Why Use Multi-Layer Boards?
Multi-layer boards using ground or power planes provide significant reduction in radiated emission over two layer PCBs.
Boards containing planes are much better than those without planes for the following reasons: a. They allow signals to be routed in a microstrip configuration. These configurations are controlled impedance transmission lines with much less radiation than the random traces used on a two-layer board. b. The ground plane decreases the ground impedance and the ground noise significantly.
When using multi-layer boards there are five points that you should try to achieve. They are:
a.A signal layer should always be adjacent to a plane.
b.Signal layers should be tightly close to their adjacent planes.
c.Power and ground planes should be closely coupled together.
d.High-speed signals should be routed on buried layers located between planes.
e.Multiple ground planes are very advantageous, since they will lower the ground impedance of the board and reduce the common-mode radiation.