Detection methods for secondary ESD discharge during IEC 61000-4-2 testing

When an ESD (electrostatic discharge) event reaches a non-grounded metallic part within a product, the voltage of this metal part with respect to ground will increase. If the isolation to ground is insufficient, a secondary ESD event can occur. As secondary ESD often leads to system upset or damage, and to poorly-reproducible results, it is important to detect the occurrence of secondary ESD. If the discharge current is monitored using an oscilloscope, the test equipment may miss the secondary discharge waveform. This is because the time delay between the primary and secondary discharge events can vary between nanoseconds to milliseconds. This delay depends on the amount that the secondary gap is overvoltaged, its shape, humidity, etc. Present oscilloscopes do not offer functionality to auto-detect a secondary discharge event. The goal of this study is to analyze different types of secondary discharge events acquired with different measurement setups. The data is analyzed with respect to the identification of parameters which allow an automatic detection of secondary ESD, with low, false or missed detection. Since ESD guns are known to produce pre-pulse and post-pulse events due to the switching of relays inside of the gun, this creates difficulty in distinguishing the secondary discharge from the pre- and post-pulses of the ESD gun. This is a necessary step toward developing methods to auto-detect secondary events while monitoring the discharge waveform either at the ESD gun tip or at the ground strap.