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Douglas C. Smith

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Technical Tidbit - January 2006
ESD, A Small Change Can Have a Large Effect

test setup

Figure 1.
Test Setup

Abstract: 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.

Discussion:  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 test 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.

discharge current, no feet

Figure 2.
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.

small feet on bottom of box

Figure 3.
Small Feet Applied to Bottom of Box

discharge current, with feet

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 box.

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 Figure 4.

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.

Summary: 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.

Other articles on this website related to this topic are:
Additional Material: An in-depth audio-visual format tutorial on this subject, covering background as well as more technical details, is available at:

If you like the information in this article and others on this website, much more information is available in my courses. Click here to see a listing of upcoming courses on design, measurement, and troubleshooting of chips, circuits, and systems.

Available now for private on-site delivery and as a public seminar: my new one day seminar titled: Failure Analysis and Prevention in Electronic Circuits (Design Troubleshooting for the Lab and Field).

Equipment used in this article includes:
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Copyright © 2006 Douglas C. Smith