Electrostatic Discharge

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Edited by Jonathan Ledet (Spring 2017)

Electrostatic Discharge (ESD) is the swift transfer of charges between objects at different potentials. ESD commonly occurs after the accumulation of static electricity on a material and can have devastating effects on solid-state electronics.

The Main Idea

All materials (both insulators and conductors) are defined on the Triboelectric series, which essentially rates that material's affinity for electrons. When an object lower on the series is touched by an object higher on the series, the lower object will acquire a negative charge. When the objects are separated, their charges are equal and opposite- just waiting for the opportunity to discharge. When the objects finally do discharge, a high voltage spark can form, turning the bridging air to plasma or frying circuits. This process is referred to as tribocharging.

You may have seen a demonstration in class involving a glass rod and a piece of cloth, or rubbed a balloon on your hair and observed its wacky behavior. Both of these are examples of the Triboelectric effect. Additionally, although friction greatly increases the magnitude of the effect, it is not required for the effect to occur. Only an initial contact and subsequent separation are required; friction only amplifies the effect due to the increased contact and separation of molecules on the microscopic level.

Electrostatic Discharge can be caused by an electrical breakdown, a short circuit, and--most commonly--Tribocharging. ESD is measured using an electrostatic voltmeter.

Sparks and lightning are visible Electrostatic Discharge events, but only represent part of the threat of ESD. For example, such a high voltage can be very damaging to the delicate pathways and components on circuit boards.

ESD events, like a spark from a human hand, allow current to travel to the ground through electronic devices, burning holes in integrated circuits and dealing heat damage to the circuit board. This can happen when working barehanded (without an electrostatic wrist strap) with circuit boards and other sensitive electronic equipment, when negatively charged synthetic materials are on or near sensitive electronic equipment, or due to the fast movement of air near electronic equipment.

There are some preventative methods taken to discourage Electrostatic Discharge. During the process in which electronic components are assembled most manufacturers implement Electrostatic Discharge Protected Areas (EPA). These areas are specially designed to prevent the build up of charge on the components, workers, and all other conductive materials. To protect against Electrostatic Discharge during transit, antistatic bags act as Faraday Cages to protect sensitive devices.

The Physics Principals and Visual Aids

Triboelectric Series

  • Celluloid
  • Sulfer
  • Rubber
  • Copper, Brass
  • Amber
  • Wood
  • Cotton
  • Human Skin
  • Silk
  • Cat Fur
  • Wool
  • Glass
  • Rabbit Fur
  • Asbestos

The objects near the top (of the list above) will tend to gain negative charges, while those below them will gain positive charges. The law of Conservation of Charge is followed.

First, a tube made out of plastic is charged when rubbed with synthetic fur (top pictures). The tube, which is now charged, is brought close to neutral paper bits on the table (bottom left). You can see that the tube and paper now attract each other and that this attraction is strong enough to lift the pieces of paper off the table.

Examples

(should be completed by a student)

Simple

Middling

Difficult

Connectedness

The concept of Electrostatic Discharge (ESD) can be applied to many practical scenarios; however, one of the most notable instances of ESD is in the electronics manufacturing industry. Due to the fact that charges can accumulate relatively easily in electronics manufacturing, companies often have to take ESD precautions by making employees wear special clothing and by designing their workbenches and flooring out of special material which prevents electrostatic build up.

Electrostatic Discharge is connected to chemical engineering in that there is a great deal of equipment in plants that has the potential to acquire electrostatic charge. As a result, it is vital to safety to be knowledgeable and cautious when dealing with such equipment in order to reduce work related injuries.

Electrostatic Discharge also has connections to Aerospace Engineering by how potentially dangerous it is to not guard against ESD in environments in which many different electronic devices are integrated into air and spacecraft. Aerospace research agencies, such as NASA, JAXA, or RKA, and corporation, such as Lockheed Martin, have implemented ESD protected areas and grounded workbenches, mandated the use of protective equipment, introduced protocol in which charge generating materials cannot be worn in protected areas, and have audits and inspections to make sure important aerospace electronics stay protected from ESD.

Additionally, ESD poses a threat in the operation of air and spacecraft due to its volatile nature and possible interaction with fuel. When fueling aircraft, it is mandated procedure to ground the air frame to the fuel truck in order to prevent a spark from jumping between the metal surfaces.

One interesting way that ESD can be used is to create sparks. When the dielectric between two oppositely charged sources is damaged or if the electric field due to the build up charge exceeds the dielectric, then ESD can occur by means of a spark through the air or other dielectric medium.

History

The phenomena of electrostatic discharge (ESD) has been known for a very long time, dating all the way back to the ancient Greeks. While ESD was mostly considered a non-factor throughout most of history, due to the growth of solid state electronics in the 1950's companies and researchers have had to account for and study in greater detail the phenomena of ESD. As the prevalence of electronics increased people began to notice the ESD could have very negative effects on certain components, causing them to short-circuit or malfunction. The 1960s and 70s were characterized by companies discovering methods and techniques to test for ESD, some of which included the Human Body Discharge Model and the Horizontal Coupling Plate. In the 1980's the release of the IBM personal computer saw an increased need for materials with resistance to ESD and thus the focus shifted towards maximizing the ability of electronic components to avoid ESD and subsequent malfunctioning. Since the 1980's and well into modern times, companies have been working to hone and refine this process in order to maximize the efficacy of their circuit components by increasing their ability to resist ESD when in use.

See also

Further reading

  • Electro Static Discharge: Understand, Simulate, and Fix ESD Problems, 3rd Edition

External links

References

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