The electronics industry have been updating a lot since the end of 1900s, therefore as the fundamental part of all electronics, PCB printing industry must be improved in order to achieve customer's goals. As nowadays electronics trends are small, light and thin, the PCB industry have developed flex PCB (FPC), Rigid-Flex PCB, and Blind&Buried PCB, etc.
While we get involved in blind&buried PCB, we must start with regular multi-layers PCB. Traditional multi-layers PCB includes inner layer circuits and outer layer circuits, by drilling and fill copper in drills to connect the circuits in different layers. As the density of circuits grows more, and the updating of components packaging, PCB designer must design thinner circuits, and the diameter of DIP packaging reduced from 1mm to SMD 0.6mm and even below to 0.4mm to put must components in restricted PCB area. But after all, this isn't enough, then blind&buried tech comes.
Drill from one outer layer and connect with one or more inner layers.
The via connect between inner layers. Invisible after pressing procedure. It doesn't cost the area surface of PCB which is as its name: buried inside the circuit board.