Small changes in the physical
design of a product can have a large impact on ESD performance both in
the lab and field. Data is presented for the case of a small switch box
charged to 250 Volts showing the effect of small insulating
rubber feet. The results show that seemingly inconsequential
differences in physical design can have a major impact on equipment
operation in the field.
: The October 2005 Technical Tidbit, Controlling Variables in High Frequency Tests and Measurements - an ESD Example
discussed the importance of controlling test variables in order to
obtain consistent test results specifically for ESD. In a related
example, this Technical Titbit shows how a small change in physical
design of a product can have a major effect on ESD performance.
Figure 1 shows a small switch box positioned directly on an insulating
layer over a ground plane. The switch box is charged to 250 Volts by
the ESD simulator and then it is discharged to the ground
plane through a 15 cm heavy brass wire passing through a Fischer F-65
current probe. A 20 dB attenuator on the oscilloscope input is used to
achieve a vertical scale of 5 Amperes/division. The small plastic pen
in Figure 1 allows quick bending of the brass wire to touch the ground
plane and discharge the switch box. A discharge voltage of 250 Volts
was chosen to improve repeatability of
the air discharge, to prevent scope overload, and avoid secondary
discharges within the switch box.
Figure 2 shows the resulting current waveform for the switch box
sitting directly on the insulating sheet with no feet on the bottom of
the box, just the flat surface of the box against the insulating sheet.
This arrangement maximizes the capacitance between the box and the
ground plane. A peak current of about 12 Amperes is achieved with a
damped oscillatory waveform having a ringing frequency of about 22 Mhz.
The current shown in Figure 2 is in some respects similar to the current generated by the machine model ESD tes
used to detremine ESD hardness of small solid state devices to a
particular stress that ocurrs during automated handling. Some companies
also have internal private
ESD tests where a small piece of equipment or a module is charged and
then discharged into a metal plane to simulate installation or handling.
Measured Discharge Current for Switch Box on an Insulating Sheet
(switch box charged to 250 Volts)
Figure 3 shows the addition of four small rubber insulating feet to the
bottom of the switch box. The switch box was then placed on the
insulating sheet, charged to 250 Volts, and discharged through the
current probe as before. Figure 4 shows the resulting current.
Figure 3. Small Feet Applied to Bottom of Box
Figure 4. Measured Discharge Current for Switch box with Small Feet on an Insulating Sheet
(switch box charged to 250 Volts)
The peak current has been reduced to about 4 Amperes from 12 Amperes
and the ringing frequency is now about 65 MHz. This current is very
much different from the current in Figure 2 and could easily
lead to a significant difference in system response. The lower current
and higher frequency are a direct result of
the reduction in capacitance between the switch box and the ground
plane caused by the small feet increasing the distance from the switch
box to the ground plane. The ringing frequency is determined by this
capacitance and the inductance of the wire used to ground the switch
The actual number of cycles in the damped waveform varied from
discharge to discharge (as it did for the case of Figure 2), but the
amplitude of the first cycle was about the same as shown. The number of
oscillations varied from a single cycle to something like that shown in
One might think that just adding small insulating feet to the bottom of an
enclosure would not affect ESD or other test results. The data
presented here shows that this is not the case. One must be careful to
analyze all changes in physical design of equipment and retest if there
is any doubt as to effects on system performance.
Small differences in physical design or test setup can result in
significant variations in ESD currents and therefore system
performance. Both test setups and system physical design must be
tightly controlled to insure consistent equipment performance in the
lab and field.
tutorial on this subject, covering background as well as more technical details, is available at:
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