ElectroStatic Discharge (ESD) tutorial [3]

- a tutorial or summary about the basics of Electrostatic Discharge, ESD and the ways in which electronic components and circuits can be protected from it affects.

This ESD tutorial is in three pages dealing which address the different topics:
ESD Tutorial detailing the basics of ESD and how it arises [1]
The sensitivity of electronics to ESD [2]
Overcoming ESD [3]

There are many ways in which the effects of ESD can be overcome. A variety of methods are employed including products including anti-static, or static dissipative workbenches, anti-static or static dissipative containers, static dissipative protection for operators and the like.

To provide the best protection the problem must be addressed from several angles:

• An area which is static free (anti-static) must be created. These areas are often known as electro-static discharge protected areas ( ESDPA ) and they must be used whenever SSDs or boards containing SSDs are to be handled.

• Any static sensitive devices, or boards containing them must be stored in conditions where they are not subjected to the effects of static.

• Any boards using SSDs should be designed so that the effects of a discharge into the board are reduced to acceptable levels.

• Finally any people who come into contact with electronic components or assemblies should be made aware of the effects of static discharges.

The decision about the number of measures to employ can be difficult because it is not always easy to determine the cause of any failures. Additionally it may take many years for some of the failures to occur. However if sufficient measures are taken then the risks of damages from ESD can be reduced to sufficiently low levels

Work Areas

To avoid static build up in the area where electronic components and boards are being handled the bench surfaces should be able to remove any static build up which occurs. If there is an existing workbench then it is possible to buy a carbon impregnated rubber ESD mat to place on the bench. These ESD mats are relatively cheap and are very cost effective. The ESD mats can simply be placed on top of the existing workbench and used without the need for expensive new furniture. However, if a new bench is being installed then special static dissipative surfaces can be used. These ESD workbenches are ideal as they provide not only the ESD surface, but also the wiring and connection points for the ESD wrist straps.

The level of conductivity of the work surfaces is important. If it is too low then it may not only affect the operation of any board or assembly placed upon it, but when a board is placed onto it, and charge that is dissipated should not be removed too quickly otherwise damage may occur. Accordingly the volume conductivity of the material used for the ESD mats and ESD workbenches should fall into the static dissipative category.

Another essential element in combating static build up on people is to use ESD wrist straps. These ensure that any charge built up on a person working on the equipment is safely dissipated. The ESD wrist strap consists of two sections. The band itself which is worn around the wrist. This is connected to earth via the lead which incorporates a large value resistor, normally in excess of 1 M Ohm. This is included for two reasons. The first is safety, and the second is again to ensure that any static is removed in a controlled fashion.

The ESD wrist straps should be regularly tested to ensure they have not become open circuit. Without a test of this nature a faulty strap could go undetected for many months. Many companies insist that every strap that is in use is tested every day. In this way any defects can be discovered before they cause too much damage.

ESD wrist straps, connections to workbench tops and any other points are normally connected together using a special junction box. These junction boxes usually have resistors of 1 M Ohm for each of the contacts. These are joined and then taken to earth. Often a special mains plug with a connection to only the earth pin can be used. These special plugs are usually yellow and have two plastic pins for the live and neutral, and a metal pin for the earth. In this way it is only possible to connect to earth.

Flooring in an electrostatic protected area, or anti-static area also needs to be considered as specialised ESD flooring is available. Flooring made out of acrylic materials is likely to generate very high levels of static. For example, acrylic carpets in the home are particularly bad whereas natural fibres like wool are much better. Even nylon is not as bad as an acrylic floor. For an electronics production area there is a wide variety of static dissipative coverings which can be installed if required to overcome any problems that might be caused.

If ESD flooring is to be used then conductive footwear must be worn. There is no point in having static dissipative flooring if peoples' shoes act as excellent insulators. Most people will want to wear their normal shoes and not have to wear 'regulation' footwear as this is not likely to be as comfortable. The solution is to use a heel strap which fits over part of the shoe. This provides an acceptable path to earth past the shoe. However ESD shoes are also available.

Clothing is another element that must be considered. Clothes of wool, cotton or even polyester cotton are normally not a problem. However some synthetic clothes can develop very high levels of static of their own even if the person wearing them is grounded by the use of a wrist strap. Acrylic ties are particularly notable. They can collect high levels of static charge, and this can be passed to nearby components and electronic boards causing damage. To overcome this type of problem special static dissipative overalls can be worn. These normally have a relatively high conductivity to contain any static fields which might be generated.

Finally chair coverings should also be investigated. They should not be of the type that generate high levels of static. In some instances they may need to be dissipative and connected to ground. It is possible to obtain special seat coverings for existing chairs, or completely new chairs. Choices can be made dependent upon the state of the chairs and the budget available.

Another approach that can be taken to help control static and ESD is to control the humidity. In dry periods of the year, especially winder when the level of water vapour held in the air drops, the possibility if ESD rises. By introducing some humidity the levels of static can be reduced. Although not one of the most commonly used methods of ESD control, there are several types of humidifiers which can be installed. Some fit into heating systems whereas others are separate units. Ideally a minimum humidity figure of 50% can be used as an aiming point. Above this the high humidity levels can lead to other problems.

Storage

Not only do work environments need to have ESD control measures introduced, but so too do the storage media. Whenever an electronic component or assembly is transported or stored it should be placed in suitable packing to ensure that it is not damaged. The dissipative ESD bags for boards and tubes and special dissipative containers for components are now common place in the electronics industry. Often the storage ESD bags have a pink or grey tint to them. The older black conductive bags are used less as they may dissipate the charge too quickly. Another problem was that they tended to discharge any on-board batteries more quickly than intended!

In addition to this, within the stores ESD bins and other ESD packing is required. This will ensure that no static build up occurs.

Soldering Irons

There is a wide variety of soldering irons available on the market today. Many are quite suitable for work with static sensitive devices. The main requirement is that the bit used for soldering should be earthed. In general it is recommended that the resistance to earth should be less than five [ohm]. Any irons which are thermostatically controlled should ideally use a zero voltage switching system. This prevents large spikes caused by the switching of the thermostat from appearing at the tip of the iron and causing damage to the equipment.

Summary

Although some of the ESD precautions may seem to be rather extreme, electronic development and manufacturing companies take ESD control very seriously in view of the damage that can be caused. Investment into equipment such as ESD wrist straps, ESD workbenches, ESD flooring,, ESD bags and the like pays dividends in terms of lower fault finding costs during the manufacturing cycle, and also lower warranty repair costs. This does not take into consideration any benefits to be gained from maintaining equipment that gives long and reliable service to customers. Accordingly investment into ESD control is deemed to be a good investment.

Further pages from this tutorial
Page:   (1) >> (2) >> (3)
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