ESD Systemsí ESD Technical Newsletter

Issue 3, September 1998: Volume 1

Reference: http://www.esdsystems.com/newsletters/v1issue3.htm


 

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

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IN THIS ISSUE:


WELCOME TO OUR 3rd EDITION ESD E-NEWSLETTER

We at ESD Systems would again like to extend our thanks for your support and enthusiasm for this Newsletter. This is a free newsletter scheduled with monthly releases, which started with Issue 1, under Volume 1 this past July. This newsletter will compliment our U.S mailed newsletter, but be distinctly different from it. It will focus more on the technical issues regarding ESD Control in the manufacturing and field arenas. We would like to incorporate our readers feedback and suggestions into our future issues. Please send your comments to: editor@esdsystems.com


20TH ANNUAL EOS/ESD SYMPOSIUM, 1998

The EOS/ESD 20th annual Symposium will be held October 4th - 8th 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 Systemsí 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

http://www.eosesd.com


ESD Q&A CORNER

The following questions and answers are selected from our FAQ WEB Page: http://www.esdsystems.com/?PageNo=QANDAINDEX concerning ESD and RH.

Q1: What is the specified humidity level for a class 100 clean room to make Hard Disk Assembly - Anonymous, Madras, India

A1: A good range would be between 40% and 55%. You want to minimize corrosion and maximize ESD protection. There is a paper on Humidity for your convenience located at the URL http://www.esdsystems.com/whitepapers/wp_humidity.asp

 

Q2: How well does relative humidity of the surrounding atmosphere control the ESD? - Rick Dibble, Palmyra, NY

A2: The relative humidity (RH) directly affects the ability of a surface to store surface charges. The higher the RH, the less time an item will hold a charge. The method of surface charge reduction [due to increased RH] can be attributed to recombination and or conduction. As the RH increases, so does the natural conductivity of the air, but even at 100% RH the increase in the natural discharge rate cannot be substituted for proper ESD control practices.

 

Q3: Does the %RH need to be at a certain level to get the maximum benefit and if so, how can that level be determined? - Rick Dibble, Palmyra, NY

A3: There is a definite and noticeable difference in the ability to generate static electricity when the air gets dryer (the % RH decreases). The event of walking across a carpet can yield a charge of 35kV at 10% RH [very dry air], but will drop significantly to 7.5kV at 55% RH. A good range for working humidity would be between 25%RH and 70%RH. This range depends on your working environments. Some clean rooms are kept at a constant RH (~50%), other environments want lower %RH due to corrosion susceptibility to humidity sensitive parts. Check with the manufacturer of your devices and ESD control products for the optimum humidity range. Some ESD control products are humidity dependent such as floor finish and topical antistats.


PRODUCT UPDATES

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SOME FEATURES


Dr. ZAP

Topics: ESD & Humidification (paper gleaned from http://www.esdsystems.com/whtpaper/humidity.htm)

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.

Need your own copy? Want to subscribe to this Newsletter? All you or your colleague(s) need to do is simply fill out the subscription form at http://www.esdsystems.com/forms/esdmail.asp

This Newsletter is never sent unsolicited. To unsubscribe from this mailing, simply reply to this e-mail and include in the subject field the following: UNSUBSCRIBE ESD_Newsletters

Let us know what you think. Tell us what you would like to see in future issues. Want to contribute articles or other related information to our Newsletter? Send your comments to the editor@esdsystems.com

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