The basic elements of a static control program are well known. The ESD Association has brought them together in the static control program designed for the U.S. military, ESD S20.20. It describes the various grounding and materials issues needed to prevent the generation of static charge and to slow the rate of discharge.

But, due to triboelectrification, primarily on insulators, it is not possible to completely eliminate charge generation from most workplaces. While eliminating all non-essential insulators is helpful, there are always insulators present in any work area. Most often they are part of the product (e.g., epoxy device packages and circuit boards), but sometimes they are required for high temperature or chemical processing of the product. Finally, there may be conductors that for some reason cannot be grounded. While S20.20 recommends isolating charged insulators from sensitive product, this is impractical in many work areas.

Figure 1.	The Triboelectric Charge. Materials Make Intimate Contact		Figure 2.	The Triboelectric Charge - Separation

ESD is a multi-step event.

Charge is generated, typically triboelectrically, accumulating on some object. At some later time, a discharge occurs.

Ionizers prevent problems that would occur in few seconds or longer from the time of generation of charge to the discharge.
Some gas-powered ionizers can reduce the neutralization time to under a second.
But ESD damage occurs in nanoseconds.

Ionizers cannot prevent ESD that occurs as soon as the charge is generated.
Ionizers are not a cure-all for all static problems, but how else are you going to deal with charge on insulators?

Ionizers are incorporated in the static control program to solve the problems caused by charged insulators and isolated conductors. Ionizers prevent the accumulation of static charge on any object that cannot be connected to ground. The shorter the time that static charge remains on an object, the less likely it is to cause an ESD event or attract contamination. Ionization is recommended only when the other parts of the static control program have failed to produce the desired results in reducing an identified static problem.

If grounding and wrist straps solve the static problem, you stop there. If the static-related losses are still too high, maybe you add dissipative worksurfaces and flooring. Finally, after you use every grounding method available, you still have a static problem due to charge on insulators that are essential to the manufacturing process or part of your product. Then you use ionizers. They are not a substitute for grounding methods, although they will neutralize charge on conductors that cannot be grounded.

Ionizer efficiency is affected by many environmental factors including airflow patterns and nearby grounds. You can always decide to use ionizers on the basis of a horsepower contest, the fastest ionizer wins. But the most important issue is whether the ionizers reduce the static problem or not. This is not a decision made on the basis of a particular device technology. All devices are at hazard to charged insulators at some level. Ionizers are needed to reduce charges on insulators below the critical level for a given device or process. It would not make sense to use wrist straps or dissipative flooring without some testing to demonstrate a reduction in the static problem. Any application of ionizers (or any other static control method) should include testing to show the impact on the static problem, whether it's contamination, ESD damage, equipment lockups, or anything else.

Once a decision has been made to use ionization, it must face the same return on investment (ROI) analysis that should be part of the decision-making process for any static control method. This is critical, because ionizers cost significantly more than wrist straps. Once it has been demonstrated that ionizers reduce or eliminate part of the static problem, and an installation of ionizers has been proposed, it should be possible to calculate how long it will take to pay back the costs of the ionizer installation. Typically, this payback period is one year or less. In critical ionizer applications, such as protecting MR heads or semiconductor photomasks, the payback might be calculated in days or even from a single ESD event that is prevented. It makes no sense to ask if ionizers are cost-effective compared to other static control methods. Other static control methods do not deal with charge on insulators. The real question should be, what is the cost of not using ionizers to solve a static problem caused by charge on insulators or isolated conductors?

All air ionization methods do the same basic thing; they move electrons between gas molecules. If a gas molecule loses an electron, it becomes positively charged, or a positive ion. Conversely, if a gas molecule gains an electron it is negatively charged, or a negative ion. For purposes of static charge control we are concerned with small air ions, which are generally molecular clusters of up to 10 atoms about a water molecule. Positively charged air ions tend to be larger and move about 20% slower than negative air ions, which must be considered when adjustable ionizer types are balanced.

Two basic methods are used to move electrons to create ions, alpha ionization and corona ionization. Alpha ionizers utilize a nuclear source, Polonium-210, which is an alpha particle emitter. The alpha particle, a helium nucleus, collides with air molecules knocking out electrons, until it comes to a stop in about 3cm in air. The gas molecules that lose electrons become positive ions. The free electrons do not exist in air for very long before they are captured by neutral gas molecules, forming negative ions. It is important to note at this point that alpha ionizers always produce balanced quantities of positive and negative ions. Each electron knocked out creates a positive ion, and when captured creates a negative ion. Ions are always created in equal numbers, advantageous in protecting sensitive components like MR heads from ESD. Equal numbers of ions means that the ionizer is always balanced to zero volts and neutralizes everything in the work area to zero.

Corona ionizers utilize high electric fields created by applying high voltage to a sharp ionizing point to move the electrons. Due to the decay of trace radioactive elements in the soil and air, there are always a few free electrons present in the air. If we create a high positive electric field, the electrons are accelerated towards the point, colliding with the air molecules and knocking out more electrons as they move towards the ionizing point. They leave behind large quantities of molecules that have lost electrons, are thereby positive ions, and are in a high positive electric field. This field repels them away from the ionizing point, presumably towards the area in which they are needed for charge neutralization. Similarly, negative electric field accelerates free electrons away from the ionizer point creating collisions with gas molecules that increase the number of free electrons available. These free electrons are captured by neutral gas molecules in a short distance from the ionizing point, creating negative ions, which are then repelled away by the negative electric field.

Corona ionization generally does not provide the intrinsic balance of alpha ionizers. Methods exist to assure that closely balanced quantities of positive and negative ions are delivered to the work area, despite differences in ion mobilities and ion production rates for each polarity. Similarly, some ionizers will include monitoring and feedback capabilities to provide adequate long-term stability of the ion balance in the work area. Ionizer balance, or offset voltage, is measured with a Charged Plate Monitor (CPM) using procedures defined in the ESD Association ionization standard ANSI ESD STM3.1. Ion balance is important because the imbalance of the ionizer can induce voltages on isolated conductors, just the opposite of why the ionizer is being used.

The ESD sensitivities of the products being protected will generally determine the types of ionization that are used. The more sensitive the product, the more precise the methods the ionizer must use to maintain balance and long term stability of the ion production. On the other hand, problems such as particle attraction to charged surfaces and ESD-related equipment problems can be solved by almost any commercially available ionizer. Solving these problems does not require ionizer balance to better then a few hundred volts, as measured with the CPM. Ionizer selection may involve several other issues such as available airflow, distance from the ionizer to the work area, and cleanroom compatibility of the ionizer. The interaction of the ionizer and its environment makes the selection process for ionizers more complex than just choosing a wrist strap.

There are several methods of using corona ionization to both create and deliver bipolar air ionization to the work area. The main difference between these methods is whether high voltage AC, DC, or Pulsed DC current is used to create ions. The following is a short description of the differences in these corona ionization technologies and a few examples of where each technology has been applied.

AC (Alternating Current) Ionization - High voltage is applied to a number of closely spaced emitter points, which cycle negative and positive at the line frequency (50/60Hz). Ionization efficiency is low, as the points remain above the ionization threshold voltage for each polarity only a small percentage of time. AC technology is widely used in bars that control static charge on low- and medium-speed moving webs. AC technology is also used for ionizing blowers and blowoff gun devices. AC ionizers are widely used in electronics manufacturing areas to protect components during assembly. Due to their dependence on the often unbalanced and noisy characteristics of the power line, AC ionizers are rarely used in applications requiring precision balance (less than +/-15 volts). Due to the high ion currents required to make up for high levels of ion recombination, particle levels from AC ionizers usually make them unsuitable for cleanroom applications.

Steady State DC Ionization - High voltage of both polarities is continually applied to pairs of positive and negative emitter points, thereby increasing the efficiency of ion production over that of AC ionizers. Lower operating currents can be used, making steady state DC ionizers more applicable to cleanroom uses. The availability of separate positive and negative high voltage supplies makes it possible to employ various schemes for monitoring and feedback control of ion balance to better than +/-5 volts. Steady State DC ionizers can be used in high airflow rooms and used in high-speed web applications. Steady DC technology is also used for ionizing blowers, ionizing bars, and blowoff gun devices. Steady state DC ionizers find wide application for controlling static charge in room systems, worksurfaces and flow hoods, and point-of-use applications in equipment.

Pulsed DC - Positive and negative high voltage to the emitter points are alternately turned on and off creating clouds of positive and negative ions which mix together in the work area. The result is a dramatic lowering of the recombination rate, allowing ionizers to be placed on the ceilings of rooms 5 meters or more in height. Pulsed DC ionizers are used in rooms with low airflow as well as being the most common type of ionizer in cleanrooms and laminar flow hoods. The advantage of this type of ionizer is its flexibility and versatility, as cycle timing can be adjusted to the specific airflow conditions. Since the polarity of the ionizer output varies with the cycle timing, it produces a "voltage swing" that must be limited to protect sensitive devices. While in common use with semiconductor devices, pulsed DC ionizers will generally not be used with sensitive components such as MR heads.

The ESD Association issued a standard on air ionization, ANSI EOS/ESD S3.1-1991, which was reaffirmed recently with minor editorial changes as ANSI ESD STM3.1-2000. It is still the only ionization standard recognized worldwide, and has been referenced in many international static control standards. As a standard test method (STM) it contains only an instrument and test methodology for comparing different systems -- or the same system over time. It does not specify required performance, due the variety of conditions under which air ionization is used. However, for a specific application in protecting 100 volt HBM sensitive devices, the static control program S20.20 recommends pulsed DC room ionization systems with less than a +/-150 volt swing, and worksurface ionization balanced to better than +/-50 volts. Discharge times should always be specified by the end user to meet the needs of solving the static charge problem.

The key instrument used in analyzing the effectiveness of air ionizers is known as the Charged Plate Monitor (CPM). The CPM has an isolated conductive plate that can be charged to a known voltage. It then measures the time required for the ionizer to reduce the charge to 10% if its initial value. Ion balance, or offset voltage, is measured by momentarily grounding the isolated plate of the CPM in the ionized area, and then measuring the voltage attained by the plate due to the balance of the ionizer. Steady state DC ionizers will reach a single value of positive or negative balance. Pulsed DC ionizers will have both positive and negative maximum voltage swings. AC ionizers will generally show a single value of offset voltage. This CPM measurement for AC ionizer balance is not accurate because the CPM is too slow to follow the rapid fluctuation of the AC ionizer output. Use of AC ionizers should be avoided in applications with extremely ESD- sensitive components.

As with most technologies, there has been considerable progress in the design of ionizers over the last twenty years. Specialized ionizers exist for protecting extremely ESD-sensitive components, high-speed industrial web applications, and high quality cleanroom applications among many other applications in electronics assembly, optics, and medical devices. Manufacturers of ionizers have learned to make the best use of the variety of ionization technologies available.

As with any static control method, one should make a decision to use ionization based on the best current knowledge available, rather than relying on information sources about technologies a decade or more out of date, or misinformed consultants. It is hoped that this article has presented information to assist you in making a decision to appropriately use air ionization. The author of this article welcomes your questions and comments at asteinman@ion.com, and invites you to attend the annual Ionization Tutorial at the ESD Association Symposium.

Source Midwest Chapter, ESD Association.