Mitigating electrostatic charges on nonconductors: eliminating ESD in the workspace can involve topical treatments and ionized air.

In December we discussed issues regarding setting up an electrostatic protected workstation or area (EPA). In that issue, we focused on the conductive items and materials, including personnel, which enter or are present in the EPA. This month we look at the necessary nonconductors in the EPA and the mitigation of electrostatic charges that may be present on those items.

Industry guidelines and standards for setting up and managing an EPA suggest that only process-essential insulators be permitted in the protected area. This means there must be a thorough and complete policing of the EPA to ensure that no nonessential insulators (nonconductors) enter the work environment of the EPA. While setting up an EPA would be simpler if everything were conductive or electrically dissipative, there will always be a need for insulating materials in electrical and electronics assembly. Component and device packaging, circuit cards, flex circuits and the insulative coating on wires are examples of process-essential insulators necessary for assembly of most electrical or electronics products. However, the nonessential insulators that must be excluded in a properly set-up EPA include personal items, food wraps, coffee cups, handbags, etc. In addition, conventional packaging materials, such as expanded polystyrene, bubble wrap, plain plastic, etc., should be eliminated from within the boundaries of the EPA whenever possible.

ANSI/ESD S20.20 offers guidance regarding the maximum electrical field level from the necessary nonconductors in the EPA. Electrical field measuring instruments usually report in terms of surface voltage at some predetermined distance, generally 1" from the surface being measured. Any item or surface of a material that measures [greater than or equal to]2000V at 1" should be kept 12" from unprotected parts sensitive to [greater than or equal to]100V when tested according to the Human Body Model (HBM). This suggested level was determined by empirical testing in several different labs during the development of S20.20. An insulative material surface with a measured 2000V electrical field at 1" will not transfer enough energy either by induction or direct contact to harm a 100V HBM device. A charged conductor will transfer nearly all its stored energy upon contact with a grounded conductor while an insulator can only discharge from a small area. Thus, a charged insulator is far less hazardous in the environment than a charged conductor. In fact, an insulator is not technically classified as being charged since it cannot discharge in the conventional sense. However, an insulator that emanates an electrical field can charge up conductors that are momentarily grounded while within the influence of the electrical field (induction). Therefore, reducing the electrical field strength from insulators is an important aspect in the design of an EPA.

Mitigating Electrostatic Charge

Reducing the electrical field strength of a charged process essential insulator requires careful forethought. Topical treatments may be applied to the necessary insulators to make them somewhat dissipative or low-charge generating (antistatic). Items treated in this manner are no longer insulative in the technical sense. It is logical, whenever possible, to apply dissipative/antistatic coatings to clear plastic windows on process equipment, and also to plastic fixtures and tools, templates, test beds and other insulating surfaces involved in an assembly or test operation. Numerous topical treatments are available, but finding the most suitable formula may require some trial and error. While some coatings and treatments are relatively permanent once applied to a surface, many conventional topical treatments wear off or change performance with time and must be reapplied with some regular frequency. This may add an unreasonable burden to maintaining the EPA, so care should be taken in selecting the topical treatment.

Ionized air can reduce the electrical field from the necessary insulators while they are in-process. Ionizers create positive and negative charges in air in approximately equal numbers. Opposite sign ions are attracted to electrostatic charges on a surface to neutralize the charge one for one. Positive ions will neutralize negative charges and negative ions will neutralize positive charges. To make certain an ionizer does not charge a surface, the ionizer must produce a balanced number of positive and negative ions. However, a reasonable off-set voltage for most ionizers is in the 10 to 50V range when measured at 12". Nuclear (alpha) source ionizers and electrical ionizers with feedback loop systems are available with lower offset voltage levels if needed in critical applications. A properly positioned air ionizer can reduce the electrical field strength of an insulator to minimal levels if given enough residence time. Any ionizer system should be checked in the intended application to ensure it is capable of reducing or eliminating the problem. Ionized air is not a cure-all but can prove beneficial when used appropriately. Ionized air may be the only available solution for many applications, especially where insulating items must be in motion within the process.

David E. Swenson is cofounder and president of Affinity Static Control Consulting LLC, a member of the ESDA standards committee, ANSI/ESD S20.20 task force and board of directors, and ESDA senior VP and professional certification manager. He spent 35 years in ESD control material development at 3M (mmm.com).

This column is a regular contribution of the ESD Association (esda.org).

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