Electrostatic Discharge

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Electrostatic Discharge
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Electrostatic discharge (ESD) is the sudden and momentary electric current that flows between two objects at different electrical potentials. 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 ESD events is static electricity. Static electricity is often generated through triboelectricity, the separation of electric charges that occurs when two materials are brought into contact and then separated. Examples of tribocharging include walking on a rug, descending from a car, or removing some types of plastic packaging. In all these cases, the friction between two materials results in tribocharging, thus creating a difference of electrical potential that can lead to an ESD event.

Another cause of ESD damage is through electrostatic induction. This occurs when an electrically charged object is placed near a conductive object isolated from ground. The presence of the charged object creates an electrostatic field that causes electrical charges on the surface of the other object to redistribute. Even though the net electrostatic charge of the object has not changed, it now has regions of excess positive and negative charges. An ESD event may occur when the object comes into contact with a conductive path. For example, charged regions on the surfaces of styrofoam cups or plastic bags can induce potential on nearby ESD sensitive components via electrostatic induction and an ESD event may occur if the component is touched with a metallic tool.

Types of ESD The most spectacular form of ESD is the spark, which occurs when a strong electric field creates an ionized conductive channel in air. This can cause minor discomfort to people, severe damage to electronic equipment, and fires and explosions if the air contains combustable gases or particles. Many ESD events occur without a visible or audible spark. A person carrying a relatively small electric charge may not feel a discharge that is sufficient to damage sensitive electronic components. Some devices may be damaged by discharges as small as 12 volts. These invisible forms of ESD can cause device outright failures, or less obvious forms of degradation that may affect the long term reliability and performance of electronic devices. The degradation in some devices may not become evident until well into the service life of some devices.

Sparks A spark is triggered when the electric field strength exceeds approximately 30 kV/cm (the dielectric strength of air). 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 (material) 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, or between two clouds, is typically hundreds of millions of volts. The resulting current that flows through the ionized air causes 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 on an aircraft.

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.

Prevention of ESD bases on Electrostatic Protective Area (EPA). EPA can be a small working station or a large manufacturing area. The main principle of an EPA is that there are no highly charging materials in the vicinity of ESD sensitive electronics, all conductive materials are grounded, workers are grounded, and charge build-up on ESD sensitive electronics is prevented. International standards are used to define typical EPA and can be found for example from International Electrotechnical Commission (IEC) or American National Standards Institute (ANSI).

ESD prevention within an EPA may include using appropriate ESD-safe packing material, the use of conductive filaments on garments 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. Humid conditions prevent electrostatic charge generation because the thin layer of moisture that accumulates on most surfaces serves to dissipate electric charges. Ion generators are sometimes used to inject ions into the ambient airstream. Ionization systems help to neutralize charged surface regions on insulative or dielectric materials. Insulating materials prone to triboelectric charging should be kept away from sensitive devices to prevent accidental charging of devices through induction. On aircraft, static dischargers are used on the trailing edges of wings and other surfaces.

Manufacturers and users of integrated circuits must take precautions to avoid ESD. ESD prevention can be part of the device itself and include special design techniques for device input and output pins. External protection components can also be used with circuit layout.

Due to dielectric nature of electronics component and assemblies, electrostatic charging can not be completely prevented during handling of devices. Most of ESD sensitive electronic assemblies and components are also so small that manufacturing and handling is made with automated equipment. ESD prevention activities are therefore important with those processes where component is touching on equipment surfaces. In addition, it is important to prevent ESD when electrostatic discharge sensitive component is connected with other conductive parts of the product itself. An efficient way to prevent ESD is to use materials that are not too conductive but will slowly conduct static charges away. These materials are called static dissipative and have resistivity values in the range of 105 to 1011 Ohm-meters. Materials in automated manufacturing which will touch on conductive areas of ESD sensitive electronic should be made of dissipative material, and the dissipative material must be grounded.

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 farad capacitor and a 1500 Ohm (unit) resistor. Other similar standards are MIL-STD-883 Method 3015, and the ESD Association's ESD STM5.1. For comportment to European Union standards for Information Technology Equipment, the IEC-61000-4-2 test specification is used. Guidelines and requirements are given for test cell geometries, generator specifications, test levels, discharge rate and waveform, types and points of discharge on the "victim" product, and functional criteria for gauging product survivability.

Charged Device Model (CDM) test is used to define device ESD withstand when the device itself has an electrostatic charge and discharges due to metal contact. This discharge type is the most common type of ESD and causes most of the ESD damages in electronics manufacturing. CDM discharge depends mainly on parasitic parameters of the discharge and is strongly depending on size and type of component package. One of the most widely used CDM simulation test models is defined by the JEDEC.

Other standardized ESD test circuits include the following:

Machine model (MM)
Transmission line pulse (TLP)

All these ESD testing standards define the testing method and procedure as well as the test circuit.

References

Danglemayer, G. Theodore, ESD Program Management: A Realistic Approach to Continuous Measurable Improvement in Static Control ISBN 0-412-13671-6