Many electronic devices operate at low voltages. Such devices typically include very small conductive pathways that can be damaged easily if subjected to high currents and relatively thin insulative layers that can be damaged easily if subjected to high voltages.
A significant cause of such damage to such electronic parts is electrostatic discharge ("ESD"). Electrostatic discharge is caused by the statically induced buildup of charge on a person or object that can raise the voltage of the person or object to several thousands of volts. If a path to a low voltage is provided to the built up charge, the charge quickly transfers or "discharges" along the path forming a brief, high current that can damage the electronic device. Also, the high voltage can break down and damage insulative layers positioned between a conductor to which the high voltage is coupled and another conductor that is at a lower voltage, such as ground.
Persons who handle electronic devices are a particularly troublesome source of electrostatic charge. As such persons move about a facility in typical fashion, such as by walking, they can build up significant electrostatic charge such that their bodies are at several thousand volts relative to ground. When the person comes near to, or in contact with, a sensitive electronic device, the voltage differential between the person and the device causes charge to transfer rapidly from the person to the device. The rapid transfer of charge can damage or destroy the sensitive part.
Electrostatic discharge is particularly troublesome before the electronic part is packaged. Before packaging, extremely sensitive circuits and circuit elements may be exposed and may come near or in contact with an electrostatically charged person or object, making unpackaged parts particularly sensitive to ESD damage. Also, devices may be on a common substrate and a single electrostatic discharge to a region of the substrate can destroy many parts simultaneously. Thus, in a production facility, a person can unknowingly damage many electronic devices.
Several ESD protection techniques have been developed to prevent persons from causing such ESD damage. For example, persons entering production facilities are typically required to wear a grounding device. One of the most common types of grounding devices is a form of conductive footwear. Conductive footwear include shoes having conductive soles or conductive booties that surround the person's shoes and grounding straps that strap around a person's shoes. The grounding strip or a conductive strap coupled to the conductive soles or booties makes contact with the person's leg to provide a discharge path from the person to a conductive floor of the facility. Thus, each of the types of conductive footwear provides a conductive path between the person's body and a conductive floor of a facility. The conductive path dissipates electrostatically induced charge from the person's body such that the person's body is maintained substantially at zero volts.
Sometimes such conductive footwear or other grounding techniques fail to perform properly. For example, conductive straps may be damaged or improperly worn such that they do not form a conductive path between the person's body and ground. Such damage to the conductive strap is typically not visible to the naked eye.
To reduce the threat of electronic parts being damaged by ESD, various grounding verification approaches have been attempted. For example, U.S. Pat. No. 4,800,374 to Jacobson describes a personnel antistatic test device that has a conductive shoe plate and a conductive touch plate. The person stands on the shoe plate and places a hand on the touch plate. The antistatic device then measures a resistance between the shoe plate and the touch plate. A lighting indicator indicates whether the resistance is within the allowable range. If the indicator indicates the resistance is within the allowable range, the person then enters a statically sensitive environment through an accessory such as a door. If the indicator indicates that the resistance is outside of the allowable range, the person adjusts or replaces the conductive footwear.