Common Terms in the Field of Electrostatics

Static Electricity

Static electricity, as the name suggests, refers to static electricity. When materials are subjected to movement, friction, or separation, electrons are transferred, generating an electric charge. These materials, such as plastics, fiberglass, rubber, and textiles, are the source of static voltage. Under certain conditions, static charges can reach as high as 30,000 to 40,000 volts.

When this happens on insulating materials, such as plastics, charge typically accumulates in a localized area. When these insulating materials come into contact with a different object, such as a person or microcircuit, and a sufficient potential difference exists between the two, this static charge can trigger an arc or spark discharge.

If a person experiences electrostatic discharge (ESD), they may experience a shock ranging from mild to painful. In extreme cases, ESD or arc flash can even be fatal. Such sparks are particularly dangerous in environments containing flammable liquids, solids, or gases, such as hospital operating rooms or explosive device components.

Even static discharges as low as 20V can damage or destroy microelectronic devices. Because humans are often the primary source of ESD in industrial production, particularly during manufacturing and assembly, they can often cause irreversible damage to sensitive electronic components. The consequences of a discharge in ESD-sensitive electrical equipment can range from false readings to equipment downtime, even requiring expensive repairs or replacement of the entire component.

Electrostatic Discharge

ESD refers to the sudden flow of current between two charged objects due to contact, short circuit or breakdown. The accumulation of static electricity can be caused by friction charging or electrostatic induction.

The electric shock felt when a person touches a door handle is electrostatic discharge.

Anti-static

Anti-static refers to measures taken to prevent the accumulation of static electricity. This is usually achieved by maintaining adequate moisture or other conductive measures, such as textiles, waxes, and polishes.

Dissipative

Compared to conductive materials, the charge in a dissipative material flows to the ground in a more controlled manner and at a slower rate. Dissipative materials have a surface resistivity between 1 x 10^5 Ω/sq and 1 x 10^12 Ω/sq, or a volume resistivity between 1 x 10^4 Ω-cm and 1 x 10^11 Ω-cm.

Conductive Materials

Conductive materials have low electrical resistance, allowing electrons to easily pass through their surface or interior to the ground or other conductive objects. Conductive materials typically have a surface resistivity less than 1 x 10^5 Ω/sq, or a volume resistivity less than 1 x 10^4 Ω-cm.

Insulating Materials

Insulating materials prevent or restrict the flow of electrons across their surface or through their interior. These materials have high electrical resistance and are difficult to ground, causing static charges to persist for extended periods. Insulating materials are defined as having a surface resistivity of at least 1 x 10^12 Ω/sq or a volume resistivity of at least 1 x 10^11 Ω-cm.

Conductive Materials and Insulating Materials

Materials used for protection and prevention of electrostatic discharge (ESD) can be divided into three categories, differentiated by their range of conductivity.

• Anti-static: Resistivity is typically between 10^9 and 10^12 ohms per square. Initial static charge is suppressed and can be a surface coating or full fill.
• Static dissipative: Resistivity is typically between 10^6 and 10^9 ohms per square. Low or no initial charge, preventing discharge from human contact. Can be surface-coated or fully filled.
• Conductive materials: Resistivity is typically between 10^3 and 10^6 ohms per square. They have no initial charge but provide a path for charge to dissipate. They often contain carbon particles or carbon fibers.

Surface Resistivity

For thermoplastic materials primarily used to dissipate static electricity, surface resistivity is a common method for verifying their dissipative capabilities. A widely used test method, based on ASTM D257, uses a megohmmeter to measure the resistance between two electrodes applied to the surface under load. Due to the heterogeneity of composite thermoplastics, measurements are typically made using electrodes rather than a point probe. Maintaining good contact between the sample and the electrodes is crucial, and considerable pressure may be required. The resistance reading is then converted to resistivity to calculate electrode size. Surface resistivity is calculated as the resistance multiplied by the electrode circumference divided by the gap distance, and is expressed in ohms per square.

Volume Resistivity

Megohmmeter with a weight can detect surface resistance and volume resistance

Volume resistivity is used to assess the dispersion of conductive additives throughout a polymer matrix. It can also correlate with the EMI/RFI shielding effectiveness of certain conductive fillers. The test method for volume resistivity is similar to surface resistivity, but the electrodes are placed on opposite sides of the test specimen. ASTM D257 also covers volume resistivity measurement, deriving the resistivity value from the resistance reading using a conversion factor based on electrode size and part thickness. Volume resistivity is calculated as the resistance multiplied by the surface area (cm²) divided by the part’s thickness (cm). Units are ohm-cm.

Static sensitive symbol

The static sensitive symbol, a hand with a slash through it within a triangle, is intended to indicate that electrical or electronic equipment or components are susceptible to damage from ESD (electrostatic discharge) events. This symbol is used to identify ESDS (electrostatic sensitive devices) and reminds workers to ground themselves when unpacking or handling these items. This symbol is also known as the static warning sign.

ESD Symbol

The ESD symbol differs from the static sensitive symbol in that it adds a circular arc outside the triangle and removes the hand and the slash from the triangle. This symbol is primarily used to identify items designed to provide ESD protection, such as anti-static packaging, ESD protective clothing, and personal grounding equipment. It can also be used to identify items designed to protect against or avoid generating static electricity, such as ESD-protected workstations, trash cans, and chairs.

Static sensitive symbol and static protection symbol

ESD Common Point Ground Symbol

This symbol represents an electrostatic common ground point, defined as “a grounding device connecting two or more conductors,” according to the ANSI/ESD-S6.1 standard. The symbol consists of a bold outer circle with the words “ESD COMMON POINT GROUND” in bold text inside. The inner circle contains two contrasting bold circles and a dot, which can fill the center or extend to the center of a snap, plug, or other fastener.