ESD Discharge Waveform Measurement, the First Step in Human ESD Simulation

Abstract

Until uncertainties concerning acceptable measurement techniques are resolved, there can't be real agreement on what constitutes the discharge current waveform of an actual human ESD event. The results of examining two different, commercially-available currentviewing resistor assemblies, along with two different commercially-available current transformer assemblies, verify their applica­ bility in ESD waveform measurement. Introduction Recent papers [1,2] have reported striking facts about the human ESD current discharge waveform: less than one nanosecond rise times combined with large (on the order of 100 to 400%) overshoots. This brings to light ques­ tions about the measurements, i.e. can some of these phenomena be attributable to the mea­ surement equipment and/or techniques used?[3] In order to resolve this issue, various time domain tests were conducted on the measurement instruments Involved. The results of these tests follow. The Oscilloscope The oscilloscope usqd was the Tektronix 7834 with EMI option, and a 7A19 vertical plug-in. The 7A19 has a 50 ohm input impedance. The combination yields an advertised bandwidth of 400 mHz (900 ps rise time). Figure 1 shows the oscilloscope response to a one nano­ second rise time pulse from a Tektronix PG502, 50 ohm pulse generator, implicitly terminated by the oscilloscope. It depicts a smooth transition with no tendencies toward under­ damped responses (I.e., overshoots). Current Viewing Assemblies Two different, commercially-available current viewing resistor (CVR) assemblies along with two different commercially-available current transformer (CT) assemblies were examined for time domain response. One CVR was a two-ohm, lEC-specifled coaxial target designed specifi­ cally for ESD measurements.[4] The second CVR was a 100 milliohm unit, Model 5BNC-5-1, available from T & M Research. Its advertised bandwidth is 1200 MHz, with a risetime of 300 ps, and it is constructed very differently from the IEC coaxial target. The current Fig. 1 Pulse Generator Output 0.5 V/half cm, 1 ns/half cm transformers examined were the models CT-1 and CT-2, both manufactured by Tektronix. Adver­ tised bandwidths and risetimes are 1 GHz and 350 ps respectively for the CT-1, and 200 MHz and 500 ps respectively for the CT-2. The first series of tests used each CVR & CT assembly In turn, to measure the current in the terminating resistor of a coaxial line connected to the PG 502 pulse generator. The CVRs were connected as shown in Fig. 2, which specifically depicts use of the coax target. Fig. 2 Connections for Checking the CurrentViewing Resistors Using the Pulse Generator (Coaxial Target as Example) 246 C H 2 1 16-2/85/0000-246 $1.00 © 1985 IEEE The CTs were connected as shown in Pig. 3The results of these tests are shown in Pigs. 4a, b, c and d.