Contact discharge is used in ESD
testing to improve test repeatability, yet air discharge can have
significantly different characteristics at higher voltages. A test
method is described that uses a modified contact discharge to
simulate the characteristics of an air discharge for some important cases but with improved
: For ESD testing using contact discharge, the discharge occurs in a vacuum relay without
the variability that air molecules, speed of approach, and other
factors cause in real air discharges. Normally such air discharges can exhibit significant
variability from discharge to discharge, especially above 4 kV. For this reason, ESD test
standards such as IEC 61000-4-2
have long used contact discharge to
improve the test repeatability, an important parameter for a test that
has legal significance. A problem arises though at higher voltages
where the discharge current of an ESD event in air can have significantly
different characteristics than contact discharge, especially for some important cases discussed below. Figure 1 shows two
"components" that can help solve this problem.
Contact discharge has the same current waveform at all voltages, including a
fast rising edge (700 to 1000 ps) to a peak current of 3.75 Amps/kV
followed by a longer duration (several tens of nanoseconds) discharge
of the human body. Figure 2 shows the current waveform abstracted from
(some elements of the figure have been removed from the
original which is available from the IEC). This current is measured by
a special calibration target
specified in the standard.
Current Waveform Abstracted from IEC 61000-4-2
(some elements of the picture have been removed and comments have been added))
IEC 61000-4-2 specifies that contact discharge is used whenever the
discharge is to metal. Air discharge is only used when metal is not
exposed, such as with a small plastic enclosure. However, some
pressure has been building in the last few years to use IEC 61000-4-2 to test solid state
devices such as ICs, even at 8 kV and higher, and since the device contacts are
exposed metal, contact discharge is used for repeatability.
The problem is that the fast initial spike of contact discharge is less
to occur in air at 8 kV and higher from small parts with sharp
edges (IC pins) and relatively slow approach speeds consistent with
handling devices. The actual characteristics of a spark are a function
of arc length. The longer the
arc length, the generally slower the rising edge. Sharp edges and slow
approach speeds lead to longer arc lengths and these conditions are
likely present for ESD occurring to devices. See this paper
(~676 kB pdf file) by David Pommerenke for details on arc length effects on ESD characteristics.
So testing a device using an 8 kV contact discharge
subject the device to a current spike that is unlikely to happen in
use and may cause useless protection to be added to the device
increasing its cost. How can one get repeatable air discharge testing
at 8 kV for small devices? I present one possible solution below.
Figure 3 shows a ~1 kV contact discharge of an ESD simulator, a KeyTek
, through a Fischer Custom Communications
F-65 current probe into a ground plane
to measure the discharge current waveform. The MiniZap shown is not fitted
with its IEC tip so the rising edge of the current spike will be
somewhat faster than the 700 ps called for in the IEC standard. Figure
4 shows the resulting current waveform. Only 1 kV was used to keep the
waveform on the scope screen without having to use external attenuators
on the scope. The waveform at 8 kV will be the same, just eight times
Figure 3. Contact Discharge Through an F-65 Current Probe
Figure 4. Current Waveform Resulting from Setup in Figure 3
(Vertical scale = 1 Amp/div)
Figure 5 shows the contact discharge being applied through a filter
composed of a free space capacitor made of aluminum foil mounted on
the ESD simulator and the ferrite choke from Figure 1. The ferrite
choke removes the initial current spike, but its characteristics are
chosen so as not to affect the slower parts of the waveform. The free
space capacitance of the aluminum foil slows the initial risetime to about 5
nanoseconds. The aluminum foil is not a
good choice for real testing as it does not keep its shape. A compact
disc sized piece of metal of just enough thickness to keep its shape
and be able to be pressed onto the tip of the ESD simulator would be
preferred. A CD covered with aluminum foil should work as well. The
ferrite core used was a Steward
part number HFB170070-100
(~140KB pdf file) core.
Contact Discharge Through "Filter" to Simulate an 8 kV Air Discharge
Figure 6 shows the resulting waveform when the filter is used. Note
that the low frequency part of the waveform is not changed much whereas
the initial peak is removed and the initial risetime is about 5
nanoseconds. The waveform shown in Figure 6 (multipled by eight for an 8 kV discharge) may be a good average for what
one might expect of an 8 kV air discharge to an IC using an IEC 61000-4-2
complaint simulator if the IC was grounded on one or more pins to a ground plane through a low inductance connection.
Resulting Current Waveform from Setup of Figure 5
(Vertical scale = 1 Amp/div)
By applying the filter to a standard IEC 61000-4-2 ESD simulator, a
pretty good approximation to a slow air discharge can be generated
without the inherent variability of a real air discharge. Designing a
device or system to survive this waveform may help insure adequate
performance at the lowest cost. This may be a very useful technique
when IEC 61000-4-2 is applied to devices.
At voltages higher than 8 kV, the leading edge of the current waveform
can become even slower. To apply the same technique to higher voltages,
the metal free space capacitor and possibly the ferrite core will need
to be modified. One can imagine, folding the free space capacitor metal
around the end of the ESD simulator body to provide a more convenient
There is another use for a filtered contact discharge. If a system
fails ESD testing using contact discharge, filtering the applied
contact discharge as above can determine whether the system is
sensitive to the low or high frequency components of the ESD waveform.
This knowledge may be useful in troubleshooting the system.
technique is shown for generating an equivalent 8 kV or higher "air"
discharge to an IC or other device with sharp edges and slow approach
by applying filtering to a contact discharge ESD simulator. This method
produces a waveform that is close to a such an air discharge and can
be reproduced from test to test. A filtered contact discharge can also
be useful in troubleshooting system level ESD problems by discovering
whether the high or low frequency components of the ESD waveform cause
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