Risks from static electricity arise in many ways in the electronics industry. People, work surfaces, garments, tools, circuit board surfaces, support fixtures, test jigs and packaging can all become easily charged by rubbing against other surfaces in the normal course of production and product-handling activities.
If these charged surfaces contact sensitive semiconductor devices or circuit-board assemblies, damage can occur. For example, contact from a person at 100 V and <1 µJ of energy can impair circuit operation. This is 200 times less than the minimum needed for ignition of common flammable atmospheres and 1,000 times less than that of a perceptible feeling of shock.
Apart from device damage, static discharges can upset the operation of microelectronic systems. Electric fields from charged surfaces can attract airborne dust and debris.
Various actions can be taken to avoid these problems. First, ensure that all conducting objects are reliably bonded to earth; for example, wrist straps for operators and conductive footwear and flooring.1
The main problem area is materials. Plastics are widely used for their many desirable properties, but nearly all are naturally good insulators that easily acquire a charge when rubbed and retain the charge for long periods of time.
The high electric fields which can arise from the retained charge may cause direct problems. Indirect problems can arise from charges induced onto conductors; for example, devices and boards retain charges and can be damaged when they contact earth. As a result, it is essential to ban all unsuitable materials from the proximity of devices and assemblies.
The question is: What do you mean by an unsuitable material and how do you determine suitability?
The simplest way to avoid problems and risks from static electricity is to ensure that any charge released by touching or rubbing materials together can easily migrate to earth. This encompasses both easy charge movement on all materials in proximity to sensitive devices and adequate electrical leakage paths to earth.
If the time scale for charge migration on materials is shorter than the time scale of actions creating charge, then dangerous levels of charge and voltage cannot arise. For manual operations, this means that time scales must be below ½ s.
Materials with long decay times are only acceptable if it is clearly shown that the levels of charge and voltage will always remain adequately low. For fast mechanical handling operations, shorter charge decay times will be needed.
Traditionally, resistance and surface resistivity measurements have been used to judge the acceptability of materials. However, if you want to know how quickly a charge moves on practical materials, the best approach is to put a charge on the material and observe how quickly it disappears.
Recent studies have shown that the method of measuring charge decay using a charge deposited from a high-voltage corona discharge matches the decay behavior of charge generated by rubbing surfaces.2 These studies support the method of measuring charge decay in which a patch of charge is deposited by a high-voltage corona discharge and a fast response electrostatic fieldmeter is used for noncontact measurement of the time for this charge to migrate away.3 The approach is shown in Figure 1 and is included in the new British Standard.
The method of measuring charge decay is important. A method which has been used for many years, the Federal Test Standard 101C Method 4046, gives much quicker decay times (by factors between 5 to 100) than are observed for corona-deposited charge. If the material really is homogeneous (for example, a layer of oil), then comparable results are obtained.5
With FTS 101C, quicker decay times are observed because the charge migration uses the fastest route available in the material, not just the charge at the surface–which is where tribocharge arises. The small print of FTS 101C says it is to be used only for homogeneous materials; however, most people don’t know if materials are homogeneous or what alternatives are available. The corona approach, part of the new British Standard, is equally applicable for homogeneous and nonhomogeneous materials.4
Surface resistivity in ohms per square is still widely used to judge the suitability of materials to control static. Figure 2 shows values of charge decay time constants and values of surface resistivity for the same size areas for a variety of materials. These materials include a number of plastic film materials treated to promote static charge dissipation, paper, fabrics and fabrics with embedded conductive fibers.
Because such a large range of values is covered, the values are shown with logarithmic scales from 105 to1013 W /sq for resistivity and 0.031 s to 320 s for decay-time values. As long as there is no grouping of results along a line, there is no general relation between resistivity and the time scale of charge decay.2 The apparent random distribution of the plotted data indicates that resistivity alone does not adequately correlate with charge mobility.
The unsuitability of resistivity measurements can easily be recognized for materials such as the types of fabrics with embedded conductive thread used in work garments. These fabrics are evidently inhomogeneous.
If electrodes are placed on the fabric to measure resistivity, contact to the conducting threads can easily give a low value. Such measurements reveal nothing about the capability of the surface between the conducting threads to retain charge.
These fabrics can charge to appreciable voltages by rubbing, and retain the charge for long periods. Surprisingly enough, within types of materials there is a wide scatter of values for charge decay and resistivity.
Regular retesting of commonly used materials is essential and can provide some unhappy surprises. Problems may not occur if charging happens to be low. But as with a precipice, you can walk awfully close to the edge without falling off–but it is very helpful to know where the edge is, and where you are.
References
1. BS CECC00015: Part 1: 1991.
2. Chubb, J.N., “Dependence of Charge Decay Characteristics on Charging Parameters,” Institute of Physics Electrostatic ’95 Conference, University of York, April 3-5, 1995.
3. Chubb, J.N., “Instrumentation and Standards for Testing Static Control Materials,” IEEE Transactions on Industry Applications, Vol. 26, No. 6, November/December 1990, p. 1,182.
4. BS 7506: Part 1 and 2: 1995, “Methods for Measurements in Electrostatics.”
5. Chubb, J.N., and Malinverni, P., “Experimental Comparison of Methods of Charge Decay Measurements for a Variety of Materials,” EOS/ESD Symposium, Dallas, 1992, pp. 5A.5.1.
About the Author
John Chubb started John Chubb Instrumentation in 1983 after 10 years of commercial contract research with the U.K. Atomic Energy Authority Culham Laboratory. He holds a degree in physics from Birmingham University and a Ph.D. on behavior of particles during electrostatic precipitation, and completed a graduate apprenticeship at English Electric. John Chubb Instrumentation, Unit 30, Lansdown Industrial Estate, Gloucester Rd., Cheltenham, U.K., GL51 8PL, (011) 441 242 573347.
ESD
Copyright 1995 Nelson Publishing Inc.
September 1995