Multilayer PCB Stack-up Planning
In order to properly design the multilayer printed circuit board, one should carefully consider the design of the layer stack-up. The need for planning the layer stack-up lies, on the one hand, in the technological capabilities of the production, and on another hand, in the requirements for electrical properties of the printed circuit board, which includes impedance control, signal integrity, noise immunity and electromagnetic compatibility. Another important thing is the optimization of the layers stack-up and interconnections from the point of view of costs. The structure of printed circuit boards can differ greatly, so the production costs will vary significantly.
One needs to remember that the multilayer PCB consists of cores, prepregs and copper foils. Thus, we have a huge amount of materials in stock for your choice.
A good stack-up can be very effective in reducing radiation from the loops on the PCB as well as the cables attached to it.
Four factors are important with respect to board stack-up:
1. The number of layers,
2. The number and types of the plane (power and/or ground) used
3. The order or sequence of the layers
4. The spacing between the layers.
Usually, not much consideration is given except for the number of layers. In many cases, the other three factors are of equal importance.
For the settlement of the number of layers, the following should be considered:
1. The number of signals to be routed and costs
3. Class A or Class B emission requirements
Usually, only the first item is considered. In reality, all the items are of critical importance and should be considered equally. If an optimum design is to be achieved within a minimum amount of time and at the lowest cost, the last item can be especially important and should not be ignored.
The above paragraph should not be construed to mean that you can’t do a good EMC design on a four-layer or six-layer board and the truth is that you can. It only indicates that all the objectives cannot be met simultaneously and some compromises are inevitable. Since all the desired EMC objectives can be met with an eight-layer board, there is no need to adopt boards with more than eight layers if you don't need to accommodate additional signal routing layers.
The standard pooling thickness for multilayer PCBs is 1.55mm.
Here are some examples of multilayer PCB stack up.