About 25 years ago, electronic manufacturers began to see unusual defects in their products. When they mounted good CMOS chips on PCBs some of the boards failed to work. What was going on? There seemed to be a silent, invisible CMOS killer on the loose.
The villain quickly was identified as the discharge of static electricity from the assembler’s body and metallic objects in the production area into the chips during the assembly process. The phenomenon soon became simply known as ESD.
Within a few months of the discovery of ESD, a group of engineers pooled their talents to decide how to minimize this problem in production environments. The bottom line of their ongoing investigation was and is to provide an electrical path to ground for every human and metallic object in the assembly and test areas. Many of the workstation improvements introduced in the past 25 years have been directed toward that goal.
The Effective Workstation in 2000
Ideally, at a well-equipped workstation in 2000, the operator wears ESD-control gloves, smock, shoes, and static-grounding wrist straps and heel straps. He or she sits on an ESD-controlled chair on a grounded floor or mat. The workbench surface is connected to ground. Materials are transported in ESD-control packages and stored on static-free shelves or bins. The tools at the station and possibly even the lamps and magnifiers are static-free.
The operator has a static-control spray for occasional use. Cleaning solutions and swabs comply with ESD regulations. An ionizer is on the worksurface or suspended nearby.
To monitor all of this static-control equipment, each workstation features a device with visible and audible alarms, possibly tied to a central computer. Another option is a check-in/check-out station, making it impossible to come on the job or back from a break without verifying the integrity of a wrist strap.
Selection Criteria
The selection of a workstation for ESD control is a many-faceted decision.
The first step is to examine how well the worksurfaces are bonded to a common point. According to the first ESD control principle listed in the foreword to ANSI/ESD S20.20-1999, “All conductors in the environment, including personnel, must be bonded or electrically connected and attached to a known ground or contrived ground….This attachment creates an equipotential balance between all items and personnel….”2
Bonding to ground at the workstation starts with the worksurface. Typically, a resistance to ground of <109 W is the basic electrical criterion. The typical soft or rigid static-dissipative worksurface has a conductive layer to control the electrical resistance to ground which means it meets the requirement with resistance to ground between 106 W and 109 W .
Looking to the future, David Swenson, technical services specialist at the 3M Electronic Handling and Protection Division and past president of the ESD Association, noted, “Probably, it is going to be necessary to reduce the upper resistance-to-ground specification. In 1995 and earlier, 1010 W was acceptable. The present limit was adopted in 1995, and there is pressure to lower it to 108 W because of increasing ESD susceptibility in the disk drive industry and some other areas.”
Once the grounding capabilities have been addressed, it’s important to look at the physical properties of the worksurface. Mr. Swenson continued: “A wide variety of materials is available that might be acceptable for an application, but mechanical and chemical properties must be considered to make the workplace fully functional.”
Most people choose a worksurface with a relatively soft surface because it is least likely to damage fragile product parts while at the same time more comfortable for the operator. Some prefer a rigid material so the operator can slide items easily.
Often a workstation is constructed with a laminate surface that provides a rigid base, and cushioning is added via a static-control mat. The mat can be replaced at low cost when it has become soiled, worn, burned, or otherwise damaged.
If chemicals are present in the assembly area, stainless steel is the worksurface of choice. This low resistance to ground is good from the ESD control standpoint. But if dangerous voltages are present at the station, the possibility of electrical shock should be considered.
Factors not related to ESD also are considered in workstation choices. These include color and price.
Industry Standards
Fortunately, there is general agreement in the industry that the ESD Association Standards are a good way to define the ESD control characteristics of a workstation. They are the result of thousands of hours of study and several review iterations.
“The typical customer wants work areas that conform to industry standards,” noted Dave Bermani, the marketing coordinator at Desco Charleswater. “They don’t want to reinvent the wheel with every purchase.” But even the highly regarded ESD Association Standards have some drawbacks. The biggest complaint relates to insufficient coverage of test methods. Some users suggest adding testing recommendations as a how-to addendum to the documents.
Another complaint focuses on the use of 5-lb probes as pickup devices for worksurface resistance measurements. Since most assembly parts are much lighter, they will have a significantly higher resistance to ground than the measurement would imply.
Using the Workstation Properly
We wouldn’t think of driving a car month after month without periodic lubrication, oil changes, and other preventative maintenance. Yet users sometimes forget to give the same attention to their ESD workstations.
So often, improper cleaning practices are the downfall of a good worksurface. Cleaning them with the wrong materials is a prime cause of damage and lowers the effectiveness and lifetime of a surface.
The first step in maintenance is to read the manufacturer’s instructions. They help define a disciplined maintenance plan to keep equipment at optimum performance levels. But as one supplier commented, “The overwhelming problem is old-fashioned ignorance. Unfortunately, reading the user’s manual is the absolute last resort.”
Effective ESD control also requires proper grounding, not just at installation, but throughout the use of the workstation or surface. “Having a wire connected to a snap in the corner of a bench mat does not guarantee that the mat has a conductive path to ground,” explained Jan Hoigaard, president of SpectraScan International. “Vinyl materials cold flow, and a snap that was satisfactory when it was installed may have a very high resistance now due to this cold flow.” Periodic checks of the integrity of the ground path are essential.
Conclusion
Yes, we have come a long way with workstations since those “helpful” remarks about bare feet and dirt floors were made more than 22 years ago. Today not only is there a full complement of mats, chairs, tables, storage bins, grounding straps, and apparel that create ESD-safe, comfortable, and pleasant work areas, but there also are some very solid industry standards that govern the specifications of workstation products and how they are tested.
References
1. “Coping With Static Electricity—Part II, Helpful Ideas and a Do-It-Yourself Solution,” EE-Evaluation Engineering, September/October 1977, p. 30.
2. ANSI/ESD S20.20-1999, Standard for the Development of an Electrostatic Discharge Control Program, ESD Association.
Workstations/Worksurfaces
Conductive Board
ECP® Electrically-Conductive Particleboard discharges static electricity as a stand-alone product or as a substrate in a static-control system. ECP35 is conductive with a surface resistivity of 103 to 105 W . ECP68 is dissipative with a surface resistivity of 106 to 108 W . The thickness is 1/4″ to 3″. DataTech Research, (920) 406-1608.
Dissipative Mat
The Type T2 Dissipative Rubber Mat has a resistance to ground of 106 W and a charge decay time of 0.01 s to 0.02 s. The two-layer, 0.06″ thick material comes in blue, gray, or beige and as benchtop sections or in 40’ rolls with 24″, 30″, 36″, or 48″ widths. Desco Charleswater, (909) 627-8178.
Swivel Chair
The 3000 Series Chair has a five-leg base, a 20″ foot ring, a tilt seat, and a two-piece back. The ESD version has surface resistance of 4.5 × 106 W /sq for fabric and 1 × 108 W /sq for vinyl. The decay time is 0.2 s for fabric and 0.35 s for vinyl. Gibo/Kodama, (714) 373-2346.
Adjustable Workbench
The Height-Right™ Adjustable-Height Workbench provides 15″ of vertical movement and features a static-dissipative worksurface and paint to meet ESD control requirements. The capacity is 1,200 lb, and the lifting capability is 600 lb. It is available with an accessory system for ESD-safe storage of tools and supplies. Lista International, (800) 722-3020.
Modular Workstations
The Modular Adjustable Workstations feature integral power strips for connecting to overhead power. The frames are welded steel and adjustable in height. The worksurfaces have a surface resistivity of 1 × 106 W . Assembled sizes vary from 30″ × 60″ to 36″ × 96″. Phoenix Workstations, (800) 270-3034.
Static-Safe Workbenches
The WS70-72 and WS70-96 Workbenches have 14-gauge steel bases, stainless-steel apron rails, and static-dissipative worksurfaces. The height of the table is adjustable. A six-gauge ground cable attaches to the building ground, and eight receptacles accommodate wrist-strap and equipment connections. The WS70-72 is 72″ long, and the WS70-96 is 96″ long. Plug-in Storage Systems, (800) 231-5952.
Chair With Drag Chain
The 5000ECR Polyurethane Chair provides a brass drag chain, ESD glides, and conductive vinyl upholstery with a surface resistance of 108 W /sq. The pneumatic lift has 5-in. travel, and the back adjusts horizontally and vertically. The chair, which weighs 24 lb, features an aluminum base. The 5000ECR is one of 32 new models for clean-room use. Bevco, (800) 864-2991.
Dissipative Mat
The 8200-Series Static-Dissipative Mat has three layers. The top layer is dissipative vinyl with typical resistivity of 2 × 108 W /sq that drains a static charge but not low enough to short the pins on a circuit board. The middle layer of conductive material serves as the main discharge path to ground. The foam backing forms a static-dissipative, nonskid cushion. The 8200 comes in six colors and several sizes. 3M Electronic Handling and Protection Division, (800) 814-8709.
SIDEBAR
40,000 Hours On ESD Workstations
How does the static-discharge enemy look to someone who spends eight hours a day on the front lines of the battle at the workstation? Is the first line of defense—the army of electrical assemblers—suitably armed and motivated?
Joan Stabenow, an electrical assembler now at Electro and previously at Sundstrand and Loral, all in Sarasota, FL, has been on that ESD battlefront for 20 years. She has some answers for us.
Joan first heard of the static-control problem and its implications in 1979 while working at Loral. “My first reaction was very negative,” she recalled. “I was sure that this was just another exercise and that everything would be back to normal in a few days. After all, at that time I had been assembling electronic equipment for 15 years, and no one had complained before. But I figured I could tolerate a silly new rule if that’s what it took to keep my supervisor happy.”
Quite frankly, probably the supervisor was a bit skeptical, too. No one had explained ESD to any of the production people.
Joan’s first ESD control device was a wrist strap. “I felt like a dog on a leash and could think of no good reason to have that wire holding me near the workstation. It gave me the same restricted feeling as my car seatbelt. Fortunately, both the wrist strap and the seatbelt become a way of life after a while.
“I have learned to keep the strap on my wrist when I leave my workstation during the day,” she continued. “I also found that the strap doesn’t make good contact some days until I use a static-dissipative hand cream on my wrist.”
Joan has worn ESD-control gloves and finger cots without problems. Smocks are another story. “This is my biggest ESD gripe,” she said. “I detest the need to snap it from top to bottom on warm, humid days. I wonder if the smock requirement is an overkill.”
ESD-protected storage is big on Joan’s priority list. She doesn’t want her products to be destroyed by the carelessness of a material handler unfamiliar with this invisible threat. “Whenever I leave the workstation, I place sensitive components and assemblies in black antistatic totes and seal them. When someone comes to deliver or pick up parts, I watch carefully to assure myself that the transit procedure is ESD safe.”
The computer terminal at Joan’s workstation is on an antistatic mat. An ionizer on the bench helps control the possible static buildup on her tools and other equipment. Since the action of the ionizer is invisible, she has to accept the assurances that it is helping in the ESD control process at face value. Visitors to the work area are Joan’s biggest challenge. It is necessary for the cleaning crew, customers, and even the plant manager to visit her workstation from time to time—but they mustn’t undo all the hard work Joan has done to keep the sparks under control.
So what does Joan do? She guards her components and workplace with a vengeance. The idea of giving visitors different-colored smocks sounds good to her because it gives an assembler an easy way to spot them.
Because Joan is an old-timer in ESD control and a natural leader, she has helped informally to educate dozens of others on the subject. There are lots of short courses, typically two hours or less, on ESD control, but new employees understand it better when they hear about ESD from someone who has been around a while and fully believes in the program.
“The most convincing motivator to learn about ESD is product quality,” Joan said. “No one wants to have the results of their labor lost because of a defect in the assembly process.”
Auditing has been a standard part of the ESD-control regimen for each of Joan’s employers. She checks in, verifies that her wrist strap works properly, and initials a form. From time to time, with no advance notice, a company auditor verifies that every assembler has signed the list.
Several other ESD control checks at the workstation are part of the audit routine. The assembly team prides itself when it passes each check or works harder when a violation is discovered.
Has this veteran assembler ever seen a dramatic example to back up the stories about the seriousness of the ESD threat? “You bet. Several years ago, I blew some PROMs. A few days later, I learned that several of the PROMs were defective and that the cause likely was ESD,” Joan explained. “Were they damaged by handling or storage before I got them, or did I damage them at my station by improper ESD control practices? Was the damage caused after they left my station?”
To this day, Joan doesn’t know the answer. But she does know the quality of valuable products was jeopardized by someone or something in the manufacturing sequence.
One of the frightening things to come from the PROM experience is the realization that latent defects can go to the customer undetected. Good components look the same as those damaged by ESD, and latent damage may not even show up in test.
Joan shudders when she thinks that some of her products are going into vehicles where failures could cost lives. “When I put an electrolytic capacitor in backward, there is black smoke and the electrolyte is sprayed all over the board. I wish the results of a static discharge were as visible, but I know that isn’t possible.”
Joan offered some advice to ESD control engineers: “Remember that ESD control is not about a set of rules at the workstation. It is about a partnership between engineers, assemblers, material handlers, and management for the overall good of the company and service to the customer.
“We electrical assemblers are eager to be part of the solution, not just employees who go to those classes and blindly do what we are told.
“And we will be more motivated if we are given specific examples of ESD problems in our plant, hopefully with visual aids, and then invited to be part of the decision-making team to initiate corrective action.”
Copyright 2000 Nelson Publishing Inc.
January 2000