Conformity Magazine - March 2009 - (Page 38) • A tote bin of circuit boards can generate a charge as it is moved about the manufacturing floor, and then discharge when an additional circuit board is placed into the bin. As the use of polymers grows in the electronics industry, even more situations will arise in which generated charges will put electronic components at risk. Polymers continue to replace metals in manufacturing areas because of their versatility, flexibility, and cost-effectiveness. However, because polymers are natural insulators, additional precautions must be taken to minimize ESD risk. Uncontrolled Instantaneous “Hard” Discharge Described by physical laws of nature, instantaneous discharge occurs when the resistance of a charged object is too low to guarantee a smooth and controlled release of the accumulated charge into the ground. The so called “hard” discharge phenomenon that typically damages electronic equipment is the same order of surface shock that a lightning bolt generates as it strikes a tree. Instantaneous discharge involves very high currents, with energies sufficient to burn or melt away the tiny structures of semiconductor IC devices. The most common and successful measure against hard discharge is to ground the whole environment. But what happens if grounding cannot be guaranteed for an electronic chip or device along its entire processing chain to the final destination? Controlled “Soft” Discharge A preferred method involves a controlled dissipation of the charge. Controlled discharge means that accumulated charges will be deviated smoothly and non-instantaneously by nonviolating currents, avoiding the lightning bolt effect. It provides the static charge with a soft “air bag” solution if grounding cannot be assured at all times. How to Measure Static Dissipative Performance Charge Decay Time Charge decay time (CDT) is a measure of a material’s ability to bleed off charges, from uncontrolled to instantaneous to smooth. CDT is incorporated into industry specifications such as EIA IS-5 and MIL-B-81705. The charge decay times shown in Table 1 were measured in LDPE film, from 5000V down to 50V, at 12% relative humidity. The LDPE film without additives held the charge significantly longer than 2 seconds, which is presumed to be the upper limit for safe discharge in all electronic specifications. Holding a static charge for the longer time means that the charge is available for any accidental release if it contacts a grounded part. Then, the discharge process cannot be controlled, which is a major issue for neat plastics, and sometimes for plastics containing migratory antistats. On the other hand, instantaneous discharge becomes a real concern with materials that contain highly conductive Charge Decay Time Additive Concentration (%) 0 15 20 Carbon Black >2 seconds instantaneous instantaneous Permanent Antistatic Agents >2 seconds 0.48 seconds 0.05 seconds Table 1: Charge Decay Time Figure 1: Correlation of surface resistivity vs. charge decay Figure 2: Comparison of charge transfer time in conductive and dissipative materials 38 Conformity marCh 2009
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