The best-equipped service bench in your shop can be a real moneymaker when set up properly. It can also be a source of frustration and lost revenue if the threat of ElectroStatic Discharge (E.S.D.) is ignored. Static electricity is nothing new; it's all around us and always has been. What has changed is the proliferation of semiconductors in almost every consumer product we buy. Couple that with the fact that as device complexity becomes greater, often it's static sensitivity increases. Some semiconductor devices may be damaged by as little as 20-30 volts!

A typical scenario might be where an electronic product is brought in for service, properly diagnosed, repaired, only to find a new and perhaps different symptom, necessitating additional repair. Damage from static electricity cannot be ruled out, unless the technician understands the E.S.D. problem and has developed methods to keep it in check.

It is important to note that we are addressing the issue of E.S.D. in terms of control, and not elimination. The potential for an E.S.D. event to occur cannot be totally eliminated outside of a laboratory environment, but we can greatly reduce the risk with the proper training and equipment. By implementing a good static control program and developing some simple habits, the problem can be effectively controlled.

As mentioned earlier, static is all around us. We occasionally will see or feel it by walking on carpet and touching something or someone, and feeling the "zap" of a static discharge. The perception level varies, but the static charge is typically 2000-3000 volts before we can feel it. Remembering that the sensitivity of some parts is under 100 volts, it's easy to see that we might never know that an E.S.D. event has occurred.

Even though carpet may not be used around the service bench, there are many other, more subtle, static "generators" frequently found around or on the service bench. The innocent-looking styrofoam coffee cup can be a tremendous source of static. The simple act of pulling several inches of adhesive tape from a roll can generate several thousand volts of static! Many insulative materials will develop a charge by rubbing them or separating them from another material. This phenomenon is known as "tribocharging" and it occurs often where there are insulative materials present.

People are often a major factor in generation of static charges. Studies have shown that personnel in a manufacturing environment frequently develop 5000 volts or more just by walking across the floor. Again, this is "tribocharging" produced by the separation of their shoes and the flooring as they walk.

A technician seated at a non-E.S.D. workbench could easily have a 400-500 volt charge on his or her body caused not only by friction or tribocharging, but additionally by the constant change in body capacitance that occurs from natural movements. The simple act of lifting both feet off the floor can raise the measured voltage on a person as much as 500-1000 volts.

Perhaps the most important factor in a successful static control program is developing an awareness of the "unseen" problem. One of the best ways to demonstrate the hazard is by using a "static field meter". Although this is not something a service center would typically purchase, it often can be borrowed from a local static control product distributor. The visual impact of locating and measuring static charges in excess of 1000 volts will surely get the attention of the skeptics.

This is an essential basic ingredient in any effective static control program. A high level of static awareness must be created and maintained in and around the protected area. Once personnel understand the potential problem, it might help to reinforce this understanding by hanging up a few static control posters in strategic locations. The technician doesn't need an unprotected person wandering over and touching things on the service bench.

Information on static control and setting up a static-safe workstation is readily available from a variety of sources. Your local electronic parts distributor will often have a variety of E.S.D. Control products and may also have literature from manufacturers and others on setting up a static safe area. The EOS/ESD Association has a comprehensive library of standards that may be used as guidelines in setting up your E.S.D. program. They cover issues such as wrist straps, work surfaces, testers, and the like. Their address is:

To minimize the threat of an E.S.D. event, we need to bring all components of the system to the same relative potential and keep them that way.

--Establish an E.S.D. Common Grounding Point, an electrical junction where all E.S.D. grounds are to be connected. Usually, a common ground point is connected to the equipment ground or green wire at the nearest electrical service outlet. For safety, the bench power circuit should be GFI protected, and proper outlet wiring should be verified and not taken for granted.

--The Service Bench Surface should be covered with a dissipative material. This can be either an E.S.D.-type high-pressure laminate formed as the benchtop surface, or it may be one of the many types of dissipative mats placed upon the benchtop surface. The mats are available in different colors, with different surface textures, and with various cushioning effects. My personal preference is one of the lighter colors - they show insulative soil better and small parts also show up more readily.

Whichever type is chosen, look for a material with surface resistivity of 1X10⁹ or less, as these materials are sufficiently conductive to discharge objects in less than one second. The E.S.D. laminate or mat must be grounded to the E.S.D. common grounding point to work properly. Frequently, a one megohm current limiting safety resistor is used in series with the work surface ground.

Several manufacturers make what is called a "common point ground" cord that attaches at one end to the equipment/green wire ground; the other end attaches to the work surface and also provides one or more banana jacks for connection of personnel wrist strap cords.

--A Dissipative Floor Mat may also be used, especially if the technician intends to wear foot-grounding devices. The selection of the floor mat should take into consideration several factors. If anything is to roll on the mat, then a soft, cushion-type mat will probably not work well. If the tech does a lot of standing, then the soft, anti-fatigue type will be much appreciated. Again, the mat should be grounded to the common ground point, with or without the safety resistor as desired.

--Workstation Tools and Supplies should be selected with E.S.D. in mind. Avoid insulators and plastics where possible on and around the bench. Poly bags and normal adhesive tapes can generate substantial charges, as can plastic cups and glasses. If charge-generating plastics and the like cannot be eliminated, consider using one of the small, low cost air ionizers available from some manufacturers. It can usually be mounted off the bench to conserve work area, and then aimed at the area where most work is being done. The ionizer does not eliminate the need for grounding the worksurface or the operator, but it does drain static charges from insulators, which do not lend themselves to grounding.

As was mentioned previously, people are great static generators. Simple movements at the bench can easily build up charges as high as 500-1000 volts; therefore, controlling this charge build up on the technician is essential. The two best known methods for draining the charge on a person are wrist straps with ground cords and foot or heel grounds.

--Wrist Straps are probably the most common item used for personnel grounding. They are comprised of a conductive band or strap that fits snugly on the wrist. The wrist strap is frequently made of an elastic material with a conductive inner surface, or it may be a metallic expandable band similar to that found on a watch.

--Ground Cords are typically made of a highly flexible wire and often are made retractable for additional freedom of movement. There are two safety features that are usually built into the cord, and the user should not attempt to bypass them. The first, and most important, is a current limiting resistor (typically 1 Megohm) which prevents hazardous current from flowing through the cord in the event the wearer inadvertently contacts line voltage. The line voltage may find another path to ground, but the cord is designed to neither increase or reduce shock hazard with voltages under 250 volts. The second safety feature built into most cords is a breakaway connection to allow the user to exit rapidly in an emergency. This is usually accomplished by using a snap connector at the wrist strap end.

--Foot or Heel Grounders are frequently used where the technician needs more freedom of movement than the wrist strap and cord allows. The heel grounder is often made of a conductive rubber or vinyl and is worn over a standard shoe. It usually has a strap that passes under the heel for good contact and a strap of some type that is laid inside the shoe for contact to the wearer. Heel grounders must be used with some type of conductive or dissipative floor surface to be effective, and should be worn on both feet to insure continuous contact with the floor. Obviously, lifting both feet from the floor while sitting will cause protection to be lost.

An effective static control program doesn't have to be expensive or complex. The main concept is to minimize generation of static and to drain it away when it does occur, thereby lessening the chance for an E.S.D. event to happen. The ingredients for an effective E.S.D. program are:

--EDUCATION--to insure that everyone understands the problem and the proper handling of sensitive devices.

--WORKSTATION GROUNDING--through the use of a dissipative worksurface material and dissipative flooring materials as required.

--PERSONNEL GROUNDING--using wrist straps with ground cords and/or foot-grounding devices.

--FOLLOW-UP TO INSURE COMPLIANCE--all elements of the program should be checked frequently to determine that they are working effectively.