A Note From The Author
The research references provided in this article occurred between 1978 and 1987, and are as applicable today as they were then, when they advanced understanding of ESD phenomena and through that, the ESD state of the art of knowledge.
These were the first reports to display that: ESD currents to a system do not increase in direct proportion to the initial electrostatic charge amplitude produced before the ESD event (e.g. low ESD voltages produce higher current than higher voltages); ESD spectra is greater than 1GHz with rise-times much faster than 1.0 nanosecond (e.g. 50 pSec to 200 pSec); the ESD “equivalent network” is not a single R-C network as previously thought, but rather a complex cascade of several networks, each with a different time constant; a “single ESD event” may be comprised of many events, even showing the approximate periodicity between each sub-event; suggested the initial impact of ESD was from boundary charge displacements of electric fields; detailed the ESD impulse waveforms and currents related to many common conditions (finger-tip direct, humans discharging through metal objects, humans discharging through furnishings); described how systems produce ESD amplitude-response dependencies owing to the different spectra of ESD events at different amplitudes; and, that compliance with a higher amplitude of ESD (e.g. 15kV) does not assure that adequate immunity will be exhibited at lower (e.g. 2kV) levels. During these research efforts the “human body concept”, the “human with metal object” concept; the “furnishings impact” equivalent values; the vertical and horizontal coupling planes for “radiated ESD equivalents” were all devised. The impact on the international community was sufficiently extensive starting circa 1979 that approximately five years of confirming work were required for the reports to achieve broad acceptance.
Since that time this work has been thoroughly disseminated and various standards and standard practices for ESD have been published, many (if not most) of which extend from these early research efforts. In the process of standardization some of the baseline foundational information delineating ESD mechanisms have been diffused. It didn’t help that when the IEEE converted published symposia archives to microfiche some years ago, much of the original photographic data for ESD event waveforms was lost as the contrast of the original printed publications became often degraded to black rectangles. Thanks to the invitation of The Editor of this magazine, you have the opportunity to travel with me, back to history, and back to this future to gather a broad understanding of the underlying boundary charge displacements that establish ESD, catch a glimpse of the propagational spectral mechanisms that impact systems-product performance, and to review foundational information that extends to this day.
W. Michael King
(For further reading, I suggest the book “Electrostatic Discharge” Third Edition by Michel Mardiguian, Wiley/IEEE Press, ISBN: 978-0470-39704-6, and ANSI, C-63.16-1994 “Guide for Electrostatic Discharge Test Methodologies and Criteria for Electronic Equipment.”)
In recent years, various research results have been published that define (for the first time) the true complexity of dynamic impulse waveforms developed through personnel electrostatic discharge (ESD) events (see reference list). These research studies have contributed significantly toward understanding the ESD susceptibility response mechanisms exhibited by systems products that previously were considered to be “mysterious”.
Examples of such system responses are performance criteria failing at low ESD initialization levels (<2 kV) and complying at higher levels (>7 kV), or complying at low initialization levels failing at intermediate levels (5 to 7 kV) and complying at higher levels (>10 kV).
This paper discusses the application of ESP impulse waveform research results toward understanding the ESD amplitude-waveform dependency of susceptibility responses from systems products, and outlines the conceptual aspects of the test methodology required for thorough product evaluation.
During the time frame of 1978 through 1983, an experimental measurement series to determine the ESD waveform continuum was performed, with results published in various forums [1, 3 and 4]. Further work that tended to confirm the earlier results continued into 1986 ). The results of these experimental measurements indicated:
- The dynamic waveform continuum of ESD events exhibited exceptionally high bandwidths of spectral level distribution at low ESD initialization levels [<5 kV] extending into the microwave domain, with risetimes measured and reported as fast as 200 picoseconds (See Figure 1);
- The spectral bandwidths of ESD resulting from higher initialization levels (>10 kV, <20 kV) ranged in the tens to hundreds of megahertz, with risetimes in the order of 4 nanoseconds to several tens of nanoseconds (See Figure 2). The spectral bandwidths of ESD initialization levels between these two extremes were found to range in between, with risetimes in the one to ten nanosecond range(See Figure 3);
- There are vast differences in ESD peak current delivered (as impulse intensity) to a product when a human discharge occurs from a finger-tip as compared to a human holding a metallic object (key, coin, pen, tool). See Figure 4.
Figure 1a: ESD Impulse Waveform Distribution: Human with Metallic Object; 2,000 Volts Initialization.
Scale: 10 Amperes/Division. Displayed: 42 Amperes to 74 Amperes (Tr = 200 pSec; Tf = 50 pSec)
Figure 1b: ESD Impulse Waveform Distribution: Human with Metallic Object; 4,000 Volts Initialization.
Examples of event-to-event distribution in nature
Scale: 10 Amperes/Division. Displayed: 15 Amperes to 58 Amperes
Figure 2a: ESD Impulse Waveform Distribution: Human with Metallic Object; 10,000 Volts Initialization.
Examples of event-to-event distribution in nature
Scale: 5 Amperes/Division. Displayed: 16 Amperes to 20 Amperes
Figure 2b: ESD Impulse Waveform Example: Human with Metallic Object; 20,000 Volts Initialization
Scale: 5 Amperes/Division: Displayed: 31 Amperes
Figure 3: ESD Impulse Waveform Distribution: Human with Metallic Object; 8,000 Volts Initialization
Scale: 5 Amperes/Division: Displayed 15 Amperes to 40 Amperes.
Figure 4a: ESD Impulse Waveform From Human Finger-tip: 500 Volts Initialization
Scale: 0.5 Amperes/Division. Displayed: 0.9 Amperes to 1.8 Amperes; (Tr = 200 pSec)
Figure 4b: ESD Impulse Waveform, Human with Metallic Object: 500 Volts Initialization
Scale: 5.0 Amperes/Division. Displayed: 34 Amperes
(COMMENT: The metal object condition yields 19:1 Greater Current than Finger-tip condition).
Independently worked from a theoretical approach, studies by William Byrne, then Senior Scientist at the Southwest Research Institute (in San Antonio, TX), generally provided theoretical confirmation of the previously described experimental results [2 and 5].
The experimental test methods developed to determine the ESD waveforms as described in the referenced research efforts have been utilized broadly in the international technical community, and form the basis of capital ESD publications of the International Electrotechnical Commission (IEC).
Based on the information provided through the research efforts noted above, a thorough understanding of systems response to ESD events becomes possible. When combined with these applications of impulse waveform research, an approach to comprehensive evaluation of product and systems performance during ESD event exposure is extrapolated.
ESD Impulse Propagation
The propagation of ESD impulse current in a product or system is significantly influenced by the distributed common-mode impedance magnitude of the product/system’s structure relative to external ground surfaces. Usually, these dynamic reference transfer impedances are simply referred to as being “coupled to ground”, although spatially they may also couple/propagate to structures that are immediately adjacent to the product being subjected to the ESD event. (When the ESD is applied to a STRUCTURE that is immediately adjacent to the product under evaluation, a radiated field displacement occurs through these same distributed transfer impedances, causing (potentially) a form of radiated susceptibility.)
The distributed transfer impedance of a unit above the reference (ground) is paralleled by the ground wire connection in the primary AC power cable, as well as the power lines themselves. The combined product-level impedance that is ultimately transferred to “ground” as represented by these segments is, in turn, paralleled by the distributed and direct impedance of any interface cables that are connected to the product under evaluation. (This is because the interface cables, in themselves, are referenced [coupled] to “ground”, and the interfaced product/system that is eventually connected to the opposite end of the interface cables are probably connected to “ground” through other primary AC power cables.) .
It may be recognized that the two axis of distributed transfer impedances (to ground and literally to adjacent structures) mutually share the ESD impulse currents within impedances that develop in the interface cables to ground and to other equipment. Further, these two axis provide current sharing interactions with the references yielded through the AC power cables.
Given the propagation of ESD energy via these multiple paths, it can be recognized that the magnitude of the ESD impulse current that circulates in the interface cables (entering or a exiting the product) must be influenced by the characteristics of the distributed transfer impedance of the product above ground. In other words, the lower the value of distributive transfer impedance formed between a product and ground, the higher value of impulse current will flow through the distributive vehicle, and lower will be the relative value of impulse ESD current that will propagate through the interface or power cables.
With these interactions of the ESD propagational “sharing” through impulse propagational references directed through the various paths noted above, substantial ESD performance (susceptibility) variances are encountered by altering the distributed impedance parameters of products through conditional/installation means. Such would be the effect, for example, of encountering ESD susceptibility response variations from a desk-top unit by placing the unit on a metal desk versus placing the unit on a non-conductive wooden desk.
This variable-response effect is brought about because the lower distributive transfer impedance provided to the unit (to “ground”) through the metal desk will, in execution:
- Move ESD energy away from the interface and power cables, resulting in less potential interface susceptibility because there will be reduced ESD current/energy propagated in the interface (and power) cables.
- Intensify the ESD current in the case and cabinet structures of the unit that are coupled to ground through distributive means,thereby increasing the potential for ESD susceptibility effects fromcase-derived (shielding or slot aperture) causes.
Accordingly, the potential of ESD performance variations available for smaller (desk-top or handheld) units may be far greater in terms of both ESD amplitude thresholds, and response characteristics, than those potential variations that may be exhibited for a larger system or floor-standing unit, in that the relative impedance ratios between the interface impedance paths and the distributive (structural) impedance is potentially more variable for the desk-top supported or hand-held unit.
On a unit level, these same interactions are responsible for variations in the ESD susceptibility performance of products with remote keyboards (or hand-held devices) if the ESD impulses are applied to the “remote” keyboard when it is supported above a non-conductive desk while the “host” product is above a common conductive (desk) support plane. The ESD impulse transfers in the keyboard cable will be significantly altered by execution of these relatively simple installation condition changes.
- King, W. Michael, “Dynamic Waveform Characteristics of Personnel Electrostatic Discharge”, Proceedings of the 1979 Electrical Overstressed-Electrostatic Discharge Symposium, Reliability Analysis Center, Rome Air Development Command, ITT Research Institute, Rome, NY (1979) pp. 78-87
- Byrne, William W., “Development of Design and Test Procedures to Meet Electrostatic Discharge (ESD)”, Proceedings of the 1982 MIDCON Convention Session 28/4, Dallas, TX (1982).
- King, W. Michael and Reynolds, David, “Personnel Electrostatic Discharge: Impulse Waveforms Resulting From ESD Of Humans Directly And Through Small Hand-Held Metallic Objects Intervening In The Discharge Path”, Proceedings of the 1981 IEEE International Symposium on Electromagnetic Compatibility (1981) pp. 577-590.
- King, W. Michael and Reynolds, David, “Personnel Electrostatic Discharge: Impulse Waveforms Resulting From ESD Of Humans Through Metallic-Mobile Furnishings in the Discharge Path”, Proceedings of the 1982 IEEE International Symposium, on Electromagnetic Compatibility (1982) pp. 212-219.
- Byrne, William W., “The Meaning of Electrostatic Discharge (ESD) In Relation to Human Body Characteristics And Electronic Equipment”, Proceedings of the 1983 IEEE International Symposium of Electromagnetic Compatibility (1983) pp. 369-380.
- Richman, Peter, “Classification of ESD Hand/Metal Current Waves versus Approach Speed, Voltage, Electrode Geometry And Humidity”, Proceedings of the 1986 IEEE International Symposium on Electromagnetic Compatibility (1986) pp. 451-460.
- King, W. Michael, “EMI And ESD Control In Commercial Devices”, Proceedings of the 1980 WESCON Session 29/2, Anaheim, CA (1980).
- Daout, B., and Ryser, H., “The Reproducibility of the Rising Slope In ESD Testing”, Proceedings of the 1986 IEEE International Symposium on Electromagnetic Compatibility (1986) pp. 467-474.
- King, W. Michael, “ESD Test Methodologies and Performance Criteria”, Hewlett-Packard DT Division Corp Standard (1979); Burroughs Computer Corp Standard (1982); Texas Instruments System Standard (1984); ANSI ESD Guide, 1st complete draft (1984). (*)
- King, W. Michael, “Systems Response To Electrostatic Discharge: Applications Of Impulse Waveform Research Toward Understanding of Product Performance”, Proceedings of the 1987 Electrical Overstress/Electrostatic Discharge Symposium, Reliability Analysis Center, EOS/ESD Association, Rome, NY (1987). (**)
(*) The Vertical Coupling Plane (VCP), Horizontal Coupling Plane (HCP), Incremental Level Test Requirements and incremental probability criteria were first devised and presented in these (and similar) documents.
(**) Note: This article is based on, and is an expansion of, the above published paper.
|W. Michael King
is a systems design advisor who has been active in the development of over 1,000 system-product designs in a 50 year career. He serves an international client base as an independent design advisor. Many terms used for PC Board Layout, such as the “3-W Rule”, the “V-plane Undercut Rule”, and “ground stitching nulls”, were all originated by himself. His full biography may be seen through his web site: www.SystemsEMC.com.Significantly, he is the author of EMCT: High Speed Design Tutorial (ISBN 0-7381-3340-X) which is the source of some of the graphics used in this presentation. EMCT is available through Elliott Laboratories/NTS, co-branded with the IEEE Standards Information Network.