As you slide out of the driver’s seat of your car on a brisk day, you experience a significant electric shock when you grab the metal door handle. What happened? Why were you zapped?
First, you were charged up with static electricity when sliding out of the driver’s seat. This is a common phenomenon experienced whenever two materials come into contact, then separate, and is known as triboelectric charging.
Secondly, your body’s skin is a fair electrical conductor. When it contacted a very good conductor (metal door), your body’s accumulated charge quickly discharged (brought to the same potential as the door), resulting in an electric shock.
Triboelectric charging, or tribocharging for short, is the most frequently occurring phenomenon for charging materials—yet one of the least understood. Tribocharging affects us whenever we move ourselves or other materials around us. It is an important concern of the electronics industry. Static-charge buildup can damage semiconductor devices upon discharge, more specifically electrostatic discharge (ESD).
To control ESD events, it is best to use materials that are low tribocharging (antistatic) as well as dissipative when handling, packaging, and storing ESD-sensitive electronic devices. By understanding triboelectrification, it may help you choose or design materials to minimize tribocharging in static-sensitive areas.
Triboelectric Series
When two materials with neutrally charged surfaces come into contact (<4 Å) and then separate, the materials have undergone tribocharging and now are at a non-neutral surface charge level. The level and polarity of this newly acquired surface charge depend partially on the triboelectric series (Figure 11,2,3).
For example, a material such as glass that contacts a vinyl material acquires a more positive charge because it is nearer the more positive position in the triboelectric series than vinyl. Alternately, the vinyl acquires a more negative charge following the same logic. The fact that these two materials are far from each other in the series may result in a larger charge level than if the glass came into contact with aluminum.
The triboelectric series is a loose ranking of a material’s polarity when triboelectrically charged with another material. Depending on factors such as surface roughness, force of contact, work function, charge backflow, or charge breakdown (of air), a material may easily change position in the series. These variables add to the confusion of understanding the tribocharging mechanism, making the triboelectric series a relative comparison of materials and not an exact science.
Theory
Several mechanisms contribute to the charge generated by the triboelectric process. Four major factors have the greatest influence:
- Surface contact effects.
- Work function.
- Charge backflow.
- Gas breakdown.
The amount that each mechanism influences the net charge is not well understood at this time.
Surface contact effects include the surface’s roughness, contact force, and frictional heating caused by rubbing, all of which influence the amount of surface area that contacts the other material during tribocharging. The greater the surface contact, the greater the resulting net charge may be when the two surfaces are separated.
Although surface contact may seem rather intuitive, some subtleties should be addressed: surface friction and surface roughness (Figure 2). When the coefficient of friction between two surfaces increases, the surface roughness between the two surfaces may be greater, which results in decreased surface contact.
For example, when two 1.0 sq. in. surfaces contact each other, the actual contact area may be only 0.2 sq. in. because of surface roughness. Now, if you press down on the surface, the contact area may increase to 0.4 sq. in. depending on this contact force and, again, the surface roughness of both surfaces.
If both surfaces were micropolished to an extremely smooth, flat area, the contact area may increase to 0.8 sq. in. The smoother either surface is, the more contact the surfaces will make with each other, resulting in the possible increase in the exchange of charges.
Surface-charge imbalance is related to friction because both depend on the adhesion between two surfaces on the molecular level. Two surfaces may stick together because chemical bonds form on the surface. When the surfaces are separated, some bonds may rupture, and any asymmetrical bonds will tend to leave imbalanced charges behind. Which surface bonds rupture depend on their work function.
The work function is the property of a material’s capability to hold its free electrons, the electrons orbiting the outermost shell of the material. The greater the material’s work function, the less likely it is to give up its free electrons during contact. The weaker the work function is, the more likely the material will acquire a more positive charge by giving up or losing some of its free electrons. In general, materials with higher work functions tend to appropriate electrons from materials with lower work functions.
Charge backflow occurs when two materials have been charged and then separated from each other. The backflow of some of this charge imbalance may return to the original material, reducing to some degree the net charge on either surface from tribocharging.
Gas breakdown can occur between two surfaces during separation. The microscopic topology of a surface has many peaks and valleys. One of these peaks may have substantial charge that yields a large electric field in a very small area, causing corona discharge or the breakdown of the air molecules that were acting as an insulator between the two separating surfaces.
During this breakdown, charge can be transferred from one surface to the other via the plasma path. The amount of charge transferred depends on the distance of separation and the gas pressure.
Examples
Standard cellulose tape is a good example of a material with a strong surface adhesion and large surface-area contact typically resulting in large charge imbalance during unwind or removal. During unwind, the process of contacting and separating the tape is called contact charging or electrification by contact and has little to do with friction.
Another contribution to the large charge imbalance of the tape during unwind is the difference in materials. The tape film is cellulose, and the adhesive may be rubber based. The two are spaced far enough apart in the triboelectric series to result in defined polarities (Figure 1).
The rubber adhesive will acquire a more positive charge and the cellulose a more negative charge due to the difference in their work functions (Figure 3). Voltages well over 20 kV are easily measured from this type of tape.
A pair of ESD training paddles also illustrates tribocharging (Figure 4). Typically, one paddle is aluminum and the other acrylic, which are well separated in the triboelectric series. When the bottoms of the paddles are brought together, rotated, and separated, an electrical charge imbalance will exist between the two plates.
Using a static field meter or charge-plate analyzer, you can measure several kilovolts on each paddle. The resulting charge imbalance on the aluminum paddle tends to be more positive and the acrylic more negative.
One paddle is very conductive and the other very insulative, again showing the types of materials that can become charged in your ESD-safe work area. Even though a surface may be conductive, it still can become charged through triboelectric generation. Only when a conductive surface is tied to ground or other reference point will it not hold a charge imbalance.
Controlling charge imbalance is important in ESD control. Conductors can be grounded, but insulators must be controlled by other methods.
Solutions
When designing an ESD control program, there are two simple rules relative to charging problems:
• Ground all conductors.
• Remove or control all insulative charge generators.
Grounding can easily be accomplished with various ESD control products.2 In lieu of not using nonconductive charge generators, controlling them may be essential to the program. Controlling process-necessary insulators can be more involved and require the use of ionization or surface treatment with a topical antistat solution or spray.
Neutralizing charged objects is accomplished by using a balanced-output air ionizer. The target is flooded with a multitude of positive and negative air ions, resulting in a near-zero voltage level relative to ground on the charged surface after just a few seconds of exposure. The charge decay time depends on several factors such as surface proximity to the ionization source, the surface area, the surface capacitance, and the level of charge imbalance.
Conclusion
Understanding how materials interact with each other during the triboelectric process can help in designing a control program to minimize charge imbalance caused by this phenomenon. Choosing materials that are either antistatic or have similar work functions in the triboelectric series will help minimize potential charge imbalances in an ESD-sensitive work area.
References
- MIL-HDBK-263B, Electrostatic Discharge Control Handbook for Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices), Department of Defense, Naval Sea Systems Command, Arlington, VA, 1994.
- Allen, R.C., “ESD Control Standards: Setting up an ESD Control Program,” EE-Evaluation Engineering, February 1999, pp. 112-123.
- Jones, T.B., Triboelectric Charging of Common Objects, University of Rochester, December 1999, www.ee.rochester.edu:8080/~jones/deoms/charging.html.
About the Author
Ryne C. Allen is the technical manager at ESD Systems.com, a division of Desco Industries. Previously, he was chief engineer and lab manager at the Plasma Science and Microelectronics Research Laboratory at Northeastern University. The NARTE-certified ESD control engineer is a member of the ESD Association and several of its standards working groups and secretary and webmaster of the Northeast Chapter of the association. Mr. Allen graduated from Northeastern University with B.S.E.E, M.S.E.E., and M.B.A. degrees. ESD Systems, 19 Brigham St., Unit 9, Marlboro, MA 01752-3170, 508-485-7390, e-mail: [email protected].
Published by EE-Evaluation Engineering
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November 2000