Electrostatic Discharge (ESD): The Hidden Threat to Electronics

It's possible your next electronic device could be deployed in an area where it is exposed to high voltages or static electricity. In these cases, a system’s vulnerability to ESD should be determined through testing and simulation. Electrostatic Discharge (ESD) is the sudden transfer of static electricity from one object to another. This phenomenon occurs when there is a difference in electrical potential between two surfaces, leading to an abrupt discharge of energy. The charge can be accumulated parasitically on a body and when transferred or by grounding when the circuit is completed a huge voltage spike with very low current appears on the other body which feels like a little shock.

There are also components that may be needed to protect a system from ESD and to ensure the system can withstand high voltage pulses. ESD can cause significant damage to sensitive electronic components, leading to product failures, reduced lifespan, and increased manufacturing costs. Know more about electronics and PCB design, see our recent blog on impedance control in PCBs.

• Direct physical contact between a charged object and an electronic device, resulting in an immediate transfer of charge.
• The transfer of charge through ionized air, often resulting in visible sparks.
• When an electrostatic field induces unwanted currents in nearby circuits without direct contact.
• Generated by friction when materials such as plastic, fabric, or glass rub against each other.

The Physics behind Electrostatic Discharge (ESD)?

Electrostatic discharge (ESD) occurs when two objects with different charges get close enough, or charged enough, to break down (ionize) the dielectric between them. For consumer products, ESD and dielectric breakdown in air usually occurs when the electric field between two points is greater than 40 kV/cm. Things like air pressure, temperature, and humidity will affect this electric field strength. For example, high air humidity in some environments can cause the air to be more conductive, which will dissipate some charge and increase the voltage required to create an ESD event.

A common misconception is that a fixed voltage is required to trigger ESD. In reality, the voltage necessary for ESD depends on the potential difference between two points and the distance separating them. This is why PCBs adhere to clearance limits between conductors (creepage and clearance) as defined by the IPC-2221 standards.

Effect of ESD on PCB and Electronic Components?

Any printed circuit board may be subject to an ESD if it’s touched or comes close enough to people, packaging, cables, or any other object that might produce a large potential difference. When this voltage difference is large enough, there will be a conductive path for current through any component, creating a massive current pulse. This current pulse can be higher than the required current ranting of any onboard device. At extreme field strengths and developed currents, the PCB can be damaged and components can be destroyed. ESD can have devastating effects on electronics, such as:

• Immediate Failure: The component becomes non-functional immediately.
• Latent Damage: Partial degradation occurs, making the device prone to future failures.
• Parameter Shift: Changes in electrical characteristics of the device, reducing performance and reliability.

How to Implement ESD Protection?

The goal in implementing ESD protection on a PCB is to prevent an ESD current from flowing into your circuits and instead divert it into ground through a low-reactance connection. The ESD protection circuit or component should only respond when an ESD event occurs, and it must do this very quickly in order to divert the ESD pulse within its rise time. In general the ESD protection is implemented to the connectors because they are more vulnerable to ESD, as they are the first point of contact.

ESD Protection Diode:

An ESD protection diode is a type of Zener diode. When a diode is reverse-biased, little current flows from cathode to anode. However, when the reverse bias exceeds a certain point (called the reverse breakdown voltage), the reverse current suddenly increases. As the reverse bias is increased, the diode reaches a region of a constant voltage regardless of the current flowing through the diode. The breakdown voltage (Zener voltage) characteristics of Zener diodes are utilized to compose a constant-voltage regulator and suppress surge voltage. Zener voltage regulator diodes are used to maintain a constant voltage whereas ESD protection diodes are used to absorb ESD energy in order to protect an electronic circuit. This should be placed very close to the source so that current can be immediately shunted to ground.

Popular Options: Littelfuse SMAJ, STMicroelectronics SMAJ series.

ESD protection for USB, Ethernet, I2C, RS-485, RS-232, and other digital interfaces will generally use a TVS diode between a connector and the IC containing that interface. Your component datasheets will often have some guidelines you should follow to ensure ESD protection.

TVS Diode Characteristics:

The most common circuit protection device is a transient voltage suppression (TVS) diode. These can be bidirectional (back-to-back) diodes, or they can be unidirectional with current flow only allowed in one direction. A Zener diode can also be used as an ESD protection component and it provides an equivalent function as a unidirectional TVS diode.

When an ESD pulse is received, the high voltage of the pulse quickly drives a protection diode deep into reverse bias until reverse breakdown occurs. The diode becomes conducting and there is now a short circuit across the terminals of the protected component. This diverts the received pulse to ground and prevents current from reaching the protected component.

Other Steps to Prevent ESD in a Circuit:

To ensure ESD protection at the PCB level, designers must implement the following strategies:

1. Ground Planes and Shielding: Large copper ground planes help in dissipating ESD energy efficiently.

2. Proper Trace Routing: Keeping high-speed signal traces away from edges and using guard traces to reduce susceptibility.

3. Use of Ferrite Beads and Filters: Helps in suppressing high-frequency noise generated by ESD events.

Conclusion:

Electrostatic Discharge (ESD) is a critical concern in electronic design, as it can lead to immediate or latent failures in sensitive components. Understanding how ESD occurs, its effects on PCBs, and the importance of implementing proper protection measures is essential for ensuring the reliability and longevity of electronic devices. By incorporating ESD protection diodes, designing PCBs with proper grounding and shielding, and following industry standards such as IPC-2221, engineers can effectively mitigate ESD risks. As technology advances and electronic devices become more sophisticated, taking proactive steps to prevent ESD will remain a crucial aspect of hardware design and manufacturing.