EMC Bonding: Types and Behavior at RF & Microwave Frequencies

EMC bonding plays a critical role in managing electromagnetic interference, especially at microwave frequencies. This guide examines various bonding types and their unique behaviors in high frequency environments.

Definition: Electrical bonding is a process where parts of a circuit or sub-modules of a system are connected together electrically, typically using low resistance media.

Function: The main function of a bond is to create a homogeneous structure that allows for a smooth flow of RF current. The bond should minimize resistance to current flow, preventing any potential differences at the joint or crossover point.

To comply with EMC (Electromagnetic Compatibility) specifications, all components of a module or all modules of a system are connected to a common chassis or reference ground via a low impedance bond. This type of bond, specifically designed to meet EMC specifications, is known as an EMC bond, and the process is called EMC bonding. The primary goal is to prevent voltage differences between the joined parts.

Bond Behavior at RF or Microwave Frequency

The figure above (Figure 1) illustrates the behavior of a bond at Radio Frequencies (RF). Due to parasitic effects, the bond is no longer purely resistive at RF. It exhibits series inductance and parallel capacitance in addition to resistance. This circuit acts as a parallel resonant circuit, providing high impedance at higher frequencies. Moreover, the skin effect at high frequencies causes current to flow via the outer periphery of the conductor, resulting in very high resistance.

Bonding Types

To perform bonding at DC or low frequencies (i.e., 50 Hz or 60 Hz), a low resistance joint can be adequate. This type of bond is often created using bolts and star washers at the contact points between two metal walls. However, this is not suitable at high RF/Microwave frequencies due to high resistance and high inductive reactance. This generates strong RF fields due to harmonics and spurious emissions.

To mitigate these issues, the bonding process is often divided into mechanical and electrical bonding. Mechanical bonding ensures the parts being joined have appropriate robust strength. Electrical bonding ensures low impedance connectivity between parts to make them electrically robust.

Bonds are classified into two main types: direct bond and indirect bond.

Direct Bond

In this bonding type, specific areas of members to be joined are placed in direct contact by permanent or semi-permanent bonds.

• Permanent bonds are typically carried out using welding, soldering, or brazing.
• Semi-permanent bonds are carried out using bolted connections.

For a satisfactory direct bond with bolted connections, the bolt/screw serves as a fastener, and pressure of about 90 to 100 Kg/Cm² needs to be maintained.

Indirect Bond

This type of bonding is used in applications where metal-to-metal contacts are not reliable, such as where parts are frequently removed, parts use dissimilar metal types, parts are exposed to corrosion, or parts will have relative motion (like hinges). In this type of bond, in addition to the primary bond achieved with a mechanical joint, an indirect bond is created using jumper wires.

Conclusion

Effective EMC bonding minimizes interference at microwave and RF frequencies, ensuring optimal device performance and regulatory compliance. Understanding bonding behaviors is key to robust system design.