What You Want Is What You Get

In choosing an ESD-safe workstation, your first concern is the electrical characteristics of the worksurface. You expect to have the correct point-to-point and point-to-ground resistance for your application, probably in the dissipative range of 1 × 10⁶ to 1 × 10⁹ W. There must be a reliable connection to the external ground. These points are not negotiable.

But what else do you need? Certainly the operator who spends several hours each day at the station is concerned about comfort. Then, depending on the application, possibly you may need clean-room compatibility or tolerance to a level of heat used in your process. If you use chemicals in the work area, the surface should stand up under the inevitable spills.

If you build sensitive products such as disk-drive heads, cushioning is necessary, and if your product is heavy, you’ll need a strong base structure. Aesthetics are significant, too.

Manufacturers know that these needs extend far beyond static-control characteristics. We asked several companies what they are developing for the work area, and possibly some of their responses will be pleasant surprises for you.

Types of Worksurfaces

The typical worksurface with a conductive backplane is not a new development. “Our company introduced a conductive backplane in 1978 that exhibited fairly consistent resistance readings from all points,” recalled David E. Swenson, senior specialist at 3M Electronic Handling and Protection. “The concept still is widely used in the industry today.”

Generally, the resistance to ground should be lower for more sensitive devices. “For most applications,” he continued, “a resistance of less than 1 × 10⁸ W is appropriate, although some will argue that this is too low and can cause charged device model (CDM) discharges. Such concerns are most valid at an automated assembly station.”

Another type of static protection is a dissipative powder coating applied to storage drawers and other surfaces that a worker contacts. “A functional thermosetting powder coating in a hybrid resin system,” according to Bill Ricker, engineering manager at the Kewaunee Technical Furniture Group, “should be applied in a 2.0-mil to 4.0-mil thickness to provide a resistance of 10⁵ W to 10⁹ W. A thicker coating provides lower resistance. Powder coatings are available in a variety of colors and can be applied safely in areas where liquid coatings would be environmentally unfriendly.”

ESD-safe worksurfaces are not limited to laminated tabletops. For the temporary implementation of static protection, ESD-safe mats are available.

“We make a laminate,” said Gene Felder, corporate marketing manager at Desco Industries, “that has a carbon inner layer to provide lifetime static dissipation. It removes 100% of the charges from conductive or dissipative materials placed on the surface. The surface is resistant to hot solder and most solvents.

“We also offer a homogeneous vinyl/rubber mat that uses 100% nitrile butadiene rubber to resist abrasion, chemicals, and heat. The alternate version is a dual-layer rubber mat with a conductive bottom and a dissipative top. Resistance to ground is 1 × 10⁶ to 1 × 10⁹ W, and point-to-point resistance is greater than 1 × 10⁶ W,” he explained.

Typically, mats are replaced when they become stained or discolored and can’t be restored to their original appearance or when they develop permanent waves or lumps. For these reasons, the true cost of a mat is based on its usable life. The heat-resistant, stain-resistant mat costs a little more but often proves to be a bargain in the long run.

Worksurfaces can be manufactured with special features. As an example, IAC Industries applies dissipative laminate to a particleboard substrate with a 180° rolled front edge. “This prevents possible cracks by eliminating air pockets,” said Don Murphy, president of IAC Industries. “After installing grounding studs in the product, we test the humidity-independent assembly.”

Improvements in Grounding

As an integral improvement in the overall static-protection task, manufacturers are developing new methods for grounding laminates and mats. One method uses serrated metal prongs that contact by digging into the conductive layers. Another technique involves soaking nonconductive laminate in a silver nitrate solution to provide a conductive path through all layers of the material.

Trends

After the proper static-control capability is built into the worksurface, what else can be done to enhance its usefulness? Manufacturers have responded to the ever-changing needs of users by introducing several improvements.

In many applications, clean-room compatibility is a characteristic of increasing concern. More and more products must be handled in a Class 100 environment. Several manufacturers offer worksurfaces with low outgassing and virtually no particulate generation.

Users typically want the ability to arrange as well as later reconfigure a work area to their needs. To meet these demands, many workstation designers offer modules that can be assembled or reassembled in dozens of unique arrangements.

Load capacity must be appropriate for the application. Some stations handle very heavy loads. Symbiote, for example, offers workstations with either a 500-lb or a 2,000-lb capacity.

Recognizing that an operator must work comfortably at the station for several hours each day, ergonomic factors are significant in the design of the work area. Static-control characteristics make the assembly comfortable for the product, but the operator also must be comfortable. 
Aesthetics have a direct bearing on operator morale. Color and a pleasing layout are important in station design and high on the priority list for workstation manufacturers.

Cushioned worksurfaces have become especially important in some applications as smaller, more sensitive products are manufactured. Workstation designers have found ways to provide a soft surface without losing the static-control feature.

Automated assembly operations have forced changes in the physical form for these applications. Work areas are smaller than before, and product handling by humans is minimal. As a result, rigid easy-to-clean worksurfaces are required.

Maintaining Workstation Quality

The ESD-safe workstation must be maintained carefully to reap the maximum benefit from its design. Daily inspections should verify that all ground connections are solid, that there are no insulators or static-generating materials at the workstation, and that the worksurfaces have been cleaned properly.

Care must be taken in the choice of cleaning materials for the ESD-safe worksurface. Harsh chemicals or those that leave a residue can degrade the workstation by changing the resistance to ground.

Dirt, the most common contaminant, typically is an insulator and will degrade the static-control characteristics of a worksurface. A typical mistake is using common household cleanser to remove grime from the surface. Such a cleaner usually contains silicone or other insulating residue. For the best long-term ESD control, surfaces should be washed regularly with a silicone-free, water-based cleaner.

The manufacturer is the best source of advice on what cleaning materials are best for a given worksurface. For example, Mr. Murphy of IAC Industries recommends cleaning his company’s worksurfaces with a damp cloth and soapy water. He said that most stubborn spots can be removed with a mild chemical cleaner, using no abrasives. To remove glue and grease, use a nonflammable adhesive solvent. Stains should be exposed to a 5% hypochloride bleach solution for two minutes, then rinsed with clean water. “Wax never is used because it can be an insulator,” he explained.

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Published by EE-Evaluation Engineering
All contents © 2002 Nelson Publishing Inc.
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without the express written consent of the publisher.

July 2002