抄録
An immunity testing method for electrostatic discharge (ESD) is being specified in the IEC 61000-4-2, in which two types of discharge method, contact and air discharges of an ESD-gun, are given. The ESD-gun is an ESD generator, which simulates ESD events from a charged human body. The IEC recommends the contact discharge method and has specified a typical discharge current waveform. The air discharge is a way applied when the contact discharge cannot be conducted, while the details of the discharge current waveform has not been given. Even in the same test levels, both the above discharges do not necessarily give the same severity for immunity testing. Though the air discharge which accompanies a spark is said to be severer, the severity has not quantitatively been evaluated. At the same time, as a strange phenomenon, it has been widely known in the related industries that in comparison with high voltage ESDs in several kilovolts, low voltage ESDs in several hundred volts generate EM fields having more broadband frequency spectra. The ESD of this kind causes more serious failure in electronic devices, while the mechanism has not yet been elucidated. From this perspective, grasping the characteristics of discharge current waveforms for contact and air discharges of an ESD-gun, we previously conducted both discharges onto a 50-Ω SMA connector, and showed that, at charge voltages below 1kV, air discharge could provide a larger current peak and shorter rise time than contact discharge. In this study, using a 12-GHz digital oscilloscope, we measured discharge currents for air and contact discharges of an ESD-gun onto an IEC calibration target, and obtained maximum rising slopes of discharge currents with respect to charge voltages from 200V to 15kV. As a result, we found that at charge voltages below 5kV, air discharges give higher current rising slopes than contact discharges, and particularly at charge voltages below 1kV, the rising slopes become larger by double figures and also have smaller variation. This means that air discharges at charge voltages below 1kV can provide a severe and reproducible immunity test.
