Multilayer boards, as the name implies, are printed circuit boards made up of 3 or more layers, generally in even numbers (4, 6, 8, 10, etc.),  which normally have ground layers (VCC, GND) and signal and interconnection layers.
Their build-up is generally specified by the customer, otherwise they are made according to ICS standards.       

One of the greatest difficulties of this process is drilling, whereby the bump contacts of the inner layers have to be centred without direct external references.

Another important factor is the printing of the inner layers: any non-conformities, not detected during this phase, result in final rejection.
For this reason, the inner layers are tested with the Orion 602 optical tester.

The materials for construction of the multilayer circuit boards are stored in cold rooms at a controlled temperature and humidity for a maximum of 6 months of shelf life.