Static electricity, formally referred to as electrostatic charge, is most commonly created by the contact and separation of materials in a process known as triboelectric charging. For example, when you walk across a carpeted floor, the interaction between the carpet and your shoes causes the carpet to lose electrons, making it positively charged, and your shoes to gain electrons, making them negatively charged. You feel a shock as the static electricity from your body discharges in an ESD event through a metal doorknob, which acts as a conductor.
The amount of electric charge created by triboelectric charging depends on many factors, including contact area, properties of the materials, how quickly the materials are separated, and relative humidity. Besides triboelectric charging, other less common sources of static electricity include induction, ion bombardment, and contact with another charged object.
Although static electricity is measured in coulombs, engineers often focus on evaluating the electrostatic potential between materials, which is measured in volts and depends on the electric charges, their spatial arrangement and the distance between the materials. As an example, someone walking across a carpet can generate 1500 V of static electricity in relative humidity (RH) of 65% to 90% and 35,000 V of static electricity in RH of 10% to 20%. In contrast, an electronic component that is electrostatic discharge sensitive (ESDS), such as a hard drive, can be damaged by a 10 V ESD event.
ESD Damage to Electronic Parts and Devices
Damage caused by an ESD event is classified as either catastrophic or latent. With catastrophic damage, the electronic device no longer functions. With latent damage, the electronic device continues to work following the ESD event, but it may become impaired over time or fail prematurely.
ESDS parts and devices are classified by their sensitivity to ESD damage using three models:
- Human-Body Model (HBM) for transfer of electrostatic charge from the human body to an ESDS device
- Machine Model (MM) for transfer of electrostatic charge from a charged conductive object, such as a metallic tool or fixture, to an ESDS device
- Charged Device Model (CDM) for transfer of electrostatic charge from an ESDS to a conductor, which can occur when static electricity accumulates on the ESDS device through handling or contact and separation with packaging materials, work surfaces, or machine surfaces
Because even small amounts of ESD can damage ESDS devices, ESD must be managed throughout the life of a product, from manufacturing and testing to shipping, handling and field service. Protection for ESDS devices includes using:
- Static-safe workstations
- Antistatic wrist straps
- Static-control garments
- Conductive or dissipative floor materials and footwear
- Antistatic paints
- Humidity control and air ionizers
- Static-protective bags
- ESD measuring tools, such as voltmeters and field meters, to monitor static charge levels
- Barcode readers for automated monitoring of devices and ESD events
Cognex Support for ESD Protection
Cognex DataMan ESD-safe handheld and fixed-mount barcode reader line is the broadest family of industrial barcode readers that combines ESD-safe plastics for all enclosure parts with the antistatic coating for all optical parts to prevent damage to components that are ESDS.
Cognex ESD-safe barcode readers meet ESD safety requirements according to IEC 61340-5-1:2016. This standard applies to activities that manufacture, process, assemble, install, package, label, service, test, inspect, transport or otherwise handle electrical or electronic parts, assemblies, and equipment with withstand voltages greater than or equal to 100 V HBM, 200 V CDM, and 35 V for isolated conductors.