EPA ESD Protected Area
- a overview or summary about the basics of an EPA, ESD PA, Electrostatic Discharge Protected Area and its setup.
In order to that ensure an environment where work on electronics components, sub assemblies such as printed circuit boards, and assemblies is carried out is safe from ESD, most companies these days set up what is known as an ESD protected area or EPA. An EPA or ESD PA is an area that is set up to minimise the generation and retention of ESD.
By setting up and correctly using an EPA or ESD protected area, the level of failures during production and later in life will be minimised. As a result, the investment in the ESD PA will soon repay itself both in terms of the cost of direct rework, and later in the life of the product where its reliability will be higher. Not only will this reduce costly call-outs, but long term reliability will result in customer satisfaction.
ESD protected area, EPA basics
When setting up an EPA, there are many different measures that can be employed to provide a safe ESD environment for the electronics components, sub-assemblies and assemblies. However when setting out to set up an EPA, it is necessary to carefully analyse exactly what measures are required. It is very easy to adopt an attitude where every measure possible is incorporated, but this may result in spending much more than is needed.
A carefully thought out EPA can result in cost savings, especially by planning how to use the facilities to their optimum. This will combine the use of many of the ESD prevention measures, along with an efficient workflow pattern.
The key element of an EPA is that it will provide an environment where any ESD build-up is minimise, and where it does occur, it can quickly dissipate naturally without being discharged through an electronic component.
The measures that can be used within an EPA can be grouped into a number of broad categories:
- ESD environment: One of the key elements of an EPA is the overall environment of the EPA. One of the key elements to be installed first is the flooring. This should be static dissipative. Suitable tiles or carpet can be installed, although carpet may need re-treating from time to time. These should have a resistance of <1.0 x 10^9 when tested as per ESD S7.1, although fully conductive tiles are not recommended for safety reasons!.
In addition to this, other items such as humidifiers to control the humidity also help. A very dry atmosphere will give rise to static. This can be noticed in a normal environment where static discharges may be noticed on a dry day, but not on a normal or damp day. Also ionisers with a +/- 50V offset tested to ESD S3.1 may be used.
- ESD products: There is a host of different ESD products that can be used to good effect to provide ESD protection. These include:
- Static shielding bags, i.e. ESD bags.
- Static dissipative IC tubes
- Other ESD packaging for components
- ESD workbench and ancillaries: The use of an ESD workbench with ESD straps, ESD seats, etc., to ensure the operator or user is grounded (via a high resistance dissipative rather than conductive path for safety reasons) is a key element in any EPA. Their use is described in a further page.
- ESD tools: Another area of importance is the use of ESD tools. A variety of tools come into this category and a particular example is the use of an ESD soldering iron. Tools, and in particular soldering irons can easily transfer static directly to the components, and having a metallic element, this can provide a good transfer path resulting in high current levels that can cause more damage. Further details of ESD tools and ESD soldering irons can be found on a further page.
- ESD clothing: The use of ESD clothing is important because items of normal clothing can generate and carry static. With many more garments being manufactured from man made fibres, the risk of static is greatly increased. Accordingly a number of items can be used:
- ESD coat: With an ESD coat covering all clothing, it will tend to dissipate any static that might otherwise reach any electronics equipment. It may also provide a form of shield against any induced static. With many clothes made from man-made fibres, high levels of static can build up. Ties worn by men are a very good example. Often being made from man made fibres very high levels of static can be carried by them, and they can easily reach sensitive electronics equipment when being worked upon at a bench. Typically a resistance of between 1 x 10^5 to 1.0 x 10^11 should be seen when tested according to ESD STM 2.1
- ESD shoes: It is important to ensure that shoes do not build up static and cause the operator to become charged with respect to ground, and hence the overall EPA. Shoes providing a leakage path to earth should be used. A resistance of <1.0 x 10^9 should be exhibite when tested to ESD S9.1.
- ESD shoe straps: Many people will want to use their own shoes and therefore an alternative to this is to use a shoe strap that fits over the operator or users shoe and provides a leakage path to earth.
- ESD shoe covers Another alternative is to use an ESD shoe cover that fits over the operator's own shoe and provides a leakage path to earth. However care must be taken when wearing these ESD shoe covers as they may form a trip hazard.
- ESD storage and transport: It is normally not possible to immediately utilise a subassembly within the final assembly, although this may happen in some particularly efficient flow-lines. As a result many components and sub-assemblies need to be stored. It is therefore essential that ESD prevention measures are adopted during storage to prevent damage. Ideally the storage area should follow the same EPA guidelines and be a static dissipative area.
Static shielding bags, i.e. ESD bags should always be used, even when transporting sub-assemblies within an EPA. Also ESD protective stands and ESD storage bins can be used for short-term location of boards. The use of ESD storage bins and other containers is equally important as the use of ESD bags as their use prevents the build up of static.
It is also very important that components themselves are stored in the correct environment prior to use. The packaging that the are contained within from the manufacturer will be ESD packaging. This ESD packaging is quite acceptable for storage prior to use.
- Exclusion of static generating material: many materials that are used in everyday life generate large amounts of static. These material should be excluded at all costs from the EPA. Materials such as expanded polystyrene generates very large amounts of static as generated by the fact that it sticks to everything. This must never be allowed into the area. Also some forms of bubble wrap are not good, although some types have been developed for use as ESD packaging - this often has a slight pink tint to it.
Plastic cups are also very bad in terms of retaining static (although drinks should not be allowed in an electronics work area anyway) and many other plastic items are not good because plastic is a very good insulator. Many other items may also be bad. Some people even use special ESD paper instead of normal paper for printouts and notes, although measures like these are not always implemented.
Note: values for resistance and test methods are defined in ANSI/ESD S20.20.
The creation of an EPA is an essential requirement for any company manufacturing or repairing electronics equipment. It is essential that any modern electronics equipment is protected from the effects of ESD that can either destroy electronics components immediately, or leave latent defects for appear later. Accordingly ESD protection is essential, and the only real way of providing a satisfactory environment is through the use of an EPA or ESD protected area.
By Ian Poole
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