Antistatic device

A label on an antistatic bag featuring the circle symbol on the left and the reaching symbol on the right

An antistatic device is any device that reduces, dampens, or otherwise inhibits electrostatic discharge; the buildup or discharge of static electricity,^[1][2] which can damage electrical components such as computer hard drives, and even ignite flammable liquids and gases.

Many methods exist for neutralizing, varying in use and effectiveness depending on the application. Antistatic agents are chemical compounds that can be added to an object, or the packaging of an object, to help deter the buildup or discharge of static electricity.^[3] For the neutralization of static charge in a larger area, such as a factory floor or workshop, antistatic systems may utilize electron emission effects such as corona discharge or photoemission that introduce ions into the area that combine with and neutralize any electrically charged object.^[4] In many situations, sufficient ESD protection can be achieved with electrical grounding.

Symbology

Various symbols can be found on products, indicating that the product is electrostatically sensitive, as with sensitive electrical components, or that it offers antistatic protection, as with antistatic bags.

Reach symbol

ANSI/ESD standard S8.1-2007 is most commonly seen on applications related to electronics. Several variations consist of a triangle with a reaching hand depicted inside of it using negative space.

Versions of the symbol will often have the hand being crossed out as a warning for the component being protected, indicating that it is ESD sensitive and is not to be touched unless antistatic precautions are taken.

Another version of the symbol has the triangle surrounded by an arc. This variant is in reference to the antistatic protective device, such as an antistatic wrist strap, rather than the component being protected. It usually does not feature the hand being crossed out, indicating that it makes contact with the component safe.^[5]

Circle

Another common symbol takes the form of a bold circle being intersected by three arrows. Originating as a U.S. military standard, it has been adopted industry-wide. It is intended as a depiction of a device or component being breached by static charges, indicated by the arrows.^[5]

Examples

Types of antistatic devices include:

Antistatic bag

An antistatic bag is a bag used for storing or shipping electronic components which may be prone to damage caused by electrostatic discharge (ESD).

Ionizing bar

An ionizing bar, sometimes referred to as a static bar, is a type of industrial equipment used for removing static electricity from a production line to dissipate static cling and other such phenomena that would disrupt the line. It is important in the manufacturing and printing industries, although it can be used in other applications as well.^[6][7]

Ionizing bars are most commonly suspended above a conveyor belt or other apparatus in a production line where the product can pass below it; the distance is usually calibrated for the specific application.^[4] The bar works by emitting an ionized corona onto the products below it.^[4][8] If then a product on the line has a positive or negative static charge, as it passes through the ionized aura created by the bar, it will attract the correspondingly charged positive or negative ions and become electrically neutral.^[8][9]

Antistatic garments

Antistatic shoes

Antistatic garments or antistatic clothing is required to prevent damage to electrical components or to prevent fires and explosions when working with flammable liquids and gases.

One of the ways to bond or electrically connect personnel to ground is the use of an ESD garment. ESD garments have conductive threads in them, creating a wearable version of a faraday cage. ESD garments attempt to shield ESD sensitive devices from harmful static charges from clothing such as wool, silk, and synthetic fabrics on people working with them. For these garments to work properly, they must also be connected to ground with a strap. Most ESD garments are not conductive enough to provide personal grounding so antistatic foot straps and antistatic wrist straps are also worn. ESD garments are considered an optional method to control ESD.

An ESD protected area is a defined location with the necessary materials, tools, and equipment capable of controlling static electricity to a level that minimizes damage to ESD susceptible items. In the ESD protected area, all conductors in the environment, including personnel, shall be bonded or electrically connected and attached to a known ground or contrived ground. This attachment creates an equipotential balance between all items and personnel. Electrostatic protection can be maintained at a potential above a "zero" voltage ground potential as long as all items in the system are at the same potential.

Antistatic garments are used in many industries such as electronics, communications, telecommunications and defense applications. As computers and electronics become ever more pervasive in consumer products so an increasing number of manufacturers will need to apply anti-static control measures. One such measure is antistatic apparel because people are the greatest source of static charge in the workplace.

Transportation of electrostatic sensitive devices also requires packaging that provides protection from electrostatic hazards in the transportation or storage system. In the case of an ESD protected area designed with continuous grounding of all conductors and dissipative items (including personnel), packaging may not be necessary.

The amount of static electricity we feel varies according to factors such as our body and foot size. A larger body and bigger feet require more charge to be stored to produce the same voltage. The material our clothes are made from and the soles of our shoes can influence static electricity too. Weather affects it as well. There is more build-up of static charge when the air is dry. Most people feel harmless shocks at around 2,000-4,000 volts. However electrical components can be damaged by as little as a few volts. It is estimated that between eight percent and 33 percent of product losses—-the proportion of products which are rendered faulty—-are due to static electricity. Static electricity is generally harmless to the individual but if not controlled, electrostatic discharge can cause product damage to electrostatic sensitive devices and lead to machinery downtime, lost man-hours, returned products and warranty costs particularly in the semiconductor and electronics industries, which caused 5 billion USD worth of damage to products each year.

Antistatic mat

An antistatic floor mat or ground mat is one of a number of antistatic devices designed to help eliminate static electricity. It does this by having a controlled low resistance: a metal mat would keep parts grounded but would short out exposed parts; an insulating mat would provide no ground reference and so would not provide grounding. Typical resistance is on the order of 10⁵ to 10⁸ ohms between points on the mat and to ground.^[10][11][12] The mat would need to be grounded (earthed). This is usually accomplished by plugging into the grounded line in an electrical outlet. It's important to discharge at a slow rate, therefore a resistor should be used in earthing the mat. The resistor, as well as allowing high-voltage charges to leak through to earth, also prevents a shock hazard when working with low-voltage parts. Some ground mats allow you to connect an antistatic wrist strap to them. Versions are designed for placement on both the floor and desk.

Antistatic wrist strap

Antistatic wrist strap connected to a bench mounted grounding socket

An antistatic wrist strap with crocodile clip.

An antistatic wrist strap, ESD wrist strap, or ground bracelet is an antistatic device used to safely ground a person working on very sensitive electronic equipment, to prevent the buildup of static electricity on their body, which can result in electrostatic discharge (ESD). It is used in the electronics industry by workers working on electronic devices which can be damaged by ESD, and also sometimes by people working around explosives, to prevent electric sparks which could set off an explosion. It consists of an elastic band of fabric with fine conductive fibers woven into it, attached to a wire with a clip on the end to connect it to a ground conductor. The fibers are usually made of carbon or carbon-filled rubber, and the strap is bound with a stainless steel clasp or plate. They are usually used in conjunction with an antistatic mat on the workbench, or a special static-dissipating plastic laminate on the workbench surface.

The wrist strap is usually worn on the nondominant hand (the left wrist for a right-handed person). It is connected to ground through a coiled retractable cable and 1 megohm resistor, which allows high-voltage charges to leak through but prevents a shock hazard when working with low-voltage parts. Where higher voltages are present, extra resistance (0.75 megohm per 250 V) is added in the path to ground to protect the wearer from excessive currents; this typically takes the form of a 4 megohm resistor in the coiled cable (or, more commonly, a 2 megohm resistor at each end).

Wrist straps designed for industrial use usually connect to earth bonding points, ground connections built into the workplace, via either a standard 4 mm plug or 10 mm press stud, whereas straps designed for consumer use often have a crocodile clip for the ground connection.

In addition to wrist straps, ankle and heel straps are used in industry to bleed away accumulated charge from a body. These devices are usually not tethered to earth ground, but instead incorporate high resistance in their construction, and work by dissipating electrical charge to special floor tiles. Such straps are used when workers need to be mobile in a work area and a grounding cable would get in the way. They are used particularly in an operating theatre, where oxygen or explosive anesthetic gases are used.

"Wireless" or "dissipative" wrist straps are available, which claim to protect against ESD without needing a ground wire, typically by air ionization or corona discharge. These are widely regarded as ineffective,^[13][14][15] if not fraudulent, and examples have been tested and shown not to work.^[16][17] Professional ESD standards all require wired wrist straps.^[13]

See also

• Electrostatic sensitive device
• Antistatic agent

References