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Assessing the Electrostatic Suitability
Of Modern Materials
by J. N. Chubb, Ph.D., John Chubb Instrumentation
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
Avoiding ESD-related risks and problems depends on four main features:
Capability of Surfaces to Drain Charge Away From Conductors in Contact
The capability of materials to drain charge from a conductor is relevant to
materials used for flooring and footwear to keep the voltages of people at a low
level during walking and similar activities. The performance of materials for
such applications is appropriately assessed by measuring resistance.
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 Ω.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
The voltages that arise on surfaces after contacting or rubbing are relevant to
the creation of electric fields on items nearby and the induction of charge that
may lead to other problems. As a general point, if surface charge on the
material that is contacted or rubbed can migrate over the surface in less time
than it takes the surfaces to separate, then no significant surface voltages
will arise from retained charge, and there will be no influence on items nearby.
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 capability of materials to provide shielding against electric field
transients is particularly relevant to packaging of microelectronic components
and assemblies for transport through areas where static electricity is not
controlled. Consider a person carrying a package of components or an assembly
over a nylon carpet and putting the package down on a grounded metal surface.
The body and package voltage might be up to perhaps 20 kV. A discharge could
occur with a rise time of a few nanoseconds, involving 3 µC of charge and 30 mJ
of energy.
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
The capability of a material to support an incendiary ESD is related to the
risks of igniting local flammable atmospheres. This is relevant to materials for
garments that protect people working where flammable gas atmospheres may arise
and with materials for fabric intermediate bulk containers.
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 Ω) 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
The logical way to assess the risk presented by the influence of charge on a
material is to put a known amount of charge on the material, measure what
surface voltage is created, and measure how quickly the surface voltage falls as
the charge moves away.
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
• Comparing these results with results obtained with corona charging of sample
areas of the same fabrics under the same environmental conditions to see how
well they match.
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)
• Whether the capacitance loading value extrapolated to zero charge, based on
the surface voltage at 0.1 s, is greater than N and that the time for the
surface voltage to fall from the 0.1 s value to 10% is less than t(b).
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)
CLq = 0 = the value of capacitance loading measured with
corona charging
extrapolated to zero charge
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
The voltage arising on materials after contact and rubbing by other materials is
a major factor responsible for the risks and problems that may arise from static
electricity. Dissipation of charge over the material in a time scale comparable
to or shorter than the time for separation of contacting or rubbing surfaces is
a prime way to avoid the elevation of any significant voltages. Where charge
decay times cannot be adequately short, then the surface voltage may be
suppressed if surface charge experiences a suitably large capacitance.
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
1. International Electrotechnical Commission, “Electrostatics–Part
5–Specification for the protection of electronic devices from electrostatic
phenomena–Section 1: General requirements,” IEC 61340-5-1 1998.
2. Butterworth, G.J, Paul, E.S, and Chubb, J.N., “A Study of the Incendivity of
Electrical Discharges Between Planar Resistive Electrodes,” Electrostatics 1983,
Institute of Physics Conference Series 66, pp. 185-189.
3. Chubb, J.N., “New Approaches for Electrostatic Testing of Materials,” J.
Electrostatics 54, March 2002, p. 233.
4. Chubb, J.N., Holdstock, P., and Dyer, M., “Predicting Maximum Surface
Voltages on Inhabited Cleanroom Garments in Practical Use,” ESTECH, 2003.
5. Chubb, J.N., “Corona Charging of Practical Materials for Charge Decay
Measurements,” J. Electrostatics 37, 1996, pp. 53-65.
6. Chubb, J.N., “Test Method to Assess the Electrostatic Suitability of
Materials for Retained Electrostatic Charge,” document prepared for discussion
as prospective British Standard 2004,
http://www.jci.co.uk/cache/JCITestMethod.pdf
About the Author
John Chubb, Ph.D., founded John Chubb Instrumentation in 1983 after 10 years of
commercial contract research work with the U.K. Atomic Energy Authority Culham
Laboratory, two years as manager of advanced technical planning with Linotype
Paul, and more than two years as managing director of IDB, a small industrial
company at the University College of North Wales. He earned a degree in physics
at Birmingham University and a Ph.D. on behavior of particles during
electrostatic precipitation, completed a graduate apprenticeship at English
Electric, and then lead development of high power vacuum interrupters there
until moving to Culham Laboratory in 1962. John Chubb Instrumentation Ltd, Unit
30, Lansdown Industrial Estate, Gloucester Road, Cheltenham, GL51 8PL, 44
(0)1242 573347, e-mail: jchubb@jci.co.uk
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