Esd Control And Handling : Human Body Model & Requirements Of Static-Free Work Station

4. Human body model:

The human body has capacitance as it can store charge to a degree dependent upon how well it is insulated from ground. The body has a resistance dependent upon muscle tone, moisture content, contact resistance, and other factors. It is generally agreed that it consists of a capacitor in series with a resistor. A small amount of inductance, probably a fraction of a micro henry is believed to have negligible effect on damage results in most cases.

One model of 100 pf capacitance in series with a resistor of 1.5kW resistance has been widely accepted. However, this may vary depending on various factors as given in previous paragraph.

5. Requirements of static-free work station:

Wrist-strap connection to ground:

Many problems have occurred through improper connection of wrist straps. Wrist straps should never be connected to the work surface as a path to ground unless the surface is metal. Most work surfaces add far too much resistance for effective personnel grounding. Using alligator clips for connection to a work surface increases the problem through excessive contact resistance. The strap connection point should never be left to chance. In general, a separate hard wired path to a connecting socket or bus for the strap connection should be provided.

Conductive flooring/ footwear:

Personnel grounding can also be accomplished through conductive flooring used in conjunction with conductive footwear. Conductive flooring can be achieved by using special conductive tiles, mats, or other conductive floor coverings or finishes.

Applications for conductive flooring/ footwear:

Conductive flooring options are commonly used in applications such as:

1. Processes where operator mobility precludes effectivity of wrist straps.

2. New or renovated buildings where installation of conductive flooring is not a major expenditure

3. High reliability or other critical static controlled areas

Grounded work surfaces:

The first requirement is that the surface in use have sufficient conductivity to be groundable. The primary purpose for a grounded work surface is to effectively ground conductive items contacting the surface. It also provides a redundant path to ground for the operator at all times when the surface is touched. An additional benefit is assurance that the surface will not acquire a static charge.

Antistatic materials are not generally recommended for work surfaces because of the excessively high resistance, dependence upon humidity, and limited life.

Static dissipative materials in the range of 10 7 to 10 9 are recommended for most applications. This range does not have the extreme sparking problems of conductive materials yet is of sufficient conductivity to effectively bleed off potentials.

Conductive work surfaces such as stainless steel are often favoured because of durability, rapid charge bleed off, and other factors. An undesirable feature is the propensity for sparking when a sufficiently charged conductive item is brought near or in contact with the surface. Thus if conductive surfaces are used, extreme care must be taken to prevent charge accumulation on susceptible items to avoid charged device failure.