Taking The Mystery Out Of Selecting ESD Flooring

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Test Against Multiple Criteria
A close reading of ANSI/ESD 20.20 should eliminate, once and for all, the argument over whether “static dissipative” or “conductive” material is better suited for static control flooring. The requirements summary section, on pages 7 and 8 in S20.20, sensibly encourages material selection based upon multiple variables and several test standards, and not on a particular range of resistance designated as conductive or static dissipative. To that end, the terms conductive and dissipative have been omitted from any section involving flooring or grounding. In section 6.2.3, under “Flooring,” the standard recommends a resistance below 1x 10⁹ for the floor by itself. Obviously, the floor cannot be used in a vacuum, and so the standard includes a recommended resistance value for the entire grounding system, which encompasses the person, protective footwear and grounded floor. In section 6.2.2, under “Personnel Grounding,” the recommended System resistance is less than 35 megohms RTC (<35 x 10⁶). Since the main purpose of a grounded floor is grounding the personnel, the upper parameter of the System threshold (less than 35 megohms) ought clearly to take precedence over the recommended Flooring (6.2.3) resistance threshold of 1000 megohms. With today’s footwear options, it would be difficult for a person to stand on a floor with resistance values over 1x10⁸ and consistently achieve system resistance values one to two orders of magnitudes lower. Anyone installing an ESD floor should carefully weigh these factors before choosing a particular range of conductivity.

In testing and comparing ESD flooring materials, one should also consider the intended choices of footwear and should include in the testing a representative statistical sampling of individual workers, since these sometimes overlooked variables often play a key role in determining the integrity and effectiveness of the ESD flooring solution. Certain ESD shoes, for example, used in conjunction with specific flooring materials, will outperform others. Similarly, weather conditions or the choice of stockings an employee wears can impact the effectiveness of heel straps. Furthermore, individual components that work well in one scenario might be totally inappropriate for a different application. For instance, the combination of a particular floor and heel strap might perform well in the lab, while in a class-10 clean-room carbon sloughing from the black rubber composite heel strap would render this same tandem unacceptable. Likewise, one particular combination might outperform another at 40% RH, while the low relative humidity of the actual manufacturing environment could present unanticipated problems for that same winning combination.

No matter which system appears most suitable, it is wise to test that combination under the full range of conditions that do or might exist in the factory situation and, as always, to utilize a broad statistical sampling before finalizing any selection.

Understanding Body Voltage
As is usually the case with far-reaching industry specifications or standards, ESD S20.20 offers options and latitude. In section 6.2.2, the standard recommends an upper limit of 35 megohms in the system resistance range. If we wish to steer clear of resistance parameters, we can design our own system around a body voltage generation limit of 100 volts. Many architects and engineers are confounded by the seemingly irreconcilable options of defining grounding parameters based on either resistance measurements or on controlling voltage to limits <100. It’s easy to see why they might be confused.

Bear in mind that the authors of this standard did not intend to create a document that automatically prevents certain manufacturers of ESD materials from selling their products. Certain less conductive materials, such as rubber, might read over 100 megohms resistance to ground (RTG), yet still control walking voltages to levels below 100 or even 50 volts. Remember that the sole purpose of grounding in any ESD program is the inhibition of harmful static voltages. The idea is to control electrostatic discharge, not ohms! It just so happens that a grounded system below 35 megohms (person, footwear and floor) cannot and will not generate walking voltages greater than 100 volts.

While this fact does not preclude the consideration of other less conductive combinations, testing for body voltage does require complicated and expensive testing equipment. To maintain the integrity of the ESD program, program managers must periodically monitor and verify performance of their ESD floor. Because of the limited number of variables and the simplicity of performing the tests, resistance testing can be easily verified with inexpensive and readily available equipment. Body voltage testing, on the other hand, requires special knowledge and elaborate equipment. For this reason, most program coordinators utilize body voltage testing only during the qualification phase of the program. In the factory, most coordinators favor flooring materials that can be monitored by resistance measuring equipment so that any engineer, technician, operator or customer can easily and comfortably test and verify the performance of their floor, the foundation of any effective ESD program.

Ergonomics
Most high-technology manufacturing involves clean manufacturing procedures. PCB assembly is no longer synonymous with the use of environmentally unfriendly chemicals like freon and trychloroethylene. Messy procedures such as wave soldering, conformal coating and paste screening comprise very little of the square footage in modern PCB assembly operations. Today, most OEMs in the computer and telecom industries assemble and test prefabricated parts in workspaces that more closely resemble office environments than the factories they once occupied. As the word “factory” has become euphemistically replaced by names like Raytheon’s “agile manufacturing operation,” so has the need for heavy-duty chemical and spill resistant floors. The advent of clean manufacturing has enabled a shift in the emphasis on flooring considerations from pure durability to ergonomic issues and employee comfort.

The most common ergonomic concerns architects and health and safety experts involve air quality, sound absorption, anti-fatigue properties and slip resistance. Slip resistance is a major growing concern and can hit companies hard in the pocketbook. Last year, according to Joe Visintin, the director of marketing for Johnsonite Rubber Company in Chagrin Falls, Ohio, the American insurance industry spent over 10 billion dollars on claims stemming from accidents involving slips and falls on floors. The industry forecast…

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