Electro-Static Discharge (ESD)

Electro-Static Discharge (ESD)

Anyone who works with electronics, especially disassembled components , needs to be very concerned about ESD. ESD is the static-electricity discharge that happens when two differently charged objects (such as your body and a computer component) come in contact with each other. ESD is an invisible killer of memory modules, interface cards, hard disks, and other computer components, because ESD buildup and discharge happens long before you actually notice it.

You might dread shaking hands with a new acquaintance in the winter because you'll get a shock , but ESD discharges far below the 3,000V level that you can actually feel can still destroy chips. As little as 30V of ESD is enough to destroy the current generation of low- powered chips, and you can build up as much as 20,000V of ESD from walking across a carpeted room in the winter if you shuffle along.

ESD damage is "invisible" for another reason: It leaves in its wake equipment that has no visible damage but simply won't work reliably.

ESD damage is a major cause of intermittent failures, which are the bane of computer technicians everywhere. An intermittent failure is the classic "it wasn't working when I called you" kind of problem that "goes away" when you examine the system but recurs from time to time later.

Preventing ESD

You can prevent ESD by taking proper precautions when you do the following:

• Install or remove components

• Store and transport components

• Use computers

Although the greatest danger of ESD occurs when you have the system open and are working with components, PC users can also cause ESD problems when working with closed-up systems. I once delivered such a big static shock to a keyboard after a coffee break that I couldn't save my document and had to power down and restart the computer to restore my keyboard to working order.

One way to prevent ESD is to equalize the electric potential of your body and the components on which you're working.

Unequal electrical potential between you and the device on which you're working is the major cause of ESD. When your body has a higher electric potential than the device or component with which you're working, an ESD from your body to the device or component equalizes the potential ”but at the cost of damage or destruction to the component.

Protection Devices

You can best equalize the electrical potential of a computer or component that is being serviced by placing the computer or component on an antistatic work mat equipped with a wrist strap ; attach your wrist strap to the mat. This will help place you and the component at the same level of electrical potential, and thus eliminate the "need" for ESD to occur to equalize the potential.

For additional safety, use the alligator clip on the antistatic mat to attach to the component or computer you are working on. The clip should be attached to unpainted metal on the chassis, such as the frame. This provides superior equalization for the mat, you, and the hardware on the mat.

Table mats connected to a grounded power supply are useful tools for preventing ESD on working computers, especially if users are reminded to touch the mat or grounded keyboard strip first. Antistatic cleaning spray and antistatic carpet spray should be used in any carpeted office to reduce static, especially in the winter when dry heat causes buildup.

Correct Storage for Equipment

Correct equipment storage should have two goals:

• Eliminate the possibility of ESD.

• Protect equipment from impact damage.

caution

Do not leave the computer plugged in while you work. This does not minimize the chances of ESD, and you could damage equipment if you attach or remove it. This is because ATX and NLX-based systems still draw power even when they have been shut down.

To protect equipment from ESD, store equipment in the Faraday cage antistatic bags originally supplied with the equipment; retain bags for installed equipment for reuse. Faraday cage antistatic bags feature a thin metallic layer on the outside of the bag, which is conductive and prevents ESD from penetrating to the components inside. Thus, metalized metallic bags should never be used for temporary mats for components; if you lay a component on the outside of the bag, you're laying it onto a conductive surface. Colored antistatic bubble wraps also work well for parts storage, and can also be used as a temporary mat, too. If you use bubble wrap, make sure it is antistatic (see Figure 13.2).

Figure 13.2. A grounded work mat, suitable for use on either a work area or under an office computer in a high-static area, and antistatic Faraday bags.

All work mats and wrist straps should have a 1-megohm resistor, as shown in Figure 13.3, to stop high voltage coming through the ground line from injuring the user .

Figure 13.3. A typical ESD kit includes a grounded work mat and wrist strap. Both have 1-megohm resistors to protect the user from electric shock and alligator clips to connect the mat and wrist strap to unpainted metal parts on or inside the system.

Store components in appropriate boxes to avoid physical damage. If the original boxes have been discarded, use cardboard boxes that have enough room for the component, the Faraday cage bag around the component,and antistatic padding.

caution

You should use a commercial wrist strap for most types of computer service, but there is one major exception: Never ground yourself when you are working with high-current devices, such as when you discharge a CRT. Grounding yourself to such devices could cause your body to receive a fatal high-current electrical charge.

Using a Commercial Wrist Strap

A typical commercial wrist strap and a grounded work mat are shown in Figure 13.3. Both the wrist strap and the work mat include alligator clips that are attached to the system chassis to equalize electrical potential between the wearer and the computer. Wrist straps use hook and loop or other types of adjustable closures; it's important to wear the wrist strap comfortably snug so that the metal plate underneath the resistor touches the skin to provide proper conductivity.

Additional ESD Precautions

A grounded wrist strap can help prevent ESD, but you should also follow these additional precautions:

• If you must handle expansion cards and other devices with chips without suitable antistatic protection, never touch the chips ! Most current products use a CMOS (Complementary Metal-Oxide Semiconductor ) design, which has practically no resistance to ESD; as little as 30V of ESD can damage CMOS-based devices.

• Hold expansion cards by the brackets, never by the motherboard connectors.

• Wear natural fibers, such as cotton and leather-soled shoes, instead of synthetics to avoid ESD buildup.

• Use an antistatic spray (commercial or antistatic fabric softener/water mixture) to treat carpeting to reduce ESD.

• Use antistatic cleaning wipes on keyboards, monitors , and computer cases to reduce static buildup. Turn off the power, and if you use a liquid cleaner, always spray the cloth, never the device!