Conformity Magazine - September 2007 - (Page 51)

possible if the device is one of the rare field-sensitive ones, it is more likely that a problem will result from a CDM model. Like the cloud in the aircraft example, the fields on charged insulators can and will charge electronic devices. If the devices are discharged to the wrong surface, the results could be catastrophic ESD damage. Voltage Suppression Voltage suppression is the product of the equation relating charge, capacitance and voltage or Q = C X V; Q is charge, V is voltage and C is capacitance. When the capacitance of a charged object is raised, its voltage decreases. The easiest way to understand voltage suppression is by performing a simple experiment. This requires two items: a static field meter and a small sheet of clear plastic like the ones used to hold paper in a 3-ring binder. Directions: 1. Place the plastic flat against a wall while rubbing it vigorously with the back of your hand. Static electricity will cause the plastic to stick to the wall. While the plastic is stuck to the wall, measure it with the static meter. Unless the wall is plastic, the static meter will read at or close to zero. Maintaining a constant distance between plastic and meter, slowly pull the plastic off the wall. The meter will suddenly register thousands of volts. Gently allow the plastic sheet to reattach to the wall. The plastic will once again measure at or near zero. Where did the charge go? objects or merely suppresses their electrostatic field. ESD table laminates, for instance, suppress but do not actually discharge items that are placed on them. Because of design flaws and contact resistance issues, it is very difficult for any hard surface table covering to bleed off voltages below 50 volts. Resilient, lower durometer work surfaces should be installed in lieu of or on top of ESD laminates. ESD floors are routinely tested for HBM discharge by attaching a person to a charge plate monitor with a wire or a wrist strap. To test the floor properly, the person standing on the flooring material should move his or her feet, periodically breaking ground. If the subject retains a static charge when the feet are lifted, the ESD control material is merely suppressing the charge. A properly functioning ESD floor should bleed accumulated charges away from the person; when his or her feet are lifted, the voltage should remain at or close to zero. The full method for performing body voltage measurements is described in ANSI/ESD STM97.2. 4th Consideration: Prevention Usually Trumps the Cure Many ESD program managers implement safety nets and redundant precautions such as ionizers, expensive ground monitors and cumbersome packaging techniques. These precautions are often unnecessary and could actually threaten rather than protect class 0 sensitive devices. 2. 3. 4. Lubrizol Conductive Polymers provides the widest selection of ESD protection materials available today. Although this may seem like an aberrant situation, it is not, since static meters do not actually measure charge. Instead, static meters measure voltage. When the plastic is stuck to the wall, the capacitance—ratio of charge to potential on an electrically charged, isolated conductor—is high, so the voltage is low. When the plastic is in free space, because it is no longer sharing space with the wall, the capacitance is low, so the voltage is high. In both cases, the charge was and is the same. It did not appear that way because of what we call voltage suppression. Voltage suppression can cause serious problems for ESD program managers. For example, troublesome insulators like sheet protectors could be highly charged; but, if they are resting on a flat surface, even if the static meter is placed directly over them, their charge cannot be detected. To ensure a correct reading with a static meter, suspicious objects should be lifted, frictioned and measured in free space to determine if they are potential problems. When evaluating ESD work and flooring surfaces, particularly those with a buried conductive layer, it is important to determine whether the material pulls static away from charged TM • • Single-source material supplier for any process or technology • Global manufacturing and distribution providing localized service • Materials meeting today’s ESD & micro-contamination standards Complete line of materials for ESD protection independent of process: injection molding, thermoforming and extrusion For more information on ESD protection materials call 909-364-1740 or visit our website at CARBON ALLOY SYSTEMS www.stat-rite.com TM ELECTRO C O N D U C T I V E C O AT I N G S Y S T E M Stat-Rite a Corporation ®Carbo-Riteisandregistered trademark of The Lubrizol Lubrizol Corporation Electro-Rite are trademarks of The TM © 2007 The Lubrizol Corporation September 2007 Conformity 51 http://www.stat-rite.com http://www.stat-rite.com

Table of Contents Feed for the Digital Edition of Conformity Magazine - September 2007

Contents Editor’s Note FCC Extends Disability Access Requirements to VOIP Commission Fines Importer $3 Million Over Non-Compliant TVs FCC Gets Tough on Retailers Over Analog TV Tuner Labeling Analog Cellular Service Sunset Date Reaffirmed Commission Adopts Recommendations of Katrina Panel Free-Space Antenna Factors Through the Use of Time-Domain Signal Processing Editorial Correction ESD Open Forum An Update on Revisions to IEC 61000-4-2, the Basic Standard on ESD Practical Advice for Meeting the DFS Testing Requirements of the FCC The EMC Impact of Embedded Software ESD Design Buyer's Guide Tate Renominated to the FCC EPA Releases Report on U.S. Electronic Waste Additional Standards For The EU’s Toy Directive Medical Device Manufacturer Enters Consent Order Updated Guidance Document on Laser Products Issued FDA Offers Advice on Defective Heating Pads CPSC Issues China Product Safety Program Report Other CPSC Actions In The News IEC Standards Update Product Reviews Product News UL Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - September 2007

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