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Douglas C. Smith

 Address:  P. O. Box 1457, Los Gatos, CA 95031
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Technical Tidbit - November 2011
Construction of a Tool for Removing Static Charges During ESD Testing
(A New Use for a Ballpoint Pen)

disassembled ball point pen

Figure 1.
Disassembled Ballpoint Pen

Abstract: During ESD testing of ungrounded equipment or parts of grounded equipment that have no connection to earth ground, the equipment or its parts can become charged, affecting the test results. Many methods used to discharge the equipment under test (EUT) generate sparks that stresses the EUT beyond what was intended in the test. A static discharge tool built from a ballpoint pen barrel is described that will not generate sparks.

Discussion: During ESD testing, it is necessary to discharge the EUT between applied discharges if there is no connection to ground. If this is not done, the EUT will charge up as successive discharges are applied. As the EUT charges, each successive discharge has less effect because the voltage between the EUT and the ESD simulator becomes less with each applied ESD stress. When the discharge polarity is reversed, a much more intense ESD stress is applied to the EUT because it is charged to a voltage of one polarity and the ESD simulator is applying the opposite polarity voltage. As much as twice the applied ESD voltage stress can result to the EUT containing up to four times the energy (E = 1/2 CV2).

Many methods of bleeding off charge from the EUT between applications of simulated ESD create ESD events themselves and can cause the EUT to fail unnecessarily. Carbon fiber or metallic brushes certainly fall into this category even with a pair of series 470K resistors placed in the lead that grounds the brush. IEC 61000-4-2 also suggests connecting a wire with two 470K resistors from the EUT to ground. But this method may not work either. This is because the brush has its own capacitance to the EUT and a 470K resistor is just a small capacitor at ESD frequencies. What is the solution?

Figure 1 shows a typical ballpoint pen disassembled into its parts. For the purposes of this article we only need the plastic barrel. A wire is assembled with a 470K resistor at each end (actually just one resistor is adequate in this case) and one end of the wire with its resistor is inserted in the ballpoint pen barrel as shown in Figure 2. The end of the wire is fastened to a small piece of ESD dissipative foam. The foam protrudes from the end of the ballpoint pen barrel like an eraser and is used to drain charge from the EUT without causing a spark.

assembled ESD pen

Figure 2. 470K resistor and ESD Dissipative Material Mounted in Ballpoint Pen Barrel

It is a good idea to use a static field meter to make sure the resistance of the foam is low enough to drain the charge in a reasonable time, generally a few hundred milliseconds, fast enough to drain charge before the next application of ESD, but fast compared to human perception. If the foam is higher than 108 Ohms, the discharge may be too slow depending on the size of the EUT. Figure 3 shows a close-up of the 470K resistor in the barrel.
close-up of 470K resistor in ESD pen

Figure 3.
Close-up of 470K Resistor and Wire in Ballpoint Pen Barrel

One way to test if the method you are using to drain static charge may be causing extra ESD events is to charge up the EUT to several thousand Volts and put a Fischer Custom Communications F-65 Current Probe around the drain wire while using the brush or tool to drain charge from the EUT. If you see much of a current peak, then your method of draining charge may be affecting the EUT.

Summary: Preventing untended ESD events during equipment testing can help avoid unnecessary failures. A tool made from the plastic barrel of a ballpoint pen, wire, two 470K resistors, and a small piece of ESD dissipative foam can be assembled in minutes and works very well.

Additional articles on this website related to this topic are:
  1. June 2009, Effect of High Voltage Pulses on Resistors - ESD and EFT

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    Copyright 2011 Douglas C. Smith