Electrostatic discharge
From Freepedia
An electrostatic discharge (ESD) is an electric current driven by the excess electric charge stored on an insulating object. The term is usually used in the electronics and other industries to describe momentary unwanted currents that cause damage to electronic equipment.
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.
Returning to the spark, this is triggered when the ambient electric field strength exceeds a certain threshold value, 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 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 is almost entirely (99%) oxygen (O2) and 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 byproducts are toxic.
ESD can also be caused by so-called 'static electricity' that people experience in dry weather, when small sparks fly between insulating materials such as hair and clothing.
ESD is a serious issue in electronics, because integrated circuits are made from semiconductor materials such as silicon and insulating materials such as silicon dioxide, which can break down if exposed to high voltages. Manufacturers and users of integrated circuits must take precautions to avoid this problem. Such measures include appropriate packing material, the use of conducting wrist straps and foot-straps to prevent high voltages from accumulating on workers' bodies, anti-static mats to conduct harmful electric charges away from the work area, and humidity control, because in humid conditions the surface layer of moisture on most objects conducts electric charges harmlessly to earth.
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:
- Machine model (MM)
- Charged device model (CDM)
All these ESD testing standards define the testing method and procedure as well as the test circuit.
See also
- Latchup - for qualification testing of semiconductor devices, ESD and latchup are commonly considered together.
- Automotive Electronics Council which defines, in some of its standards, ESD test qualification requirements for electronic components used in vehicles.
External links
- Fundamentals of ESD by the Electrostatic Discharge Association
- "Breakdown and sparking" by Richard Fitzpatrick, University of Texas
