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 may cause damage to electronic equipment.
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.
Another cause of ESD damage is through electrostatic induction. This occurs when an electrostatically charged object is simply placed near a conductive, but electrically neutral object. The presence of the nearby charged object causes electrical charges to redistribute themselves on the surface of the neutral object. Even though the object is still electrically neutral, it now develops regions of excess positive and negative charge, and a potentially damaging discharge may now occur when one of these charged regions is suddenly grounded. Charged regions on the surfaces of styrofoam cups or plastic bags can cause ESD damage to nearby sensitive components via electrostatic induction.
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 that affect the reliability of a device.
Sparks
Returning to the spark, this is triggered when the electric field strength exceeds approximately 30 kv/cm. This may cause a very rapid increase in the number of free electrons and ions in the air, temporarily causing the air to abruptly become an electrical conductor in a process called dielectric breakdown.
Perhaps the best known example of a natural spark is a lightning strike. In this case the potential difference between a cloud and ground is typically hundreds of millions of volts, and the resulting current that flows heats the air causing an explosive release of energy. On a much smaller scale, sparks can form in air during electrostatic discharges from charged objects that are charged to as little as 380 volts (Paschen's law).
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 for nitrogen fixation.
Prevention
ESD is a serious issue in solid state electronics. Integrated circuits are made from semiconductor materials such as silicon and insulating materials such as silicon dioxide. Either of these materials can suffer permanent damage when subjected to high voltages. Manufacturers and users of integrated circuits must take precautions to avoid this problem. Such measures include special design techniques for device input and output pins, using appropriate ESD safe packing material, the use of conductive tracks on cleanroom clothing worn by assembly workers, conducting wrist straps and foot-straps to prevent high voltages from accumulating on workers' bodies, anti-static mats or conductive flooring materials to conduct harmful electric charges away from the work area, and humidity control because, in humid conditions, the surface layer of moisture on many objects conducts electric charges harmlessly to earth. Ion generators are sometimes used to inject ions into the ambient airstream. This helps to neutralize charged surface regions that may be "stranded" on dielectric materials. Insulating materials prone to triboelectric charging should be kept away from sensitive devices to prevent accidental charging of devices through induction.
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 high voltage from an external source, and then suddenly 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)
- Transmission Line Pulse (TLP)
All these ESD testing standards define the testing method and procedure as well as the test circuit.
See also
External links
Book
- Dangelmeyer, E, ESD Program Management [1]