Electromagnetic Field Suppression in Multilayer Printed Circuit Power Plane Stackups
Thin dielectric power plane pairs suppress high frequency cavity modes and radiated emissions by lowering target impedance and shifting resonant frequencies.
Stitching via Fence
Shielding Structure
Vertical interconnect structures arranged in a linear or grid pattern create an electromagnetic shield around sensitive signal traces or board edges. Constructing a stitching via fence involves a series of plated through holes that connect multiple ground planes together to form a conductive wall. This structure prevents electromagnetic fields from escaping the circuit or entering sensitive regions.
It is a standard requirement for high speed designs that must meet strict emissions standards. The spacing between the vias is calculated to be smaller than a fraction of the shortest wavelength present in the system.
Isolation Purpose
Implementation of a fence around the perimeter of a board reduces edge radiation by shorting the parallel plate modes. It also provides a clear return path for signals that transition between different layers. If the fence is incomplete, the gaps can act as slot antennas that increase interference.
High frequency circuits often require multiple rows of vias to provide sufficient isolation between different functional blocks. This technique effectively contains the electromagnetic energy within a defined volume of the dielectric.
Physical Constraint
Fabrication of the fence adds to the drill count and can impact the mechanical strength of the board. Proper clearance must be maintained between the vias and the board edge to prevent copper exposure during the routing process. Electrical testing confirms the continuity of the fence to the common ground.