The abbreviation HDI stands for High Density Interconnect and refers to multilayer PCBs with four or more layers that are built up sequentially in several press cycles (Sequential Build Up, abbreviated: SBU). With this structure, high wiring and assembly densities for high-pole active components can be realised on an electronic assembly.
Typical for an HDI PCB is a high wiring density in connection with a very high number of microvia holes, microvias for short. One speaks of HDI with conductor track structures from 150 µm and finer as well as 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, plugging or resin filling, play a decisive role in the choice of an optimal rewiring strategy.
The advantages are:
Maximum packing densities, miniaturisation and best possible space utilisation,
Greatest possible freedom in component placement,
Shortening of the track lengths and improvement of the assembly EMC behaviour,
Simplified unbundling of the PCB layout, reduction of the number of layers,
Density, single-sided assembly and associated reduction of assembly costs