As we mentioned before, there are five points that you should try to achieve when design multi-layer boards. In this article, we will tell you more about why it is important to achieve these five points.
Often we are faced with the choice between close signal/plane coupling and close power plane/ground plane coupling. With normal PCB construction techniques, there is not sufficient inter-plane capacitance between the adjacent power and ground planes to provide adequate decoupling below about 500 MHz. The decoupling, therefore, will have to be taken care of by other means and we should usually opt for tight coupling between the signal and the current return plane. The advantages of tight coupling between the signal layers and the current return planes will more than outweigh the disadvantage caused by the slight loss in interplane capacitance.
An eight-layer board is the fewest number of layers that can be used to achieve all five points. On four and six layer board some points will have to be compromised. Under those conditions you will have to determine which points are the most important to the design at hand.
Another point, from a mechanical point of view, is to have the cross section of the board symmetrical in order to prevent warping. For example, on an eight-layer board if layer two is a plane, then layer seven should also be a plane. Therefore, all the configurations presented here use symmetrical, or balanced, construction. If a non-symmetrical, or unbalanced, construction is allowed additional stack-up configurations are possible.