Copyright (c) 1995, Desco Industries, Inc.

How Grounded Work Areas
are Still Susceptible to Charges

Ryne Allen
December 1995

 

Your work area can still harbor dangerous charges even after applying the proper grounding techniques. Grounding is a very effective ESD technique for conductive or dissipative materials, but is ineffective for non-conductors, i.e., insulators which are materials with resistance greater than 1 x 1012 Ohms.

 

Examples of insulative materials include human hair, man-made fibers in clothing, some hand tool handles, latex and vinyl gloves, various tapes, test equipment, computer video monitors, key boards, and many packaging and storage items. Even the main constituent of electronic circuit boards are themselves insulators.

 

The presence of insulators in the work area is the root cause of hard-to-control-charge build up. Insulators cannot effectively be grounded since they severely restrict the flow of charges. But, resident charges on insulators can be neutralized by the introduction of air ions. Air ions, as generated from air ionizers, are molecular groupings of large numbers of both positive and negative charges that can effectively neutralize the surface charges on insulators.

 

Charges can be transferred to insulators through frictional or proximity contact with another object or media, e.g., solid, liquid, gas or plasma. Once an insulator bears a substantial charge, over a few thousand volts, it can more easily find an opportunity to release its burden, trying to attain a state of stability or equilibrium, via a charge transfer (discharge) to another object such as a human or electronic circuit board. This unplanned charge transfer, from insulator to conductor, also know as an ESD event, can damage expensive components -- even if the worker and work surface are properly grounded.

 

Ionized air is the most effective method to neutralize charged insulators. High voltage (corona) ionization is the most common ionization technology in the semiconductor and electronics industries. The most widely used commercial method for ionization is the high voltage corona technology, of which there are three types: AC, steady state DC, and pulsed DC. Which type of ionization technology should you use to neutralize charges in your work area? This depends on your application and working conditions and can be a recommended addition for your ESD control program.

 

Classical applications for these ionizers are; automated machinery, test equipment, wave solderers, clean room areas, conveyor belts, component kitting, and in shipping and receiving. ESD Work benches can greatly benefit from the application of Overhead Ionizers such as Plastic Systems' distinctive line of Overheads, [refer to tech briefs PS-2063 - PS-2065]. Isolated areas such as a tape dispenser, solder pot, or printer can be spot protected with a single ionizer such as ESD Systems' Eliminator JR., Item 43156, [refer to tech brief PS-2066]. Critical environments such as clean rooms, wafer fabrication, and film production can greatly benefit from the use of widespread ionization using ESD Systems' family of Overhead Ionizers.

 

When determining which Ionizer correctly suits your application it's important to evaluate the units characteristics such as charge decay rates, charge balance, calibration, adjustability, maintenance requirements, safety, and cost.

 

Since insulative materials can be found in most electronics assembly operations, the applications for air ionization are widespread. One way to evaluate the suitability and effectiveness of ionizers is to test them in your facility before buying. A number of manufacturers, including ESD Systems, offer "demo" units for this purpose. Check out our catalog pages 38-41 for ionizers in our July 1997 catalog or our on-line catalog