COPING WITH ESD

For an ESD control program to be effective, it must be designed around the electrostatic discharge (ESD)-sensitive devices it is protecting. Most importantly, all levels within the company—from the company officers through all managers to the operators and technicians—must support the program. This top-down approach ensures that all the elements of the program are properly deployed.

One of the main reasons a company deploys an ESD control program is to save money. Increased throughput and decreased scrap can yield a return on investment of up to 3,000% per year for successfully deploying an ESD control program.¹

Companies also establish an ESD control program to comply with the demands of their customers. Whatever the motivation, an ESD control program almost always pays for itself within the first year.

Approximately 60 of the more commonly used standards for ESD control are listed in Table 1 and Table 2. These standards and specifications originate from many different organizations and serve different purposes:

Military and government, including FTM, MIL, and OSHA.

Commercial organizations, such as AATCC, ANSI, ASTM, AT&T, CECC, IEC, IPO, and ISO.

Associations including EIA and the ESD Association.

Standard testing and classification of semiconductor devices.

Standard test methods.

Standards for qualifying and monitoring ESD control programs or items.

Technical reports or advisories for understanding the standards or technology.

The ESD Association provides the most comprehensive and up-to-date industry-accepted standards for controlling ESD and is the first place to look for guidance in developing your ESD control program. The current standards are listed in Table 1 by application.

Several years ago, the ESD Association became an American National Standards Institute (ANSI)-accredited standards development organization. This is reflected in some of the ESD Association standards carrying the ANSI accreditation.

The association has refined the definitions of standards by specifying the following categories:

Standard (S)—a precise set of requirements for a material, product, system, or process that specifies the procedures for determining whether each of the requirements is satisfied.

Standard Test Method (STM)—a definitive procedure for the identification, measurement, and evaluation of one or more qualities, characteristics, or properties of a material, product, system, or process that yield a reproducible test result.

Standard Practice (SP)—a procedure for performing one or more operations or functions that may or may not yield a test result.

Technical Report (TR)—a collection of technical data or test results published as an informational reference on a specific material, product, system, or process.

As a new standard evolves and becomes ready for industry review, it is classified as a Draft Standard (DS), represented by the designation Draft Standard Practice (DSP), Draft Standard Test Method (DSTM), and Draft Technical Report (DTR). An Advisory (ADV), which may be replaced by the TR, can be educational in nature. It consists of general information and guidelines to help you understand the use of standards and related technology.

A sound ESD control program begins by classifying the sensitivity of your devices to ESD damage. Classification should include all simulation models (human body model, machine model, and charged device model) that will characterize the sensitivity of the devices when handled at various locations within the facility. Remember that sensitivities may vary according to different locations within the facility.

The ESD Association standards that aid the sensitivity testing process are ESD STM5.1-1998, ANSI/ESD S5.2-1994, ESD DS5.2-1996, and ESD DS5.3.1-1996. These documents are the most recent in the industry.

The military standards that can be used to determine device ESD sensitivity are MIL-HDBK-263B, MIL-STD-883D Method 3015.7, MIL-STD-750C/4 Method 1020, and MIL-STD-785. The IEC standards to help classify device sensitivity are CISPR 24 (1997-09) and IEC-61000-4-2 (1995). ANSI also has a document, ANSI C63.16, that can aid in device sensitivity classification.

If you do not classify the devices, then you can assume the worst case for all three models. However, this makes the ESD control program difficult and expensive to design.

Once ESD device sensitivities for the various areas have been determined, this information can be mapped over the complete facility. This map serves as a guide to designing the ESD control program.

Now, expand the location/sensitivity map of the facility by determining what standards you will use to evaluate the success and monitor the progress of the program. This map also should include the transportation systems and traffic flow of the sensitive devices.

Additional design criteria to ensure device protection are listed as follows:

Minimize voltage or field exposure by removing nonessential charge generators.

Minimize voltage or field exposure by using protective packaging during transportation or storage.

Minimize voltage or field exposure from machine to device contact; for example, automated equipment.

Provide dissipative worksurface materials.

Provide grounding (power ground distribution) for common-point grounds/worksurfaces.

Provide grounding for floors and traffic areas.

Provide grounding for personnel using wrist straps/foot grounders/smocks/gloves.

Provide air ionization for essential nongrounded or insulative materials/equipment/tools.

Provide environmental control of temperature and humidity.

Provide training of employees within various affected areas.

Which standards should you use when building, updating, or evaluating your ESD control program?

Table 3 will help you design and develop an ESD control program for your facility using either MIL-STD-1686 or the more recent ESD ADV-2.0-1994. Table 4 lists various ESD control products and the associated ESD Association standards that can be used to qualify them.

For example, an ESD-sensitive workstation designed for worst-case criteria may have an ESD floor; grounded floor mats used with ESD footwear, such as foot grounders; grounded and monitored table mats covering all exposed surfaces; a common-point ground with monitored wrist-strap connections; and air ionizers covering all areas on the worksurface where the devices would be exposed.

In addition, replace all exposed insulators and metal surfaces with grounded, dissipative materials. Remove all nonessential items, especially insulators, from an ESD-sensitive area. Most importantly, staff the ESD-sensitive workstation with well-trained operators.

Begin your workstation design with the floor. An ESD floor must be capable of conducting charges to ground.

The documents that help in choosing a floor are ANSI/ESD S7.1-1994, AATCC Step Test—Method 134-1979, ANSI/EIA-625-1994, MIL-STD-1686, MIL-HDBK-263B, and the AT&T Electrostatic Discharge Control Handbook.

Protection for an ESD-sensitive device should start at receiving, continue to the inventory storage, and then travel through its production flow until it reaches the shipping department. Only grounded personnel should handle the device.

Mobile grounding methods are best for people who travel from one station to the next. Wearing foot grounders (one on each foot) in conjunction with a conductive floor can ensure that the operator is grounded and protected from delivering or receiving an ESD event.

Several ESD Association standards can help in testing and verifying foot grounders and shoes: ESD DSTM54.1-1997, ESD DSTM54.2-1997, and ESD S9.1-1995.

The surfaces where ESD-sensitive devices are handled should be both conductive (in the dissipative range) and properly grounded to the equipment grounding conductor. There are several materials to choose from, such as rubber mats, vinyl mats, and both single and multilayered rigid or permanent bench surfaces. Discontinue the use of or cover any existing conductive metal worksurfaces with dissipative material.

Control your discharge time by using resistive materials to ground.⁴ The ESD Association standards to help characterize a worksurface are ESD STM4.2-1998 and ESD ADV53.1-1995.

The human being, as the most dynamic part of a working environment, is one of the most important objects to ground. A wrist strap, a conductive wristband with a connecting ground cord, is the most popular and effective way to ground a person. EOS/ESD S1-1987 can aid in qualifying your wrist straps before implementation.

Treat materials that must stay in an ESD-sensitive work area but are neither conductive nor groundable with air ionization. Ionizers come in several types; the most popular is the corona discharge air ionizer. Corona discharge air ionizers have emitters powered by AC, DC, or pulsing DC high voltage. Air ionizers can be qualified by applying ANSI-EOS/ESD, S3.1-1991ADV3.2-1995, and ESD SP3.3-1998.

Always store ESD-sensitive devices in an enclosed, antistatic shielding bag or a closed, conductive tote or bin when not being handled. This includes inventory storage, transportation, and work in process. During transportation of ESD-sensitive devices in shielded containers, use dissipative carts with conductive wheels or drag chains in conjunction with conductive flooring.

The standards to help characterize and qualify packaging materials are ANSI/ESD S11.31-1994 for shielding bags, ANSI/EOS/ESD S8.1-1993 for proper use of package markings, and ANSI/EIA-541-88 and ANSI/EIA-583-91 for packaging materials.

Using the ESD Association and other related standards will help your ESD control program comply with industry-accepted requirements and procedures that govern the materials, products, systems, or processes. Acceptability, repeatability, and dependability can be expected from an ESD control program that uses a good design based on the appropriate standards along with proper personnel training and program monitoring.

1. Dangelmayer, G.T., ESD Program Management, Chapter 14, Van Nostrand Reinhold, 1990.

2. The ESD Association, 7900 Turin Rd., Building 3, Suite 2, Rome, NY 13440-2069.

3. MIL-STD-1686B, Department of the Navy, Defense Printing Service Detachment Office, Dec. 31, 1992.

4. Allen, R.C., "Controlling Workstation Discharge Times," EE-Evaluation Engineering, January 1998, pp. 88-92.

Ryne C. Allen is the technical manager at ESD Systems, a division of Desco Industries. Previously, he was chief engineer and lab manager at the Plasma Science and Microelectronics Research Laboratory at Northeastern University. Mr. Allen is a NARTE-certified ESD control engineer and the author of 27 published papers and articles. The ESD Association member also is a graduate of Northeastern University with B.S.E.E, M.S.E.E., and M.B.A. degrees. ESD Systems, 19 Brigham St., Unit 9, Marlboro, MA 01752-3170, (508) 485-7390.