Sender : ESD SYSTEMS, 19 Brigham Street #9, Marlboro, MA 01752-3170

Phone : 508-485-7390

This is a free monthly newsletter, which specializes on issues in static control in the workplace.

The EOS/ESD 20th annual Symposium will be held October 4^th - 8^th in Reno Nevada. This Symposium is an excellent opportunity to learn more about ESD Control from tutorials, technical papers, workshops, speakers, presentations, exhibitors, etc.

We encourage you to stop by the ESD Systems exhibit to learn more about our wide-variety of Electrostatic Discharge (ESD) Control products. This is a great opportunity to meet some of our ESD Systems’ team. Our Booth number is 106. We look forward to seeing you there.

For more information about the EOS/ESD Symposium, contact:

1998 EOS/ESD Symposium

c/o ESD Association

7900 Turin Rd. Bldg. 3, Suite 2

Rome, NY 13440-2069

Phone: (315) 339-6937

Fax: (315) 339-6739

• 1/4 Watt, 1 Megohm blue molded external resistor.
• 3/4" wide royal blue elastic loop material with return hook tipped with a red loop-lock for quick and easy adjustment and wearer's comfort.
• 3/8" wide, 18" long royal blue polyester tab with 8 electrically conductive carbon suffused fibers.
• 0.06" thick, 1.50" wide, dual layer, black conductive exterior with a beige non-marking interior

In very dry areas, humidification is desirable because it makes antistatic materials with "sweat layers" function better as well as an overall reduction (not elimination) in triboelectric charging for all materials. Do not let high humidity levels build a false confidence, and beware of corrosion problems with interconnects and other electrical interfaces.

A high relative humidity, over 30% relative humidity (RH), reduces the resistance of most dielectrics resulting in an increase in return current, which is the current that opposes a charge buildup. When an object is undergoing tribocharging in a high humidity environment, the object will reach an equilibrium point where the tribocharging current equals the return current. For objects that undergo charging to a high potential, the primary impact of humidity is to encourage or discourage corona, and effect the rise time of the discharge current.

Normally, the moisture content in the air tends to lower the surface resistance of floors, carpets, table mats, etc., by letting wet particles create a vaguely conductive (or less than 10^-9 Ohms/square) film over an otherwise insulating surface. If the relative humidity decreases, this favorable phenomenon disappears.

The air itself, being dry, becomes a part of the electrostatic build-up mechanism every time there is an air flow (wind, air conditioning, blower) passing over an insulated surface.

Even a high relative humidity environment can be susceptible to triboelectric charging. Case in point, someone walking across synthetic carpet or arising from a foam cushion can tribocharge 1.5 kV at 80% RH. The fact remains that triboelectric charging becomes troublesome below 20 to 30% relative humidity, for the high voltages attained at 20% RH for the identical case above is 35 kV and 18 kV respectively. According to Koyler et.al. [1], Relative humidity values should include an associated temperature because a temperature factor is involved in surface resistivity.

SUMMARY

Humidity control does limit the triboelectric charging process, but does not eliminate any of the conventional safeguards; it is strictly a backup or "safety net" measure. Also, humidity control may give personnel a false sense of security and cause a relaxation of operator disciplines, thus lowering overall ESD safety. Humidity control is also expensive and can cause corrosion or other adverse side effects. Humidity control is a backup that should be implemented only after careful consideration of benefits vs. cost and hazards.

Humidification to 30 or 40% RH, minimum, at 70 degrees F, is surely desirable, but drawbacks include (1) expense of facilities for adding water to the air, (2) possible adverse effects such as delamination of polyamide circuit-board laminates or corrosion of metals if the humidity becomes too high, (3) the psychological factor of false confidence inspired in operators and even engineers, and (4) personnel discomfort. An ESD control program should still be employed using conventional grounding, shielding, ionization, and training products and techniques.

REFERENCES

[1] ESD From A to Z; Electrostatic Control for Electronics, Kolyer, John M.; Watson, Donald E., Van Nostrand Reinhold, New York, NY, 1990

[2] Electrostatic Discharge and Electronic Equipment, Boxleitner, Warren, IEEE Press, New York, NY, 1989

[3] Electrostatic Discharge; Understand, Simulate and Fix ESD Problems, Mardiguian, Michel, Interference Control Technologies, Inc., Gainesville, VA, 1986

[4] "Exploding the Humidity Half-Truth and Other Dangerous Myths", Moss, R., EOS/ESD Technology Magazine, page 10, April 1987

This is a free monthly newsletter, which specializes on issues in static control in the workplace.

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