Effectiveness of PCB perimeter Via fencing: Radially propagating EMC emissions reduction technique

Via fencing around the periphery of a printed circuit board is a practice to attempt to eliminate radially propagated energy from the edge of a board. This study comparatively analyzed via fenced boards against unfenced boards as well as investigated other emission related design variables including trace distance from the PCB edge, termination loads, and via fence spacing. The S2i measurements were taken with a network analyzer and a horn antenna with the PCB trace as the source. Frequency sweeps from 1–8.5GHz showed that via fencing attenuated as much as 21dB at some frequencies, concluding that via fencing was definitively beneficial. Trace distances to the edge of the board showed mixed results at lower frequencies, but gave higher emissions with smaller distances to the edge at higher frequencies. Varying loads yielded results showing emissions for given loads were highly dependent on frequency with the exception of the dominant short-circuit load. Via fence spacing was shown to be significant, but only loosely followed the 1/8^th wavelength rule of thumb. This study proved to be very important for the electronics industry as EMC regulations are increasingly harder to meet with shrinking die process technologies and rising clock frequencies.