While there is general agreement on the use of passive grounding methods in most standards, there still is controversy over the use of air ionization. Air ionizers have one very obvious application: They control the static charge on insulators and isolated conductors to assure there is no ESD hazard to products or the equipment that handles them.
Until recently, the many industrial, national, and international standards-making organizations hadn’t satisfactorily addressed the role of air ionization in a static-control program. Then along came ANSI/ESD S-20.20.1
ANSI/ESD S20.20
ANSI/ESD S20.20 is a guidance document, developed under contract to the Department of Defense to replace an out-of-date military standard, MIL-STD-1686C.2 The document was created using industry experiences with the ISO 9000 standard. Rather than describing a rigid static-control program that must be followed exactly, it specifies the elements of the program and allows the user to define the implementations:
- The static sensitivities of the products that are being protected.
- How the program is to be set up and the static-control methods to use.
- How the performance of static-control methods is being verified.
- How deficiencies in the program are corrected.
- How personnel are trained as part of the program.
- How records of performance, corrective actions, and training are retained.
Implementing a successful S20.20 static-control program will be a requirement for doing any electronics business with the Department of Defense, NASA, and eventually for many other government agencies whose operations are concerned with electronic components. It also could simplify the way contract manufacturers satisfy the varied static-control requirements of diverse customers. Conversely, companies doing business with the government agencies may require their vendors to comply with the requirements of S20.20.
S20.20 and Air Ionization
S20.20 specifically recognizes the existence of both essential and nonessential insulators in the workplace. Nonessential insulators must be kept out of any electrostatic protected areas. Nonessential clothing, personal items, plastic packaging materials, and Styrofoam cups are examples of insulators to avoid.
S20.20 also acknowledges that insulators are essential parts of the product and some manufacturing operations, particularly those involving high temperatures or caustic chemicals. Circuit-board substrates, component epoxy packages and coatings, Teflon carriers and equipment parts, and polycarbonate enclosure panels are examples of insulators that cannot be removed from the workplace.
Many electronic components must be produced in clean rooms where the elimination of insulators is even more difficult. S20.20 recommends that any insulator or isolated conductor on which a field of more than 2,000 V/in. can be measured with a fieldmeter should be isolated from sensitive products by at least 12 in. at all times.
While such a specification might be possible to implement in a strictly disciplined military equipment repair depot, how would you carry it out in a large contract manufacturing facility? It most cases, this would be impractical simply due to the number of employees moving about the facility and the need to transfer work materials from one place to another.
The motion of insulators inevitably results in triboelectric charge generation. Contact and separation, friction, and low humidity contribute to charge on insulators. In addition, the distance essentially is zero when the charged insulator or isolated conductor is part of the product itself. The S20.20 specification of 2,000 V/in. assumes that a 12-in. separation is maintained. Product sensitivity to fields may require lower limits if the separation is smaller.
S20.20 recommends the use of air ionization when the isolation cannot be maintained. Ionizers charge the gas molecules in the air, providing balanced quantities of both positive and negative air ions. Whatever charge is present on objects in the work area, there are opposite polarity ions available in the air to neutralize the charge.
Wherever the ionized air goes, it eliminates the charge. In reality, this always is the correct approach. Doesn’t it make more sense to eliminate the cause of the static problem by using ionization to neutralize the static charge rather than leave the static charge and try to isolate the product from harm?
In manufacturing situations, it is not possible to achieve S20.20 compliance without neutralizing the static charge on process-essential insulators and isolated conductors. A properly designed S20.20 program will include ionization of some type targeted to where the insulators are being handled. This may involve the use of one or more types of ionization.
For general coverage in an uncontrolled manufacturing environment, room ionizers provide charge neutralization over both personnel and equipment areas. Typically, pulsed DC ionizers mounted on the ceiling of the room are used for this purpose.
Occasionally, pulse DC bar-type ionizers also are used. The alternation of ion-generation polarity of the pulsed DC system assists in covering large areas. For this type of ionizer, located a significant distance from the ESD-sensitive products, the offset voltage (balance) is specified by S20.20 to be ±150 V as measured with the test methods of ESD STM3.1-2000 (formerly ANSI EOS/ESD S3.1-1991).3
Workstation ionizers, including bar ionizers, blowers, and compressed-gas blow-off devices, are used in defined work areas. Whether mounted above or directly on the worksurface, the distance separating the ionizer from the ESD-sensitive product is much smaller than with room systems, typically less than 1 meter.
Steady-state DC ionizers, AC ionizers, and alpha ionizers are used in these situations. Alpha ionizers are particularly useful in the most ESD-sensitive applications where components such as magnetoresistive (MR) heads for disk drives must be handled. S20.20 specifies offset voltages of ±50 V for ionizers used in close proximity to 100-V human body model ESD-sensitive products. The intrinsic zero balance of alpha ionizers makes them suitable for use around the £10-V sensitivities of MR heads.
Closed equipment enclosures require the use of a variety of ionizers, such as ionizer bars, fans or gas-assisted ionizers, or alpha ionizers. These ionizers would have the same balance requirements as workstation ionizers because they would likely be located close to the ESD-sensitive product. An increasing number of electronic components, circuit boards, and systems are produced by automated equipment. Although the major emphasis of S20.20 is personnel-related, it does mention equipment as an area in which the static-control program should be considered.
It is unlikely that air ionization in the room will find its way into the equipment enclosures. Additional ionization will be needed in automated equipment to realize the full potential of a static-control program to improve product quality and reduce production costs.4
Conclusions
A properly designed and executed static-control program is vital to achieving high quality products. In some areas, notably disk drive manufacturing, it is essential.
The ESD Association has directed its attention to static-control programs in military/government procurement and operations through ANSI ESD S20.20. The areas covered by this standard will expand and overlap as industries progress along their technology roadmaps.
Standards compliance will help in improving product quality and factory output and should be part of an overall electrostatic management program. Besides installing, auditing, training, and operating the program, ESD-control management strives to understand the impacts of static charge, find solutions to existing problems, and anticipate and mitigate problems in new product designs and manufacturing processes. As long as insulators and isolated conductors continue to be present in all parts of the products and the equipment used to produce them, air ionization will be an essential part of an electrostatic management program.
ESD-control management ultimately is mandated by customer needs for assurance that what they are buying is not damaged by static charge. For everyone’s customers, whether military, government, equipment users, computers, or telecommunications, static control is not an option.
References
- ANSI ESD S20.20-1999, Standard for the Development of an ESD Control Program for Protection of Electrical and Electronic Parts, Assemblies and Equipment, ESD Association, 1999.
- MIL-STD-1686C, Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies and Equipment, U.S. Department of Defense, October 1995.
- ESDA STM 3.1-2000, Ionization, ESD Association, 2000.
- Levit, L.B. and Steinman, A, “It’s the Hardware. No, Software, No It’s ESD!,” Solid State Technology, May 1999, pp. S-7-S-12.
About the Author
Arnold Steinman is chief technology officer at Ion. He joined the company in 1983 after 12 years as an electronics consultant specializing in digital, analog, and microcomputer design. Previously, he served at the Lawrence Radiation Laboratories in the biomedical electronics and heavy ion linear accelerator groups. Mr. Steinman earned his bachelor’s and master’s degrees in electrical engineering from the Polytechnic Institute of Brooklyn. Ion, 1005 Parker St., Berkeley, CA 94710, 510-548-3640, e-mail: [email protected].
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April 2001