A superior ESD protection plan is essential for any electronics manufacturer wanting to gain an advantage over competitors. The development of the plan requires attention to every detail of the ESD-safe area including the ESD workstation and worksurface. But where do you start when you are ready to specify an ESD-safe workstation, and how do you know if it meets your electrostatic and functional requirements?
A practical ESD-safe workstation has three parts, said Algis Norkevicius, an engineer at Charleswater. It must provide grounding, shielding and neutralizing capabilities to be an adequate solution in an ESD-sensitive area.
Grounding is an extremely potent line of defense against static generation, said Mr. Norkevicius. ESD-control is accomplished by providing common-point grounding for devices such as wrist straps, work mats, floor mats and heel grounders. It is crucial to have both worksurface and personnel grounded to the same potential.
Shielding static-sensitive components from an ESD event during storage and movement is another crucial aspect of a static-safe workstation, said Mr. Norkevicius. The components must be protected using conductive bags or tote boxes. It is important that these containers only be opened at a static-safe workstation by properly grounded personnel.
Grounding and shielding alone will not drain charges from insulators such as circuit boards, synthetic clothing or common plastics, added Mr. Norkevicius. Ionization is needed to neutralize static charges on insulators. It is an effective tool to reduce the risk of an ESD event, but it is not a substitute for grounding or shielding.
An effective workstation must be made up of ESD-safe parts including the worksurface via a hard-surface laminate and grounding hardware, drawers, cabinets, shelves, uprights and parts-cup rails, said Don Murphy, president of IAC Industries. Static-safe materials are also needed for chairs, carts, footrests, CRT platforms and document holders. For easy access, cabinets below the worksurface need right- and left-handed doors, while cabinets above the worksurface usually have flipper doors.
Before choosing a workstation and accessories, know what standards they must meet, said Harry Smart of Plastic Systems. For example, does the equipment need to meet the military or commercial requirements for an ESD-safe work area? Without this knowledge, it is impossible to make a proper decision.
Workstations also must be flexible and offer preconfigured combinations to meet most technical applications, a material warranty and features that will prevent static-related problems.
For example, workbenches should have a static-dissipative plastic-laminated top with a grounding bolt, a wrist strap and a grounding cord with a 1 M-W resistor. Static-dissipative equipment shelves, storage bins and pedestals should also be used.
Ergonomics is another important aspect not to overlook. An ergonomic workstation allows you to select an appropriate working height for your tasks.
The chosen adjustment method depends on how often height corrections are desired, said Dr. Joy Ebben, human factors and ergonomics specialist at IAC Industries. For example, if you find it convenient to stand occasionally, it will probably be more advantageous, perhaps even necessary, to elevate the working height.
Some companies, such as IAC, offer answers to questions you may have about specific ergonomic situations. They can advise you on specific ergonomic risks to health and safety in your workplace. Suggestions about workstation and accessory modifications and process changes are offered to improve productivity by making the human/workstation interface more efficient.
AC electrical distribution systems on workstations are another important consideration for proper operation of sensitive equipment. The distribution should meet Underwriters Laboratories standards and be Electrical Testing Laboratories approved, said Mr. Murphy.
For example, the Power Distribution System (PDS) from IAC meet these requirements. The PDS has three independently operated 20-A circuits that allow sensitive test equipment to be connected to outlets on different circuits, preventing possible electrical interference.
Acceptability Criteria
When you look for an ESD-safe workstation or worksurface, you need to know what industry-wide acceptability criteria they should meet. The first place to look for help is the ESD Association. It has a group of standards that encompasses the safety of the whole work area including wrist straps, grounding, packaging and worksurfaces.
Many ESD workstation manufacturers can prescribe the appropriate requirements to help you make a static-safe workplace. For example, Charleswater recommends
that your workstation and worksurface areas meet the current standards for the United States, including ANSI EOS/ESD S4.1-1990—Worksurfaces-Resistive Characterization; ANSI ESD S11.11–1993—Surface Resistance Measurement of Static-Dissipative Planar Materials; MIL-HDBK-263B ESD Control Handbook for Protection of Electrical and Electronic Parts, Assemblies and Equipment; MIL-W-87893A Workstation, ESD Control; the Electronic Industries Association EIA-541 Packaging Material Standards for ESD-Sensitive Items, and FED-STD-101C, Method 4046 Electrostatic Properties of Materials.
For European requirements, Charleswater suggests the International Electrotechnical Commission IEC-93 Methods of Test for Volume Resistivity and Surface Resistivity of Solid Electrical Insulating Materials; ISO 2878 Rubber, Vulcanized-Antistatic and Conductive Products, Determination of Electrical Resistance; and EN 100 015 Issue 1, Protection of Electrostatic-Sensitive Devices, Part 1: Minimum Requirements.
No organization defines the quality of the material used to make a workstation or worksurface, said Mr. Murphy. IAC recommends that you include all relevant guidelines for which there are industry specifications for worksurface substrates, laminates, fasteners, paint and electrical distribution systems.
For example, request industrial-grade particleboard (45 lb) that is 1-3/16-in. thick with a wax-based sealed backer sheet, said John Notti, director of operations and engineering at IAC. The specification is Type 1, Grade B, Class 2 particleboard as defined by ANSI A208.1-1993 Particleboard, Mat-Formed Wood. Substrates meeting this requirement have a more solid base for the attachment hardware than medium- density particleboard that is £ 1″-thick.
The specification for plastic laminates is ANSI/National Electrical Manufacturers Association LD-3 which defines the thickness, appearance and physical properties of the material bonded to the substrate. For the paint, make sure it meets the American Society for Testing & Materials test specifications D3363 for pencil hardness, D2794 for impact resistance and B117 for salt-spray corrosion resistance. Fasteners including studs, bolts and screws should meet the Society of Automotive Engineers’ J429 Grade 5 specification to ensure they have sufficient tensile strength and hardness. Additional aspects you should consider for the workstation include a floor-leveling system, metal roller bearing drawer slides and aluminum and steel connectors for suspended worksurface attachment points.
Workstation and Worksurfaces
Workbench Offers Several
Options and Styles
The Agility™ Industrial Work Benches are available in four styles and many options. The four types include a fixed-height surface, and electric-, hydraulic- or mechanical- height adjustment. The worksurfaces measure 30″ ´ 60″ or 30″ ´ 72″ and are made of a static-dissipative material. Accessories include articulating arms, VDT and keyboard holders, a tool holder, overhead lighting, a work organizer and a power bar. BioFit Engineered Seating, (800) 597-0246.
Three-Layer Material Provides
Discharge Path to Ground
Type Z2 is a three-layer material for ESD-protective workbenches. It has a dissipative vinyl top layer, a conductive middle layer and a cushioned dissipative bottom layer. Charge decay of the dissipative layers, per ANSI EOS/ESD S4.1, is <0.1 s; surface resistance is 2.5 ´ 107 W . The 0.125″-thick mat is available in kits of 24″ ´ 36″ or 24″ ´ 48″ and roll stock of 2’ ´ 50’ or 4’ ´ 50’. Charleswater, (617) 821-8370.
Chairs Offer Improved
Adjustment Mechanisms
The Advanced Seating Systems line now is available with larger seat pans, taller backs, improved adjustment mechanisms and a variety of stationary and movable arm options. The line includes models for production, ESD and clean-room use. It consists of 16 basic models in fabric or vinyl. The chairs have a 25″-dia base and ESD grounding casters. They meet static-control requirements per DoD HDBK-263. IAC Industries, (714) 990-8997.
Workstation Line Offers
Variety of Heights and Lengths
The Technical Workstation line can be configured into a variety of heights and lengths. A variety of modular components is available, such as shelves, parts bins, lights and electrical accessories. Worksurfaces are offered in maple, plastic laminate, chemical resistive and static dissipative. Ground blocks and wrist straps also are available. Drawers can be coated with static-dissipative paint, and 16 7/8″-wide cabinets are provided for extra leg room. Lista International, (508) 429-1350.
Rubber Matting Material
Has Dual Purpose
Type T2 is a dual-purpose rubber matting used as a dissipative or conductive matting. The soft dissipative layer is used in soldering and assembly areas. The conductive layer provides a slip-resistant surface. The 0.06″-thick material resists abrasion and chemicals, is heat resistant from 32° F to 290° F, and does not produce toxic fumes. Charge decay on the dissipative layer is 0.01 s to 0.02 s per FTMS 101C, Method 4046. Resistance-to-ground on the conductive layer is 103 W to 104 W . Plastic Systems, (508) 485-7390.
Workstation Series
Offered in Four Designs
Each of the four workstation configurations in the StaticGard® Series features laminated static-dissipative tops with compatible ESD-protective bench legs, cabinets and worksurface shelves. A central personnel and equipment grounding point is included. Point-to-point resistance and point-to-common-point grounding resistance are 106 W to 109 W . The series includes configurations for maintenance, technical assembly, inspection and packaging applications. Stanley Storage Systems, (800) 523-9462.
Dissipative Rubber Mat
Resists Heat and Solvents
The 8800 Series of table rubber mats consists of a static-dissipative blue or gray rubber layer laminated to a bottom layer of black conductive rubber. The synthetic rubber worksurface is unaffected by oil, grease and common solvents. It resists heat from soldering irons and hot solder flux. The mat includes a wrist-strap grounding system with snap fasteners. Surface-to-ground resistance is 1 ´ 106 W to 5 ´ 107W . The 0.065″-thick mat is available in sizes ranging from 2’ ´ 4’ to 3’ ´ 24’. 3M Electrical Specialties Division, (800) 665-7862, ext 74.
Copyright 1997 Nelson Publishing Inc.
January 1997