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Electrostatic discharge

May. 06, 2024

Electrostatic discharge

Sudden flow of electric current between two electrically charged objects by contact

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Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two differently-charged objects when brought close together or when the dielectric between them breaks down, often creating a visible spark associated with the static electricity between the objects.

ESD can create spectacular electric sparks (lightning, with the accompanying sound of thunder, is an example of a large-scale ESD event), but also less dramatic forms which may be neither seen nor heard, yet still be large enough to cause damage to sensitive electronic devices. Electric sparks require a field strength above approximately 4 × 106 V/m in air, as notably occurs in lightning strikes. Other forms of ESD include corona discharge from sharp electrodes, brush discharge from blunt electrodes, etc.

ESD can cause harmful effects of importance in industry, including explosions in gas, fuel vapor and coal dust, as well as failure of solid state electronics components such as integrated circuits. These can suffer permanent damage when subjected to high voltages. Electronics manufacturers therefore establish electrostatic protective areas free of static, using measures to prevent charging, such as avoiding highly charging materials and measures to remove static such as grounding human workers, providing antistatic devices, and controlling humidity.

ESD simulators may be used to test electronic devices, for example with a human body model or a charged device model.

Causes

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One of the causes of ESD events is static electricity. Static electricity is often generated through tribocharging, 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, rubbing a plastic comb against dry hair, rubbing a balloon against a sweater, ascending from a fabric car seat, or removing some types of plastic packaging. In all these cases, the breaking of contact 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 the 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 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.

ESD can also be caused by energetic charged particles impinging on an object. This causes increasing surface and deep charging. This is a known hazard for most spacecraft.[1]

Types

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Electrostatic discharge (ESD) phenomena vary in complexity and magnitude, with the electric spark being the most visible and dramatic example. This occurs when a strong electric field ionizes the air, creating a conductive channel that can convey an electric current. People may experience this as a small jolt of discomfort, but ESD can inflict severe damage on electronic components, potentially leading to malfunctions and failures. In hazardous environments where flammable gases or dust particles are present, ESD can trigger fires or explosions.

Not all ESD events, however, are accompanied by a visible spark or noise. It is possible for a person to carry a charge that, while undetectable to the human senses, can still be potent enough to harm delicate electronics. Some components can be compromised by discharges as faint as 30 V, with such damage sometimes not becoming apparent until significant usage has occurred, thus affecting the lifespan and performance of the devices.[citation needed]

Cable discharge events (CDEs) are discharges occurring when connecting electrical cables to a device.

Sparks

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A spark is triggered when the electric field strength exceeds approximately 4–30 kV/cm[2] — the dielectric field 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 in a process called dielectric breakdown.

Lightning over Rymań. Northern Poland.

Perhaps the best known example of a natural spark is lightning. In this case the electric potential between a cloud and ground, or between two clouds, is typically hundreds of millions of volts. The resulting current that cycles through the stroke channel causes an enormous transfer 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 V (Paschen's law).

Earth's atmosphere consists of 21% oxygen (O2) and 78% nitrogen (N2). During an electrostatic discharge, such as a lightning flash, the affected atmospheric molecules become electrically overstressed. The diatomic oxygen molecules are split, and then recombine to form ozone (O3), which is unstable, or reacts with metals and organic matter. If the electrical stress is high enough, nitrogen oxides (NOx) can form. Both products are toxic to animals, and nitrogen oxides are essential for nitrogen fixation. Ozone attacks all organic matter by ozonolysis and is used in water purification.

Sparks are an ignition source in combustible environments that may lead to catastrophic explosions in concentrated fuel environments. Most explosions can be traced back to a tiny electrostatic discharge, whether it was an unexpected combustible fuel leak invading a known open air sparking device, or an unexpected spark in a known fuel rich environment. The result is the same if oxygen is present and the three criteria of the fire triangle have been combined.

Damage prevention in electronics

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A portion of a static discharger on an aircraft. Note the two sharp 3/8" metal micropoints and the protective yellow plastic.

Many electronic components, especially integrated circuits and microchips, can be damaged by ESD. Sensitive components need to be protected during and after manufacture, during shipping and device assembly, and in the finished device. Grounding is especially important for effective ESD control. It should be clearly defined, and regularly evaluated.[3]

Protection during manufacturing

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ESD Jacket

In manufacturing, prevention of ESD is based on an Electrostatic Discharge Protected Area (EPA). The EPA can be a small workstation 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 and dissipative materials are grounded, workers are grounded, and charge build-up on ESD sensitive electronics is prevented. International standards are used to define a 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.

Ionizers are used especially when insulative materials cannot be grounded. Ionization systems help to neutralize charged surface regions on insulative or dielectric materials. Insulating materials prone to triboelectric charging of more than 2,000 V should be kept away at least 12 inches from sensitive devices to prevent accidental charging of devices through field 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 cannot be completely prevented during handling of devices. Most of ESD sensitive electronic assemblies and components are also so small that manufacturing and handling is done with automated equipment. ESD prevention activities are therefore important with those processes where components come into direct contact with equipment surfaces. In addition, it is important to prevent ESD when an 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 below 1012 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. These special materials are able to conduct electricity, but do so very slowly. Any built-up static charges dissipate without the sudden discharge that can harm the internal structure of silicon circuits.

Protection during transit

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A network card inside an antistatic bag, a bag made of a partially conductive plastic that acts as a Faraday cage, shielding the card from ESD.

Sensitive devices need to be protected during shipping, handling, and storage. The buildup and discharge of static can be minimized by controlling the surface resistance and volume resistivity of packaging materials. Packaging is also designed to minimize frictional or triboelectric charging of packs due to rubbing together during shipping, and it may be necessary to incorporate electrostatic or electromagnetic shielding in the packaging material.[4] A common example is that semiconductor devices and computer components are usually shipped in an antistatic bag made of a partially conductive plastic, which acts as a Faraday cage to protect the contents against ESD.

Simulation and testing for electronic devices

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Electric discharge showing the ribbon-like plasma filaments from multiple discharges from a Tesla coil.

For testing the susceptibility of electronic devices to ESD from human contact, an ESD Simulator with a special output 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 1,500 ohm resistor. Other similar standards are MIL-STD-883 Method 3015, and the ESD Association's ESD STM5.1. For compliance to European Union standards for Information Technology Equipment, the IEC/EN 61000-4-2 test specification is used.[5] Another specification referenced by equipment maker Schaffner calls for C = 150 pF and R = 330 Ω which provides high fidelity results. While the theory is mostly there, very few companies measure the real ESD survival rate. 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.

A charged device model (CDM) test is used to define the ESD a device can 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 in electronic devices and causes most of the ESD damages in their manufacturing. CDM discharge depends mainly on parasitic parameters of the discharge and strongly depends 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 machine model (MM) and transmission line pulse (TLP).

See also

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References

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Essential Guide to Controlling ESD (Electrostatic Discharge)

Plastics and elastomers are great electrical insulators. By default, they just don’t conduct electricity in their unaltered bulk material state. The vast majority of the ESD-safe (or static-dissipative) products such as coatings, clothing and specialized ESD handling products are polymer-based. 

But by nature, a product cannot be both an insulator and ESD-safe at the same time for reasons that will become clear. So what is it that makes some polymers insulators, while others are classed as ESD-safe, or even electrically conductive?

In this article we'll be examining what makes an ESD product safe, and of equal importance from an industrial perspective, we’ll look at the standards for defining and qualifying an ESD-safe product.

 

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Effects of ESD (Electrostatic Discharge)

Just to remind you, ESD, or electrostatic discharge, is the electrical discharge between a statically charged object, and another object of a different potential.

One example you may be familiar with is the case of walking across a carpeted floor in a pair of sneakers and touching a door. If you had experienced triboelectric charging as you walked across the floor, you may have felt a zap on your fingers as you grabbed the metal door handle.

That was the zap of ESD, and while it's annoying when you get shocked by a door handle, the consequences arising from ESD in an industrial setting such as aviation maintenance or electronic assembly work range from being financially expensive (at best) to potentially catastrophic.

Those shocks you feel on the skin when you touch a door handle are of a fairly significant voltage (yet thankfully a small and non-lethal Amperage). If you feel the zap on your skin, it means that the ESD discharge has been at least 2000 to 3000 volts.

Electronics components, such as transistors and integrated circuits however, are significantly less tolerant to voltages, even at low current. A voltage of just 10 volts of static electricity can obliterate transistors on a chip. That’s very small comparatively speaking. And this is why ESD control exists-- to provide permanent protection against even the slightest of discharges.

ESD damage typically occurs from three main types of events:

  1. Discharge to the device (from the body)

  2. Discharge from the device (when the device charges through contact from packaging and surfaces)

  3. From field-induced charge (from regional static fields).

These discharges bring another risk from the sparks themselves. Not only is the hardware at risk from the electrical arcs caused by ESD damage, but the arcs are a potential source of ignition, which can be catastrophic in an environment with explosive vapors, liquids and even solid particulate matter such as coal dust.

Nobody likes electrostatic discharge events, and so an entire industry with its own governing standards has arisen to control ESD damage.

 

ESD Standards and Definitions

There are a range of standards governing the management and implementation of ESD-safe materials. But before that, we need to define what exactly is an ESD-safe material in the wider context of surface resistivity.

Conveniently, there is a standard that defines this also, and this is ESD ADV1.0-2017, which is published by the Electrostatic Discharge Association (ESD). [1] The document’s full title is the “ESD Association Advisory For Electrostatic Discharge Terminology - Glossary”, and it defines conductors, insulators and ESD-safe materials in terms of their surface (and volume) resistivity.

The association defines surface resistivity in the glossary thus:

“For electric current flowing across a surface, the ratio of DC voltage drop per unit length to the surface current per unit width. In effect, the surface resistivity is the resistance between two opposite sides of a square and is independent of the size of the square or its dimensional units. Surface resistivity is expressed in Ω/square.”

Or to put it another way, surface resistivity is the resistance to leakage current along the surface of an insulating material. The higher the surface resistivity value, the lower the leakage current and the less conductive the material is.

Note that resistivity is described as being independent of the size of the sample. This is because resistivity is a property of the material, unlike resistance, which is a property of an object’s geometry as well as being dependent on the material type. Strain gauges demonstrate the relationship between geometry and resistance rather nicely.

ESD ADV1.0-2017 divides materials into three categories according to their surface resistivity:

  1. Conductive material -

    A conductive material is one that has a surface resistivity of less than 1 x 10E5

    Ω/square, or a volume resistivity of less than 1 x 10E4 Ωcm. Conductors permit electrons to flow quickly across the surface, or through the volume of the material. To use an automotive analogy, a conductive material is analogous to a racetrack with fast cars zooming around unimpeded by speed limits or traffic.

  2. Static dissipative (ESD-safe) -

    A static dissipative material has a surface resistivity of at least 1 x 10E5

    Ω/square, but less than 1 x 10E12 Ω/square. In terms of volume resistivity, a static dissipative material falls in the range of 1 x 10E4 Ωcm and 1 x 10E11 Ωcm. Charge flows more slowly with static-dissipative materials. When an arc occurs, it has lower energy as it tries to reach ground. This is what you want when protecting devices from ESD damage. Back to our automotive analogies, and a static dissipative material can be thought of as a journey in regular traffic, with all the limitations that goes with it.

  3. Insulative materials -

    A material that has a surface resistance or a volume resistance equal to or greater than 1 x 10E11

    Ω. Plastics are insulators. They have a high electrical resistance, and charge does not flow through the material. This can be changed by addition of conductive particles into the base plastic. If an insulative material were to have an equivalent driving scenario, it would be stuck in traffic in rush hour, with no exit ramp to get off the highway.

In summary, to be classed as an ESD safe material, the surface resistance of that material must fall within the range of 1x 10E5 Ω/square and 1x 10E11 Ω/square according to ESD ADV1.0-2017.



And that clear definition illustrated in the graphic above, shows why from a regulatory point of view, a material can only exist as either a conductor, insulator or static-dissipative (ESD-safe) material. Resistivity exists as a spectrum, and in this context, a material can only exist at one point in that spectrum.

 

ANSI/ESD S20.20

Now that we have defined materials in terms of their surface resistivity value, let’s look at the main set of standards that determines if a product such as a coating is indeed ESD-safe. The most recent set of commercial standards relating to ESD protection in this context is named “ANSI/ESD S20.20-2021: Protection Of Electrical And Electronic Parts, Assemblies And Equipment (Excluding Electrically Initiated Explosive Devices)

…or “S20.20” for short.

This document covers a range of ESD management and control topics including training, product qualification, compliance verification, grounding/equipotential bonding systems, personnel grounding, ESD protected area (EPA) requirements, packaging and marking.

This set of standards is used extensively in the aerospace, automotive, electronics manufacturing and medical industries. It covers electrical or electronic parts, assemblies, and equipment susceptible to damage by discharges greater than or equal to 100 volts human body model (HBM) and 200 volts charged device model (CDM). [2]

 

MIL-STD-1686

For military applications, the set of standards governing ESD control is MIL-STD-1686. The military standard is named “MIL‑STD‑1686-Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies and Equipment (excluding electrically initiated explosive devices)”.

It was first released in 1980 and was one of the first standards pertaining to ESD management and control. It is currently under the control of the US Navy. You’ll notice some similarity between the full titles of the commercial and military standards. The similarities don’t just end there.

There is a big overlap in the Venn diagram of ANSI/ESD S20.20 and MIL-STD-1686 which we won’t cover in this article. But if you are interested in the similarities and differences between the commercial and military standards, this document from ESD Association highlights them side by side for comparison.[3]

Suffice to say that there is enough commonality between the two standards that a product certified to the ANSI standard would likely pass the military standard testing in most aspects. That’s not to say that it automatically qualifies - you still have to test it to the specific standards!

 

What Makes Polymer-based ESD-Safe Products Safe?

Resistivity-Altering Additives

The surface (and volume) resistivity of the polymer can be altered by the use of conductive additives during the manufacturing process.

Traditionally in ESD applications, this was achieved by the addition of carbon black to the polymer, which would result in a darker colored coating. Metal powders such as Al, Au, Ag, and Cu and stainless steel can also be added to make polymers more conductive.[4]

So how exactly do these additives alter the electrical properties of polymers? In the case of carbon additives, manufacturers utilize what is known as conductive carbon black, which is manufactured in industrial furnace processes. Carbon black is intrinsically a semiconductor, so from an electrical point of view, addition of such an additive results in the formation of two interpenetrated networks: the conductive additive network, and the resistive polymer network. This principle applies to polymers filled with other conducting fillers too.

The electrical conductivity of filled–polymer compounds such as ESD-safe coatings depends on the structure of the additive particles, the size, and also depends on process parameters such as mixing time.[5] A higher mixing time results in better dissipation of the additive into the polymer, and hence a more even and consistent conducting matrix.

The other ways of achieving the ESD safe property is by using antistatic additives such as Static-dissipating polymers and Surfactant-based ESD agents.  Static-dissipating polymers provides long-term ESD levels by forming a clear coat on the surface such as Techspray Licron. Surfactant-based ESD additives, which have a partially hydrophilic structure that attracts a film of surface water that lowers the resistivity of the part, allowing excess electrons to dissipate. Both of these antistatic materials can be applied directly to the surface of the finished article from an aqueous and/or alcohol solution as a spray or dip.

 

Summary

We know what resistivity is and how it relates to ESD protection, we know that additives alter the electrical properties of polymers due to the formation of conductive networks formed from the dissipated additives in the polymer.

And from a regulatory perspective, we are now clear on the main standards applying to ESD control for both industrial and military applications.

That’s the who, what, where and why as far as manufacturing and defining requirements of ESD-safe materials.

If you’d like to know more about specific ESD-safe, clear coatings for your applications, then head on over to the ESD products page for more information.

 

 

References

[1] ESD ADV1.0-2017 - ESD Association Advisory for Electrostatic Discharge Terminology – Glossary

[2] ANSI/ESD S20.20-2021: Protection Of Electrical And Electronic Parts, Assemblies And Equipment (Excluding Electrically Initiated Explosive Devices)

[3] ESD Standards Direct Comparison

[4] Resistivity and thermal reproducibility of carbon black and metallic powder filled silicone rubber heaters, Eun-Soo Park,Lee Wook Jang,Jin-San Yoon, Journal of Applied Polymer Science

[5] A comprehensive picture of the electrical phenomena in carbon black–polymer composites, I. Balberg, Carbon Journal

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