ESD testing and standards responsibility shifts to users
By Carolyn Mathas
Sparks, NV — For the past 20 years, the EOS/ESD Symposium presented critical data covering the effects of electrostatic discharge (ESD) and electrical overstress (EOS) on electronic devices, assemblies and systems, to growing ranks of attendees. This year, the Symposium, appropriately held in Sparks, NV, broadened its coverage to include the effects of electrostatics in such areas as contract manufacturing, cleanrooms, disk drive manufacturing, explosive environments, and emerging industries.
More than 50 technical papers covered myriad industry topics — from giant magneto-resistive (GMR) advances to the effects of outgassing in the electronics workplace. For those subjects with a direct bearing on cleanrooms, a subtle, but recurring theme could be seen — the importance of a hands-on approach to contamination control. For example, in the past, ESD and EOS were viewed from a 50,000-foot level as to how they generally affected tasks and equipment. Today, the view is changing. On the way to attaining zero product defects, no longer is it safe to use off-the-shelf solutions, but to view each cleanroom environment as a separate entity. This shifts the responsibility for testing and standards to the users.
Three presentations in particular demonstrate this shift in viewpoint. While discussing the advances in GMR, for example, a technology even more susceptible to ESD than is MR technology, one presentation suggested the importance of internal testing of ESD-safe fabrics. Another presentation discussed the need for changing categorizations and standards that measure the efficacy of cleanroom flooring. Yet another discussed testing of each component of wrist strap systems to determine benefits vs. cost. The presentations did not discuss solely the features and benefits of fabrics, flooring, or wrist straps, but centered on the testing, measuring, and standards used to evaluate them.
A paper written by Wayne Boone of Maxtor, for example, walks the audience through the testing and evaluation of 40 fabrics to identify those that will consistently outperform on an ESD basis. What was unique in this presentation is that the goal was to identify a material to provide the best overall ESD performance by performing comparative testing of numerous fabrics; and, to make testing methods both simple and repeatable, using moderately priced and commercially-available equipment. What was also unique was that all of the testing was performed on fabrics, when, ordinarily, testing is performed on constructed garments themselves. The tests resulted in Maxtor identifying three fabrics as suitable for use by the company — critical information for a hard-disk manufacturer where the results of a tribocharge event are catastrophic failure of MR heads with as little as 5 nJ of energy.
Similarly, a presentation entitled, “Discussion on electric parameters of standard for anti-electrostatic floor,” delves into the classification of floor systems. The authors, representatives of universities, research laboratories, factories and Institutes in Nanjing and Changzhou, China, looked at existing standards for anti-electrostatic environments. One standard applies to the design of the environment where manipulation or equipment minimizes electrostatic damage; and the second standard applies to decreasing the frequency of damage in a running state. Whereas flooring today falls into the first classification, the authors maintain that ESD flooring should be considered part of the anti-electrostatic environment system as a whole, falling into the second category.
By performing tests on flooring samples, the authors conclude that the resistance of floors under hundreds of volts is different from those under tens of volts, making the flooring that is suitable for anti-electrostatic purposes in a computer room no longer suitable in microelectronic production workshops. Their recommendation: there should be a standard stipulating the resistance of an anti-electrostatic floor under certain voltages, or the non-linear relation of voltage and resistance. Present standards that stipulate only resistance values are incomplete. The group suggests that manufacturers publish resistance values, and electrostatic voltage half-decaying time periods so that customers are more informed at the onset. In addition, some indication of the time of usage — given the fact that the resistance of the floor`s surface will also change in time — should also be published.
A third example of how taking a closer look at systems is apparent in a paper delivered by Toshikazu Mamaguchi of Sumitomo and Hideka Uchida of 3M Ltd. Citing the role people play as the major source of ESD-generated damage to electronic components, the team reviewed existing wrist strap methods for the grounding of stationary personnel. Wrist straps when non-functioning were stated to destroy a bipolar amplifier if the worker touches the IC package cover after little more than moving from a sitting to a standing position.
After reviewing the two components that make up a wrist strap grounding system, the cuff and the cord that connects the wearer to ground, the authors reviewed each type of wrist strap and each type of cord variation under ANSI-EOS/ESD S1.1 and Mil-Prf-87893B commercial and military standards. Conventional wrist straps and tin-plated copper foil wire in ground cords, and testing wrist straps at least one time per shift were recommended. Flex life tests performed under the two standards could assist in rating the useful life expectancy of cords, but not pinpoint the actual failure.
Consistently, the view of presenters is to take the available data and continue testing in-house where the conditions affecting EOS/ESD are constant and unique. No longer is a 50,000-foot view appropriate. Performing its own due diligence on products substantially enhances both the effectiveness and knowledge of each company`s manufacturing environment and needs.
Carolyn Mathas is a freelance writer and public relations consultant in Fremont, CA. She can be reached at [email protected].