ESD Control/Prevention

ESD Controls (Page 3 of 4)

Grounding of Moving Equipment

Moving equipment such as carts, chairs, and lifters can likewise easily generate static charges. Thus, these moving equipment also need to be grounded to the common ground. One way of doing this is by providing these equipment with conductive wheels, casters, or drag chains that creates an electrical path between the equipment and the conductive flooring. Thus, moving equipment must only be used over conductive floors, since this is the only way they can be grounded with a drag chain or conductive wheels or casters.

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Grounding Work Stations and Work Surfaces

A work station must be equipped with materials and equipment to limit ESD events. The key ESD control elements used in most work stations are: 1) properly grounded static dissipative surfaces; 2) a means of grounding personnel (usually a wrist strap); 3) appropriate signage and labeling; and 4) air ionizers if there are insulative materials in the work station. As usual, the static dissipative surfaces and personnel grounding ports must be properly grounded to the common grounding system.

Static protective work surfaces must have a resistance to ground of 10⁶ to 10⁹ ohms to provide a surface that is at the same electrical potential as other ESD protective items in the workstation. They also provide an electrical path to ground for the controlled dissipation of any static build-up on materials that contact the surface.

Fig. 6. Examples of conductive mats and flooring

Grounding Production Equipment

Although personnel-generated static is usually the primary source of ESD in many environments, automated manufacturing and test equipment can also pose an ESD problem. For example, a device may become charged from sliding down an input track. If the device then contacts a grounded conductive surface, a rapid discharge occurs from the device to the metal object. Such a discharge is known as a Charged Device Model (CDM) ESD event.

Proper grounding is the primary means of controlling static charge on production equipment. Electrical equipment are required by the National Electrical Code to be connected to the equipment ground (the green wire) in order to carry fault currents. This ground connection can also serve as a means to dissipate static charges on the equipment.

All electrical tools and equipment used to process ESD sensitive hardware require the 3 prong grounded type AC plug. Hand tools that are not electrically powered, i.e., pliers, wire cutters, and tweezers, are usually grounded through the ESD-protective work surface and the (grounded) person using the conductive tools.

Holding fixtures should be made of conductive or static dissipative materials as much as possible. A separate ground wire may be required for conductive fixtures not sitting on an ESD-protective work surface or handled by a grounded person. Again, this wire must be connected to the common ground system.

For those items that are composed of insulative materials, the use of ionization or application of topical antistats may be required to control the generation and accumulation of static charges.

Ionization

It is not possible to eliminate all insulative materials or isolated conductors that cannot be grounded from the production area. Thus, there must be a way to control their ESD generation tendencies because these items can not discharge to the common ground. Air ionizers serve this purpose.

Air ionizers neutralize the static charge on insulated and isolated objects by charging the molecules of the surrounding air. Whatever static charge is present on objects in the work environment will be neutralized by attracting opposite polarity charges from the air. Because it uses only the air that is already present in the work environment, air ionization may be employed even in clean rooms where chemical sprays and some static dissipative materials are not useable.

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