An explanation of the ESD keepout distance for 2000V products.

Why does ANSI/ESD S20.20 suggest keeping items with an electrical field greater than 2000V at 1" just 12" away from ESD-susceptible parts? What does this mean?

Charged objects emanate an electrical field, much like the lines of force surrounding a magnet. Most modern electrical field measurement instruments report volts at 1". If an object measures 2000V or more at 1", S20.20-1999 recommends the object be kept 12" or more from any unprotected ESD-susceptible parts or steps be taken to reduce the electrical field strength. Please note this is a recommendation and not a requirement at this time. The S20.20 requirement is for the organization to have a plan to deal with electrical fields when present in a process. The March 2007 revision of S20.20 changes all formerly recommended items into requirements, including the electrical field specification. S20.20 applies to 100V HBM (human body model) rated parts and above. If parts are handled within processes that are sensitive to less than 100V HBM, then the organization must establish its own risk level. For instance, a disk drive industry process standard from IDEMA specifies a 500V electrical field requirement because of the greater susceptibility level involved with magneto-resistive heads and related newer technologies.

An extensive study done in the late 1990s established the 2000V level for general electronic assembly (Figure 1). The right vertical axis represents a fixed electrical field of 2000V at 1" from a 6" x 6" square plate. Distance from the electrical field is shown on the x axis. At 0.625" the induced electrical field to a 20 pF plate (6" x 6" plate of a charged plate monitor – CPM) is less than 90V. This is the voltage induced to the CPM receiving plate when it was grounded in the presence of the 2000V electrical field from a close distance. At 12" away, the voltage induced onto the CPM plate is less than 40V. Therefore, a 2000V charged source with a size of 6" x 6" is not very dangerous, at least by way of induction to sensitive items, if kept 12" away. S20.20 does not suggest any controls for items with less than a 2000V (at 1") electrical field. There has been much discussion about this suggestion. This means that an object with a measured 1999V or less electrical field at 1" is of no concern for 100V HBM parts. This column provides guidance as to why this is considered acceptable.


Conductors and insulators. S20.20 correctly considers conductors and insulators separately. This is an important distinction. The S20.20 requirement for conductive materials is to electrically interconnect them as a minimum (equipotential bonding) and attach them to ground when possible. Bonded conductors will share a charge and will balance that charge between them, depending on their capacitance. If one item is large and the charged item is small, the larger item could actually emulate a ground connection. A grounded conductor cannot hold an electrostatic charge. However, an isolated conductor is dangerous to ESD-sensitive items because any stored charge can be transferred by contact at a rate limited only by the contact resistance between the two items and the resistance to ground of the system. A charged conductor with an electrical field of 2000V at 1" represents a dangerous item in a process if it is permitted to contact a sensitive item. This would be considered a machine model (MM) event that is very energetic and generally more damaging to ESD-susceptible parts than a similar level HBM discharge.

A charged insulator does not have electrical potential (voltage) in the technical sense because it cannot lose a charge by contact with ground. An insulator can only transfer a small amount of charge from a small area by contact. However, the 2000V discussion in S20.20 applies to the electrical field from the necessary insulators that must be involved in a process. These include, but are not limited to, circuit boards, the insulated portion of component packaging, and elements of process or handling equipment.

Danger of electrical fields. From this point, the discussion involves only process-essential insulators, as S20.20 requires electrical bonding of all conductors (includes personnel) and grounding when possible, as well as removal of all nonessential insulators. These actions reduce most ESD risk in any process where ESD-susceptible items are stored, processed or otherwise handled. If the electrostatic-protected area (EPA) is set up correctly, there will be no electrical field issues involving conductors. If any electrically isolated conductors are found in a process, they need attention using bonding and grounding techniques. Only in rare instances should the 2000V electrical field specification apply to a conductor.

The size of a charged object has a major influence on the risk to sensitive parts. Figure 1 involves relatively large items – 6" x 6" plate and the induced voltage to a 20 pF 6" x 6" CPM plate. Figure 2 shows the same type of data, but with several smaller plates charged by a 2000V potential. The 6" plate follows the data from Figure 1 closely, but the smaller plates represent almost no induction risk to a 20 pF plate. This simple observation could have significant implications within any process. As an example, if there are no large charged insulators in a process, the allowed 2000V electrical field could be significantly increased if needed. While it is doubtful that the 2000V level would need to be changed, as it is already conservative, S20.20 permits tailoring if data can be shown that permit an adjustment of the risk.


Small insulative items have a small inductive risk, but most electronic components have conductive and insulative elements within the same part. An electronic component approaching a circuit board in an automated assembly operation is at risk for a direct discharge, if the conductive elements are charged. In addition, a field-induced charged device model (CDM) event is likely if the component is carrying an electrostatic charge on the insulating elements. If the insulative portion of the device has a charge, some level of charge will be induced to the conductive portion of the device at the moment of contact with a grounded conductive item (e.g., bonding pads on a PWB). Thus, keeping the measured charge on a device below a specified threshold level is important during assembly. Monitoring this area requires some new tools and equipment that can detect the electrical field or charge within the process environment.

S20.20 does not specifically address rapidly changing electrical fields, but this aspect should be considered in any process. As an example, assume there is a 1000V positive initial charge on an object moving through a process. S20.20 would not consider this a direct risk. However, the positive charge could cause an inductive charge of perhaps nearly 1000V negative polarity in some close operations. This results in up to a 2000V absolute voltage differential. Any sensitive item passing in or near this event will see some of that larger potential difference, even though the initial electrical field was well below the recommended threshold. If the inductive event takes place over a long period of time, it may not be an issue. If a device sees a rapid change in electrical field, it has a higher level of risk.

Figure 3 shows the induced voltage level to the various sized plates from the 6" x 6" 20 pF CPM plate. Here the charged source is large and the receiving plates are of various sizes. In this case, the 6" x 6" plate accepted a similar charge as before by induction from the CPM at 1". A 3" square plate was somewhat less, the 1.5" square lower, and the 0.75" square was less than half. As shown in the figure, the induction level drops off at an expected distance. At 12", plates smaller than 6" x 6" received no measurable inductive charging.


Minimizing risk from electrical fields. S20.20 suggests using topical treatments, ionized air or separation distance as tools or techniques in a process to minimize electrical field strength and the risk of inductive influences from process-essential insulators. Topical treatments usually create an electrically conducting surface on an insulator so that grounding will permit a charge to dissipate. While some new treatments are fairly permanent, the organization will need a plan for monitoring all treated surfaces to ensure they function as intended. Included in the topical treatment category are paints, clear coatings and antistat solutions. Coatings are particularly useful on plastic windows and clear shields on process equipment. Standard acrylic-type plastics are high static generators, and clear dissipative coatings can reduce or eliminate the charge accumulation ability of these process essential insulators. Ionized air is useful in many process areas to reduce the charge on insulators; however, care must be taken to use the correct type of ionizer in any given application. Separation distance may not be possible in some automated processes where charged items are moving, so the judicious use of ionized air is the only practical way to reduce charge and associated electrical fields.

Process monitoring. New tools are coming available for remote measurement and monitoring of the electrical fields within process equipment. Figure 4 shows an example of the data collected in a semi-automated process. The large voltage spikes are where plastic pieces and wire harnesses were assembled to a metal panel. Various operators install parts, including a piece of tape at the 20-sec. mark. A circuit card installation is shown at the 100-sec. mark. At approximately the 210-sec. mark, another operator installed a wire harness. Voltage stayed high until another operator installed additional parts at the 260-sec. mark. With attention to the timing of operations in a process, high charge generation activities can be identified and reduced by proper application of charge mitigation techniques.


Charge generation in a process can lead to excessive electrical fields. Electrically isolated conductive objects touched to ground while in the presence of an electrical field will obtain a charge by induction. Isolated, charged conductors are the source of MM discharges and field-induced CDM events. Conductors must be grounded within a process to avoid these events. Insulators do not lose a charge by grounding, and therefore do not represent a risk for MM-type discharges. The electrical field from a charged insulator may result in a field-induced CDM event depending on several factors, including electrical field strength, separation distance, size of the charged insulator and whether the electrical field is rapidly changing.

Ed.: This article is copyright 3M Static Digest, 2006, and is used with permission.

Dave Swenson is senior vice president of the ESD Association (esda.org) and founder of Affinity Static Control Consulting LLC (affinity-esd.com); static2@swbell.net.