Putting Static Control Flooring to the Test

Q: The resistance measurements of our static control flooring do not agree with the specifications supplied by the manufacturer of the material; the resistance is much higher. What are the proper procedures and test methods for evaluating static control floor materials?

A: Differences in the resistance measurements of flooring materials usually occur for at least one of these four reasons: different procedures, different electrodes, different applied voltages, or different resistance-measurement instruments. Eliminating measurement differences that result from variations in these factors is one of the key goals of standards development.

For electronics applications in the United States, the primary standards used to measure the resistance of floor materials are ANSI/ESD 7.1 and ESD STM97.1. In Europe, the primary standards are EN 100015 and IEC 1340-4-1, Part 4.

Developed specifically for static control flooring in electronics environments, these standards replace such general resistance standards as NFPA 99, ASTMF-150, or DIN 51953.

Although the standards differ somewhat in their procedures, electrodes, applied voltages, or instrumentation, the goal of each is to provide a standard methodology for the evaluation of floor materials. For example, the ANSI/ESD 7.1 document specifies the use of 63.5-mm-diam, 2.27-kg (5-lb) electrodes for measuring resistance. Conductive rubber is used for the contacting surface of the electrode. The meter applies a specified 100 V to the sample under test. Laboratory tests are conducted under humidity conditions of 12 and 50%, while in situ testing is performed under ambient conditions.

While volume and surface resistivity have sometimes been used to evaluate flooring materials, resistivity is not a proper parameter for them. Both volume and surface resistivity are applicable only to electrically homogeneous materials. In general, the measurements are not applicable to flooring materials, which are typically composed of conductive and insulative materials, or which may also include conductive backings or be installed with conductive adhesives.

In addition to the resistance of the material itself, an evaluation of static control flooring should include a measurement of the combined electrical resistance of the floor material, the footwear, and the personnel functioning together as a system. The new standard test method ESD STM97.1-1999 provides a procedure for making this measurement. The method can be used for laboratory evaluation of the system under different temperature and humidity conditions, as well as on installed systems. It can be used in conjunction with the new ANSI/ESD 20.20 ESD Control Program standard, which recommends that a flooring-footwear system have a resistance to ground of < 35 x 10⁶ (omega) through the person.

To complete the evaluation of flooring materials, one would also include measurement of body-voltage generation using ESD STM97.2-1999. The new test procedure measures the voltage generated on personnel during a prescribed walking cycle with specified shoes and floor materials under specified temperature and humidity conditions. Unlike such procedures as AATCC-134 that are more concerned with the human comfort aspects of ESD, ESD STM97.2 specifies equipment and procedures capable of measuring the relatively low voltages that are of concern in the electronics manufacturing industry. ANSI/ESD 20.20 recommends that this measurement be below 100 V.

Evaluating static control floor materials means using commonly accepted standards, such as ANSI/ESD 7.1, and performing a complete evaluation that includes both body-voltage generation and resistance, but not resistivity. This evaluation will yield the most precise evaluation of the floor materials, helping to ensure adequate ESD protection for specific applications.

To submit your questions to the ESD Help Desk or to browse the archives of past questions and answers, go on-line at http://www.ce-mag/esdhelp.html. Information on the ESD Association may be found at http://www.eosesd.org.

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