Meaning
The electrical resistance to high-frequency currents across the joint where two parts of a shielding enclosure meet determines the overall effectiveness of the electromagnetic barrier. Maintaining a low seam impedance prevents electromagnetic energy from escaping the enclosure and causing interference. This important parameter is critical in the design of multi-part enclosures where metal-to-metal contact is interrupted by assembly joints.
The impedance is bounded by the contact area, the pressure applied to the joint, and the conductivity of the mating surfaces.
Contact Resistance
Direct metal contact must be maintained across the entire length of the joint to keep the resistance as low as possible. Any non-conductive coatings or oxides on the surface will increase the impedance. This can turn a protective seam into an active slot antenna.
Gasket Application
Conductive gaskets are often compressed within the seam to bridge any physical gaps and ensure a continuous electrical path. These gaskets are made of conductive elastomers or metal fingerstock that can conform to surface irregularities. The amount of compression must be controlled to prevent damage to the gasket while ensuring a tight seal.
This assembly technique is used in high-frequency devices where even microscopic gaps can allow signal leakage.
Joint Design
Overlapping joint geometries like tongue-and-groove or rebate joints help increase the path length for escaping signals. This design increases the capacitive coupling across the seam. It reduces the dependency on high mechanical compression to achieve low impedance.
