ESD Workstation Systems in Practice
Increasing component density in electronic control switches and low-capacity control technology mean a higher sensitivity of the components to the effects of electrostatic discharge. Therefore when handling these components, conductive workstations are essential for protection purposes.
Components or assemblies at risk from electrostatic dissipation (ESD) are among those construction elements which can be destroyed or damaged by charges or discharges of static electricity if the permitted levels of current or impact energy are exceeded. As a rule, these are always semi-conducting components and most are also thick- and thin-film construction elements. This type of construction element is mostly damaged through human mis-handling. A human-being can charge several thousands of volts just by walking. A discharge can be felt by human beings from approximately 2000-3000 V, a current which is already well above the 'tolerance level' of many ESD components.
ESD Workstation Requirements
Steps taken to protect these construction elements must be orientated towards avoiding charges and bleeding off discharges gently, i.e. not abruptly.
IEC norm 61340-5-1 gives important instructions for the requirements of ESD protection zones and ESD workstations: All worksurface materials must be suitable for grounding laying on ground and must have a surface resistance between 1E4 and 1E10 Ohm. Alternatively the bleeding-off resistance of materials for work areas should lie between 7.5E5 and 1E10 Ohm. Several company norms in major German electrical companies also contain the requirement that ESD components should not be exposed to electrical fields > 1E4 V/m. This requires additional ESD measures to be applied to the workstation system; a dissipative worksurface alone is no longer sufficient.
The most important element of a ESD workstation system is the dissipative tabletop. The first conductive work surfaces still consisted of a normal plate coated with a layer of copper to enable contact. The actual table covering was then stuck onto the copper film - mostly a conductive floor covering. The floor coverings used had several distinct disadvantages: they were neither heat-resistant to liquid tin-solder, nor scratch-proof, difficult to clean and visually not very attractive; even the earth connection was difficult to set up.
Modern workstation systems like ELABO's are fitted with constantly conductive support surfaces. The ESD covering is applied with conductive adhesive onto the support surface. Today's ESD coverings are heat-resistant (20 mins. at 180?C), extremely scratch-proof, resistant to most chemicals and available in light, attractive colours. With ELABO work surfaces, the ESD covering is mainly rounded ergonomically along the front edge of the table using a postforming process and lightly textured to eliminate glare and reflections. Since the support surface is always delivered in a constantly conductive form, the earth connection can be set up very easily: all that is needed is a simple wooden screw on the under side of the table and a connecting cable.
However, the equipment of a standard table frame with a dissipative work surface meets only the absolute minimum requirements imposed on ESD workstations. The frame of a work desk is mostly lacquered or powder-coated; this finish covers the metal with an insulating surface, on which powerful electrical fields can build up.
Therefore the first generation of ESD table frames had a conductive coating with graphite-containing lacquer. Due to the proportion of graphite, the table frames were only available in dark colours; also, the lacquer is not very abrasion-proof or scratch-proof.
That is why ELABO applies to all their ESD table frames an environmentally friendly, conductive powder coating, whose conductivity is achieved by means of the metal alloy powder added. The powder coating is very robust and also available in light colours. All the table-frame assemblies and accessory parts such as shelf surfaces, chests of draws, foot supports and swing arms are joined together by means of electric controls. Equipping of ESD bracelets with a connector box will become standard in ESD workplace. The ESD contact points contained in the box are mainly connected for reasons of personal safety to the ESD earth installation with a resistance of 1 MOhm. They should not be mechanically compatible with power plugs; therefore 4 mm laboratory sockets are not permitted.
An ESD workstation which complies to the regulations must be additionally earthed in order that it can function. For this, there are several common procedures:
A very commonly used method is to earth of the ESD table using an already existing conductive floor. The workstation is electrically connected by conductive footplates to the conductive floor. The disadvantage of this method is a relatively undefined transition impedance between footplate and floor, which is guaranteed to worsen over time through collection of dirt (wet floor cleaning) In new proposed norms and various company norms by major companies this method is no longer to be used.
A much better procedure is to earth the workstation (ground wire) by the steel frame or the table top. The earth connection can be attached with a simple metal or wooden screw to any part of either the steel tube or table top. If the earth wire is connected to the metal frame, all metal parts are 'hard-earthed ' (ground wire PE potential), the work surface is above the internal resistance (volume conductivity) on 'soft' earth (ESD potential), likewise the earthing system above the built-in protective resistor. The most reliable method is to connect the workstation to a separate ESD earth. This procedure however requires a very large amount of installation work. With this method, the table is insulated from the conductive floor by non-conductive footplates and is connected to an ESD ring earth on the steel tube; this ring earth must be provided by the customer. In order to achieve a trouble-free disconnection of ESD and protective earth, all electrical installations on the table must be equipped with protective insulation. Using this method, injections of interference via the ground wire (PE) on the workstation are eliminated.
Source: Elabo GmbH