Benefits of Multilayer Board

The first basic advantage of a multilayer board over a single layer or a two layer board is that it provides more routing space. Experiences PCB designers know how easy is it to drop down a via for power and ground nets. It would not have been easy without defining power and ground planes. If a board is not very dense and does not has BGA ICs, you can probably do with a 4 Layer PCB. If it has a BGA of pitch greater than or equal to 0.8 mm pitch you can probably do it with 6 layers, assuming that the power nets are not many and BGA is not very big. For BGAs of pitch 0.65 mm or lower and many power nets 8 layers or more may be required. 

This is just a general guidance. We would like to minimize the number of layers to keep the cost of PCB down. However, the extra layers in the multilayer PCB fulfills some important High Speed PCB design requirements. We must ensure that these requirements are fulfilled. 

Power Plane for Impedance Requirements

Some high speed signals need their characteristic impedance to have a certain specified value. For the characteristic impedance to have a fixed value like 50 Ohms, it needs to refer to a metallic place (power or ground) at a certain distance from it. The value of the characteristic impedance depends upon the distance of the power plane from the trace (besides trace width and relative permittivity). We must place the power plane beneath the signal requiring a certain defined value for characteristic impedance. 

Power Plane for Power Supply Requirement

The second important function of a power plane is that it provides a return path for the power signals. Without power planes the power signals would have to travel to the thin traces. The traces have inductance, because of which fast returning current may induce noise in the power system. The power and ground planes placed very close to each other provide a very low impedance ac path for very high frequency noises on the power supply rail. This helps mitigate the high frequency power supply noise issue in the board.

Tight Coupling and Crosstalk requirements

Keeping the high speed signals close to the power and ground layers keeps the electromagnetic signals tightly coupled. Keeping high speed signals in a stripline fulfills two important functions – first it reduces the crosstalk. Secondly, it shields any potential radiation. It is therefore, recommended that you rout your high speed signals in inner layers.

Power Plane and Stitching Capacitors

It is often happens that we have many power islands in a single power plane. In this scenario we must be careful while routing the controlled impedance traces which pass across the power islands junctions. We must place the stitching capacitors at the junctions of the power islands. The stitching capacitors provide ac return path and keeps the impedance constant during the flight of the controlled impedance nets.