Conformity Magazine - September 2007 - (Page 52)

One good example of an over-prescribed cure is the use of bench-top air ionizers. Air ionizers produce positive and negative ions that neutralize charged objects if the charged object has an electrostatic field. Air ionizers effectively eliminate static only on stationary or slow-moving, lowcapacitance, charged objects at close range. Air ionizers will not reduce static on fast-moving pick and place equipment if the air curtain is obstructed or if the ions must travel into a small space surrounded by grounded metal. Air ionizers also do not reduce static on human beings or on charged objects such as sheet protectors that are resting on a flat surface (capacitance is too high; there is no field to neutralize). Worse, when they malfunction, air ionizers can pose unanticipated threats to class 0 devices. Contamination or mechanical problems can cause air ionizers to become unbalanced. An unbalanced air ionizer can deposit a charge equal to the amount of the imbalance, causing a CDM failure. A 20 volt imbalance that had no impact on older ESDhardened components would be unacceptable in a class 0 ESD environment. Ninety percent of all ionizers are installed to neutralize charged objects that find their way into the workplace. A solution that’s better than neutralizing charged insulators is not to use them. Before buying ionizers, ESD program managers should consider eliminating specific static generators, such as plastic tweezers, masking tapes and the interfaces on certain placement equipment. Most insulative production aids are available in antistatic or dissipative form. Plastics and other non-essential insulative items should be removed or banned from places where ESD-sensitive devices are being handled, used or repaired. Ionizers should be purchased only after all other preventive alternatives have been exhausted. 5th Consideration: When Buying, Look Beyond Spec Sheets Often, specifications are derived from test methods that do not predict performance for a particular application. Reliance on inappropriate test methods can lead buyers in the wrong direction. For example, carpet manufactures use the AATCC134 test method to specify the antistatic properties of their product. This standard textile industry test method, designed for conventional carpet, is irrelevant for applications involving electronic components. The test evaluates newly manufactured carpet (one time) for walking body voltage, and tests voltage generation only on a person wearing leather or neolite shoes. This test does not measure conductivity, evaluate the longevity of a carpet’s antistatic properties, or test the static generated by the soles of more popular footwear, such as athletic shoes, hiking boots or flip-flops. Most standard ESD testing is done with static control footwear. These tests, conducted under perfect conditions, do not predict what will happen if a visitor or an employee wanders into an area with standard footwear, or if heel or sole straps are worn improperly. In a class 0 application, people should always wear ESD protective footwear, and the only ESD flooring materials that should be used are those that, with ESD footwear, keep charges below 20 volts. Obviously, it is not always possible to monitor or control the use of static control footwear. For this reason, ESD program managers should also evaluate the static generating potential of the floor under various real-world conditions—e.g., with conventional footwear, by asking the subject to lift his or her feet or to wear one heel strap instead of two, etc. Having a full picture may enable a buyer to justify investing in an ultra lowvoltage generating conductive rubber floor instead of a lesseffective epoxy or urethane floor. This same scrutiny should be used when purchasing air ionizers. Most ionizer manufacturers use ANSI/ESD STM 3.1-2006 to provide data on charge decay based on discharge times of a stationary charged plate of a known low capacitance. If the ionizer will be used to neutralize charges on moving placement heads on automated equipment, exposure time should also be considered. The ionizer may be “best of class,” but that does not mean it can neutralize a small object moving at high speeds. 6th Consideration: Buy the Right Instruments to Evaluate New Purchases and to Audit the Program Most ESD program managers use test equipment to audit the performance of ESD controls. Because there are numerous instruments on the market, these purchases are almost always costdriven. This low-cost equipment is described as portable or easyto-use. Portable, easyto-use instruments frequently do not store data and are often Figure 3: Ideal resistance to ground of person wearing static control footwear in combination with ESD flooring, per ANSI/ESD STM97.1-2006 (Copyright 2007, Staticworx) 52 Conformity September 2007

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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