ESD Control/Prevention

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ESD Controls (Page 2 of 4)

Grounding of Personnel

People are one of the primary generators of static electricity. The simple act of walking around or repairing a board can generate several thousands of volts on the human body. If not properly controlled or dissipated, the accumulated static charge on a person can easily discharge onto a static sensitive device. Such an event is known as human body model (HBM) discharge.

Wrist straps are generally the primary means of controlling static charge build-up on personnel. When properly worn and connected to ground, a wrist strap keeps the person wearing it near ground potential. Because the person and other grounded objects in the work area are at or near the same potential, there can be no hazardous discharge between them. In addition, static charges are safely dissipated from the person to ground and do not accumulate.

Fig. 2. Examples of wrist straps

Wrist straps have two major components, the cuff that goes around the person's wrist and the ground cord that connects the cuff to the common point ground. Most wrist straps have a current limiting resistor molded into the ground cord head on the end that connects to the cuff. The resistor most commonly used is a one mega-ohm, 1/4 watt resistor with a working voltage rating of 250 volts. This resistor would protect the person wearing it from electric shock in case the point to which the wrist strap is grounded accidentally gets 'live.'

Wrist straps should be tested on a regular basis. Daily testing or continuous monitoring is recommended.

Fig. 3. Examples of wrist strap monitors

A second method of controlling electrostatic charge on personnel is with the use of conductive floors in conjunction with conductive shoes or foot straps. Any charge build-up on a person wearing conductive shoes will be dissipated to the conductive floors through the sweat layer between each foot and shoe. The conductive floors must be properly grounded to the common ground system.

In addition to dissipating charge, some floor materials (and floor finishes) also reduce triboelectric charging. The use of floor materials is especially appropriate in those areas where increased personnel mobility is necessary. When used as the primary personnel grounding system, the resistance to ground including the person, footwear and floor must be the same as specified for wrist straps (< 35 x 10E6 ohms) or the voltage accumulation on a person must be less than 100 volts.

The use of ESD-protective clothing is another way to control charge build-up on a person.

Fig. 4. Examples of heel and sole grounders, also known as foot straps

Fig. 5. Conductive shoes and slippers

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