What is electrostatic discharge (ESD)? Simply put, it is a sudden and violent redistribution of electrons between objects. As most of us know too well, if the path for this sudden electron rush includes sensitive electronic devices, destruction or latent damage can result.
Unfortunately, once ESD occurs, it is generally too late. It is imperative to know about the potential for damage at the work site before ESD occurs. So the real problem here is electrostatic charge (ESC). Unless ESC is monitored and recorded, the best ESD protection—shoes, smocks, and heel and wrist straps—might not give adequate protection against ESD damage.
Background and Definitions
The time during which an ESD event takes place depends on many parameters. For example, if the interacting bodies are excellent conductors suddenly touching each other, the electrons are probably redistributed in a matter of picoseconds. The resulting peak current during the sudden massive flow is substantial—up to 100 A is not unusual.
On the other hand, if there is resistance or impedance in the path between the interacting bodies, the electrons will require more time to work themselves through these obstacles. The peak current will be lower and the duration of the discharge longer.
Most ESD-protective products work in this manner; that is, by inserting known resistive paths. Consequently, the currents are kept at microampere levels and the durations are extended to the millisecond range or longer, resulting in less likelihood of product damage.
Probably the most important property of a charged body is its capacitance. A capacitor seen through the eyes of a physicist is totally different than the capacitor seen by most electrical engineers. For a physicist, a capacitor is basically a physical body immersed in a dielectric. As a result, there is no need for a second plate in parallel with a first one to make a capacitor—which is the device that the electrical engineer usually thinks of when he hears the word capacitor.
This is a very important distinction, which has resulted in a fair amount of ESD-related products and test-equipment designs based on debatable or erroneous concepts and models of capacitance.
Charge, Capacitance and Potential Difference
If charge is stored in a body, it will develop a potential difference with respect to the body from which the charge was extracted. For example, when a person with conductive footwear walks over a nonconducting floor surface, the shoe sole-to-floor interaction will result in electrons being extracted by one from the other. Assuming it is the shoe soles that get the electrons, the extracted electrons will be distributed over that person’s body; that is, in body capacitance.
The potential difference on an unprotected human body in motion will vary continuously for several reasons, including:
The body bobs up and down above the ground plane.
The dimensions and the geometry of the body are continuously changing.
Different amounts of charge are extracted at each shoe sole-to-surface interaction. Predicting the potential difference of a human body in motion and interacting with its environment is nearly impossible. On the other hand, the potential difference excursion on the human body can be easily measured with a suitable noncontact electrostatic voltmeter or electrometer as shown in Figure 1.
An electrometer provides real-time electrostatic charge measurements for monitoring, auditing or training. A proximity antenna permits the addition of noncontact electrostatic charge monitoring at a workstation.
In the electronics industry, charged human bodies are the component or system killer, and the charges come almost invariably from the interaction between shoe soles and the floor. Seldom does charge come from friction in the clothing itself. Clothing becomes the effective conductive outer skin of the human, or where the excess charges will ultimately end.
The Solution?
The best protection against ESD is to eliminate ESC. If there are no electrons lost or gained in an interaction between bodies, there can be no electron rush between them.
How can we reliably and acceptably protect ourselves from ESC? The easiest solution is to rely on the electron backflow phenomenon. When two materials physically interact, one is likely to lose electrons to the other. The number lost is, among other things, a function of a materials property called the work function. If the interacting materials are both conductors, then electron backflow occurs at the moment of separation, and no net charge exists on the interacting bodies.
Foot Straps and Conductive Shoes
The interaction between conductive footwear and conductive flooring is an example of electron backflow in action. For example, a pair of conductive shoes on a conductive floor works by dissipating ESC. Conversely, conductive shoes on a nonconductive floor, nonconductive shoes on a conductive floor, and nonconductive shoes on a nonconductive floor do not work.
Conductive shoes work well as long as the person wearing them does not use insulating inserts. Foot straps on both feet on a conductive floor work as long as the foot straps are touching the conductive floor. Heel straps do not work when wearers stand on their toes, and toe straps do not work when wearers stand on their heels. Velcro on the foot straps often interferes with the shoe laces, and foot straps frequently loosen on the heels of personnel sitting at workbenches.
These conditions happen more often than you might think. The best foot straps cover both toes and heels.
Wrist Straps
Nothing rivals a wrist strap when it comes to protection against ESC. Unfortunately, wearers frequently forget to connect them when they arrive at their workstations. The wrist-strap grounding cords also are a frequent source of aggravation. They tend to be in the way, snag and sweep tools and components off the workbench.
Smocks
A smock is basically a conductive outer skin. Smocks become the equivalent of a concentric spherical capacitor around the human body, storing enough charge with sufficient capacitance to become a threat of comparable magnitude to that of an unprotected human body. This occasionally happens when they are worn over thick winter clothing in dry weather or when the smock does not make reliable contact with the skin of the person wearing it.
Friction between the smock and underlying clothing causes the smock to become charged with respect to the human body which, in turn, can have a charge of its own. Smock contact with the skin of the human body is very important and is usually done at the cuffs of the smock.
Conclusion
For many years ESC and ESD were neglected, poorly understood, underfunded and stigmatized niches of physics and electrical engineering. This is changing rapidly and dramatically because dense IC structures and magnetic recording heads are more prone to ESD-induced damage.
Still, much of the remedial effort has concentrated on the ESD portion of the problem. While this effort is sorely needed, it is too often misplaced, while the ESC side of the equation is ignored. Remember, if there is no ESC, there is no ESD.
Copyright 1996 Nelson Publishing Inc.
July 1996