Development of an Electrostatic Discharge Model for Electronic Systems

Abstract

Electrostatic Discharge (ESD), as discussed in this paper, is limited to the buildup and flow, or redistribution of charge between a human being, an electronic system, and earth. Like lightning, it is really an electrostatic/electrodynamic event. The background includes electrostatics or charges at rest on the sur­ faces of a system of conductors, consisting of a person, an elec­ tronic system, and earth. These charges have a surface density from which the electric field intensity is derived using the Gauss Law of electrostatics, provided the charged surfaces are suffi­ ciently isolated from each other so that the surface charge density is uniform over the.surfaces of the bodies. The actual charge distrubution is discussed later. Flow of the charges (current) takes place slowly during the charging phase, and very rapidly during the charge redistribution phase. Charges are built up on the per­ son usually as a result of triboelectric action between the shoe soles and the floor covering. The sudden release of built-up elec­ trostatic energy in the person, machine, and earth system causes potentially devastating voltages, currents, and electromagnetic fields to occur in vulnerable components in the electronic system. Upon ionization and air dielectric breakdown (arcing), the rapid redistribution currents easily propagate down interconnecting cables and other units before finally bleeding off through the higher RF impedance of the ground connection. The person has a strong capability to retain the built-up charge due to his high degree of insulation from the floor covering and earth. Also, there is a significant capacitance between his body and earth, and a potentially larger capacitance (depending upon separation) to the usually grounded electronic enclosure or enclosures. There­ fore, a large amount of energy may be available for discharge. Depending on the condition, potentials in the order of 30,000 volts may be present on the person. Potentials higher than this value are usually discharged into the atmosphere by corona effects. This occurs when the electric field exceeds approximately 3 x 106 volts per meter. The problems with ESD are as follows: