ESD Workbench and ESD Mat
- a summary or overview about the basics of an ESD workbench and mat and how an ESD work bench is constructed and used.
ESD tutorial includes:
One of the key elements of any ESD protected area is the use of an electrostatic workbench. An ESD work bench will provide a static dissipative area which will provide ESD protection for electronics assemblies being worked upon.
ESD workbenches are widely available and they are used within the electronics industry for providing a suitable work surface for undertaking electronics construction, repair, test, and any work associated with electronics equipment.
ESD work benches are a key element in any EPA, ESD protected area. They are the surface on which much of the work on the electronics equipment will be undertaken.
Static dissipative surface
The key to a operation of the ESD work bench is the surface of the bench. Original concepts for ESD protected workbenches used conductive surfaces. However these are far from ideal from two viewpoints:
- The conductive surface will short out any connections making operation and testing of the circuit impracticable.
- The conductive surface will provide a low impedance discharge path that will give a very rapid discharge, resulting in high currents flowing and possible damage to the circuitry.
In order to provide the correct environment for an ESD workstation, an ESD work bench should have a static dissipative surface.
A further reason for having a high resistance dissipative surface rather than a fully conductive surface is that of safety. If high voltages were present in a system, then accidentally touching a conductive work surface and a high voltage source could be dangerous. Accordingly safety precautions necessitate that a high resistance dissipative surface is used.
Typically an ESD protective worksurface should have a resistance value of <1.0 x 10^9 ohms when tested according to ESD S41. (ANSI/ESD S 20.20), but having a lower limit of 1.0 x 10^6 ohms as recommended in ESD-ADV 53.2. This is for two reasons, namely for preventing a rapid discharge of any static that may cause damage, and secondly for safety reasons as mentioned above.
An ESD work bench is one of the central elements within a full ESD protected area for use within an electronics manufacturing or repair facility. It is necessary that the ESD work bench is able to provide a full protected environment for any electronics assemblies that require work on the ESD work bench. Accordingly its performance is critical within the overall facility.
The ESD workbench can be seen to consist of a number of different elements:
- Static dissipative work surface: The primary element of any ESD work bench is the static dissipative work surface.
- Shelves: As static can build up anywhere on a work bench, it is essential that all elements of the bench are static dissipative. As items, either electronics assemblies, or tools may be placed here the shelves need to be part of the ESD protected area. Also if tools are placed on a non-ESD dissipative surface, attract static, and then they are used to work on an electronics assembly, then damage could result.
- Metalwork: While the metalwork itself is conductive, the paint used is not normally conductive. For an ESD workbench it is necessary to use static dissipative paint to prevent any sudden discharges.
- Cupboards: Many work benches have cupboards attached to them as an integral element for tidy storage. In view of the fact that electronics assemblies or other items such as tools for working on the assemblies may be stored in these cupboards, it is essential that the cupboard conforms to the same ESD standards as all other areas. Shelves should have static dissipative surfaces and any painted metalwork should have a static dissipative paint finish.
- ESD wrist straps: When working at an ESD work bench, it is necessary to ensure that the operator is also held at the same potential so that no discharges occur. ESD wrist straps are generally used for this.
- Wiring: It is necessary to ensure that all static dissipative surfaces, wrist straps, etc are all connected to a common grounding point, and wired in a suitable manner. It is essential that this is achieved effectively and in the correct manner from the viewpoint of electrical safety.
- Monitoring system: With wires such as the connections to ESD wrist straps, etc being in almost constant use there will be periodic failures. One method that has been adopted is to test ESD wrist straps and their wires each day. However when a problem is detected, it is possible that the operator has been causing damage to assemblies since the last time the ESD wrist strap was tested. To overcome this problem, continuous monitoring systems are available which are considerably more satisfactory. These are typically incorporated into the ESD work bench assembly.
ESD workbench care and maintenance
With a good dissipative surface on an ESD workbench it is necessary to use and maintain it for it to provide a long effective life. Apart from testing which is covered in the paragraph below there are a few simple steps that can be taken to ensure the ESD workbench provides a long and useful life.
- Avoid damaging the ESD dissipative surface of the work bench with sharp objects as these can significantly reduce the effectiveness of the surface, particularly if small areas become isolated by cuts, etc.
- Clean the surfaces periodically to prevent build-up of grease or other layers that will impede the static dissipation. A plain cotton rag or cloth, with diluted ammoniated detergent is ideal. Using this, ensure that all dirt films are removed from the surface.
- Never use wax based cleaners or solvents that may attack the surface or modify its properties.
- Never use abrasive cleaners. This can include abrasive cleaning agents as well as abrasive cleaning pads such as kitchen scourers. These can remove or damage the ESD dissipative surface and considerably impair its action.
It is worth implementing a regular care and maintenance schedule as dirt and grease can build up on an ESD dissipative surface over time and its performance gradually reduce. As it is difficult to visibly detect any performance degradation, and any effects may come on gradually and not be noticed, the only way to overcome this is to regularly clean and maintain the surface. Testing should also be incorporated into this schedule.
Although a complete ESD workbench is the ideal solution, this may not always be possible, especially for field engineers who may be working in remote locations where full ESD protection facilities may not be viable. In these cases an ESD mat is an option that provides effective ESD protection, while not occupying the space of a full ESD workbench, and also being portable so that they can be used by field engineers.
Like full ESD work benches, ESD mats must be used in conjunction with ESD wrist straps to which they should be connected.
An ESD work bench is a core element of any ESD protected area. It is therefore essential that it offers the correct level of performance so that ESD is adequately dissipated without causing high levels of current to flow in any static sensitive devices. Accordingly ESD work benches are essential for electronics assembly, manufacturing and repair facilities, and ESD mats can be used for field engineers where full ESD protected areas are not feasible.
By Ian Poole
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