What is ESD?
Electrostatic discharge (ESD) is when an electrical charge is suddenly transferred from one object to another. It is well-known that many non-metallic materials will build up an electrical charge in normal use, known as static electricity, and when they meet a metal or a material of the opposite charge, this build-up suddenly dissipates, with a flow of electricity. Sometimes, this can be felt (static shock) or even seen (a tiny spark), but in most cases, it happens without us knowing about it. Except for lightning, these discharges are almost always harmless, doing no significant damage to living things or materials. So why is there so much study, standards and products related to ESD?
When is ESD a Major Problem?
There are two significant instances when ESD can cause a huge problem. The first is when working with explosive gases or materials; a tiny spark can ignite a massive explosion. The second is when working with electronics. Electronic circuits and chips are designed to work with low voltages, and the thousands of volts present in a static build-up can instantly damage them.
As the static charges are not visible but almost always present, this issue can be a real plague to manufacturers or anyone working with electronic devices. For some companies, ESD can cause a nuisance if a component fails. For others, this could be life-threatening or hugely expensive, if the component is part of a critical safety system, as used in aerospace, automotive or medical industries. ESD can cause some components to fail completely, and this can usually be discovered during the testing and quality control processes.
However, ESD can also cause systems to degrade partially, and this can be far worse, as the symptoms might not show up initially in testing or commissioning but sometime later. They can cause failure or erratic behaviour, the latter being especially difficult to diagnose and repair. Because of the critical nature of some of these processes, special procedures, standards, and products have been developed.
What can be done to avoid ESD problems?
There are many things that can be done to reduce the impact of ESD in industry, and a major factor is the type of materials used on workbenches. Some materials are less prone to causing static build-up and are known as anti-static materials. Any electrically conductive materials will dissipate electrical charge instead of allowing it to build up. A metal is a great conductor; however metal will dissipate a static charge immediately, and it is this sudden discharge that is the problem. The best solution is usually materials that are developed that have a conductive substance or coating, such as carbon. They are not good electrical conductors like metals are, but they conduct sufficiently to quickly eliminate any static build-up, but not as suddenly as a metal would. But how conductive do they need to be, and what is considered an acceptable standard for materials that reduce ESD?
Industry Standards that are Involved with ESD Materials
Given the critical need to control ESD in the electronics industry, a number of government standards have been created that govern the procedures and products suitable for the processes. The main one in the UK/Europe is BS EN 61340-5-1 Protection from electrostatic phenomena requirements. In North America, it is ANSI ESD S20.20-2021 for the Protection of Electrical and Electronic Parts. A global voluntary group known as the ESD Association researches, teach and helps formulate standards. These standards set out the way things should be done, what materials should be used, and how to certify these materials. A training plan is required, so that all operatives can work safely, and various ESD awareness booklets are produced. The standards also define certain terms and names for key items, such as Static Dissipative Materials.
What is a Static Dissipative Material?
A static dissipative material is neither a conductor nor an insulator but somewhere in between. It conducts enough to prevent static charges from building up but not as quickly as a conductor, which might cause a problem. They also allow high charge concentrations to spread over the entire surface of the material, thus reducing the concentration. The way a material is classified as Static Dissipative might depend on which standard is involved, but typically it is one that has a surface resistivity between 1 x 10 4 Ω and 1 x 1011 Ω.
What is Surface Resistivity and Surface Resistance?
These terms have to do with the electrical resistance of the surface of flat materials. This is important for ESD in proving which materials can be classified as Static Dissipative. The surface resistance is simply the electrical resistance measured between two points on the material. This, of course, will vary according to the distance between the points. From this, the surface resistivity can be calculated, which is an inherent quality of the material and should not vary regardless of how the electrodes are placed.
This constant value for a given material allows similar materials to be compared and upper and lower limits for suitability in preventing ESD. The units of surface resistance and surface resistivity are both Ohms and for this reason, the values can easily be confused, so surface resistivity is sometimes expressed as ohms per square to distinguish it from surface resistance.
How is a material assessed for ESD suitability?
To meet the standards required for ESD, carefully calibrated test procedures are used. These involve placing a special test probe with concentric electrodes on the material. A low voltage is applied to the electrodes, and the resultant current flow is measured, from which the surface resistance can be calculated. The dimensions and spacing of the electrodes are known, and from this, the surface resistivity can be calculated. The electrodes have a relatively soft face, with a specified hardness, so that the contact with the material surface is consistent in all tests. Examples of test probes, meters, the formulae and the theory behind defining surface resistivity are readily available online, such as this document about Surface Resistivity by Advanced Energy.
What is the difference between Static Dissipative and Anti-Static materials?
As mentioned earlier, materials that are less prone to creating static charges in the first place are known as anti-static materials. They usually have a medium-high electrical resistance, which means that charges will gradually dissipate over several seconds. Although this is better than very-high resistance materials, which store charge for a long time, the level of charge present, even in those few seconds, can still be sufficient to cause damage or decay to sensitive circuits, so they are not classified as ESD safe. Surfaces such as Supaseal AS help avoid nuisance issues due to static build-up, but for ESD-safe work surfaces, you should only use products that have been tested and certified as static dissipative materials, such as the ESD bench mat collection. Another difference is that ESD-safe work surfaces must have a grounding stud to allow them to be connected to earth. Without this, the charges could build up and cause damage, almost as much as in a non-ESD-safe work surface.
More about Rhino Xtratuff ESD
The Rhino Xtratuff ESD workbench protection surface has been developed to provide a certified ESD-safe work surface for the electronics industry. It has been independently evaluated and classified according to BS-EN 61340-5-1 as “Static Dissipative” and has a calculated surface resistivity of 9.6 x 10³ Ω. It is stocked in four standard sizes, from 1.5 x 0.75m to 2 x 1m, and can be cut to size either when ordered or after delivery. A grounding stud is fitted in one corner or other positions on request. This stud is compatible with the Rhino StaticSafe ESD wrist strap. The product is a rigid board 4.5mm thick with a black, textured surface, which is hard-wearing and easy to clean. The product can be ordered online or a sample requested, from the Xtratuff product page.
