ANSI/ESD S20.20 explained: an explanation of the ESD keepout distance for 2000V products.

Ed.: For the complete column, please see circuitsassembly.com/cms/content/view/4612.

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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.

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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.

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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.

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); [e-mail].