The ESD Control Program Handbook. Jeremy M. Smallwood
subjecting the part to discharge currents
Generation of an ESD event subjecting a part to induced transient electric or magnetic fields, or some other stress
At the root of any ESD event there is an object or surface that has a voltage that is different to its surroundings. Without this voltage difference, no electric field is present, and no ESD current can flow. Hence, the objective of ESD prevention measures has been to keep surface voltages and electric fields to a low level, below which damaging ESD cannot occur.
A review of the explanation of electrostatic charge build‐up and ESD sources included here quickly reveals many ways in which ESD risks can be generated in the real world. These are summarized in brief in this chapter.
2.2 Contact Charge Generation (Triboelectrification)
The first thing to state is that charge is never generated, nor is it ever destroyed. The phenomenon that we often describe lazily as the “generation” of charge is more correctly “separation.” Some practitioners speak of the charge being “liberated” or “set free.” The charge is initially present in the atoms that make up all materials. There are positive charges as protons in the atomic nucleus, and there are negative charges as electrons around the nucleus. Normally these are present in equal numbers so that in an uncharged atom the number of positively charged protons equals the number of negatively charged electrons present. Static electricity arises when an imbalance is created and the local amounts of positive and negative charge become different.
In practice, the amount of charge imbalance required to give strong electrostatic effects is surprisingly small. The limit of the amount of charge that can be built up on a surface is governed by the electrical breakdown field strength of air, around 3 × 106 V m−1. The surface charge density required to give this field is only 2.64 × 10−5 Cm−2 (Cross 2012). This is equivalent to about 1.7 × 1014 electrons m−2, or 8 atoms per million on the surface acquiring or losing an electron!
One common way in which static electrical charge imbalances can arise is when two materials make contact and then are separated. While in contact, electrons move from one material to the other at points of contact; this material gains a net negative charge, and the donor material gains a net positive charge. When the objects are separated, the negatively charged object can take its charge with it, leaving an equal positive charge on the other object. Although it is really charge separation that takes place, it is common to refer to the “generation” of static electrical charge.
2.2.1 The Polarity and Magnitude of Charging
The polarity of charge left on a material can be positive or negative and depends on a range of factors, especially on the other material with which it made contact. Materials may be arranged in a table according to the polarity of charge they take in contact with other materials, called the triboelectric series (see Table 2.1).
A material in the table (e.g. aluminum) can be expected to charge positively against another material below it in the table (e.g. polytetrafluoroethylene (PTFE)) and negatively against a material above it (e.g. wool). The amount of charge generated is a function of the separation of the materials on the table; aluminum and paper can be expected to charge relatively little against each other, but polyvinylchloride (PVC) and nylon can be expected to charge strongly against each other.
In practice, triboelectrification is a variable phenomenon and is highly dependent on surface conditions, contaminants, and humidity. Small amounts of surface contaminants can have a large effect on triboelectrification. One result is that the order of triboelectric series is not unique. Different experiments and samples of the same materials may produce different results especially if the experimental conditions are varied. While it could be assumed from the triboelectric series that contact between two surfaces of the same material would not generate charge, this is generally not what happens in practice.
2.3 Electrostatic Charge Build‐Up and Dissipation
Any two materials in contact give charge separation that can lead to static electrical charge build‐up. This may or may not lead to charge and voltage build‐up, depending on the circumstances.
The key to this build‐up is the balance between charge generation and charge dissipation (or neutralization). If charge is dissipated (or neutralized) more quickly than it is generated, no static electricity builds up, and no effects are noticed. If charge is generated more quickly than it is dissipated (or neutralized), then high voltages and static electricity effects are quickly built up.
Table 2.1 An example of a triboelectric series.
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2.3.1 A Simple Electrical Model of Electrostatic Charge Build‐Up
Static electricity can be modeled as a charge generator, and a simple electrical model can be used to understand many practical situations (Figure 2.1).
The separation of charge is effectively a small electrical current represented by a current source I. The capacitor C represents the charge storage properties of the system and could be a material surface or a conducting object with a capacitance to earth. The resistance R represents charge dissipation processes (other than ESD) and can vary from less than 1 Ω to more than 1014 Ω for good insulators. (See Sections 1.7.3 and 2.3.4 for discussions on the meaning of insulators and conductors.)
If the current is constant (i.e. the effect of capacitance can be neglected), it's easy to see by Ohm's law that the voltage developed is highly dependent on the resistance R. If a charge generation rate of 1 nA (1 nCs−1) is present, with a resistance of 109 Ω, a steady state voltage of 1 V is produced. If, however, the resistance was 1012 Ω, a voltage of 1 kV would be produced, and for a resistance of 1014 Ω, a voltage of 100 kV would theoretically be produced! For a charge generation rate of 1 μA, a resistance of 1010 Ω would yield a voltage of 10 kV. In practice, electrostatic sources rarely generate charge at this rate or on a steady current basis unless there is steady movement involved (e.g. in a conveyor system).
Figure 2.1 A simple electrical model of electrostatic charge build‐up.
The rate of electrostatic charge generation is affected by many factors. Some of the key factors are as follows:
Relative position of the materials in the triboelectric series
Rate of separation of contact area (high rates of movement)
Condition of the surfaces that make contact
Rubbing of the contacting surfaces
Ambient humidity and temperature
The many factors involved in triboelectric charge separation make it a highly unpredictable phenomenon.
2.3.2 Capacitance Is Variable
The capacitance C represents charge storage. There is a simple relation between charge Q and voltage V, and capacitance is the ratio of charge and voltage.