The high density interconnect (HDI), is a nowadays demand on printed circuit boards, can be done with technology sequential multilayer build up (SBU).
This technology allows to add sequentially inner core layers. Dielectric (pre-preg) with copper foil are added on both sides of the inner core and build-up sequentially such structure like a normal multilayer, with holes running through it completely (through hole) or blind and / or buried holes performed with depth control or laser drilling to the layer required.
Nowadays CIPSA is prepared to make this SBU technology up to 4 + N + 4, that means we can add up to 4 layers pressed to a central core.