Electrostatic discharge

Sudden flow of electric current between two electrically charged objects by contact

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Electrostatic discharge (ESD) is a sudden flow of electric current between two differently charged objects when they come close or when the dielectric between them breaks down. This often creates a visible spark indicative of static electricity interactions.

ESD can produce impressive electric sparks, such as lightning (which is a large-scale ESD event), but also cause less noticeable yet harmful discharges. These can be strong enough to damage sensitive electronic equipment without making any visible or audible sign. Electric sparks require a field strength above roughly 4 × 10⁶ V/m in air, a condition prevalent in lightning strikes. Other types of ESD include corona discharge from sharp electrodes or brush discharge from blunt ones.

ESD can result in harmful consequences for industry, like explosions in gas, fuel vapor, and coal dust environments, or the failure of solid-state electronic components. High voltages can cause permanent damage to items like integrated circuits. Consequently, electronics manufacturers create electrostatic protective areas, free from static, to mitigate these risks. Preventive measures include using antistatic materials, grounding workers, and controlling humidity levels.

ESD simulators, such as those using human body models or charged device models, may be employed to test the resilience of electronic devices.

Causes

Static electricity is one of the main causes of ESD events, often generated through tribocharging—the separation of electric charges when two materials are in contact and then separated. This occurs in everyday situations like walking on a rug or rubbing a balloon against a sweater. In all these cases, breaking contact generates tribocharging, creating an electrical potential difference that can lead to an ESD event.

Another cause of ESD damage is electrostatic induction, which happens when a charged object is near a grounded, conductive object. This creates an electrostatic field, redistributing electrical charges on the other object's surface. If this object touches a conductive path, an ESD event can occur. For example, charged regions on the surfaces of styrofoam cups can induce potential on nearby ESD-sensitive components, and ESD might occur when a metallic tool touches the component.

Energetic charged particles striking an object can also cause ESD by increasing surface and deep charging, a known risk for most spacecraft.

Types

ESD events vary in complexity and intensity. The electric spark, a visible effect of ESD, occurs when a strong electric field ionizes the air, creating a conductive channel. ESD can be an unpleasant surprise for people, but it's particularly damaging to electronic components, leading to malfunctions and failures, and can even cause fires or explosions in hazardous environments. Even invisible discharges can compromise delicate electronics. Some components can be damaged by discharges as low as 30 V.

Cable discharge events (CDEs) occur when connecting electrical cables to a device, posing another ESD risk.

Sparks

A spark is triggered when the electric field strength exceeds approximately 4–30 kV/cm—above the dielectric field strength of air. This rapid increase in free electrons and ions causes the air to temporarily become an electrical conductor, a process known as dielectric breakdown.

Lightning is the most well-known natural example of a spark, where the electric potential between a cloud and the ground, or between clouds, is typically hundreds of millions of volts. The resulting current causes a significant transfer of energy. Sparking can occur in air during electrostatic discharges from charged objects, even at just 380 V. In the atmosphere, an ESD event can split diatomic oxygen molecules, forming ozone, which is unstable, or nitrogen oxides, both of which are toxic. Ozone is used in water purification and attacks organic matter by ozonolysis.

Sparks can ignite combustible environments, leading to explosions from a tiny electrostatic discharge.

Damage Prevention in Electronics

Many electronic components, especially integrated circuits and microchips, can be damaged by ESD. Protecting these sensitive components during and after manufacture, shipping, and device assembly is crucial. Effective ESD control often centers around proper grounding.

Protection During Manufacturing

In manufacturing, preventing ESD involves creating Electrostatic Discharge Protected Areas (EPA), which range from small workstations to large manufacturing areas. EPA principles include avoiding highly-charging materials, grounding conductive and dissipative materials, and preventing charge build-up on sensitive electronics. International standards from organizations like the IEC and ANSI define these practices.

Within an EPA, prevention measures may include using ESD-safe packaging materials, conductive filaments on worker garments, wrist and foot straps to prevent high voltage accumulation, antistatic mats, and humidity control. Ionizers are used when insulative materials can't be grounded, neutralizing charged surface regions.

Manufacturers and users must take precautions to avoid ESD, including built-in prevention measures in devices and using external protection components.

Due to the nature of electronics, complete prevention of electrostatic charging during handling is impossible. Most ESD-sensitive components are small and handled by automated equipment, necessitating precautions to prevent ESD when these components come into contact with equipment surfaces or conductive parts. Using static dissipative materials, which conduct electricity slowly, effectively manages ESD risk. These materials are grounded to ensure any static discharge happens gradually, protecting the internal structure of silicon circuits.

Protection During Transit

Sensitive devices need protection during shipping and storage to minimize static build-up and discharge. Packaging materials control surface resistance and triboelectric charging through rubbing, sometimes incorporating electrostatic or electromagnetic shielding. For example, semiconductor devices and computer components are often shipped in antistatic bags made of partially conductive plastic, acting as Faraday cages.

Simulation and Testing for Electronic Devices

Testing electronic devices for ESD susceptibility from human contact often involves an ESD Simulator with a human body model (HBM). This model, defined in standards like JEDEC 22-A114-B and MIL-STD-883 Method 3015, uses a capacitor and resistor to simulate human touch. The IEC/EN 61000-4-2 test specification applies to Information Technology Equipment in the EU.

A charged device model (CDM) test simulates ESD when a device with an electrostatic charge discharges due to metal contact, the most common type of ESD in electronic devices manufacturing, defined in the JEDEC standards.

Other standardized ESD tests include the machine model (MM) and transmission line pulse (TLP).

See Also

References

Essential Guide to Controlling ESD (Electrostatic Discharge)