According to the classics, two-layer PCBs consist of several elements
— a substrate of fiberglass, which is covered on both sides with foil, a layer of dielectric solder mask that protects the board from external physical impact and short circuits.
These components, plus the electrical components soldered on top, make up the thickness of PCB. To put it simply. it’s one plane and everything that’s on it.
The development of Stackup
The development of Stackup (layer structure) of PCB consists precisely in deciding how each layer will be located and how the components will be located on it.
An interesting fact is that there is no correct shape of layers and their stacking, as in the above example with a sandwich, and it all comes down to finding the best solution for the specific task of the project. Of course, manufacturers and designers have their own developments and solutions on how to arrange all the layers in the best way.
While multilayer PCBs resemble a cake or sandwich with a variety of fillings in structure. And, just as a sandwich has assembly rules – do not put wet sauce after bread, make a layer of salad, etc. – so a PCB has similar rules that make the whole device "tasty" and usable.
It is also important to complete the documentation in a clear and accessible language for everyone, so that when it comes from the designer to the manufacturer of PCBs, there are no difficulties. The easiest way to solve this problem is to find one company that can both design and manufacture PCBs.