Ionization probably is the most misunderstood ESD-control technique used by the electronics industry today. We can relate to dissipative worksurfaces, wrist straps, and chairs with grounding chains. Why not ionization?
Probably because ions are invisible, and their presence feeds our imagination. Unfortunately, air ionization often—and erroneously—is blamed as the root of all evil, causing everything from cataracts to nuclear irradiation.
To put the principles of air ionization into perspective, let’s start with the basics. An air ion merely is a molecule that has lost or gained an electron. Ions are present in normal outside air. For example, every storm front creates huge waves or pools of these harmless particles.
Why Use an Ionizer?
When we bring air into a building, we strip out most of the natural ions by filtration and air conditioning. This process is not favorable in an environment where static-sensitive devices are present. As a result, electronics manufacturers must reintroduce natural ions. This is where an ionizer comes into play. It produces a concentration of ions that can be directed toward components and subassemblies in the manufacturing process.
Today, most consumer electronics are housed primarily in plastic, one of the biggest static generators in manufacturing environments. We need ions at the workstations to eliminate charges on these and other insulators. In fact, ionization usually is the only effective way to keep the plastic housings from damaging their ESD-sensitive component parts.
Ionizers haven’t always been successful in production environments. Their early attempts to control static charge in manufacturing facilities were largely unsuccessful.
On early models of ionizers, the emitter points were positioned directly in the process. This placed the ionizer too close to the product and to ground, forcing the ions to ground before they neutralized unwanted charges. Now we know that, generally speaking, a product should be at least 12 inches from the ionizer’s emitter points.
Technological improvements in emitter design and materials, along with the use of blowers in nonlaminar flow units, have increased ionizer effectiveness and efficiency. A modern ionizer can be very effective even when it is located up to 1 meter from the product. This helps an operator to feel less cramped while working in an ionized airflow.
The most common types of ionizers—AC, DC, or pulsed DC—use a phenomenon called corona discharge, where a high voltage is applied to an emitter point. Both positive and negative ions are generated, hopefully in approximately equal quantities. In nonlaminar flow applications, a blower distributes them to locations on a workstation where charges might exist.
Ionizers accomplish their intended purpose. But in the process, do they degrade the air and present a potential hazard to operators? Let’s see.
A Case History
A major manufacturing facility had an air contamination problem. The symptoms of the contamination were skin, eye, and throat irritation. Immediately, the invisible, unknown ions were suspected because the facility used AC ionizers with fan-forced airflow.
I was asked to investigate the problem. Originally, the company thought radiation might be the culprit as evidenced by their questions: “What amount of ozone do these ionizers emit during normal operation?” and “Is there any possible radiation hazard?”
I assured them there was no radiation. The electrodes in the ionizers were made of stainless steel and didn’t contain any alpha-particle emitting thorium, a radioactive material.
As for ozone generation, it is not known to increase with the corona discharge ionizer, even when the polarity balance is out of calibration. Regarding the possible interaction of the electrical field near the ionization electrode with airborne contaminants, particularly hydrocarbons, the possibility was very remote. Studies in other electrostatic applications, such as particle precipitation in gaseous streams and contamination control, addressed these issues and eliminated the possibility of such a problem.
Corona discharge ionizers operate at about 5,000 V, far below the 50,000 V used in pulsed plasma applications. Sure, the extremely high voltages can create chemical reactions. However, the opportunity for free radical formation at the low voltage levels in these particular air ionizers was extremely unlikely.
The energy levels are orders of magnitude lower than those needed to generate ozone. The free radicals generated by these devices are so low that they would be undetectable. The lifetime of airborne free radicals, even in a pulsed plasma, is only a few nanoseconds.
In my best engineering judgment, corona-discharge ionizers do not degrade the air quality. Nor do they cause air contamination.
If ionizers are to be criticized, it is because many of the earlier models did nothing to improve the air quality. Many older ionizers were not fitted with air intake filters. As a result, they recirculated airborne contaminants and did not remove any particulates as the air passed through them.
Newer ionizers have filters. If kept clean, they do remove some of the airborne particles.
In the manufacturing facility I studied, ionizers were absolutely essential. The facility operated several automated production lines and used a large number of insulators and ungrounded conductors.
Were it not for the generous use of ionizers, the production lines would have performed like enormous Van de Graaff generators. The lines would have stored large amounts of static electricity, causing damage to the components in the manufacturing process and posing a high-voltage hazard to operators. Consequently, the ionizers not only were used for product protection, but also for personnel safety.
Vindication of the Ionizers
In the final analysis, ionizers were indeed vindicated from guilt in the contamination problem. So, what did cause the difficulties?
Airflow studies inside the building showed that the recent removal of a 30-ft section of wall between the production area and the PC fabrication shop had changed the airflow patterns. This allowed airborne fiberglass particles from a PCB router operation to circulate throughout the building, causing the physical problems for the workers.
Conclusion
In today’s electronics assembly facilities, a consolidated team effort and spirit of cooperation between the ESD and safety support groups are critical. This provides the best and safest possible factory work environment. The safety team can help this effort by thoroughly understanding air ionization and then relaying that knowledge to the production workers.
Ionization is perfectly safe. Working in an ionized area is no riskier than breathing the outside air as a storm approaches.
About the Author
William Metz is the corporate ESD program manager for Hewlett-Packard/Agilent Technologies and has produced numerous ESD training videos and workmanship standards for the company. Previously, he was the ESD control manager at the Hewlett-Packard Washington Printer Division. Mr. Metz is a member of the EOS/ESD Association and a technical advisor to the Northwest Chapter and has a B.S.E.E. from Sacramento State University. Hewlett-Packard, Bldg. 6U, 1501 Page Mill Rd., Palo Alto, CA 94304, 650-857-8948, e-mail: [email protected].
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September 2001