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Is your workbench ESD safe? With a few inexpensive tools and proper precautions, it can be.
Last month, I discussed the generation of ESD, its hazards and some ways of avoiding damage to your electronic equipment and components. This article wraps things up by telling you a bit more about ESD and what you can do to make your workbench ESD-safe.
Some ESD considerations
Chances are, most of us will eventually work on a piece of solid-state gear. Without the appropriate antistatic equipment, (see "Supply Sources" in the Appendix) you'll subject that gear to unnecessary ESD risk. Many ESD products (such as static-dissipative gloves, hats, garments, shoes, and floor and wall finishes) are impractical for most amateurs to consider. Other products, however, such as antistatic tools, desktop air ionizers, wrist/ankle straps and conductive mats are both readily available and affordable. Even the more-expensive and specialized ESD equipment can sometimes be found at swap fests, so it's important for you to become aware of what's out there and what it can do.
Tools
A number of antistatic tools are available for working with static-sensitive components. Antistatic IC insertion and extraction kits, including antistatic DIP pliers and tweezers, are useful additions to any amateur's toolbox. Antistatic soldering systems, both stand-alone soldering irons and temperature-controlled stations, are available from Weller and Ungar. The major difference between the antistatic models and the conventional irons available from these manufacturers is the composition of the plastic used to construct the handles and stands of the soldering irons. Instead of a nonconductive blue plastic, the antistatic irons use a black, carbon-impregnated plastic that can bleed static charges to ground. Prices for antistatic soldering irons from both Weller and Ungar start at about $70, with temperature-controlled models ranging from $75 to almost $300.
Mats
Static-dissipative plastic mats, available for both floors and desktops, are useful for transforming any flat surface into an ESDsafe environment (see Fig 4). These mats, composed of carbon-impregnated plastic, are especially handy when you don't have access to a permanent work area that can be ESD-proo fed (when your workbench is a kitchen or dining-room table, for example). For proper operation, static-dissipative mats must be connected to ground through a several-megohm resistor from a single point on the mat. Mats are relatively inexpensive, with prices starting at about $30.
Fig 4 - Inexpensive, easily obtained protection against ESD-induced damage includes static-dissipative bench and floor mats, as well as wrist straps. To bleed off static charges from personnel and devices on the mats, wrist straps and mats must be connected to ground through a 1- to 10-MΩ-resistor.
Wrist straps
Conductive wrist and ankle straps (see Figs 5 and 6), when connected through a 10-megohm resistor to ground, provide a low-resistance path for dissipating the static charge, but not so low as to present an electric shock hazard. Commercial units can be had for about $10. For instructions on how to build a low-cost wrist-strap system, see the article by George Peacock, W4WYV.(6)
Fig 5 - Conductive wrist straps are an inexpensive and effective means of dissipating static charges from the body. In this assembly, a 1-MΩ resistor has been molded into the plastic cap on the ground-cord snap fastener. When the alligator clip is connected to ground, static charges on the body are bled to ground through the resistor.
Fig 6 - A conductive wrist strap in use. The strap should fit snugly to ensure good electrical contact.
Aerosol coatings
Antistatic sprays (such as Staticide or Static Free) are useful in neutralizing and eliminating static buildup. These sprays - about $8 to $12 for 32-oz pump bottles of liquid or $6 for 16-oz aerosol cans - leave a clear film that neutralizes existing static charges and minimizes subsequent static accumulation.
Air-ion balance and health
Unlike antistatic ionizers, which are designed to produce an equal number of cations and anions, residential air ionizers, often marketed as air-purification systems, are designed to produce anions. These anions in turn become attached to dust particles in the air, which also become negatively charged. Walls, floors, furniture, and other surfaces with a net positive or neutral charge attract these charged dust particles, precipitating them from the air.
In addition to removing dust particles from the air, room air ionizers also have a significant effect upon the ion balance in a room. Environmental pollution, ducted air conditioning, air heaters, and many types of electronic equipment decrease the air-ion density. Although air in the countryside tends to have a net negative ion balance, polluted city air is likely to have a very low to nonexistent ion density, suggesting that a negative ion balance is somehow beneficial. Although controlled medical studies have not shown conclusively that this is true, the research to date suggests that a high cation density is associated with headaches, irritability, and reduced psychological performance (see note 7). In short, the lack of negative ions is thought to lead to human stress.
Because ionizers designed for ESD applications cause the formation of equal quantities of cations and anions in the air, there is no evidence that their use can result in physiological stress. However, because the significance of ion balance and concentration on human health is not yet fully understood, exercise caution when using an antistatic ionizer. That is, use it when you work with static-sensitive devices, and turn it off as soon as you are finished. NU1N
Air ionizers
Air ionizers, although somewhat extravagant for an amateur's workbench, are extremely effective in neutralizing static charges on nonconducting objects in their immediate vicinity, such as tools and components within three or four feet of the ionizer unit.
Normally, room air is composed predominantly of electrically neutral atoms (neutral atoms have an equal number of negatively charged electrons and positively charged protons) of nitrogen, oxygen, helium, etc. Because it's relatively rare for any atoms comprising room air to lose electrons (thereby becoming positive ions or cations) or gain them (becoming negative ions or anions), typical indoor ion density is only a few hundred ions/cc. In comparison, ion densities in the 50,000 to 500,000 ions/cc range can be realized with ionizers designed for ESD protection. Ionized atoms conduct electricity with greater ease than do electrically neutral atoms.
Though the ionization of gases in the air can be accomplished through the use of heat or other types of radiation, commercial antistatic ionizers rely on electron-atom collisions.(7) Electrons, accelerated by a high-voltage discharge (5- to 10-kV), collide with atoms in the air, knocking electrons off some atoms (forming cations), and donating electrons to others (forming anions). Although RF noise may be associated with these high-voltage discharges, it should not pose a problem to most amateurs, because ionizers should be used only when ESD-sensitive devices are actually being handled (you'll be at the workbench and not on the air).
Properly designed antistatic ionizers produce an equal number of cations and anions. This balance is important in ensuring that there are enough positive and negative ions in the air for effective static neutralization of objects with a net positive or negative charge. If a tool has a net negative charge, cations from an ionizer will attach to the tool surface, neutralizing the charge. Similarly, an object with a net positive charge will be neutralized by anions.
Residential room air ionizers, sometimes sold as air-purification devices (see the sidebar, "Air-ion balance and health"), should not be confused with commercial ionizers designed for ESD protection. Residential ionizers are designed to spew free electrons into the air, increasing the concentration of anions and neutralizing any cations in the air. Therefore, if an object has a net negative charge, a residential air ionizer will not help neutralize the charge. Ionizers sold for home air purification should not be used for ESD prevention.
Smoking depletes the air of ions, diminishing the efficacy of any ionizing system. If you're going to rely on an ionizer to protect sensitive components, don't smoke. Even if you don't care about your health, the tar and other airborne products of smoking can compromise the function ality of your communications equipment, computers, and peripherals.
Although good for neutralizing electronic components and tools, ionizers will not neutralize charges that have accumulated on your clothing and body. Ionizers should therefore be used in conjunction with a good wrist- or ankle-strap system. Desktop antistatic ionizers prices start at about $200, with room ionizers costing considerably more.
Summary
Radio amateurs working with solid-state components, servicing microprocessor-controlled communications equipment or working with microcomputer hardware, should be aware of the threat of ESDinduced damage and know how to avoid it. By observing a few inexpensive and easily implemented precautions during component storage and handling, potential ESD damage can be minimized, if not eliminated entirely. Is your workbench ESD safe?
Notes
- G. Peacock, "The Spark Zapper, A Static Electricity Discharge Device," CQ, Mar 1989, pp 36-38.
- The New Illustrated Science and Invention Encyclopedia (Westport, CT: H. S. Stuttman, Inc, 1987).
NU1N, Bryan P. Bergeron