Electrostatic discharge
Electrostatic discharge (ESD) is the sudden and momentary electric current that flows when an excess of electric charge, stored on an electrically insulated object, finds a path to an object at a different electrical potential (such as ground). The term is usually used in the electronics and other industries to describe momentary unwanted currents that cause damage to electronic equipment.
Types of ESD
The most spectacular form of ESD is the spark, which occurs when a strong electric field creates an ionised conductive channel in air. This can cause minor discomfort to people, severe damage to electronic equipment, and fires and explosions if the air contains combustible gases or particles. However, many ESD events occur without a spark, when a person carrying an electric charge touches a sensitive electronic component. Even these invisible forms of ESD can cause device failures or less obvious forms of degradation.
Sparks
Returning to the spark, this is triggered when the electric field strength exceeds a certain threshold value in the air, causing a rapid increase in the number of ions in the air (electrical breakdown), and these free ions temporarily cause the air to become a conductor.
The best known example of a spark is a lightning strike. In this case the potential difference between a cloud and ground can be millions of volts, and the resulting current that flows, heats the air causing an explosive release of energy.
Earth's atmosphere consists of 21% oxygen - O2, and 78% nitrogen - N2. During an electrostatic discharge, the intervening atmosphere can become electrically overstressed. The diatomic oxygen molecules can be split, and then recombine by probability into ozone (O3), which is unstable, or react with metals and organic matter. If the electrical stress is high enough, nitrogen oxides (NOx) can form. Both products are toxic to animals, but ozone is essential for the ozone layer, and nitrogen oxides are essential to nitrogen fixation.
Causes of ESD
One of the causes of charge separation that creates an ESD event is the triboelectric effect, in which certain materials become electrically charged after coming into contact with another different material and then being separated. This is why people experience ESD events after walking on a rug, descending from a car, or removing some types of packaging. In all these cases, friction between different materials causes triboelectric charging that, when discharged, becomes ESD.
Simulation and testing
For testing the susceptibility of electronic devices to ESD from human contact, a simple test circuit called the human body model (HBM) is often used. This consists of a capacitor in series with a resistor. The capacitor is charged to a specified voltage from an external source, and then discharged through the resistor into an electrical terminal of the device under test. One of the most widely used models is defined in the JEDEC 22-A114-B standard, which specifies a 100 picofarad capacitor and a 1500 ohm resistor. Other similar standards are MIL-STD-883 Method 3015, and the ESD Association's ESD STM5.1.
Other standardized ESD test circuits include the following:
Human Body Model (HBM)
Machine model (MM)
Charged device model (CDM)
All these ESD testing standards define the testing method and procedure as well as the test circuit.
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