HDI = High Density Interconnections. A printed circuit board with small vias, so-called microvias (<0.15mm), and with a resolution of conductive tracks and insulation patterns equal to or less than 0.1mm.
A microvia is a relatively small hole (<0.15mm) through a thin dielectric (<0.10mm). After plating they connect conductive patterns through dielelectric. They are typically both outer surfaces in one or multiple layers. There are several technologies used, such as laser drilling, plasma etching and photo-mechanical to produce microvias.
Product consisting of multiple layers of electrical conductors separated from each other by insulating supports and fabricated into a solid mass. Interlayer connections are used to establish continuity between various conductor patterns.
Printed circuit board (also known as a PWB, printed wiring board). A PCB is the principal sub-assembly in virtually all electronic-based products. The board itself represents the basic platform used to mount and then interconnect the various components, including microprocessors and other integrated circuits, resistors, capacitors and other standard or specialized devices. It typically consists of an insulating (non-conductive) substrate which is covered with metallic copper circuit patterns that provide the electrical interconnections for the mounted components.