Measurements and phenomenon study on discharge currents with small gap in electrostatic discharge immunity test

In electrostatic discharge (ESD) immunity tests, it has empirically been known that contact discharge currents are enhanced by the existence of small air gaps between an ESD generator and equipment under test; however, its mechanism has not been well examined so far. In this study, to clarify the phenomena, we measured discharge currents through small air gaps from 30 μm to 1 000 μm in contact discharges of an ESD generator to an IEC current calibration target. As a result, we found that multiple current peaks appear depending on the gap length and test voltage as well, which are unlikely to occur in conventional contact discharge tests. It was also found that the contact discharge current with a small gap provides a faster rise time and larger current peak in comparison with normal contact discharge tests even at the same test voltage. A circuit approach was used to qualitatively explain the peculiar finding and revealed that the stray capacitances of the discharge electrode are first charged by a contact discharge test and then the accumulated charges are abruptly discharged due to the spark that occurs across a small gap, which gives faster rise time and larger current peaks. © 2015 The Institute of Electrical Engineers of Japan.