Jeremy M. Smallwood - The ESD Control Program Handbook

In ESD control, it is convenient to make simple measurements to evaluate the surface properties of a material or item of equipment. One simple way of evaluating a surface is to place two electrodes on it and measure the resistance between them. The electrodes are often cylindrical in form. This is called a point‐to‐point resistance measurement. Standard test methods based on this approach are often used. Examples of point‐to‐point resistance test methods are given in Chapter 11.

As explained earlier, in ESD control work, voltages on conductors are often eliminated or controlled by providing an electrical connection for the charge to pass to earth (ground). It is often required to know the resistance from an object or surface to ground to help understand the charge dissipation paths. This is known as resistance to ground . Examples of measurement methods for this are given in Chapter 11.

In practice, the resistance of a ground path may be due in part to several components. If these are effectively in series ( Figure 1.4), the effect is to add the resistance of all component contributors R 1 …R nto get a total resistance R tot.

If resistance of ground paths is in parallel ( Figure 1.5), they are combined as

Figure 1.4Resistances in series.

Figure 1.5Resistances in parallel.

The voltage V on a conductor is related to the stored charge Q as

The variable C is the capacitance of the conductor. In electrostatics, any conductive object has capacitance; it is just the relationship between the stored charge and the object's voltage.

In practice, the capacitance of an object can vary with proximity of other conductors and materials (see Chapter 2).

A charged capacitor stores energy. The energy W stored in a capacitance C at voltage V is given by

This can also be expressed as

An object in free space (with nothing in the near vicinity) still has capacitance. For a spherical conductor of radius r in air or a vacuum, this capacitance C is

In practice, the capacitance of an object may be due in part to the proximity to several objects. If these are effectively in parallel ( Figure 1.6), the effect is to add the capacitance of all component contributors C 1 …C nto get a total capacitance C totas

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

Figure 1.6Capacitors in parallel.

Figure 1.7Capacitors in series.

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.

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 −1or 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)).

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