The PCBs generally categorized as single layer, double layer and multilayer PCBs that actually describe the number of conductive layers or copper layers in a PCB. The PCBs with more than two layers such as 4, 6 and 8 etc., are known as multilayer. In a multilayer the top and bottom layer are called the outer layers and the layers within these two are called inner layers. The layer count roughly defines the complexity of designed circuit, total number of pins and density of signals. A simple PCB is an insulator sheet of FR-4 material with copper sheet on at least one side. As the circuits are becoming more complex, dense SMD chip packages, operating at high frequencies and multiple power supply rails the PCB design and fabrication is becoming more and more complex. The circuits are laid out on two and higher layer count PCBs. The multilayer PCBs are more costly compared to double layer PCBs.
A multilayer PCB is not only to increase the number of routing layers to make routing easy. There are many factors that should be considered during multilayer PCB design that ultimately define the number of layers, placement of ground-plane or power-plane layer, high frequency and low frequency signal layers and the issue related to signal integrity and power integrity etc. The proper positioning of type of layer is called the layer stackup.
In layer stackup other than the PCB layers order, substrate material, copper thicknesses are also considered. There are different layer stackups for different types of circuit designs. A good planned PCB stackup ensures best performance of the PCB such as decreasing the electromagnetic interferences, signals crosstalk, coupling, overshoot and undershoot, and signal dissipation etc. The design to be completed and qualified first time means that it can reduce costs and the design cycle time significantly. It is possible if the signal integrity issues are eliminated before they arise.
Oher than signal layers, the power planes do also play very important role in a successful product development. The signals either digital or analog are possible to route through microstriplines or striplines that reduce crosstalk and consequently improve signal integrity. The low frequency signals are routed on the inner layers whereas the high frequency signals are routed on the outer layers. It is a good practice to place ground plane layer adjacent to each signal layer, however, to reduce layer stackup and manufacturing cost the ground layer is placed after each two signal layers. The power planes should also be adjacent with the ground planes that make tight coupling. The power planes are split into more than one part in case of multiple power supply rails. Normally, the PCB thickness is 1.6mm but it is difficult to maintain 1.6mm thickness in case of more than 12 layers PCB.
In the figure 1 below two 8-layer PCB stackup schemes are shown that emphasize over the changing order of layers. In figure 2 10-layer PCB stackup scheme is depicted.