Investigations Into Unintended ESD Generator Artifacts: Prepulse and Postpulse

An electrostatic discharge (ESD) generator may create pre- and postpulses in addition to the main pulse. These artifacts can cause additional device under test (DUT) failures. The origin and magnitude of these pulses were investigated in four commercial generators using measurements and simulation. Two types of unwanted pulses were identified. One is caused by the reignition of the spark, and the other relates to the relay contacting for charging the main capacitor. These pulses cause strong high-frequency signals, partially surpassing the radio frequency content of the main pulse. The spectrum of the pre- and postpulses varies greatly between different generators. These differences and the fact that some DUTs may react to the pre- or postpulse instead of the main pulse lead to repeatability problems between test sites. The other artifact that has been investigated is the leakage current in contact discharge mode. The charge accumulated on the gun tip by the leakage current can contribute to a short (few nanoseconds) fast-rising pulse (<500 ps) that occurs the moment the tip is contacted, thus, before the main pulse in contact mode.

I. Introduction

Although the contact mode discharge currents are well defined in the IEC 61000-4-2 [1], the test result repeatability is low. The frequency content of the transient fields from the main pulse varies strongly between different brand electrostatic discharge (ESD) generators [2]–[9]. Correlations between the failure level and the induced noise spectrum of the ESD currents have been well studied in [6]–[9]. Besides the main pulse of the ESD gun, there are other possible causes for failures during ESD testing. One of the artifacts that need to be considered is the leakage current. Others are noise-like pulses before and after the main pulse of the ESD generator, as reported in [10]–[14]. As these pulses can cause soft failure and damage, they need to be characterized with respect to their current, spectrum, and origin. The characterization of those artifacts helps the test engineer to better understand the root cause of ESD failures. It helps to avoid mistakenly associated a device under test (DUT) failure with the main ESD gun pulse, while it actually was caused by a secondary effect of the ESD gun.