Demonstrating charge dissipation with antistatic materials

Thomas B. Jones
Professor of Electrical Engineering
University of Rochester

Introduction

The need to protect sensitive electronic components and computer boards from electrostatic discharge during handling, shipping, and assembly provided the driving force for development of an entire new class of antistatic packaging materials. Key developments in plastics -- notably conductive polyethylene and sophisticated laminates with very thin metallized films -- have created a new multi-million dollar packaging industry. This enterprise undoubtedly saves many hundreds of millions of dollars each year for the electronics and computer industry and dwarfs all other commercial and industrial antistatic abatement activity. Further impetus for new developments in materials stems from increasing demand for antistatic plastics in other manufactured items such as carpets, automotive upholstery, fuel-handling components in engines, etc. In the lecture hall, demonstrations based on these materials provide excellent opportunities to show that electric charge is not static at all, but instead is always moving and redistributing itself.

The demonstrations described on this page focus on static charge dissipation. They are designed to reveal the importance to chip protection of controlling the rate at which charge is dissipated. The demonstrations described below use either the dissectible capacitor or the electrophorus. The choice between the two is best made based on convenience in a given lecture situation. A very useful accessory is the electroscope attachment because it very nicely reveals the transient decay of electrostatic charge before an audience.

The demonstrations are listed below:

Please click here for a summary of the overlapping and sometimes confusing definitions used for antistatic and static dissipative materials. The classic text on charge relaxation by Woodson and Melcher lays out all the important assumptions and attributes of charge decay in materials with electrical resistivity r and dielectric permittivity e [Woodson and Melcher, 1968]. It describes both temporal charge decay and charge convection phenomena. More recent contributions have focussed on the closely related problem of surface charge accumulation and decay when a free surface is present [Jones and Chan, 1989; Pazda and Jones, 1992].


All these demonstrations rely on the semi-insulative properties of antistatic materials, and so they are quite sensitive to ambient humidity. For RH levels above ~60%, certain countermeasures can be taken to improve the chances of success.


Resistive electrode

Both the classical electrophorus and the dissectible capacitor ordinarily use a conductive (metal) electrode. An interesting variation results when the metal electrode is replaced by a plate of antistatic material (usually plastic) having resistivity in the range of r ~ 109 W-m. To perform this demonstration with the electrophorus, the "electrode" is set down upon the charged plate and connected to ground for ~10 seconds or more. When the electrode is lifted up by its insulating handle, the leaves of the electroscope separate, but if the plate has not been connected to ground for sufficient time, the electroscope reveals less than full charge. The explanation of this phenomenon is that an RC circuit time constant governs the charging rate. This time constant is made far more evident to a lecture audience by using the electroscope attachment. When the electrode is grounded, the leaves take 5 to 10 seconds to settle instead of dropping down instantly as they do when a metal electrode is used. Using much the same procedure, the finite charging/discharging time constant may be demonstrated using the dissectible capacitor.

An implication of this RC time constant -- one with crucial implications for the protection of sensitive electronic chips and computer boards -- is revealed when one attempts to draw a spark from the fully charged resistive "electrode." The discharge that results is either quite anemic or even undetectable. Attempts to achieve ignition of flammable vapors in the ignition chamber are unsuccessful. The reason is that the resistivity of the plate impedes the current flow so much that a capacitive spark discharge is impossible. Refer to the figure showing an equivalent circuit for the charged "electrode" with its distributed capacitance and resistance. Simple identification of a static dissipation time constant for the resistive electrode is difficult because the effective time constant depends on where on the surface the discharge occurs.

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This method of passive spark suppression is designed into electronics packaging materials to protect sensitive electronic components during handling and assembly. To achieve an RC time constant of ~10 seconds in the demonstration, the "electrode" should be made from a sheet of antistatic material having a surface resistivity of ~1012 W/square. For a plate ~1 mm thick, a volume resistivity of ~109 W-m works quite well. The capacitance of the electrode when held by its handle far from other conductors is of the order of 10 pF. The estimated relaxation time constant is then

tckt= RC ~ (1012 W)(10-11 F) = 10 seconds.


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The antistatic handle

A second demonstration of static charge dissipation relaxation can be achieved by replacing the insulating handle of the electrode with another handle made of antistatic material. Either the dissectible capacitor or the electrophorus apparatus can be used with the conductive electrode and the leaf electroscope attachment. The electrode is charged in the usual way, but when it is lifted, the charge on the electrode starts to decay immediately, flowing through the slightly conductive handle to the person's hand, which for the most convincing effect should be grounded. The leaves of the electroscope separate when the electrode is first lifted, but then start to settle immediately.

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To identify the governing time constant for this apparatus, refer to the figure depicting the equivalent RC circuit. The resistance R is easily estimated using

R = r L / p r 2

where L is the length of the handle, r the radius, and r the resistivity. Choosing L ~ 20 cm and r ~ 5 mm for the handle's dimensions, estimating the capacitance of the electrode at C ~ 10 pF and requiring the time constant to be RC ~ 10 seconds, the handle resistivity must be r ~ 4.108 W-m.

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The resistive ground plane

If a sheet of antistatic material is substituted for the lower conductive electrode, the charging time of the dissectible capacitor is affected because the capacitance is now distributed and coupled through finite resistance. To achieve full charging, the electrode must remain connected to the battery for the time required to make the resistive layer an equipotential surface. The figure shows a simple distributed capacitance and resistance model that explains the finite charging time.

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available

To realize a time constant of ~10 seconds, the surface resistance of the layer Rs should be ~1010 W/square. It is important to note that, in contrast to the resistive electrode above, an increase in the charging time caused by the resistive ground plane has no effect at all on the capacitive sparking ability of the upper electrode, once it has been charged up and separated from the lower electrode.


Electrostatic shielding can be demonstrated with antistatic bags made of various conductive plastics and laminates. Please Click here to see this demonstration.

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Library references

T.B. Jones and S. Chan, "Charge relaxation in partially filled vessels," Journal of Electrostatics, Vol. 22, 1989, p. 185-197.

R.J. Pazda and T.B. Jones, "Effect of surface conduction on charge relaxation in partially filled vessels," Journal of Electrostatics, Vol. 28, 1992, p. 175-185.

H.H. Woodson and J.R. Melcher, Electromechanical Dynamics," part II, (Wiley, New York), 1968, section 7.2.

ELECTROPHORUS | DISSECTIBLE CAPACITOR | ELECTROSCOPE

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Last modified: Wednesday, 21-Feb-2007 20:31:53 EST