Digital Circuit Boards: Mach 1 GHz

4.16 Traces Through Conducting Planes

When a trace crosses through a single conducting plane, the field associated with any signal must cross to the other side of the plane. The only path for the field is the hole (via) used by the trace. This transition will have very little effect on the transmission, as the characteristic impedance of the path will be fairly constant. When a trace crosses more than one conducting layer, the problem is more complex. The problem can best be viewed by locating the path the return current must take. This path will usually be through a nearby via that interconnects the two planes. The result is the field pattern that spreads out between the conducting planes, which means that there is a poorly defined stub associated with the transition of signal between layers. This spreading of fields can result in wave reflections and cross talk. To limit the problem, the vias used for the transition should be close together so that the space used by the fields is limited. Ideally, the path should be coaxial, but this is not practical with simple vias.

A trace that crosses more than one layer might be routed near power on one layer and near ground on another layer. The return path for signal current must cross in the power and ground plane space and through the nearest decoupling capacitor. The transmission line character of this capacitor is in series with the return path. This capacitor supplies field energy for many signals and can be a source of cross coupling. ...