Electrostatic Discharge (ESD) Control

Lightning is an extreme form of ElectroStatic Discharge (ESD). Lightning is caused by many millions of water particles rubbing electrons off each other as they rush up and down through a thundercloud - the result is a massive build-up of static electricity in the thundercloud that eventually discharges to earth in the familiar lightning stroke. ESD in the form of lighting will discharge through the easiest path to earth - often by way of an electrical conductor, a steel tower, etc. Frequently the discharge path is through trees, buildings and even people.

Very large static electricity charges can be found during everyday occurrences. Static electricity will be produced when walking across a carpet, when getting up off a chair, when lifting a hand from the desk, when opening a plastic bag, etc.. Static electricity is produced whenever any two objects or surfaces separate or rub together. Often the discharge of static electricity (ESD) is heard and felt - when touching an earthed metal light switch a spark may be seen and a "crack" heard - in effect a miniature lightning stroke. Sometimes there may not be a spark or a crack and the discharge will go unnoticed.

The build-up of static electricity will discharge through the easiest path to earth. When working with electronic components, circuit boards and sub-assemblies there are possibly many different paths the discharge (ESD) may be able to take. A build-up of static electricity may occur, but the discharge may not be noticed as it may pass through one or more of the components being handled.

When ESD is accidentally applied to an electronic component, severe damage may occur either immediately or in the future.

If the ESD cannot find earth it will flow to a point that has a lower charge. As this area is often a circuit board containing electronic components, the sudden surge caused by the discharge can be sufficient to damage components either totally or to a degree where failure is imminent (commonly known as latent failures).

Static Sensitive Devices are components commonly found on circuit boards and computer sub-assemblies which are particularly prone to damage by ESD. Typically, these are:

Integrated Circuits - processors, gate arrays, RAM chips, EPROMs.

Transistors - Bi-polar and FETs.

Diodes and voltage/current regulators.

Crystal oscillators.

In order that damage does not occur to Static Sensitive Devices, it is essential to take anti-static precautions when handling electronic equipment and sub-assemblies during repair or maintenance, etc..

ESD Precautions and Preventing ESD Damage.

Ensure you have an earthed anti-Static mat laid out on your workbench - make sure it is laid flat and is free from any unnecessary items (such as food and drink).

The mat should be connected to mains earth ground. (Mains earth ground should be checked for good earth path - there should be less than 0.1ohm resistance between earth and neutral)

Make sure the mat is clean and not damaged in any way.

Place the equipment on the anti-static mat - on its side if it is a desk-side shape (Apricot FTe for example).

Make sure that you wear an appropriate wrist strap having a wrist cord fitted with a 2 Megohm resistor at the wrist end.

Connect the mat to the grounding point and then yourself to same grounding point on the mat using the wrist strap/cord. It is essential that only one point is used as a central ground point to maintain the best possible path of least resistance to earth.

If possible follow all points in (1) above.

If you are unable to move the equipment you are working on, ensure it is powered off and then connect your wrist strap/cord to a suitable earthed point on the equipment.

Ensure all sub-assemblies are handled to avoid ESD - don't take them out of their anti-Static bags until actually required, always place them on surfaces covered by anti-Static mat.

If you are working on a deskside model such as an Apricot FTs, connect your earth strap to the earth connector provided.

Check for physical damage

Check the combined resistance of the wrist band and coiled cord is 2 Megohm.

Check the dissipative anti-static mat is free from damage and contamination.

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