How to Select Ionization Systems

The laws of physics are the same everywhere. Static-charge generation is unavoidable. And without a static-control program, the problems caused by static charge also are unavoidable.

The most common problems are caused by electrostatic discharge (ESD) or electrostatic attraction (ESA). ESD is the rapid, uncontrolled transfer of charge between objects at different potentials. This results in damaged semiconductor ICs, destruction of magneto-resistive (MR) heads in disk drives, and failure of the drive circuits in flat-panel displays (FPD). Technology trends to smaller device geometries, faster operating speeds, and increased circuit densities can compound ESD problems.

Charged surfaces attract and bond particles, similar to what happens on your TV or computer screen. This is ESA. Since many electronic and medical products must be produced in clean rooms, anything increasing particle contamination is a serious problem.

If the charged surface is small or lightweight, ESA also may cause it to move or stick to another surface. This causes jamming and other machinery malfunctions as well as product breakage.

Static-control programs are designed to eliminate the problems caused by ESD and ESA. Common elements of these programs include static awareness training, grounding, static-dissipative materials, and ionization. Choosing an ionizer for a given application always is more complex than choosing wrist straps, table mats, or shielding bags. This article tells how to select ionizers.

Do We Need Ionization?

You bought wrist straps, table mats, conductive flooring, and dissipative packaging and maintained the integrity of all the ground paths for these items. Haven’t you solved the static problem?

There would be fewer static-charge problems if you could ground everything and eliminate all insulators from the work area. Unfortunately, this is not possible. Most static-sensitive products contain insulators or require insulators for their manufacture.

Almost all electronics assembly occurs on some type of insulated circuit board using components with insulated packages. Charges on insulators cannot be controlled by grounding. A static-control program that does not neutralize charge on insulators is incomplete at best.¹

Ionization is the only universally acceptable method for eliminating charges on insulators. Ionizers use a variety of techniques to supply positive and negative charge to the molecules of gases in the air, which then are referred to as air ions. Such techniques include corona discharge, radioactive sources, and soft X-ray generators. Ionizers flood the air of the work environment with balanced quantities of both negative and positive air ions, neutralizing charges on insulators or isolated surfaces.

What Is the Right Ionizer?

No single type of ionizer effectively controls static charge for all applications. Commercially available ionizers use AC, steady-state DC, and pulsed DC corona ionization as well as soft X-ray and nuclear ionization sources. Clean rooms, mini-environments, and laminar flow hoods with high levels of airflow use ceiling or bar-type ionizers. Fan-powered blowers or compressed gas blow-off guns are best for test or assembly areas that lack a source of airflow. Specialized point-of-use ionizers are used in production tools where small dimensions often are involved.

Ionizers are used in test and assembly areas where large numbers of static-sensitive components are handled. With the exception of rework areas, most test and assembly operations are performed by automated equipment.

Overhead or benchtop ionizing blowers and ionizing compressed gas nozzles are used because of the lack of ambient airflow and the short times available to neutralize components in automated operations. These blowers achieve the fast static-discharge times and precision balance without occupying tabletop space.

Small equipment dimensions often require the miniaturization possible with nuclear ionizers or high-efficiency, steady-state DC ionization. Where ionizer size and airflow rate are not critical, AC ionizers also are used in test and assembly areas.

Clean-room applications demand that the design of the ionizer itself be clean-room compatible. Highly efficient steady-state and pulsed DC corona ionization and nuclear or soft X-ray ion sources are found most often in clean-room applications.

All of these types work when the distance from the ionizer to the work area is less than 1 meter. At distances from 1 to 5 meters, pulsed DC ionizers are commonly used. Clean-room ionizers must work with the available laminar airflow, as turbulence from ionizing blowers makes contamination control more difficult. Ozone and particle production from ionizers are concerns in clean rooms.

The interiors of production equipment pose special challenges for ionizer selection. Small dimensions mean that ionizers may be close to sensitive products or the grounded surfaces of the equipment. Exposed ion emitter points will charge nearby products or allow ions to be wasted without affecting charge neutralization. Isolated, steady-state DC ionization in blowers or compressed gas ionizers, nuclear ionizers, and X-ray ionizers all provide high levels of ionization in equipment without these problems.

How Important Are Discharge Time and Balance?

Ionizers interact with their environment. The rate of charge neutralization (discharge time) primarily depends on the available airflow and the distance from the ionizer to the charged surface. It also depends on the efficiency of ion production and the recombination rate for ions of opposite polarity. An ionizer with the fastest discharge time may not always be the best choice.

For example, ionizing blow-off guns or fan ionizers using AC, steady-state DC, or nuclear ionization technology provide fast discharge times, but stir up particles in clean-room applications. A better choice would be a ceiling-mounted, pulsed DC ionization system that works with the available laminar airflow. Discharge times might be slower, but probably would be adequate to solve static problems without compromising contamination control.

In assembly areas, high airflows can cause solder cooling and operator discomfort. The higher ionization efficiency of steady-state DC ionizers produce comparable discharge times at lower airflow rates. For particle blow-off applications using compressed gas ionizers, discharge times are not dependent on ionizer technology. The design of the gas nozzle for high blow-off force is more important.

Ionizer balance levels are measured using the charged-plate monitor (Figure 1). Balance is a concern for applications with extremely ESD-sensitive components like MR heads, where acceptable ionizer balance usually is 10 V or less. Few other electronic components are sensitive at this level.

In most electronics assembly applications, ionizer balance below 100 V is sufficient. FPD assembly and medical or plastics applications tolerate even higher balance levels. When very low values of ionizer balance are required, sensor monitoring and feedback control will be required, adding to the cost of the ionizer.

What Does Ionization Accomplish?

Laboratory testing of ionizer discharge or balance performance may not confirm that a static problem is controlled by the ionizer. Buying the most costly ionizer with specialized features does not guarantee success of the static-control program. Evaluating the ionizer in the actual use location is recommended.

After ionization is installed, tests or observations should demonstrate a measurable effect on the static problem. The results may be fewer ESD-damaged devices, increased equipment availability, or less contamination on critical product surfaces. Whatever the problem, evaluating ionizers in the actual use area demonstrates that ionization is an effective part of a static-control program (Table 1).

Conclusions

Static charge and the problems it causes will not go away. And ignoring them is becoming increasingly difficult and costly. As products and the equipment used to make them become more sophisticated, the causes of static charge increase. As a result, a static-charge control program becomes essential.

Since you cannot eliminate insulators from products and production areas, ionization is required in a static-control program. Choosing an ionizer includes analyzing the static problem in the work area, reviewing ionizer performance characteristics, and evaluating the effect an ionizer has on solving the static problem. Remember, there may be a best ionizer to use for a given application, but no single type of ionizer is best for all applications.

References

1. Steinman, A. “Why Use Ionization in a Static Control Program?,” EE-Evaluation Engineering, November 1997, pp. S-42-S-45.

NOTE: This article can be accessed on EE’s TestSite at www.nelsonpub.com/ee/. Select EE Archives and use the key word search.

About the Author

Arnold Steinman is chief technology officer at Ion Systems. Before joining the company in 1983, he was affiliated with Lawrence Livermore and Lawrence Berkeley Laboratories and later was an independent consultant. Mr. Steinman is a member of the ESD Association Ionization Standards Committee and several other standards work groups, leader of the SEMI ESD Task Force, a senior member of the Institute of Environmental Sciences, and a member of the Electrostatics Society of America.

He graduated from the Polytechnic Institute of Brooklyn with B.S.E.E. and M.S.E.E. degrees. Ion Systems, 1005 Parker St., Berkeley, CA 94710, (510) 548-3640,

e-mail: [email protected].

Table 1.

Copyright 1998 Nelson Publishing Inc.

June 1998