount of moisture present remains the same, increasing the air temperature will result in a reduction in relative humidity. Conversely, lowering the temperature will increase the humidity.

If the temperature is lowered sufficiently, the water vapor pressure eventually becomes equal to the saturated vapor pressure, and the air becomes saturated. Any further reduction in temperature may result in moisture condensing from the air on to surfaces in contact with it or in fog forming. This temperature is called the dew point.

References

      1 Cross, J.A. (1987). Electrostatics Principles, Problems and Applications. Adam Hilger. ISBN: 0‐85274‐589‐3.

      2 EOS/ESD Association Inc. (2014) ANSI/ESD S20.20‐2014. ESD Association Standard for the Development of an Electrostatic Discharge Control Program for – Protection of Electrical and Electronic Parts, Assemblies and Equipment (excluding Electrically Initiated Explosive Devices). Rome, NY, EOS/ESD Association Inc.

      3 EOS/ESD Association Inc. (2015a) ANSI/ESD STM 11.11‐2015. ESD Association Standard for Protection of Electrostatic Discharge Susceptible Items – Surface Resistance Measurement of Static Dissipative Planar Materials. Rome, NY, EOS/ESD Association Inc.

      4 EOS/ESD Association Inc. (2015b) ANSI/ESD STM 11.12‐2015. ESD Association Standard for Protection of Electrostatic Discharge Susceptible Items – Volume Resistance Measurement of Static Dissipative Planar Materials, Rome, NY, EOS/ESD Association Inc.

      5 International Electrotechnical Commission. (2012) IEC/TR 61340‐1: 2012. Electrostatics – Part 1: Electrostatic phenomena — Principles and measurements. Geneva, IEC.

      6 International Electrotechnical Commission. (2013) PD/IEC TS 60079‐32‐1. Explosive atmospheres Part 32‐1. Electrostatic hazards, guidance. Geneva, IEC.

      7 International Electrotechnical Commission. (2015) IEC 62631‐3‐2. Dielectric and resistive properties of solid insulating materials ‐ Part 3‐2: Determination of resistive properties (DC methods) ‐ Surface resistance and surface resistivity. Geneva, IEC.

      8 International Electrotechnical Commission. (2016a) IEC 61340‐5‐1: 2016. Electrostatics – Part 5‐1: Protection of electronic devices from electrostatic phenomena ‐ General requirements. Geneva, IEC.

      9 International Electrotechnical Commission. (2016b) IEC 61340‐2‐3:2016. Electrostatics. Methods of test for determining the resistance and resistivity of solid planar materials used to avoid electrostatic charge accumulation. Section 3: Methods of test for determining the resistance and resistivity of solid planar materials used to avoid electrostatic charging. Geneva, IEC.

      10 International Electrotechnical Commission. (2016c) IEC 62631‐3‐1. Dielectric and resistive properties of solid insulating materials ‐ Part 3‐1: Determination of resistive properties (DC methods) ‐ Volume resistance and volume resistivity ‐ General method. Geneva, IEC.

      11 Jonassen, N. (1998). Electrostatics. Chapman & Hall. ISBN: 0 412 12861 6.

      12 Lawrence, M.G. (2005). The relationship between relative humidity and the dewpoint temperature in moist air. A simple conversion and applications. Bull. Am. Meteorol. Soc.: 225–233. https://doi.org/10.1175/BAMS-86-2-225 [Available from htt.s://journals.ametsoc.org/doi/pdf/10.1175/BAMS-86-2-225. Accessed 15th Aug. 2018.].

      13 Wikipedia (2018) Ion, viewed 17 October 2018, [Available from https://en.wikipedia.org/wiki/Ion]

2 The Principles of Static Electricity and Electrostatic Discharge (ESD) Control

      2.1 Overview

ESD stands for electrostatic discharge or, according to some, electrostatic damage. This chapter provides the basis of how static electricity arises and can lead to ESD in the real world. It also provides the principles that underlie ESD control techniques and equipment design.

      Electrostatic charge can build up in a variety of ways. The charged object has an electrostatic field that could conceivably lead to an ESD event in several ways:

       Direct breakdown of sensitive parts due to high electric field

       Generation