Electrostatic Discharge

Electrostatic Discharge

Electrostatic discharge (ESD) isn't an everyday topic of discussion, so it's easy to put it out of mind. However, careless work habits or the rightin this case, maybe the wrongenvironmental factors can create a lot of trouble for you.

Recognize this experience? You walk across a carpeted room on a day when the humidity is very low, reach for a doorknob, and get zapped. Here's how it happens. Electrostatic charges, also known as static electricity, build up when two insulating materials are rubbed together. Electrons are rubbed off of one surface and onto another. So, one surface will now have an excess of electrons and thus be negatively charged, while the other will have a relative deficiency of electrons and be positively charged.

The magnitude of these charges is impressiveas much as tens of thousands of volts . Specific magnitudes of electron flow, or current, expressed in amperes, pose an electrocution hazard to people. Fortunately, the current in the situation just mentioned is very low. Although the shock that we get when we touch a doorknob or other conductive material is startling, it doesn't do any damage.

Even though people aren't harmed by such ESD incidents, transistors and the integrated circuits built from them are extremely sensitive to minute currents and any voltages that exceed the 5 volts to 12 volts from which they typically are powered .

The Unseen Threat

Manufacturers of computers and other electronic devices, such as telephones, copiers, fax machines, and so on, take great pains to design their systems so they're armored against ESD hazards. In general, they're mostly successful in that regard. Most computers are enclosed in a grounded metal chassis that will shield the internal electronics from any discharge. The chassis is further surrounded by an insulating plastic outer casing . About the only vulnerable points of most computers are serial and parallel ports, and users are unlikely to be touching these in everyday use.

The part of a computer most likely to get zapped is the one you come into contact with most frequently: the keyboard. 3M (St. Paul, MN) has come up with a self- adhesive static dissipative strip that you can apply to a keyboard, just below the space bar. It includes a ground wire that you connect to a known electrical ground (the screw on the faceplate of a grounded AC wall outlet, for example).

By touching the grounding strip before touching anything else on your computer, you'll dissipate any electrostatic charge that you may have built up by walking across the carpet. I've also seen other types of keyboard wrist rests and related devices that incorporate some type of anti-static feature.

Perhaps the situation where you must take the most precautions against ESD is when you're servicing a computer or handling circuit boards. Electronics manufacturers are keenly aware that unmounted integrated circuits (ICs) and circuit boards are extremely vulnerable to ESD hazards, and they take elaborate precautions in their manufacturing operations.

Most do not install carpeting on shop floors, preferring linoleum or other floor tiles, polished with electrically conductive floor waxes. Assembly tables are covered with conductive laminates. Workers wear cotton anti-static shop coats and eschew nylon or other nonconductive synthetic textiles . Parts bins and trays are molded from special conductive plastics. And finished circuit boards are shipped in anti-static plastic bags.

Minding The Shop

A completed circuit boarda network adapter or modem, for exampleis most vulnerable to ESD right when it arrives in your shop, ready for installation in a computer. But you eliminate ESD hazards by taking a few simple precautions and investing in a couple of low-cost anti-static components .

A good start is to obtain conductive anti-static table mats for your service bench. These have a ground wire for connection to a reliable ground point. It's a good idea to place any computer that you're going to service on such a mat before opening it up. You can also get anti-static wrist straps. These come with a coiled cord that connects to ground through a 1-megohm resistor. The resistance is for personal safety, in the event you should come into contact with 110-volt power. The coiled cord shouldn't interfere with your work. And, most use a snap attachment to the wrist strap, so if you need to walk away from the bench to fetch something, it's a simple matter to unsnap the coiled cord, then quickly snap it back onto the wrist strap when you return.

If you refuse to be chained to your workbench, you can get elastic grounding straps that go around your ankle and under your shoe. (Perhaps the pinnacle in geek attire , they could make a great conversation starter if you wear one out to lunch . Maybe you'd like to rethink that wrist strap now.) For the grounding straps to work, you need to be standing on a grounded, conductive surface. A grounded anti-static mat is best.

For those times when you need to work on a computer at the user 's site rather than in the shop, you can get static dissipative field service kits that consist of a wrist strap and a roll-up flexible plastic mat.

If you need to work on a computer, but don't have an anti-static mat and wrist strap, there are still some precautions you can take. For example, before opening up the computer, make sure the power is turned off, but leave the power cord plugged in. This will ensure that the computer's chassis is grounded through the power cord.

Alternatively, use a wire terminated with alligator clips to connect the chassis to an earth ground. Touch some point on the metal chassis, the back panel, for example, before opening the computer. This will drain off any static charge that may have been on your body. When you've opened up the system and before you reach in to remove any circuit boardstouch the chassis again.

Before opening the anti-static bag on any new board you're installing, hold the board in its protective bag in one hand while you touch the chassis with your other hand. By doing this, you'll drain off any static charge that may exist on the outside of the bag. You can then open the bag and remove the board. It's good practice to handle circuit boards by their edges or mounting brackets, and avoid touching ICs or circuit traces. Obviously, if the board has jumpers that need to be set, you're going to have to touch the board. But it's still a good idea to avoid handling circuit boards to the extent that you can.

Walking around is a good way to build a static charge, so if you need to step away from the system, make sure you again ground yourself by touching the chassis upon your return.

By using these techniques, you can work on a system with very little likelihood of doing any ESD damage. Clearly, though, using an anti-static wrist strap is a far more positive way to guarantee that you aren't harboring a static charge.

Be Sensitive

If a circuit board arrives in an anti-static plastic bag, you should probably conclude that the component is static-sensitive. So, don't pull it out of the bag until you're ready to install it in a computer. First, place the computer and all the boards you're going to install (still in their conductive plastic bags) on the grounded anti-static mat. Then, put on your wrist strap before pulling any circuit boards out of their bags. Any boards you remove should be put into conductive anti-static bags. (It's a good idea to hang onto anti-static bags that come with new boards.)

Handling polystyrene cups, polystyrene peanuts and packing material, and the plastic shrink-wrap film that new software comes in can expose you to a high-voltage static field that can zap components, so keep these things out of your server rooms, wiring closets, and service shop.

To understand why nonconductive plastics are such an ESD hazard, consider the polystyrene cup. Let's assume it has a high-voltage static charge on its surface. Touching the cup to a grounded electrical conductor will drain off the charge only at the point of contact. Because the plastic in the cup acts as an insulator, the charge that exists over the rest of the cup's surface remains in place.

Ionized-air blowers are available for service benches. These produce a stream of ionized air (both positive and negative ions) that neutralizes any static field that may exist on nonconductive plastic surfaces. The blowers can be a nice insurance policy, in case nonconductive materials come into contact with sensitive electronic components. Even if you take this precaution, however, it's still good practice to have a policy of keeping plastic cups and other materials away from the service bench.

On The Carpet

You'll also want to give some attention to the type of floor covering that's being used in your offices. Many synthetic carpets are complete insulators and promote static build-up. If you're outfitting a new office or putting new carpet into an existing one, look for carpets that are conductiveeither by nature of the material they're made of, or from some coating that's been applied. You can also treat existing carpeting with anti-static sprays. (Repeated carpet cleaning tends to remove anti-static treatments and sprays, so you'll need to periodically reapply them.)

Many people have trouble rolling their office chairs across carpeting ( especially if it's deep pile), so they put a hard plastic or rubber chair mat on top of the carpet, to get a nice, smooth rolling surface. The only problem with this is that most of these mats are electrical insulators, which again means there's potential for static charges to build up. An alternative is to buy special static-dissipative chair mats. These are made of conductive plastic and come with a ground wire that you can connect in the same fashion as the keyboard anti-static strip mentioned previously.

Also, avoid low relative humidity. Humid air conducts electricity, to a certain extent. By contrast, very dry air is an insulator. Heaters and furnaces not only heat air, they reduce the relative humidity as they do so. You may want to add a humidifier to your heating system to add moisture back into the air. Obviously, you don't want to go overboard with thisrelative humidity in excess of 90 percent can result in condensation that can short-circuit electronics. It's ultra -low relative humidities (15 percent or less) that you want to avoid.

How Bad Can It Be?

Am I going overboard with this ESD stuff? Somewhere between paranoia and oblivion lies a happy medium. Some organizations don't have much of an ESD problem. Others, particularly those in dry or cold climates, can have severe problems. What's so insidious about ESD is that an organization may have a problem and not even know it. It takes fairly high voltages (up in the thousands of volts) to produce a noticeable spark when you touch a metallic object. You can be carrying a static charge of a few hundred volts, which is enough to damage a circuit board, and not know it, because you don't feel the discharge when you touch metal.

One way to determine whether hazardous ESD levels exist in your environment is to use a static field meter. These devices, which typically sell for $300 to $500, can measure the static charge on a surface, when placed within a few inches of the surface.

Making changes to your office building's heating and air conditioning equipment is not a trivial expense. If you're not willing to take that plunge, there are still a number of easy, inexpensive steps you can take. The logical move is to eliminate the points of greatest vulnerability. So, focus your efforts on those areas where computers are being worked on and circuit boards are being handled.

For the most part, it doesn't cost a fortune to purchase the equipment and supplies needed for ESD protection. Anti-static wrist straps cost $25 or less. Most table mats are priced at less than $100. Wrist strap resistance testers are in the $100 to $300 range. The cost of all of this probably adds up to less than that of a single computer. (I can hear it now: "If it saves even one computer's life, it's worth it." But it's true.)

For anti-static materials and equipment, check out suppliers of electronic assembly equipment and computer furniture. If you don't know of any sources, here are a couple you can use as a starting point. This is by no means an exhaustive list, however.

Contact East, a distributor of test, repair, and electronic assembly products has a catalog that lists numerous static protection products, including anti-static mats, wrist straps, parts trays, static-shielding bags, anti-static sprays and floor waxes, and ionized-air blowers. It even carries a few videotapes and books on ESD awareness and techniques. (I haven't reviewed the tapes or the books, so I can't vouch for their merit. But they may well prove to be quite informative).

Misco, a distributor of computer peripherals, supplies, and furniture, carries anti-static chair mats and the 3M keyboard strip previously mentioned. Jensen Tools, which distributes a broad range of electronics toolkits, carries anti-static wrist straps, floor mats, table mats, and roll-up field service kits.

Sources Of Supply

Here are some vendors of anti-static equipment and supplies:

Contact East

335 Willow St.
North Andover, MA 01845
[phone]
Fax: [phone]

Jensen Tools

7815 S. 46th St.
Phoenix, AZ 85044
[phone]
[phone]
Fax: [phone]

Misco

1 Misco Plaza
Holmdel, NJ 07733
[phone]
Fax: [phone]

This tutorial, number 85, written by Alan Frank, was originally published in the September 1995 issue of LAN Magazine/Network Magazine.