opposite polarity ions. That is, a negatively charged surface repels negative ions and attracts positive ions. A positively charged surface attracts negative ions and repels positive ions. Opposite polarity ions will drift to the charged surface at a rate proportional to the field strength and in numbers proportional to the ion concentration. An opposite polarity charge on reaching the charged surface neutralizes an equal charge, reducing the net surface charge and electrostatic field. The ion drift represents a neutralizing current, limited by the ion concentration and field strength.
2.8.3 Ionizers
Ionizers are devices used to generate air ions for neutralization of surface charges on charged materials and objects (Jonassen 2016a, 2016b). These are available in various types based on passive, radioactive, electrical, and other principles of operation.
Passive ionizers rely on high electric fields developed around sharp points or edges on earthed conductors to generate air ions by corona discharge. These will always generate ions of the opposite polarity to the voltage producing the field at the point or edge. Unfortunately, corona discharges do not occur below a threshold field strength, and this means there is a threshold voltage, known as the corona inception voltage, below which ions are not produced, and neutralization does not occur. This threshold voltage may be several kilovolts (kV). This means that passive ionizers cannot be used to reduce voltages to below this threshold, and they are seldom useful in ESD control in electronics manufacture. They do find major applications in industrial processes such as plastic film manufacturing, printing, copiers, and other processes involving insulating materials.
Electrical ionizers also use the high electrostatic fields at points, usually needles, to generate air ions. In this case, however, a power supply is used to raise the needle to a voltage above the corona inception voltage to ensure that sufficient ions are produced. A balanced ion source with equal numbers of positive and negative ions is usually required. This can be produced either by using an AC driving voltage or by using two sets of needles, one at positive voltage and the other at negative voltage. This can produce an ion stream that is nearly balanced, but it is difficult to produce an exact balanced ion stream by this method.
Radioactive ionizers use a radioactive source to ionize the air by impact of radioactive particles with molecules in the air. Exactly balanced ion streams are produced by this means, as the molecule splits into two ions, one positive and one negative ion. The rate of production of ions is small and limited by the level of radioactivity of the ionizing material.
2.8.4 Rate of Charge Neutralization
Each ion that reaches a charged surface will add its charge to the surface. As this is usually the opposite polarity to the surface charge, the arriving ion will normally neutralize an equal and opposite polarity surface charge. The rate at which ions arrive at the surface depends on the electrostatic field strength, the concentration of ions in the air, and the mobility of the ions (Jonassen 2016b). This can be thought of as an ion current flow. Different air ions can have different mobility depending on the size, polarity, and charge of the ions.
As the charged surface neutralization process continues, surface voltages and field strength reduce, and the drift of ions to the surface reduces with the reducing field strength. The neutralization process continues ever more slowly until the electrostatic fields and thus forces on the ions are insufficient to attract further charge from the air.
If the surface voltage is monitored, the voltage is seen to reduce at a reducing quasi‐exponentially decaying rate. This is used in a charge plate monitor (CPM) instrument to measure the effectiveness of ionizers in neutralizing charges on surfaces. A typical voltage decay curve obtained is shown in Figure 2.25.
Figure 2.25 Electrostatic voltage on a CPM plate reducing during charge neutralization showing decay time and offset.
Charge neutralization by ionizers is typically quite a slow process and can take tens of seconds or longer, depending on the electrostatic field strength and ion density in the region surrounding the charged surface. This charge density, and the speed of charge neutralization, can be affected by many factors.
2.8.5 The Region of Effective Charge Neutralization Around an Ionizer
The ion density in the air typically reduces with the distance from the ionizer. This is in part because the ion cloud tends to spread out due to repulsion forces between like polarity ions. In addition, attraction between positive and negative charges in the cloud causes them to move toward each other and recombine to neutralize each other (Jonassen 2016a).
As the ion cloud is drifting in air, movement of the air can have a significant effect on the location of the cloud and the local ion concentration. Ions can be blown toward or away from the region where they are wanted (e.g. by drafts from a fan or open window). Conversely, in many situations a fan can be used to blow ions to the location they are needed to improve charge neutralization effectiveness.
2.8.6 Ionizer Balance and Charging of a Surface by an Unbalanced Ionizer
An ion cloud is rarely accurately balanced in terms of ion charge polarity density. One consequence of this is that any surface within the ion cloud tends to reach a state of excess charge reflecting the ion density balance in the ion cloud. If, for example, the ion cloud has excess positive ions, then surfaces within the ion cloud will tend to become positively charged until electrostatic fields are sufficient to prevent further ion deposition. This effect will create residual voltages on insulating surfaces and isolated conductors within the ion cloud. With a well‐balanced ionizer, this effect will be small, and the excess voltage achieved may be only a few volts or tens of volts. This “offset voltage” is an important parameter of an ionizer specification (Figure 2.25).
A poorly maintained ionizer can become seriously out of balance. This can happen in service due to erosion or contamination of the ionizer needles. If this occurs, then surfaces within the ion cloud may become charged to hundreds or even thousands of volts. Monitoring this effect over time and performing appropriate maintenance of an ionizer to prevent the imbalance from reaching an excessive level is an important aspect of ionizer maintenance (Simco Ion TN‐003 2019).
2.9 Electromagnetic Interference (EMI)
ESDs can give waveforms that have very fast (nanosecond or subnanosecond) rise times, high peak current levels of tens of amps, and oscillatory waveforms. Discharge current rise rates can be of the order of 109 A s−1, and electrostatic fields may collapse with rates of the order of 1012 V s−1. The ESD source may radiate or conduct strong transient electromagnetic fields over a wide frequency band up to GHz frequencies.
These induce transients in nearby conductors, especially tracks or wires of significant length that can act as efficient antennas at the frequencies radiated in the discharge. This radiated or conducted interference can cause upset or malfunction to nearby equipment such as test equipment. Giant magnetoresistive (GMR) heads have been demonstrated to be damaged by this type of ESD transient (Wallash and Smith 1998).
EMI can also be an issue in highly automated facilities where automated test equipment (ATE) can test large numbers of production items. EMI may cause the ATE to register a product fail or to otherwise
