An electrostatic discharge (ESD) is a sudden flow of electric current through a material that is normally an insulator. A large potential difference across the insulator generates a strong electric field, converting the material's atoms into ions that conduct a current.
The best known example of ESD is a lightning strike. In this case the insulator that breaks down is air, 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.
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 insulating materials such as silicon, 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.
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