Choosing an ESD workstation is more than deciding what size, color, and special features are best for your facility. Really effective workstations and surfaces are those that comply with today’s many ESD standards—but perhaps not.
Buying workstation products that meet current industry and military standards is a good start, but they don’t guarantee an ESD-safe workplace. In addition to understanding the standards that are most relevant to your applications, pay particular attention to:
Worksurface characteristics.
Grounding.
Monitoring.
ESD standards that apply to workstations and worksurfaces are listed in Table 1. They focus on key issues governing the effectiveness of ESD workstations and worksurfaces: surface resistivity, the surface charge dissipation capability, surface resistance to ground, and proper equipment grounding. Most of these properties are designed into the products before you buy them and verified according to the ESD standards.
“We recommend that you use ANSI/EOS/ESD S4.1 as the basis for evaluating materials, installation, and periodic auditing of worksurfaces,” said David Swenson, applications development manager at 3M Electronic Handling and Protection Division and president of the ESD Association. “ESD ADV 53.1 also gives guidance for evaluating fixed workstations that is similar to S4.1, but it provides help on the placement of electrodes and how to measure a variety of workstation designs as well.”
Worksurface Characteristics
Surface resistances measured according to ESD standards usually are between 106 and 109 W . But, that doesn’t tell the whole story. There is a contact resistance between the surface and the object from which charge is being drained. This contact resistance is affected by how much moisture is retained by the surface and how much of the surface actually is in contact with the object.
“ESD STM4.2 specifies a method for evaluating charge-drainage and charge-suppression characteristics,” said 3M’s Mr. Swenson. “Residual charge is measured on a test object after it is placed on a surface at a defined pressure and then removed. If a charge remains, the worksurface does not provide complete charge drainage.”
Softer surfaces generally have lower contact resistance than hard surfaces. On the other hand, hard surfaces typically are more durable and offer less friction, which means that it is easier to remove objects from them. Of course, the choice of a surface may be somewhat dependent upon other factors besides ESD properties, such as color and resistance to abrasion, chemicals, and stains.
What Is a Good Ground?
The science and practice of grounding lie at the heart of controlling ESD.
“Workstations should use the single-point ground method with wrist-strap jacks integrally mounted in the worksurface edge and hardwired directly to the mounted ground bolt,” said Les Thompson, president of Phoenix Workstations. “We recommend that you check the proper path to ground by measuring the difference between electrical ground and true earth ground.”
An ESD standard dedicated to grounding further emphasizes its importance. “Our workstations are grounded in accordance with EOS/ESD S6.1,” said Bill Ricker, engineering manager at Kewaunee Scientific. “The worksurface is attached to a common-point ground, which is attached to the equipment ground.”
Monitoring for Best Results
Remember, even the best worksurfaces can be improperly installed, misapplied, or just stop working. The ground connection could be improperly installed, work loose, or break. Or, it may not even be a true earth ground. Surfaces get dirty or marred and stop draining charge. Perhaps the product doesn’t work well below 20% relative humidity, or the surface resistivity or resistance to ground is inconsistent along the surface.
To protect your investment and properly control ESD, set up a regular auditing and maintenance program. Both ESD ADV-2.0-1994 and MIL-STD-1686A have a section on auditing, and the book ESD Program Management devotes two chapters to it.
How often should the ESD effectiveness of your workstation be checked?
“We recommend checking wrist straps for continuity and surfaces for dirt buildup daily,” said Mr. Thompson of Phoenix Workstation. “There should be weekly checks of the path to ground, the surface resistivity, and the surface-to-ground resistance in accordance with EOS/ESD S4.1 and EIA-625.”
Kewaunee’s Mr. Ricker favors checking all ground connections daily, removing unnecessary static-generating and insulative items from the work area, and cleaning the workstation according to the manufacturers instructions. He also prefers quality audits on a periodic basis to look for surface damage or wear.
Continuous monitoring of the workstation is a very popular trend in the battle against ESD damage. “The main risk of worksurface failure is associated with loss of ground connection,” said Mr. Swenson of 3M. “As a result, it is important to monitor the connections to the worksurface and common-point ground very often.
“Workstation monitors are becoming more and more accepted in critical applications. They can verify wrist-strap performance, and some models monitor the electrical connections to the worksurface,” he continued.
Even if the equipment is perfectly intact, there always is the opportunity for human error, such as the operator forgetting to plug in a wrist strap. “Constant monitoring of the workstation is becoming as common as grounding a worksurface in today’s production facilities,” said Mr. Ricker of Kewaunee. “More and more of our customers are interested in continuous monitoring when they buy ESD workstations.”
Standards and Reference Materials
Copies of most of the ESD standards in Table 1 may be obtained from the ESD Association, 7900 Turin Rd., Building 3, Suite 2, Rome, NY 13440-2069, (315) 339-6937, www.eosesd.org. Some standards will tell you what must be accomplished but not how to do it. Here are some books that may help you:
Dangelmayer, T., ESD Program Management, 1990, Van Nostrand Reinhold;
a how-to of successful ESD programs developed by addressing human and technical factors.
Kolyer, J.M., and Watson, D.E., ESD From A to Z, 1996, Chapman and Hall;
a summary of the results of ESD experiments in many categories and how they link to the standards.
McAteer, O., Electrostatic Discharge Control, 1989, McGraw-Hill; a scientific and mathematical approach to ESD and an analysis of different failure mechanisms.
ESD Workstation/Worksurface Products
Welded Workstation
Supports 1,000 Lb
The Model 1105 ESD Workstation is fully welded from 16-gauge cold-rolled steel and supports 1,000 evenly-distributed pounds. The 60″ × 30″ unit can adjust from 30″ to 36″ in height. Adjusting levelers compensate for uneven floors. A minimum surface resistivity of 105 W /sq ensures good static-dissipative properties. The workstation, which comes in three colors, includes a flush ground bolt and jacks. Phoenix Workstations Division, General Woodworking, (800) 270-3034.
Cabinet Provides Safe Storage
For ESD-Sensitive Components
Sentry 100™ Storage Units and Workstations perform within the static-dissipative range of 106 to 109 W /sq. They are tested and certified at an open circuit voltage of 100 V. The test criteria was established by the ESD Association and follows the ANSI Accredited Test Standards and Advisories (reference ESD S4.1, 6.1, and 8.0, and ADV 53.1). All interior cabinet partitions, dividers, and labels are static-safe treated. Cabinet drawers can be subdivided so that interior layouts achieve maximum storage efficiency. Stanley Storage Systems, (800) 523-9462.
ESD Workstation Monitor
Checks Two Operators
The 724 Workstation Monitor provides instant notification of static- control equipment failures at an ESD workstation. It checks the resistance of two operators and the worksurface ground. A slide switch sets the applied voltage to 9 or 16 V, and another adjusts the loop resistance to 10 or 35 MW . Connections are available for a third wire ground, an audible alarm for each condition, and a quick connect/disconnect dual remote jack. 3M Electronic Handling and Protection Division, (800) 814-8709.
Modular ESD Workstation
Accommodates Accessories
The Evolution™ ESD Workstation is designed in accordance with ESD ADV 53.1 and EOS/ESD S6.1. The worksurface meets ANSI/EOS/ESD S4.1 with a resistivity of 106 to 109 W /sq . The surface also provides a controlled path to ground. The workstation can be configured to accommodate single or multiple applications, and options includes a complement of ESD accessories. Heights of 30″, 36″, and 76″ are offered. Kewaunee Scientific, (800) 824-6626.
Three-Layer Worksurface
Dissipates Static
Models 50060 and 50065 Type L Static-Dissipative Mats are made of three layers of rubber. A smooth surface makes cleaning and maintenance easy, and the reflective white color is suitable for use under microscopes and inspection equipment. The 0.060″ material also is heat resistant for use in soldering and assembly areas. Charleswater, (781) 821-8370.
Ergonomic Workbench
Is Height-Adjustable
The Height-Right™ Adjustable Workbench is suitable for testing, repairing, and assembling electronic equipment. The electostatic paint and worksurface dissipate static at a safe rate. The workbench adjusts in height over a 15″ range, has a 1,200-lb static-load capacity and a 600-lb dynamic capacity. The center panel of the bench is removable and comes in a variety of colors. Lista International, (800) 722-3020.
Workbench Automates
Height Adjustment
The Agility™ Industrial Workbenches offer three types of rugged height adjustments for industrial applications, including electrical, hydraulic, and mechanical types. Fixed-height and tilt-top versions also are available. The worksurfaces measure 30″ × 60″ or 30″ × 72″ and are made of a static-dissipative material. The accessories include articulating arms, VDT and keyboard holders, a tool holder, overhead lighting, a work organizer, and a power bar. BioFit Engineered Products, (800) 597-0246.
Ergonomic Seating
Reduces Fatigue
The 7000E Series Seating features a self-skinned ESD polyurethane seat and backrest that resists stains, punctures, and damage from water, grease, and chemicals. Surface resistance is 108 W . Twelve models in the series cover seat heights from 16″ to 35″. All models come with pneumatic seat-height adjustment, a 360° swivel, an adjustable backrest, and stationary ESD glides or heavy-duty ESD casters. Bevco Ergonomic Seating, (800) 864-2991
Trash Receptacles Have
Static-Dissipative Surfaces
Two sizes of ESD-safe trash receptacles are available for electronic workstations and production areas. The containers have an impregnated conductive shielding layer and a permanently static-dissipative surface with a surface resistivity ranging from 107 to 1010 W /sq. The receptacles have a steel perimeter frame and conductive handles. Static-dissipative liners also are offered. Brick Container, (714) 994-4140.
Standard
Title
ESD ADV-2.0-1994
ESD Handbook
MIL-STD-1686A
Electrostatic Discharge Control Program
EOS/ESD S1-1987
Personnel Grounding
ESD DS1.1-1996
Wrist Straps
ANSI/EOS/ESD S4.1-1998
Protective Worksurfaces Resistive Characterization
ESD STM4.2-1998
ESD Protective Worksurfaces Charge Dissipation Characteristics
ANSI/EOS/ESD S6.1-1991
Grounding—Recommended Practice
ESD ADV 53.1-1995
ESD Protective Workstations
MIL W-87893A
Workstation, Electrostatic Discharge Control
ANSI/EIA-625-1994
Requirements for Handling Electrostatic Discharge-Sensitive Devices
EN 100015/1-1991
Protection of Electrostatic-Sensitive Devices
Table 1.
Copyright 1999 Nelson Publishing Inc.
January 1999