The need to solve static generation and dissipative problems has grown, not because of the increase in the computer chip use, but in the increased density of the chips. As industry puts more and more on a single chip, the layers inside the chips are getting smaller and smaller, making them more susceptible to the effects of ESD and EOS. When static charge of less than 10 volts will destroy a class 1 ESD Sensitive Device and, it is possible to create a static charge of over 35,000 volts walking across a carpet on a dry day, the problem is immediate and severe.

Static charge is generated between materials through friction, pressure, or separation of two materials, one of which is usually non-conductive. This process is called the Triboelectric Effect [tribo means rubbing].

Material composition, applied forces, separation rate, and relative humidity determine the magnitude of the static charge.

When humidity is low, higher static charges are generated. Static becomes more noticeable in the winter months, in dry climates, and in air-conditioned environments. Increasing humidity to 60% limits static build-up as surface moisture on materials makes a good conductor. Unfortunately, 60% relative humidity is extremely uncomfortable, can cause equipment problems and introduce contaminants into your system.

There are two sources of static damage to electronics: EOS [Electrosatic OverStress] and ESD [ElectroStatic Discharge].

EOS occurs when an electronic device is exposed to a strong static electric field generated by a static charge. Internal electronic components stressed beyond their designed tolerances may fail immediately or have reduced service lives.

ESD occurs when two objects of different electrostatic potentials are brought close enough together to allow a charge transfer. ESD is the sudden discharge of this electrostatic potential from one body to another. If the discharge current exceeds designed tolerances, damage may cause immediate failure or result in reduced service life.

One common misconception is that conductive materials do not generate charges. This is because the dissipation of static charges from grounded conductive material tends to be complete and rapid. Ungrounded conductors can generate and hold static charges.

A material that inhibits the generation of static charges [generally less than 200 volts] from triboelectric generation is classified as antistatic. An antistatic material can be conductive, dissipative, or even insulative. Only conductive or dissipative antistatic materials should be used in ESD safe areas.

Insulative materials are more commonly understood to generate and hold a static charge. Since they are insulators they do not allow the charge to move or distribute throughout the object. Grounding is not an effective method of neutralizing insulators. Static fields on insulators are not necessarily permanent either; they will eventually be neutralized by gradual recombination with free ions.

ESD/ EOS Brushes

An ESD/EOS brush can be an active or a passive static eliminator.

The active ESD/EOS brush works because, in the process of mechanically brushing the potentially charged surface, the use of conductive brush fibers and a conductive core allow the static charge to flow to ground through conduction.

An ESD/EOS brush can also be constructed as a passive static eliminator with an air gap between the fiber ends and the charged surface. A charged surface will have an electric field emanating from it. In most situations the electric field will be uniform and perpendicular to the surface. However, if a sharp point is brought into the vicinity of the field, the field lines will concentrate at the point.

In situations where the point is sufficiently sharp, the value of the electric field gradient at the point tip will be greater than the breakdown strength of air [3,000 volts per millimeter]. This causes the air molecules in the vicinity of the point tip to ionize. This is called the corona effect.

Since the ions are also subjected to the electric field, ions of one polarity [opposite the charge polarity of the surface] will travel along the electric field lines to the surface. This will reduce the static charge on the surface. The opposite polarity ions will travel through the passive device to ground. This process will continue until the field has been reduced to the point where ionization stops.

For contact operation [conduction], performance is best with minimal spacing between fibers. For operation with an air gap [induction], performance is best with a space between the fibers and with the filaments spread out at the fiber ends.

Some ionization will take place even when the fibers are in contact with an insulated surface. Some charge will be removed through direct contact and the surrounding charge will induce ionization at the fiber tips.

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