ESD ElectroStatic Discharge Tutorial
- a tutorial or summary about the basics of ElectroStatic Discharge, ESD and why ESD products including anti-static bags, ESD mats, ESD wrist straps, etc are needed.
ElectroStatic Discharge or ESD awareness is particularly important for anyone associated with electronics.
As integrated circuits become more compact, and feature sizes shrink, active devices as well as some passive devices are becoming more prone to damage by the levels of static that exist in a normal environment.
To combat its effects, industry is spending very considerable sums of money to prevent damage to electronic components from the effects of static. Anti-static areas using protective antistatic workbenches, as well as measures for ensuring people are not carrying static are all used. Using what are termed EPAs or Electrostatic Discharge Protected Areas, the destructive effects of static on electronics equipment during manufacture can be virtually removed.
In view of the fact that ESD is so important electronics manufacturing and development companies go to significant lengths to overcome the effects of ESD. Specially protected areas using a variety of ESD products including anti-static mats, ESD benches, ESD bags and packaging, ESD wrist straps, soldering irons adapted to absorb static, and much more are all implemented. These ESD precautions enable the effects of static to be overcome, and ensure the long term reliability of the products being developed and manufactured.
Although awareness has grown considerably in recent years, the problem has existed for a long time.
The effects of ESD were noted in military applications where its effects could have devastating effects on munitions and especially gunpowder.
However later, around the mid-1800s paper mills installed basic grounding systems and they also used steam to reduce the effects of static which had been attributed to catastrophes where paper dust in these mills had been ignited.
Today, many industries need to be aware of the effects of electrostatic discharge.
For the electronics industry, the drastic effects of ESD came to light in a major way with the introduction of the first MOSFET devices. In view of the very high gate impedances that existed it was found that they were easily damaged. Originally it was thought that only devices such as MOSFETs were at risk, but studies soon revealed that far more damage was being done that had been originally imagined. The problem of ESD became more acute as feature sizes on ICs dropped and they became more prone to damage.
What is ESD?
Static electricity is a natural phenomenon which occurs as part of everyday life. Its effects can often be felt when touching a metal door handle having walked across a nylon carpet. Another effect can be seen when hair stands up after it has been combed. The most dramatic effect is lightning. Here the scale is many orders of magnitude greater than those seen in and around the home. Colossal powers are dissipated in every strike, and its effects can be heard for many miles around. This is a particularly impressive form of ESD.
Static is created when there is movement. When objects rub together there is friction and this causes the surfaces to interact. An excess of electrons appears on one surface while there will be a deficiency on the other. The surface with the excess of electrons becomes negatively charged, whereas the surface with the deficit becomes positively charged.
These charges will try to flow and neutralise the charge difference. They may leak away slowly, or the discharge may take place more quickly. However as many substances exhibit a very high resistance these charges can remain in place for a very long time and wait until a suitable path is created for the discharge to take place. When charges find a path through an electronic circuit, the high instantaneous currents can give rise to damage. As a result ESD is of great importance.
ESD and the tribo-electric series:
The size of the charge which is generated is determined by a variety of different factors. One is obviously the conductivity of the two materials and also whether the charge between them can leak away. However one of the major influences is the materials themselves and their position of the two materials in what is called the tribo-electric series. The position of the two materials which are in rubbing against one another in this series governs the size of the charge and the relative polarities. The further apart they are in the series, then the greater the charge. The material that is higher up the series will receive the positive charge, whereas the one lower in the series will receive the negative charge. Materials such as human hair, skin, and other natural fibres are higher up the series and tend to receive positive charges, whereas man-made fibres together with materials like polythene, PVC and even silicon are towards the negative end. This means that when combing hair with a man-made plastic comb, the hair will receive a positive charge and the comb will become negative.
Practical examples of ESD
One of the most commonly visible examples of generating charge is when walking across a room. Even this everyday occurrence can generate some surprisingly high voltages. The actual voltages vary considerably dependent upon a variety of factors, but estimates can be given to illustrate the extent f the problem.
To illustrate the extent of the problem, a variety of instances are detailed in the table below:
|Likely ESD voltages caused by everyday actions|
|Cause of charge generation||Likely voltage generated (kV)*|
|Walking across a carpet||30|
|Picking up a polythene bag||20|
|Walking on a vinyl tiled surface||15|
|Working at a bench||5|
* These are approximate figures and assume a relative humidity of up to 25%. As the humidity rises, so these levels fall: with humidity of around 75%, the static levels can fall by a factor of very roughly 25 or more. All these figures are very approximate, because they are very dependent upon the particular conditions, but they give an order of magnitude guide to the ESD levels to be expected.
Although the resulting from ESD appear very high, they usually pass unnoticed. The smallest electrostatic discharge that can be felt is around 5kV, and even then this magnitude of discharge may only be felt on occasions. The reason is that even though the resulting peak currents may be very high, they only last for a very short time and the body does not detect them because the charge behind them is relatively small. Voltages of this magnitude from electronic or electrical equipment where the more current can be source and for much longer will have a much greater effect and can be very dangerous.
ESD effects on electronics
With most electronics ICs and components being designed to operate at voltage of 5 V or less, it is hardly surprising that electrostatic discharges can cause damage. As a result ESD is of major importance to all who are involved in the electronics industry. For any electronics area manufacturing, repairing, maintaining, or working on electronics equipment in any way, it is imperative that the effects of ESD are taken seriously. To this end ESD measures ranging from full ESD protected areas using ESD workbenches, ESD flooring, ESD clothing, ESD wrist straps and the like are used. Further pages in this ESD tutorial will detail different aspects of ESD, electrostatic discharge and how to combat its effects for electronics equipment.
By Ian Poole
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