The best-equipped service bench in your shop can be a real money-maker when set up properly. It can also be a source of frustration and lost revenue if the threat of ElectroStatic Discharge (ESD) is ignored.
A typical scenario might be where an electronic product is brought in for service, properly diagnosed and repaired, only to find a new symptom requiring additional repair. Unless the technician understands the ESD problem and has developed methods to keep it in check damage from static electricity cannot be ruled out as a potential source of the new problem.
Static electricity is nothing new; it’s all around us and always has been. What has changed is the spread of semiconductors in almost every consumer product we buy. As device complexity increases, often its static sensitivity increases as well. Some semiconductor devices may be damaged by as little as 20-30 volts!
It is important to note that this post is addressing the issue of ESD in terms of control, and not elimination. The potential for an ESD event to occur cannot be completely eliminated outside of a laboratory environment, but we can greatly reduce the risk with proper training and equipment. By implementing a good static control program and developing some simple habits, ESD can be effectively controlled.
The Source of the Problem
Static is all around us. We occasionally will see or feel it by walking on carpet, touching something or someone and feeling the “zap” of a static discharge. The perception level varies but static charge is typically 2000-3000 volts before we can feel it. ESD sensitivity of some parts is under 100 volts – well below the level that we would be able to detect.
Even though carpet may not be used around the service bench, there are many other static “generators” may not be obvious and frequently found around or on a 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-ESD 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.
Setting up a “Static Safe” Program
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 ESD hazard is by using a “static field meter”. The visual impact of locating and measuring static charges of more than 1000 volts will get the attention of skeptical individuals.
Education of Personnel
ESD education and awareness are essential basic ingredients 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, reinforce the understanding by hanging up static control posters in strategic locations. The technician doesn’t need an unaware and/or unprotected person wandering over and touching things on the service bench.
Workstation Grounding
To minimize the threat of an ESD event, we need to bring all components of the system to the same relative potential and maintain that potential. Workstations can be grounded with the following options:
- Establish an ESD Common Grounding Point, an electrical junction where all ESD grounds are connected to. Usually, a common ground point is connected to ground, preferably equipment ground.
- The Service Bench Surface should be covered with a dissipative material. This can be either an ESD-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. Whichever type is chosen, look for a material with surface resistivity of 1 x 109 or less, as these materials are sufficiently conductive to discharge objects in less than one second. The ESD laminate or mat must be grounded to the ESD common grounding point to work properly. Frequently, a one Megohm current limiting safety resistor is used in series with the work surface ground. This blog post will provide more information on how to choose and install your ESD working surface.
- 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 ESD 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 It can usually be mounted off the bench to conserve work area, and then aimed at the area where most of the work is being done. The ionizer does not eliminate the need for grounding the working surface or the operator, but it does drain static charges from insulators, which do not lend themselves to grounding.
Personnel Grounding
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 grounders. Personnel can be grounded through:
- 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. For more information on wrist straps, check out this post.
- 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 for 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 allow. 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.Don’t forget to regularly check and verify your personnel grounding items:
Summary
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 ESD event to happen. The ingredients for an effective ESD program are:
- Education: to ensure that everyone understands the problem and the proper handling of sensitive devices.
- Workstation Grounding: use a dissipative working surface material and dissipative flooring materials as required.
- Personnel Grounding: using wrist straps with ground cords and/or foot-grounding devices.
- Follow-up to ensure Compliance: all elements of the program should be checked frequently to determine that they are working effectively.
The ESD “threat” is not likely to go away soon, and it is very likely to become an even greater hazard, as electronic devices continue to increase in complexity and decrease in size. By implementing a static control program now, you will be prepared for the more sensitive products that will be coming.
