As microelectronic components have become smaller and more sophisticated, the danger of their being damaged by electrostatic discharge (ESD) has become proportionally greater. Even very small charges—so small that an operator may not even notice them!—can seriously degrade or destroy many static-sensitive devices.
Making sure that operators are electrically discharged before they begin handling and assembly procedures isn’t enough to assure product safety. Even the slightest movements, such as shifting in a seat, can cause the regeneration of potentially disastrous charges. As a result, it has become crucial to provide proper grounding for operators at every manufacturing and packaging stage.
Conductive wrist straps provide the most reliable, economical safeguard against this regeneration of electrostatic charge. Because they maintain a continuous path to ground, they should be an integral part of the microelectronics work station. Terra offers a complete line to suit applications in even the cleanest environment.
Terra highly recommends that personnel in static-sensitive areas wear two heel straps, one on each foot, to increase the integrity of the body-to-ground connection. Wearing a foot grounder on each foot ensures contact with ground via the ESD floor, even when one foot is lifted. This will more reliably remove static charges generated by human movement and more reliably protect ESDS devices.
For enhanced safety from accidental contact with equipment line voltage, we recommend using foot grounders with 2 megohm resistors. Underwriters Laboratories and OSHA recommend a minimum of 1 megohm resistance-toground (RTG) in order to limit electrical current exposure to a maximum of 0.00025 amperes. When an operator has both feet on the floor, with each foot grounder having a 1 megohm resistor in series with the ground path, then the operator’s combined RTG, in parallel, is only 1⁄2 megohm. That is, his equivalent resistance is halved and he is at risk of exposure to 0.00050 amperes in case of contact with 250 VAC line voltage.
By wearing two foot grounders, each with a 2 megohm resistor in series to ground, operators comply with the UL and OSHA recommendations at all times. The operator’s RTG is 2 megohms when one foot only is grounded, and he meets the minimum recommended resistance of 1 megohm RTG when both feet are grounded.
All our foot grounders with resistors have been tested by Underwriters Laboratory and are UL listed.