(Today’s sensitive components are really sensitive!)
Electronic components are getting smaller and more sensitive every day, and with this comes a greater possibility of damage from electrical stresses in the environment. Some devices are so sensitive, they can be destroyed by an ESD event across the room because the radiated fields from the ESD event generate enough current in wires to do this. Measurement of currents in a one meter cable from a remote, small ESD event are presented and related to device damage thresholds.
Figure 1 shows the test setup used for the experiment and Figure 2 shows a close-up of the test table. A one meter Cat 5 Ethernet cable was used and the current in the cable was measured by a Fischer F-65 current probe.
Figure 1: Test setup for measuring induced current in a one meter cable
Figure 2: Close up of test setup for measuring induced current in a one meter cable
The ESD source is shown in Figure 3. It is described in the June 2001 Technical Tidbits, “A Static Field Powered EMI Source” (www.emcesd.com). ESD is generated in the very small gap between the points of the strips of copper foil tape when a charged object is brought close to one of the copper foil strips. This is caused by electrons jumping across the gap due attraction or repulsion by the applied electric field. Figure 4 shows a charged strip of Teflon™being passed back and forth along the plastic backing of the copper foil strips. This generates hundreds of ESD events between the copper foil strips as electrons are chased back and forth across the gap. I estimate the breakdown voltage of the gap to be several hundred volts. The rising edge of the radiated field is on the order of 200 to 300 ps.
Figure 3: Spark gap EMI source for experiment
Figure 4: Test setup for measuring induced current in one meter cable with ESD source shown [1]
Three distances were used between the ESD events and the one meter cable/current probe: 3 m, 1 m, and 30 cm. Figure 5 shows the induced current at 3 m. The ringing in the waveform is due to the resonant frequency of the copper foil strips (~500 MHz) and to some extent the lower resonant frequency of the 1 m cable. Note the peak current of about 17 mA. One of the most sensitive devices in use today are the read heads in disk drives. They have an ESD Human Body damage threshold of just a Volt or two (not thousands or even hundreds of volts, a few volts)! [2]
Figure 5: Discharge at 3 m From Cable and Current Probe – #1
(Vertical scale ~ 5 mA/div, Horizontal scale = 2 ns/div)
It takes just a milliamp or two for a nanosecond to destroy the head. [2] Clearly, if such a head was not in a shielded enclosure and connected to wires (for testing possibly), it could easily be damaged from across the room. If one were to split the 1 m cable in the middle to form a dipole, the two terminals at the middle of the dipole would have enough voltage between them at a 70 Ohm source impedance to damage a disk drive head from a voltage point of view as well.
Figure 6 shows another typical current waveform generated showing greater than 20 mA flowing in the 1m cable, an even worse case. This is enough to damage even the older GMR heads of 15 years ago. Those heads were damaged by 20 mA flowing for a nanosecond.
Figure 6: Discharge at 3 m From Cable and Current Probe – #2
(Vertical scale ~ 5 mA/div, Horizontal scale = 2 ns/div)
Figures 7 and 8 show the measured currents at a distance of one meter from the ESD events. The peak current for this distance was over 40 mA!
Figure 7: Discharge at 1 m from cable and current probe – #1
(Vertical scale ~ 10 mA/div, Horizontal scale = 2 ns/div)
Figure 8: Discharge at 1 m from cable and current probe – #2
(Vertical scale ~ 10 mA/div, Horizontal scale = 2 ns/div)
Figures 9 and 10 show the measured currents at 30 cm. The peak current is greater than 80 mA in Figure 10.
Figure 9: Discharge at 30 cm from cable and current probe – #1
(Vertical scale ~ 20 mA/div, Horizontal scale = 2 ns/div)
Figure 10: Discharge at 30 cm from cable and current probe – #2
(Vertical scale ~ 20 mA/div, Horizontal scale = 2 ns/div)
The induced current from a remote discharge having more energy could be much larger than this.
In order to check for EMI from the ESD directly into the measurement, a null experiment, the 1 m cable was removed from the current probe, leaving the probe alone on the table as shown in Figure 11. ESD was generated as before at about 30 cm and the results are shown in Figure 12 and 13. The peak reading recorded was about 4 mA. This represents less than 5% error compared to the greater than 80 mA displayed in Figure 10, an acceptable amount of error.
Figure 11: Null experiment discharge at 30 cm from current probe
Figure 12: Null experiment discharge at 30 cm from current probe – #1
(Vertical scale ~ 5 mA/div, Horizontal scale = 2 ns/div)
Figure 13: Null experiment discharge at 30 cm from current probe – #2
(Vertical scale ~ 2 mA/div, Horizontal scale = 2 ns/div)
There are many devices very sensitive to ESD in use today. They carry an ESD damage threshold rating of Class 0, less than 50 volts Human Body Model. For such devices, especially the very sensitive devices like modern disk drive heads, damage from ESD remote from the device is a real possibility. ESD must be controlled not just at the component, but in the nearby environment as well.
Summary
Devices have gotten so sensitive that we now have to worry about ESD damage from ESD events that may be several meters from the device in question. The implication for ESD control is obvious, much stricter ESD controls must be employed and not just at the workstation where the device is handled. ESD must be controlled for a distance of several meters from the sensitive device. This is especially true for disk drive read heads.
For more Technical Tidbits, please visit Doug’s site, http://emesd.com. 
Notes
- Thanks to my youngest son, David, for generating the ESD events while I measured the result. He just finished four years in the Army in electronic repair and will be going into EE or CE towards the end of this year.
- From a discussion with Al Wallash, HGST, a Western Digital Company, about published data.
Equipment used:
- Fischer Custom Communications F-65 Current Probe
- Agilent DSO5054A
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Douglas C. Smith Mr. Smith held an FCC First Class Radiotelephone license by age 16 and a General Class amateur radio license at age 12. He received a B.E.E.E. degree from Vanderbilt University in 1969 and an M.S.E.E. degree from the California Institute of Technology in 1970. In 1970, he joined AT&T Bell Laboratories as a Member of Technical Staff. He retired in 1996 as a Distinguished Member of Technical Staff. From February 1996 to April 2000 he was Manager of EMC Development and Test at Auspex Systems in Santa Clara, CA. Mr. Smith currently is an independent consultant specializing in high frequency measurements, circuit/system design and verification, switching power supply noise and specifications, EMC, and immunity to transient noise. He is a Senior Member of the IEEE and a former member of the IEEE EMC Society Board of Directors.His technical interests include high frequency effects in electronic circuits, including topics such as Electromagnetic Compatibility (EMC), Electrostatic Discharge (ESD), Electrical Fast Transients (EFT), and other forms of pulsed electromagnetic interference. He also has been involved with FCC Part 68 testing and design, telephone system analog and digital design, IC design, and computer simulation of circuits. He has been granted over 15 patents, several on measurement apparatus. |
Mr. Smith has lectured at Oxford University, The University of California Santa Barbara, The University of California Berkeley, Vanderbilt University, AT&T Bell Labs, and internationally at many public and private seminars on high frequency measurements, circuit design, ESD, and EMC. He is author of the book High Frequency Measurements and Noise in Electronic Circuits. His very popular website, http://emcesd.com (www.dsmith.org), draws many thousands of visitors each month to see over 150 technical articles as well as other features.
He also provides consulting services in general design, EMC, and transient immunity (such as ESD and EFT), and switching power supply noise. His specialty is solving difficult problems quickly, usually within a couple of days. His work has included digital and analog circuits in everything from large diesel powered machinery to IC chip level circuits. His large client base includes many well known large electronic and industrial companies as well as medium sized companies and start-up companies.
