ESD Safety in Cleanrooms: Natural vs. Man-made Materials

Electrostatic charge build-up in cleanrooms can produce higher levels of surface contamination, electrostatic discharges that damage integrated circuits, MR and GMR heads, and electromagnetic pulses that can disrupt robotics.

Electrostatic charge build-up in cleanrooms can produce higher levels of surface contamination,
electrostatic discharges that damage integrated circuits, MR and GMR heads, and electromagnetic pulses that can disrupt robotics. [The July/August 1998 issue of INSIGHT was largely devoted to electrostatic discharge and its effects.] Cleanroom fabrics and document materials (“papers”) made of natural fibers are somewhat hygroscopic and are often assumed to be static dissipative, in contrast to materials made from artificial fibers (e.g., polyester and nylon). To handle special situations that involve low humidities, the ESD Association test for surface resistivity (S11.11, ESD Association, 1993) is run at 12 ± 3 percent relative humidity (RH).

“Humidity and Temperature Effects on Surface Resistivities” was reported by Kolyer and
Rushworth. (Kolyer and Watson, 1996). They studied antistatic polyethylene, antistatic nylon,
polyethylene with radiation cured coating, cellophane (bare, plasticized, coated and plasticized), filter paper, paper, static limiting floor finish, detergent, and leather. They noted that surface resistivity has been assumed to follow relative humidity (H) exponentially, with R(H) proportional to exp(-bH), but that this behavior was seen only for paper, cellophane and leather in their study.

On their graphs of log(R) vs. H graphs, a straight line would correspond to:
R(H) / R(0) = exp (-bH).

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