Surface Potential Helps Assess ESD Properties of Materials A Tutorial

Many materials involved in manufacturing, handling, and using microelectronic parts easily become electrostatically charged when rubbed. If charged surfaces come near static-sensitive devices, the device could be destroyed or suffer function degradation. The risk may originate from direct electrostatic discharges or indirect effects by the induced charges from the local electric fields created.

Where charged surfaces are metal, such hazards can be avoided by ensuring an electrical bonding path to ground. With materials that are insulating such as plastics or artificial fibers, this is not so easy.

So, how do you know if materials will be suitable for use where static electricity could present a risk? Go back to the basics, and look at questions from the point of view of an end user. Then, you can get a general appreciation of how to assess materials and how the performance of materials may be improved.

The Basics

Static electricity arises when surfaces in contact are separated. If the charge generated from differences between the surfaces cannot travel to ground quickly enough, then it is trapped. The term quickly enough relates to the time for the charge to spread over the surface of a material or bleed off to ground.

If this time for charge movement is very short, the material is a conductor. If the discharge is very slow, then it is an insulator.

Electrostatic charge retained on the surface of a material creates a potential at the surface, which generates electric fields on nearby items. These fields attract dust, dirt, and thin films and induce charges that can cause electrostatic discharges. As a result, surface potential is important to many of the problems that static electricity can cause.

The quantity of electrostatic charge transferred to materials by rubbing and sliding is limited by the intensity of the mechanical operation (speed and pressure) and the characteristics of the materials involved. Then, the highest surface potential that will arise for the maximum quantity of charge likely to occur is important.

Surface potentials will be limited to a low value:

If the time scale for dissipation of the charge over the surface and away to ground is short compared to the time taken for the rubbing surfaces to separate.
If charge on the material experiences a high capacitance since this suppresses the surface potential.

Either or both of these aspects may control the surface potentials likely to occur in practice. They determine whether materials will give rise to ESD problems or be suitable for particular applications.

Assessment of Materials

To assess the capability of materials to dissipate static charge and show the capacitance experienced by charge on the surface, put a known amount of charge on the material. Then, see what surface voltage is created and 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 decreases to a low value.

Studies on a variety of materials by this scuff-charging approach have been conducted.^1,2 They indicate that decay times below 0.25 s are needed to limit surface voltages to low values during rubbing actions.

With tribocharging studies, observed voltages vary from test to test depending on the pressure and speed of the rubbing action. By measuring the quantity of charge transferred to create individual values of initial peak voltage, you must perform much more consistent and meaningful measurements.

The charge transferred to the surface can be measured by putting an initially charge-neutral Teflon rod into a Faraday cage immediately after the scuff-charging action. Figure 1 illustrates how the surface voltage created by scuff charging varies linearly with the quantity of charge transferred in the rubbing action.

The charge needed to achieve a certain surface voltage has the dimensions of a capacitance. The slope of the lines shows that surface charge experiences different values of capacitance with different materials.

In Figure 1, the surface voltages achieved per unit quantity of charge are much less for materials with conductive threads. These threads enhance the capacitance experienced by the surface charge.

This capacitance arises from the spatial distribution of the charge and the influences of the dielectric constant of the surface material and of nearby ground or conductive surfaces that may be within the material. To avoid worrying about details of the area and distribution of charge, the term capacitance loading has been proposed.^1,2 This is the ratio of the capacitance experienced by a charge on the test material compared to the capacitance a similar distribution of charges experiences on a thin layer of a good dielectric.

The scuff-charging approach has created some useful insights into the behavior of materials.^1,2 However, a more suitable approach uses a high-voltage corona discharge to place a local patch of charge on the surface of the material. A fast-response electrostatic field meter measures both the initial peak voltage and how quickly it dissipates.³

Figure 2 illustrates the practical arrangement in which a small cluster of corona discharge points is mounted on the underside of a light moveable plate with an electrostatic field meter. After a corona pulse of a few kilovolts, typically in the range 3-kV to 10-kV positive or negative for 20 ms, the plate is moved quickly away in less than 20 ms. Then the field meter observes the initial peak voltage generated by the charge and follows how quickly the surface voltage decays.

To calculate the effective capacitance experienced by charge on the surface, combine the initial peak voltage measurement observed in charge-decay studies with the measurement of the quantity of charge transferred to the test surface.² Studies have shown that the capability of materials to dissipate triboelectric charge is well matched by the decay of charge deposited from a high-voltage corona discharge.^1,2

Practical Studies on Clean-Room Garments

One area of recent interest has been clothing for clean-room use. Measurements of charge-decay and capacitance-loading characteristics were conducted on a variety of garments. Then measurements were made on the local voltages generated on these garments when inhabited and scuff charged by striking an area with a charge-neutral Teflon rod.

Clean-room garments usually are made of a polyester fabric that includes a stripe or grid pattern of surface or core conductive threads. A surface antistat also may be applied.

The quantities of charge used in corona-charging studies should be comparable to the quantities from scuff charging. When corona-charging measurements were made with low quantities of charge, capacitance-loading values varied essentially linearly with the quantity of charge. The best prediction of the maximum local surface voltage with inhabited garments is based on the capacitance-loading value that would be observed with zero charge deposited. Then,

V_max = f q/(CL_q=0)

where: q = the quantity of charge transferred, typically 20 to 80 nC
f = a factor around 75
CL_{q = 0} = the capacitance loading value extrapolated to zero charge

A reasonable relationship exists between the surface voltages per unit charge observed on inhabited garments and capacitance loading values measured with corona charging on the fabric. Figure 3 shows the average peak values of voltage observed on garments for a quantity of charge of 10 nC and the maximum surface voltage predicted from the capacitance-loading measurements.

The form of variations matches reasonably well. Differences in values are the result of the comparison between average and predicted maximum values. The predicted maximum values are above the highest value likely to occur.

Measurements during studies also showed that garment voltages did not relate to charge decay times or resistivity values. Where charge decay times are short, surface voltages can be expected to be below the levels predicted by capacitance-loading measurements.

Conclusions

The surface potential created by retained electrostatic charge is most significant in assessing the suitability of materials for avoiding problems with static electricity. These measurement methods give results relevant to end users. In particular, surface voltages on inhabited clean-room garments can be predicted from measurements of capacitance loading made using corona charging on sample areas of fabric.

In general, there is no matching between surface voltage values and resistivity values or corona charge decay times.⁴ If charge-decay times are very short, less than 1 s, then surface voltages remain low. However, the surface voltages created by charge retained on materials have no relationship to resistivity measurements.

Measurements of charge decay time and capacitance loading must be made using appropriate test methods. Charge-decay measurements based on the use of corona charging provide the basis for easy-to-use instrumentation suitable for a variety of materials and give results that match well to data observed with tribocharging.^1,2

A number of standard test methods are available for assessing materials, but few have matched the practical experience of charge dissipation and been suitable for use with a variety of materials. In particular, studies have shown that FTS 101C Method 4046, the federal test standard for measuring charge decay, is not appropriate for measuring the charge-decay characteristics of materials.⁵

References

Chubb, J. N., “Measurement of Tribo and Corona Charging Features of Materials for Assessment of Risks From Static Electricity,” Trans IEEE Ind Appl 36 (6), November/December 2000, pp. 1515-1522.
Chubb, J. N., “New Approaches for Electrostatic Testing of Materials,” J. Electrostatics 54 (3/4), March 2002, p. 233.
Chubb, J. N., “Instrumentation and Standards for Testing Static Control Materials,” IEEE Trans Ind. Appl. 26 (6), November/December 1990, p. 1182.
Chubb, J. N., “Avoiding Risks From Static Electricity,” Evaluation Engineering, September 1995, p. 57.
Chubb, J. N. and Malinverni, P. “Experimental Comparison of Methods of Charge Decay Measurement for a Variety of Materials,” EOS/ESD Symposium, 1992, 5A.5.1.

About the Author

John Chubb started John Chubb Instrumentation 20 years ago after spending 16 years with the U.K. Atomic Energy Authority. He earned a degree in physics from Birmingham University and a Ph.D. on electrostatic aspects of dust collection. In addition, Mr. Chubb has published more than 70 papers, organized a number of meetings for the Electrostatics Group of the Institute of Physics, and been involved in the development of British Standards documents. John Chubb Instrumentation, Unit 30, Lansdown Industrial Estate, Gloucester Rd., Cheltenham, GL51 8PL, U.K., (011) 44 (0) 1242 573347, email: [email protected]

Return to EE Home Page

Published by EE-Evaluation Engineering
All contents © 2003 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.

September 2003