The Impact of Discharge-Point Geometry on Air-Discharge ESD Testing: Current-Waveform Shape and Intensity

Hand-held electrostatic discharge (ESD) generators (colloquially, ESD Guns) are often used in air-discharge mode to deliver ESD stress to electronic devices and systems. In spite of the perception that air-discharge is only intended for insulating surfaces, there are numerous cases where the governing standards require or encourage air-discharges to conductors; even when not required, it has become a common specification on device datasheets. Air-discharge mode relies on the formation of a spark between the discharge electrode (or tip) of the ESD generator and a discharge-point (the object receiving the discharge). While the geometric shapes of both electrodes are important factors in determining the characteristics of the spark, only the ESD Generator tip is controlled by the governing standards. The characteristics of the spark strongly influence the shape (current versus time) and magnitude of the current-waveform injected into the device being tested, thus influencing the actual severity of the ESD event. Current-waveforms are rarely monitored during actual testing and the test voltage (the programmed voltage of the ESD Generator) is the only measure used to judge the severity of the ESD stress. As a result, many air-discharge tests are conducted without a meaningful measure of the actual ESD stress severity. Because the shape of the discharge-point is uncontrolled (and rarely reported), it is difficult to repeat or reproduce results. This article will demonstrate the variation in current-waveforms for common discharge-point shapes and will provide a categorization method for classifying and reporting them. There is an urgent need to clarify the allowed geometric shapes of conductive discharge-points and to specify the expected current-waveform shapes and intensities. In the meanwhile, it is recommended that both the shape of the conductive discharge-point and the shape and magnitude of the resulting current-waveform be included in any test report, using the categorization method proposed herein.