Static Protection-Personal
(881)What does ESD (electrostatic discharge) protection mean?
Electrostatic discharges (ESD) are with us every day, and they are becoming increasingly important, especially in the electronic industry. Years ago, when we used vacuum tubes, they didn’t cause us any problems, nor did they even when transistors were introduced. However, the situation changed when MOSFET transistors became popular, as the failure ratio grew. When the problem was researched, it was discovered that the build-up of electrostatic discharge was enough to cause a malfunction of the oxide layer in the device. From this time onwards, the awareness of ESD rose significantly, as it has been proved that the phenomenon has an influence on many electronic components. Currently, many manufacturers treat all components as sensitive to electrostatic discharge (not only MOS devices, which are the most susceptible to damage) and introduce EPAs in their factories. In relation to the importance attached to ESD, manufacturers of electronic devices invest a lot in EPAs (Electrostatic Protected Areas). Thanks to those special areas, they can make sure that their products’ malfunction rate is not high during production tests and are able to demonstrate high reliability for a long time.
Static transfer and ESD sensitivity. Why do electronic devices need ESD protection?
Seeing how ESD affects electronic devices, it is worth having a look at the devices themselves to check how they react to ESD. It was found that some electronic devices are more sensitive to ESD than others, and the most sensitive were those employing MOS technology (Metal Oxide Semiconductor) solutions. There are many ways in which static charges can be transferred to semiconductor devices and cause damages due to electrostatic discharge. The most obvious way is physical contact with the object which is charged and conductive. Charges can be found on the hands or clothes of the person at the workbench. Unsecured tools can be even more dangerous. For example, metal screwdrivers conduct charges faster than the human body, which results in higher peak current. That is why EPAs should be equipped only with devices which are carefully picked for such areas. This also applies to other elements like plastic trays for documents, stationery or even mugs made from plastic.
ESD protection
There are many ways to protect ourselves from ESD. Is it worth pointing out the main elements of effective protection:
Build and test devices in EPAs: while working with electronic devices, it is crucial that all components are treated in a way that prevents exposure to ESD. To make it easier, companies often treat all components as ESD-sensitive devices. The ESD protection required for this environment is usually achieved by using EPAs, or ESD Protected Areas, where all means of protection are used to prevent damage. In EPAs, tight control measures are implemented to ensure the dispersion of ESD, and the environment itself ensures protection of all electronic components. Using EPAs or ESD Protected Areas is currently the standard in all factories producing electronics.
Keep components in ESD Protected Areas: It is crucial to not only ensure that electronics are built in an environment where ESD protection is the priority, but also to ensure that all components are stored and transported in an environment where ESD protection is implemented. Similarly, ESD protection should also be applied to all subassemblies stored.
Carry out the ESD control process: While installing equipment and generating physically a static-dissipative environment are the first step in creating an ESD Protected Area, it is also crucial to introduce appropriate work processes and ensure training for all personnel. The area will only be efficient, if the personnel using it knows and understands the rules of work there, i.e. knows how to handle the components and equipment to prevent damage caused by ESD and understands the effects of ESD. The key to this is training, as it is absolutely crucial for people to know, understand and use the right processes.
Personal protective equipment against electrostatic discharge for employees:
Wristbands
Usually, wristbands are the basic way to ground the personnel and can also ensure equipotential bonding during maintenance and repair works in field. If the wristband is worn correctly and the wire is properly connected to ground or the point of equipotential bonding, it keeps the person wearing it at the ground potential or the same potential as the object touched when grounding is impossible. Because the person and other grounded objects that are in the work area have the same potential, no dangerous discharge may occur between them. Additionally, static charges are removed from the person to the ground and do not accumulate.
Wristbands are made up of two main elements: a band for your wrist and a ground wire which connects the wristband to a shared ground point.
Most wristbands are equipped with a resistor which is placed on the ground wire. Its job is to limit the current. According to Ohm’s law, the current is equal to the voltage divided by the resistance; that is why adding the resistance of the operator’s “circuit” to ground will limit the amount of electricity flowing through the wristband to the operator’s wrists. The resistance of most resistors is usually 1MΩ, their power is at least 1/4 watt and the operating voltage is 250V. For safety reasons, if the operator is exposed to electric circuits of 250V or more, this type of protection must not be used. Wristbands should be regularly tested to guarantee effective protection. Is it recommended to test the wristbands daily on dedicated test stands, or by using a continuous monitor on the workbench.
Floors, floormats and floor finishes protecting from ESD.
Other two pieces of personal protection equipment are the floor and footwear system obtained by using ESD-protected floor systems together with ESD-protected shoes. This combination of conductive or dissipative materials ensures a safe ground path for electrostatic dissipation, and in this way reduces the build-up of electrostatic discharge. Apart from charge dissipation, some floor materials (and floor finishing materials), also reduce triboelectric charging. Using a floor and shoe system is especially recommended in those areas in which increased personnel mobility is needed. Additionally, floor materials can help minimise the accumulation of charges on chairs, mobile equipment such as trolleys, lift trucks and other objects that move on the floor.
ESD clothing
Proper clothing is one of personal protective measures in EPA environments. Textiles, especially those made from plastics, generate electrostatic charges which can discharge on elements or create electrostatic fields which induce charges. Because clothing is usually isolated electrically or isolated from the body, charges on clothing materials are not always dispersed over the skin and later in the ground. Antistatic clothing can suppress or in another way influence the electric field from clothing worn underneath the external layer. According to the ANSI/ESD S20.20 norm and the ANSI/ESD STM2.1 clothing norm, there are 3 categories of ESD clothing:
ESD Category 1 clothing is static control clothing without being attached to the ground. Without grounding, a charge can accumulate on conductive or dissipative clothing elements, if present, resulting in a charged source.
ESD Category 2 clothing is groundable static control clothing which, when connected to the ground, ensures a higher level of suppression of the effects of an electric field from clothing worn underneath such a garment.
ESD Category 3 clothing is groundable static control clothing system which also bonds the person’s skin to an identified ground path. The total resistance of the system, including the person, clothing and ground wire should be lower than 35MΩ.
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