Radiation from printed circuit board edge structures

Abstract

One of the primary modes of radiation from printed circuit boards are due to emissions along the edges of printed circuit boards (PCBs). The problem is exasperated by PCB resonance effects. A variety of edge treatment techniques are used to minimize these two effects including adding a string of vias around the periphery of the PCB to short out ground planes (fencing), and pulling back power planes from the edges by a factor of 20 times the plane-to-plane separation distance (20H role). Both methods have drawbacks that compromise their overall effectiveness, especially when applied to more complex multi-layer ground-power-signal PCB structures commonly found in today's high performance computing devices. This paper describes how vias excite the edges, and the relative effectiveness of fencing and the 20H rule.