Choosing the Right ESD Floor for Laboratory and Technical Environments
by David Long, President of Staticworx®
Overview
Previous studies have illustrated the significant benefits of a comprehensive employee grounding program for effective control of unwanted static charges.
Between 1978 and 1982, G. T. Dangelmayer, AT&T, conducted factory studies which documented dramatically different yields between side-by-side identical circuit board assembly lines with only one variable: during two year long, with-in plant studies, assembly lines equipped with personnel grounding devices and dissipative table mats produced significantly better yields than lines that did not utilize any grounding procedures. Of greater import, AT&T also noted statistically relevant correlations between compliance with ESD grounding procedures and overall improvements in product quality. Despite 25 years of overwhelming evidence supporting the benefits of personnel grounding, noncompliance with grounding procedures remains a major obstacle to many ESD program managers.
Noncompliance is most prevalent in R & D labs, server rooms, engineering offices, test areas, box assembly and other scientific spaces where more technically trained people perform daily but non-repetitive tasks. Many facilities install some form of conductive flooring in technical work spaces assuming incorrectly that a grounded floor will contribute some benefits despite the lack of conductive footwear usage. The present study investigated the possible value of conductive flooring without heel strap compliance and whether, under certain conditions, ESD flooring materials might behave like insulative flooring and contribute to walking body voltage generation. During a one month period we measured peak body voltage on individuals wearing several common types of insulative footwear while they walked on six different grounded flooring materials that complied with the recommended resistance range of ANSI/ESD S20.20 tested per S 7.1 in original lab testing.
We found no relationship between ohms to ground measurements and tribocharging characteristics and several functional ESD flooring materials demonstrated excessive tribocharging and voltage accumulation on test subjects. These data suggest that certain types of two layer conductive rubber and ESD carpet tile with conductive thermoplastic backing significantly attenuate walking body voltage even though heel straps were not used. This is most likely attributed to a general triboelectric compatibility of specific flooring compositions with most common shoe sole materials and an extreme lack of compatibility among other triboelectrically active materials. These results also question the validity of choosing conductive flooring solely based on controls, specifications and procedures that will not be duplicated in the actual static sensitive environment.
Background: ESD Flooring Materials, Test Methods and Standards
The ESD Association in Rome NY has generated many useful standards and test methods for evaluating grounding procedures, conductive flooring and personnel grounding devices. The Association’s test methods for evaluating flooring can be traced to Stephen A. Halperin’s, “A Proposed Methodology For Evaluating The Electrostatic Characteristics Of Flooring Materials” presented at the 1990 ESD annual symposium. In his paper, Mr. Halperin proposed measuring both resistance and body voltage characteristics of flooring materials and his summary concludes that ESD flooring performance is a function of flooring/footwear combinations but not necessarily electrical resistance. ANSI/ESD S20.20 has absorbed those finding by recommending test methods that evaluate ESD flooring by characterizing resistive properties (ESD-STM 7.1-2001), total system resistance (ESD-STM 97.1-1999) and body voltage generation (ESD-STM 97.2-1999).
By consulting ESD standards, organizations that use or handle sensitive electronic equipment can intelligently select and install effective ESD flooring solutions without fear of exposing their employees to dangerous low resistance ground connections. But safety isn’t the only concern; excessive electrical resistance can diminish the performance of a grounded floor – footwear system. While a low electrical resistance might expose employees to potentially harmful leakage currents, too much resistance inhibits the decay of static charges allowing potentially harmful walking body voltage generation. For this reason ANSI/ESD S20.20 recommends a maximum system “resistance to ground” of 35 X 106 ohms for either a wrist strap system or an ESD flooring-footwear system.
Armed with lab data derived from ANSI/ESD as well as AATCC test methods, flooring manufacturers advertise and promote their products using several common performance references:
“All (our) flooring meets the minimum recommended requirements of ANSI/ESD S20.20 – tested per ESD S7.1”
- Or -
“This flooring will not generate more than 100 volts when tested per ESD STM97.2-1999,”
- Or -
“Low KV generation less than 1.5 kilovolts per AATCC Test Method 134-2001.”
Although these statements cite mandatory product criteria, meeting these specifications should be viewed a minimum starting points for purchase consideration. Specifications found on product sheets are almost always lab results measured on new flooring under carefully controlled conditions. Controls allow for simple comparisons between various new materials in a non-abusive environment. Specifiers also need to know what they can expect for performance in a real-world environment that includes traffic, chair caster damage, chemical spills, regular maintenance, and material handling abuse as well as normal product aging. Purchase considerations should always include a contingency plan in the event that the flooring material is used either improperly or in a way it was not intended to be used.
For example; a floor that generates body voltages below 100 volts when tested per ESD STM-97.2 may generate several thousand volts on a person who forgets to wear heel straps. A dissipative carpet that measures between 1.0 X 106 ohms and 1.0 X 108 ohms may become insulative from conductive fiber fractures in situations involving chair casters constantly roll back and forth over the surface. These potential scenarios represent serious liabilities to any ESD program and yet they are an everyday reality. As previously stated, engineers and lab technicians are least likely to comply with proper ESD personnel grounding policies and yet these individuals test and handle their company’s most important and expensive items. For any of these reasons it may make sense to identify “contingency or adaptive features” during any product selection process. In the case of ESD flooring that logic may constitute identifying ANSI/ESD S20.20 compliant floor systems that also demonstrate low tribocharging characteristics on people wearing standard insulative footwear. Low tribocharging characteristics (triboelectric compatibility) may not entirely prevent walking body voltage but a conductive floor with antistatic properties will significantly minimize the liability associated with peak voltages in uncontrolled environments.
