Fowler Associates Labs

ESD Control Standards:
Setting up an ESD control program

Ryne C. Allen
ESD Systems

INTRODUCTION

This paper will help an ESD Coordinator, Manager or Engineer apply the proper ESD standards when developing an ESD Control Program. For an ESD Control program to be effective it should be designed around the ElectroStatic Discharge Sensitive (ESDS) devices it is protecting; and most importantly, it should be supported at all levels within the company, from the company officers through all managers to the operators and technicians. This top-down approach, when fully adapted, ensures that all the elementary elements of the program are properly deployed.

One of the main reasons companies deploy an ESD Control program is to save money. Increased throughput and decreased scrap can yield a ROI (Return On Investment) of up to 3000% per year for successfully deploying an ESD Control program [1]. A secondary reason is to comply with their customers’ demands and ISO 9000 programs. Whatever reason, setting up and implementing an ESD Control program will almost always pay for itself within the first year.

There is an array of ESD Control products on the market. Which products should you choose when developing your ESD Control program? The products that you select are determined by how you have defined your ESD control program.

If your company supplies various electronic components or is a board house, your customers may require you to follow certain prescribed standards to which they adhere. If you are establishing a new ESD control program then the Electrostatic Discharge Association’s Standards listed in Tables I through IV will aid you to establishing the proper criteria.

The Electrostatic Discharge Association (ESDA) standards are the most comprehensive and up-to-date industry accepted standards for the control of ESD. Their current standards are listed in Table I by application.

Six years ago, the ESD Association became an American National Standards Institute (ANSI) accredited standards development organization, and this attainment is reflected in some of the ESDA standards carrying the ANSI accreditation.

The first place to look for guidance in developing your ESD Control program would be from within the ESDA standards. As of February 1998, the ESDA has refined their definitions of standards by specifying the following categories.

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

The ESD Association has recently accepted the task of refining MIL-STD-1686 Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies and Equipment as a commercial document and currently has a draft standard ESD DS20.20-1998. The ESDA is also assisting the update of EIA-625, Requirements for Handling Electrostatic-Discharge-Sensitive (ESDS) Devices.

If you do not classify the devices then you can assume the worst case for all 3 models, (Classes 0, M0, C0 - refer to the ESD STM5 series standards in Table I), making the program design critical and expensive.

Now, the location/sensitivity map of the facility needs to be expanded upon by determining what standards you will use to evaluate the success and monitor the program’s progress. This map should also consider the transportation systems and traffic flow of the sensitive devices between various working areas. Additional design criteria to ensure device protection that needs to be broadened are listed as follows.

IV Selecting General Product Criteria for each Area

Table IV will help aid the design and development of an ESD control program for each area using either the ESD ADV-2.0-1994 or the MIL-STD-1686. The former is more recent at this time. Table III lists various ESD Control products and the associated ESDA Standards that can be used to qualify them.

As an example, an ESD Sensitive (ESDS) workstation that is designed for worst case criteria may have the following ESD Control products: an ESD floor; grounded floor mats with use of 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 work surface to which the devices would be exposed. In addition, all exposed insulators and metal surfaces would be replaced with grounded dissipative materials; all non-essential items, especially insulators, would be removed the ESDS area; and most importantly, the ESDS workstation would have ESD Control Trained operator(s) at the helm.

 

1. Floors

Starting from the ground up, your floor would be the first place to start. One of the most important characteristics of an ESD floor is its ability to conduct charges to ground. The second most important aspect is its anti-static property. One of the main mechanisms of charge generation is triboelectric generation or tribocharging. Some examples of tribocharging are people walking along a floor and carts carrying sensitive devices rolling across a floor. Depending on where the materials in contact with the floor are in the triboseries, voltages of over 30,000 Volts can be attained. If a floor has the property of being anti-static, tribocharging becomes a much smaller concern. The standards documents to help choose 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.

 

 

2. Mobility

Typically protection on an ESDS device should start at receiving, continue to inventory storage, and then travel through its production flow usually from one workstation to the next before ending up in shipping. All throughout its handling, the device should be handled by grounded personnel. The easiest way to ground people who travel from one station to the next, delivering or picking up sensitive materials, is through mobile grounding. Wearing foot grounders (one on each foot) in conjunction with a conductive floor is one way to ensure that the operator is grounded and protected from delivering or receiving an ESD event. There are several ESDA standards to help in the testing and verification of foot grounders and shoes: ESD DSTM54.1-1997, ESD DSTM54.2-1997, and ESD S9.1-1995.

 

 

3. Work Surfaces

The surfaces where ESDS devices are handled should be both conductive (in the dissipative range) and properly grounded to the equipment grounding conductor to be an effective ESD control element. There are several materials to choose from such as rubber mats, vinyl mats, both single and multi-layered and FRP and Micastat® for rigid or permanent bench surfaces. Conductive metal work surfaces should be discontinued or covered with a dissipative material because it is highly susceptible to causing an ESD event from a metal-metal contact. It is very important to control your discharge time by minimizing the energy transfer by employing resistive materials to ground [4]. The ESDA standards to help characterize a work surface are ESD STM4.2-1998 and ESD ADV53.1-1995.

 

4. Personnel Grounding

The human being can be the most dynamic part of a working environment and consequently should be considered one of the most important objects to ground. Wrist straps, a conductive wristband with a connecting ground cord, is the most popular and effective way to ground a person. Wrist straps should always be properly employed when working with ESDS devices. The ESDS standard EOS/ESD S1-1987 can aid in qualifying your wrist straps before implementation.

 

5. Ionization

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

 

 

6. Transportation & Packaging

ESDS devices should always be stored in an enclosed antistatic shielding bag or conductive closed tote or bin when not being handled. This includes inventory storage, transportation, and WIP. Further precautions during transportation include using dissipative carts with conductive wheels or drag chains in conjunction with a conductive floor when transporting ESDS devices in their shielded containers. 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, ANSI/EIA-541-88 and ANSI/EIA-583-91 for packaging materials.

Standards Summary Tables

Table III-ESD PROTECTIVE PRODUCTS & RELATED ESDA STANDARDS

 

1. MIL-STD-1686B, Department of the Navy, Defense Printing Service Detachment Office, Philadelphia, PA, 31 December 1992

2. R. C. Allen, "Controlling Workstation Discharge Times", Evaluation Engineering, Jan. 1998, pp. 88-92.

FOR EXTERNAL COLORED TEXT BOX (related info)

There is always some confusion between the terms conductive and dissipative. Conductive can imply at least two different things: (1) the basic property of being able to move charges along a surface and (2) the range in ESD Control that differentiates very conductive materials (classified as "conductive") from lesser conductive materials (classified as "dissipative"). A "dissipative" material is conductive in that it will move charges along its surface. A "dissipative" material also has more resistance to the charge flow and will move these charges slower than a "conductive" material will, but much faster than an "insulative" material. Figure 1 graphically displays the ESD Control resistive ranges of "conductive", "dissipative" and "insulative". In ESD Control, one of the main keys of this control is to control the charge flow (current) during a discharge. Increasing the time for an ESD discharge will result in a reduction of the peak energy transferred over any given point in time, which minimizes the chance for an ESD event to occur and cause damage to a sensitive device. This is best accomplished with a conductive material that has sufficient resistance. The resistance range considered for this type of control starts at the high end of the "conductive" range through a good part of the "dissipative" range, refer to Figure 1.

About the Author
Ryne C. Allen is the technical manager at ESD Systems, a division of Desco Industries, Inc. (DII). Previously, he was chief engineer and lab manager at the Plasma Science and Microelectronics Research Laboratory at Northeastern University. Mr. Allen is an NARTE-certified ESD control engineer and the author of 27 published papers and articles. He is a member of the ESD Association and an active ADCOM associate member of the Northeast Local Chapter. He graduated from Northeastern University with B.S.E.E, M.S.E.E., and MBA degrees. ESD Systems, 19 Brigham St., Unit 9, Marlboro, MA 01752-3170, (508) 485-7390, resume: http://ryne.hotresume.net/, e-mail: rcallen@esdsystems.com, URL: http://www.esdsystems.com.