The abbreviation HDI stands for High Density Interconnect and refers to multilayer PCBs with four or more layers, which are built up sequentially in several pressing processes (Sequential Build Up, abbreviated: SBU). With this structure, high wiring and placement densities for high-pole active components can be realized on an electronic assembly.
A high wiring density in conjunction with a very high number of microvia holes, or microvias for short, is typical of an HDI PCB. We speak of HDI with conductor structures from 150 µm and finer and microvias with a drill hole diameter <0.2 mm. Modern production technology enables layouts from a line/space of 75 µm. The filling technologies of holes, e.g. microvia copper filling, plugs or resin filling, play a decisive role in the selection of an optimal rewiring strategy.
The advantages are:
Maximum packing densities, miniaturization and the best possible use of space,
Maximum freedom in the placement of components,
Shortening of conductor lengths and improvement of the EMC behavior of the module,
Simplified unbundling of the PCB layout, reduction in the number of layers,
Tight, one-sided assembly and associated reduction in assembly costs
English 
German