ESD, or electrostatic discharge, comes in two forms: contact discharge and air discharge, with air discharge generally being more severe. Below are several protective measures derived from the ISO 10605-2008 standard ESD test setup, which the author has summarized to assist with ESD issues in electronic product design.
1. TVS Diodes
In electronic design, one or a group of TVS (transient voltage suppression) diodes are commonly placed at ports like low-speed signal lines, high-speed signal lines, and RF antennas. The diodes protect the circuits that follow by clamping voltage during avalanche breakdown.
2. MLCC Ceramic Capacitors
For cost-sensitive products, it is common to place MLCC (Multi-Layer Ceramic Capacitors) at I/O connectors or critical signal points. Connection leads are kept as short as possible to minimize their inductance.
3. Spark Gap Method
Note: This method is not recommended because it tends to accumulate carbon and has a short lifespan. As illustrated, ESD can generate an arc (similar to creepage, which becomes ineffective if the gap is too large) that discharges to the ground, dissipating the electrical energy.
4. Metal Shielding
When static electricity flows through metal conductors, the existing parasitic inductance at high frequencies can generate a changing electromagnetic field. This can induce noise voltage on signal routing near metal backplanes, causing system malfunctions. Shielding is a method commonly employed to resolve this issue.
5. PCB Stack-up Design
The key principle in PCB stack-up design is to ensure high-frequency signals have a complete reference ground plane and minimal loop area. The goals are twofold: firstly, having the power layer adjacent to the reference ground plane allows for rapid current discharge; secondly, a smaller loop area weakens the capacity of induced currents.
7. PCB Antistatic Ring
Around the perimeter of the PCB, a line without a solder mask is drawn and connected to the chassis where possible, with the loop opened to avoid the antenna effect. This antistatic ring is particularly evident in devices like automotive domain controllers, where the exposed copper around the PCB acts as a protective feature.
8. Grounding and Bonding
Grounding and bonding provide a low-impedance path for static electricity discharge and are essential for establishing a low-impedance current path and reducing parasitic inductance between different reference ground planes. This setup helps to maintain equipotential reference ground planes across various operating frequencies.
Conclusion
By carefully protecting the power and signal lines of circuits, one can effectively prevent ESD currents from penetrating the PCB, ensuring the reliability and integrity of electronic devices against ESD challenges.
