|
Are today�s ESD test methods up to the challenges presented by modern materials? The suitability of materials to avoid the risks of and problems from static electricity traditionally has been assessed by measuring resistance. This technique, unfortunately, may not be appropriate for many modern materials. The Basics Capability of Surfaces to Drain Charge Away From Conductors in Contact As walking involves times of a few tenths of a second, and a body capacitance typically is around 150 pF, the resistance to ground from the person must be well below 109 ?.1 This is not, however, the same situation as controlling the voltage on a person getting out of a car or up from a chair. Voltages Arising on Surfaces When Contacted or Rubbed by Other Surfaces Considered in this way and appreciating that many modern materials such as clean-room garment fabrics are not very homogeneous, the definition of the suitability of materials must be in terms of charge decay time�not resistance. As times for separation of surfaces with manual activities typically are at least 0.1 s, this means that a dissipative material is one in which the decay of surface voltage to 10% of its initial value takes no more than a few tenths of a second. The maximum time permissible does depend on the application, and � s to 2 s may be acceptable.1 Capability of Materials to Provide Shielding Against Electric Field Transients The ESD Association has developed a standard method to test the shielding performance of packing and bags. This is based on using a human-body model discharge across the outside of the package and measuring the energy observed by pick-up electrodes inside. While this is a practical test for microelectronic packaging, it does not provide any information relevant to other areas where shielding may be needed. It also does not cover the very fast transients around 1 ns in duration that arise with metal-to-metal sparks at a few kilovolts. Information on shielding performance over a wide range of frequencies would enable the suitability of materials to be judged for various applications. Shielding may be needed, for example, to prevent electric fields generated by undergarments from penetrating through clean-room garments. Capability of a Material to Support an Incendiary ESD Spark discharges between metal electrodes can cause ignition, and with common hydrocarbon gases, the minimum ignition energy is around 0.2 mJ. With dissipative and insulating materials, the situation is less clear. A surface voltage needs to be at least 20 to 30 kV for ignition; with metal-to-metal sparks, a minimum voltage of only a few kilovolts is needed. It has seemed plausible that incendiary sparks can be inhibited when the accessible resistivity of the material is fairly high (>108 ?) but not so high that discharges can propagate through the air above the surface.2 Not much work seems to have been done in this area, and there is need for clarification for fabrics that include surface or core conductive threads. Assessing Materials for Retained Charge A suitable approach would be to rub the surface of a material, quickly remove the rubbing surface, and then observe without contact how quickly the surface voltage created by rubbing falls to a low value. Studies using this scuff-charging approach have been carried out on a variety of materials. They indicate that decay times below 0.25 s are needed to limit surface voltages to low values against rubbing actions and that maximum initial peak voltages can be held to low values if the surface charge experiences a high capacitance.3 The appropriateness of corona charge decay and capacitance loading measurements to predict the behavior of a variety of clean-room garments and other fabrics has been validated by: � Measuring the charge decay and capacitance loading performance of a variety of inhabited garments by localized tribocharging.3,4 These studies have shown that comparable results for decay times and capacitance loading are obtained by these two methods of testing.4 Measurements based on corona charging are much easier to make and more suitable for industrial testing use. The studies comparing results to those with tribocharging give confidence in their practical relevance. A number of charge decay measurement techniques in use are not appropriate but included in formal standards. Questions have been raised about whether the surface of materials may be damaged by exposure to corona. Repeat testing at the same location with a variety of materials has shown constant values for decay times and capacitance loading.5 This indicates that corona exposure produces no significant changes in characteristics. On the basis of the comparative experience between tribo and corona charging, a prospective standard test method was prepared for the British Standards Institution.6 It describes design features for corona charge decay test instrumentation, the test procedure, the analysis and presentation of results, and methods for formal calibration. The test method provides measurements of the time for decay of surface voltage from an initial value to a selected percentage of this: 37% and 10% are recommended end points. The initial voltage is measured 0.1 s after the end of a short 20-ms period of charging. This delay is to emulate the time for separation of surfaces rubbed together. In addition, the quantity of corona charge transferred to the sample surface is measured so the effective capacitance experienced by surface charge can be calculated from the initial surface voltage value. Because the distribution of corona charge on the material is not easily known, the capacitance effect is expressed as capacitance loading. This is defined as the ratio of the effective capacitance calculated with a thin layer of good-quality dielectric divided by the effective capacitance measured with the test material, where the distribution of charge is expected to be similar. The suitability of materials is based on either of these performance features: � Whether the time for the surface voltage to decay from the value observed at 0.1 s to 10% of this with open and with grounded backing is less than a specified time, t(a) For general applications t(a) shall be 1 s, N 40, and t(b) 20 s. If the time for charge decay after the 0.1 s is short compared to the time of separation of surfaces and if there is a route available for the charge to leak away to ground, then no significant surface voltages can arise. If it is clear from initial measurements that capacitance loading values are too low for effective control of surface voltages, then only charge decay time measurements are relevant. With installed surfaces, it is only practical to measure the charge decay time with the material as it is. This measurement will be sufficient as long as the decay time is less than the acceptance time t(a). The maximum surface voltage Vmax that may arise in practice for a quantity of charge q can be obtained from the capacitance loading values extrapolated to zero charge as: Vmax = fq/(CLq = 0) where: f = a factor (typically around 75) In practice, values for q are likely to be no more than 50 nC. For critical assessment of materials, testing must be done under well-controlled and standard values of temperature and humidity and with adequate time to stabilize to test environmental conditions. Conclusions Studies have shown that the characteristics of materials with tribocharging are well matched by measurements based on the use of corona charging. A draft standard document has been prepared for making reliable measurement of corona charge decay and capacitance loading. From these measurements, it is possible to predict the surface voltages that can be expected in practical situations for likely quantities of charge transfer. References About the Author |
September 2006