Figure 1 shows one
frame of a short video taken of sequential contact discharges from an
ESD simulator. The measurement was made in a fashion similar to that
shown in Figure 2, although the ESD simulator used to generate the data
for this article was not
the one shown in Figure 2, but a different one measured at a later time.
In Figure 2, the ESD simulator is discharged in the contact
discharge mode into the Horizontal Coupling Plane, HCP, of an ESD test
table through a Fischer Custom Communications F-65 current probe
To get maximum current, the simulator ground lead was connected to the
HCP (as done in the ISO10605 ESD standard) rather than the ground
reference plane on the floor to which the simulator ground lead is
normally attached for IEC testing. The test was performed near the edge
of the table for convenience, so the waveform will not be exactly a
calibration waveform, but similar. The object was to look at variations
from discharge to discharge.
Current Waveform Measurement of ESD Simulator
Figure 3 is identical to Figure 1 except if you click on Figure 3 a
Flash video should play in your browser. You can see significant
variation from discharge to discharge. The scope was sampling at 8
GSa/sec, so the variation was not due to sampling errors. That sampling
rate gives 400 samples per horizontal division, more than adequate for
Current Waveform from ESD Simulator
(Vertical = 1/2 Amp/div, Horizontal = 50 ns/div)
Click the picture or here
to see the video.
I suspect a defective relay in the
simulator may be the root cause of the variable discharge waveforms.
Also possible are simulator design problems such as using an inadequate
discharge relay. An unintended discharge within the simulator itself
cannot be ruled out at this point. The waveforms in the video
point out that ESD simulators should be checked frequently, especially
if the simulator is rented or you don't know the service history of the
simulator. Being calibrated once per year is not nearly enough for ESD
simulators. I personally check the simulator waveform at the start of
Measuring ESD current from a simulator before ESD testing starts is critical in order to reveal problems with the
ESD simulator itself. This is especially important if you are using a
rented ESD simulator.