Clipping the ESD problem at the knees
10/01/2003
Methods of measurement lend added confidence in the assessment of materials before you establish an ESD management program
By John Chubb
The quantity of electrostatic charge transferred by rubbing and sliding type actions is limited in practice by the intensity of the mechanical action and by the character of the materials involved. What is important is the surface potential created for the maximum quantity of charge likely to occur in practice.
Most people know that static electricity can present risks and cause problems in a variety of industrial situations. The electronics industry is particularly susceptible because of the low energy and low voltages at which damage can occur. While much attention is paid to body voltage in relation to direct discharges, it needs to be understood that indirect effects from charges induced by electric fields from nearby charged surfaces can cause damage via the charged device model.
Static is a materials question—and risks and problems arise if materials are not suitable. In this article, methods of measurement will be considered that can give cleanroom operators confidence in the assessment of materials—clothing, work surfaces, packaging, jigs and fixtures—anything that may come into proximity to sensitive devices or assemblies.
The risks presented by static can be tackled in a number of ways. With conducting items, and in particular people, it's necessary to ensure reliable and continuous earth bonding. With materials that are insulating (such as plastic items and artificial fiber fabrics), or include insulating features, this is not so easy.
Bipolar air ionization may be used to make the local atmosphere slightly conducting to allow surface charges to dissipate. While this is fine as a secondary control action, it will not remove charge fast enough to avoid the occurrence of high surface voltages. The best approach is to ensure that materials are suitable.
Static electricity arises when surfaces in contact are separated. If the charge that arises from differences between the surfaces cannot run away to earth quickly enough, then it is trapped, or static. The "quickly enough" relates to the time for the charge to spread out over the surface of a material and/or to leak away to earth. If this "range of time" for charge movement is very short, the material is described as a conductor; if very slow, an insulator. Performance of materials in this "range of time" has traditionally been assessed by measuring resistivity. We shall learn that this is not an appropriate approach.
Electrostatic charge retained on the surface of a material creates a potential at the surface. It's this potential that creates electric fields at nearby items. These fields cause attraction of dust, dirt and thin films, inducing charges on items nearby that can cause direct electrostatic discharges. Thus, it is surface potential that is the parameter of prime practical importance.
With this is mind, the two primary features of importance to end users are:
- The surface potentials that may arise;
- The time that significant surface potentials are present.
Many materials are non-homogenous; paper, for instance. A prime example is the fabric used for cleanroom garments and for static-controlled clothing. Such fabrics are basically polyester that include a stripe or grid pattern of conductive threads. If these threads are surface-conducting, then it's easy to see that a resistivity measurement from contact electrodes on the fabric surface will only give information about the conductive threads.
In this case, this measurement tells you nothing about the opportunity to retain static charge on the fabric between the threads. It is the retention of charge between the conductive threads that may create an average high surface voltage.
 Set-up for measuring surface voltages on inhabited cleanroom garments. |
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The way to assess whether significant surface voltages will arise on materials and also learn how long voltage is present is to put electrostatic charge on the surface and see what happens. This can be done experimentally by simply rubbing a local area, measuring how much charge has been transferred, and measuring both the initial peak voltage created and how quickly this decays away.
The quantity of electrostatic charge transferred by rubbing and sliding type actions is limited in practice by the intensity of the mechanical action—speed and pressure—and by the character of the materials involved. What is important then is the surface potential created for the maximum quantity of charge likely to occur in practice.
Practical measurements
Experimental measurements have been made on a variety of materials. These studies have shown that the initial peak voltage at the end of a sharp rubbing action is proportional to the charge transferred.1, 2, 3 Results from a number of materials are shown in Figure 1 (page 23).
Practical studies, like those in Figure 1, show how measurement of the charge transfer associated with individual values of surface voltage provides a simple way to get consistent and meaningful measurements from tribocharging experiments.
 Studies have shown that the initial peak voltage at the end of a sharp rubbing action is proportional to the charge transferred. |
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This charge per unit voltage is equivalent to a capacitance; however, since the surface voltage is not a single value but a value averaged over the area charged, it's best to think of the capacitance effect as a capacitance loading, or the capacitance experienced by charge on the test material compared to that which would arise with a similar distribution of charge on a thin layer of a good dielectric.1, 2, 3
Studies have shown that the effect experienced by the tribocharge on the test surface varies greatly between materials. As one might expect, the capacitance loading provided by a grid or stripe pattern of conductive threads can be quite large on some cleanroom garment fabrics. This is shown by the points near the charge axis in Figure 1, compared to the points with best-fit trendlines shown.
Practical measurements have been made on charge decay times and capacitance-loading values on inhabited cleanroom garments. In the test arrangement, illustrated in the photo (page 22), the operator stands on a charge measuring support plate while a fieldmeter is used to measure the local surface voltage arising when an area of the garment surface—in this case, the upper arm—is struck with a charge-neutral Teflon rod. Although such measurements are not easy, and may not give the "neat" results illustrated in Figure 1, they show the same linear relationships between surface voltage and quantity of charge.
Studies to measure capacitance loading can also provide information on how quickly the surface voltage decays as the charge migrates over the surface and away to earth. From measurements at a single test, one gets two parameters relevant to illustrating the practical electrostatic performance of materials.3, 4
Corona charging test procedure
Testing materials by rubbing surfaces is fine for laboratory studies; however, it's quite unsuitable for industrial testing.
Corona charging-based instrumentation developed over the last 15 years is compact, portable and easy to use for consistent measurements.5 Studies have shown that the capacitance loading and the charge decay time performance exhibited when surfaces are rubbed are well matched by those observed with corona charging.1, 2 Test methods based on corona charging are well established, and there is now an International Electrotechnical Commission (IEC) Standard in place for corona charge decay time measurement.6
At the IEST's ESTECH Conference this past May, measurements were reported showing how voltages of several hundred volts can arise on the surfaces of some inhabited commercial cleanroom garments when they are rubbed (as illustrated in the photo).4 It was also shown how these voltages could be predicted from corona charging measurements of capacitance loading and charge decay time on the basic garment fabric. The basic approach is to use a short period of high-voltage corona discharge—for example, 20 ms—to deposit a local patch of charge onto the material.
A fast response electrostatic fieldmeter measures the initial peak voltage created by this charge as soon as the corona discharge electrodes are moved out of the way (within 20 ms) and how quickly this surface voltage falls as charge migrates away. Measurement of the quantity of charge transferred enables the capacitance loading to be calculated.3, 4
It is shown that the maximum surface voltage expected on an inhabited garment, Vmax (volts), can be predicted from the extrapolated value of capacitance loading at zero charge, CL (q=0), and the quantity of charge likely to arise in practical rubbing actions, q (nC), as:
Vmax = f q / (CLq=0)
A value of f=75 has seemed appropriate to ensure that the voltages observed in practice, with allowance for their errors, do not exceed the predicted voltage of Vmax. In the studies on cleanroom garments, we found that the quantity of charge transferred has not exceeded 50 nC. To limit the maximum allowable surface voltage to say 100 V, the capacitance loading at q=0 needs to be over 40.
Care is needed in measuring performance features for materials. In many cases, antistat additives or topical antistats are used to achieve short charge decay times. These usually work by absorbing atmospheric moisture; however, performance will deteriorate at low humidity. When selecting and purchasing materials, it's imperative to know that those materials have been tested over the full range of likely application environments.
Requirements for avoiding static electricity
The risks and problems presented by static electricity can be avoided; it requires that suitable materials be chosen for the particular application and that all conducting items are earth-bonded.
Suitability of materials is assessed by a combination of charge decay time and capacitance loading measurements. Materials can be judged from whether the charge decay time is adequately short—less than 1s to 10% of the initial peak value at cessation of charging—and/or whether the capacitance loading is sufficiently high (typically, over 40).
It's very important that the test methods used are shown to relate to end user requirements. It is to be noted that there are many "standard" methods for charge decay measurement—and they give very different results. In the work presented in this paper, care was taken to ensure that measurement methods match to end user requirements and actually validate the test method proposed.
JOHN N. CHUBB is president of John Chubb Instrumentation in Cheltenham, England. He can be reached at: [email protected].
References
[1] J. N. Chubb, "Measurement of tribo and corona charging features of materials for assessment of risks from static electricity," Trans IEEE Ind Appl 36 (6) Nov/Dec 2000, pp. 1515-1522
[2] J. N. Chubb, "New approaches for electrostatic testing of materials," J. Electrostatics 54 (3/4) March 2002, p. 233 (Presented at ESA meeting, Brock University, Niagara Falls, May 2000)
[3] J. N. Chubb, P. Holdstock, M. Dyer, "Predicting the maximum voltages expected on inhabited cleanroom garments in practical use," Inst Phys Conference 'Electrostatics 2003' Heriot-Watt University, Edinburgh, March 2003
[4] J. N. Chubb, P. Holdstock, M. Dyer, "Predicting the maximum surface voltages expected on inhabited cleanroom garments in practical use," ESTECH 2003, Contamination Control Division, Phoenix, Arizona, May 18-21, 2003
[5] J. N. Chubb, "Instrumentation and standards for testing static control materials," IEEE Trans Ind Appl 26 (6) Nov/Dec 1990, p. 1182
[6] "Measurement methods—Ability of materials and products to dissipate static electric charge," EN 61340-2-1: 2002