The Newest ESD Standard

A Review of ANSI/ESD S20.20-1999

From EE , February 2000

ANSI/ESD S20.20-1999, the eagerly awaited document defining the method for developing an electrostatic discharge (ESD) control program, now is ready for use. The official name of S20.20 is The ESD Association Standard for the Development of an Electrostatic Discharge Program for Protection of Electrical and Electronic Parts, Assemblies, and Equipment (Excluding Electrically Initiated Explosive Devices).

Many ESD experts anticipate that ANSI/ESD S20.20 will become the most important document on static control in North America, and it should have a significant influence in the global community. With such an enthusiastic endorsement from the ESD community, this standard deserves a closer look at all that it encompasses.

S20.20 is part of a long-range plan by the U.S. military services to get out of the standards-writing loop. The Department of Defense asked the ESD Association to convert MIL-STD-1686 Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies, and Equipment, Excluding Electrically Initiated Explosive Devices into a commercial document and update the handling procedures to reflect current and state-of-the-art thinking.

The standard was initiated under the guidance of the late Joel P. Weidendorf of IBM when he was chairman of the ESD Association Standards Committee. It went through 18 versions before the final approval by the ESD Association and then was reviewed and approved by the American National Standards Institute.

Scope of Coverage

According to the foreword to the document, "The standard covers the requirements necessary to design, establish, implement, and maintain an ESD control program for activities that handle electrical or electronic parts, assemblies, and equipment subject to damage by ESD equal to or greater than a 100 V human body model (HBM)." These are the three fundamental control principles presented in the standard:

  • All conductors in the work environment, including personnel, must be electrically connected to ground.
  • Ionization systems must be used to neutralize charges on necessary nonconductors in the work environment.
  • ESD-sensitive items must not be transported outside a protected area except when they are enclosed in static-protective materials.

 

Organization of the Standard

The standard is divided into administrative and technical requirements. For each topic, a requirement is presented along with guidance on how to implement it.

A total of 35 other publications, most of them ESD Association standards, forms a part of S20.20-1999. The 12 U.S. government documents and nine industry standards reviewed during preparation of this standard also are listed for reference.

 

Administrative Requirements

Recognizing that ESD control will be ineffective without a well-thought-out plan, the administrative requirements outline the basics:

 

Program Plan

The using organization shall prepare an ESD control plan to address each requirement of the program. It will be the principal document for implementing and verifying the program and should include a list of the ESD-protective materials and equipment to be used. A major element of the plan is an assessment of the ESD susceptibility of parts, assemblies, and equipment, and the required protection level for each.

 

Training Plan

A training plan shall be developed to define how ESD awareness and prevention training will be provided on a recurring basis to all personnel who handle or otherwise have contact with ESD-sensitive items. New technologies and correction of discovered deficiencies should be included in this plan.

 

Compliance Verification Plan

In a compliance verification plan, formal audits or certifications shall be defined, including the frequency at which verifications must occur. Both internal and external audits must be performed.

 

Technical Requirements

The standard identifies and describes key measurement processes to qualify a company's ESD control program. Each process is marked as required or optional for permanent or temporary protected areas. For each test, the document also shows the source of an appropriate test method and presents the range within which the test results should fall. The following excerpts highlight the recommended limits for various tests; specific test methods and qualifying notes can be found in the document.

Grounding/Bonding Systems

Grounding/bonding systems shall be used in a protected area to ensure that ESD-sensitive items, personnel, and other conductors are at the same electrical potential. In most cases, the green wire for AC equipment is the preferred choice for ground reference. Here are the recommended limits for:

  • Equipment Ground: < I W AC Impedance.
  • Auxiliary Ground:< 1 W AC Impedance.
  • Equipotential Bonding: <1 x 109 W.
  • Common-Point Ground:<1 W AC Impedance.

 

Personnel Grounding

All personnel shall be bonded or grounded when handling ESD-sensitive items. A log should be maintained to verify that personnel have tested their grounding devices. Constant monitoring devices, if used, must be checked to verify they are functioning properly.

ESD-protective flooring, used with approved footwear, may serve as an alternative to wrist-strap grounding for operators who stand to perform their duties. The recommended limit for a wrist strap system is <35 x 106 W and <35 x 106 W or <100 V for a floor/ footwear system.

 

Protected Areas

A protected area is any area with designated boundaries that contains materials and equipment to limit electrostatic potentials. It can be a fixed or a portable workstation, laboratory, room, building, or other area.

ESD-sensitive parts, assemblies, or other equipment shall only be handled in a protected area. Caution signs must clearly denote such areas. Access to protected areas without an authorized escort shall be limited to persons who have completed ESD training.

Nonessential insulators shall not be allowed in a protected area. Charges on process-essential insulators must be neutralized by ionization or some other technique if an electrostatic field is deemed to be a threat. Any field of greater than 2,000 V should be kept at least 12" from ESD-sensitive items. These are the recommended limits for:

  • Work Surface: <1 x 109 W or <200 V.
  • Wrist Strap Cord: 0.8 x 106 to 1.2 x 106 W.
  • Footwear:< 1 x 109 W.
  • Flooring:< 1 x 109 W.
  • Seating: <1 x 109 W.
  • Ionization (other than room systems): < 50 V offset.
  • Ionization (room system): < 150 V offset.
  • Shelving: <1 x 109 W.
  • Mobile Equipment: <1 x 109 W.
  • Garments: 1 x 105 to 1 x 1011 W.
  • Humidity: >30% RH <70%.

 

Packaging

If specified by a procurement contract, ESD-protective packaging for transporting or storing ESD-sensitive material outside of protected areas shall be in accordance with the requirements of the contract. In the absence of contractual requirements, the organization must specify acceptable packaging as part of the overall ESD control plan. Some types of protective packaging are low-charge-generating bags and wraps, air space, dissipative wrap, static shielding, conductive shunts, and EMI/RFI shielding.

Some packaging materials may be humidity-dependent and have a limited shelf life or lose their shielding properties when crumpled, punctured, or folded. Following are the recommended limits for several packaging types:

  • Conductive:<1 x 104 W.
  • Dissipative: 1 x 104 to <1 x 1011 W.
  • Shielding: <50 nJ.

 

Marking

All ESD-sensitive assemblies and equipment must be marked with an ESD caution symbol in a readily visible position. The symbol should still be visible when the assembly is incorporated into its next higher assembly. ESD-protective packaging also must be marked.

 

Equipment

The working parts of AC-powered tools in a protected area must provide a conductive path to ground. Battery-powered and pneumatic hand tools should have a resistance to ground of less than 1012 W. Automated handlers, whether stationary or in motion, must provide a continuous conductive path to ground. The recommended limit for AC powered tools is < 1 W and < 1 x 1012 W for battery-powered and pneumatic tools.

 

Handling

Handling procedures shall be established and enforced in all protected areas as well as in locations where ESD-sensitive materials may be stored in protective packaging.

 

Sensitivity Testing

Sensitivity testing is addressed in the appendix to S20.20-1999, with the reminder that ESD failures are due to a complex series of interrelated events. Some of the factors that influence the sensitivity of a device to a given ESD threat are the current and energy envelope of the ESD event, its rise time, the design and fabrication technology of the device being subjected to the event, and its package style.

An energy-sensitive device will be damaged by excessive current through a bipolar junction, a protection resistor, or a protection MOS transistor. A voltage-sensitive device will be damaged by excessive voltage. Devices can be tested to determine their probable energy or voltage sensitivity by simulating a human body, machine, or other device discharge.

A charged HBM is a current source that can be modeled by a 100 pF capacitor discharging through a 1,500 W resistor into the device being tested. The discharge is a double-exponential waveform with a rise time of 2 to 10 ns and a duration of about 150 ns.

The charged conductor, or machine model, is a voltage source such as aboard assembly, cables, or the conduction arm of an automatic tester that can be modeled by a 200 pF capacitor discharging through a 500 nH inductor into the device being tested. The discharge is a sinusoidal decaying waveform with a rise time of 5 to 8 ns and a duration of about 80 ns.

The charged device model is an energy source that can discharge through its leads into another device of a different potential. The rise time often is less than 0.2 ns, and the duration can be less than 2 ns.

 

For More Information

Click here to download your copy of ANSI/ESD 20.20 standard now in PDF format (129K)

Copies of ANSI/ESD S20.20-1999 can be obtained from the ESD Association, 7900 Turin Rd., Building 3, Suite 2, Rome, NY 13440-2069, (315) 339-6937,