HDI is an abbreviation for High Density Interconnect. It is a type of printed circuit board (PCB) technology that uses micro-blind buried vias to achieve a relatively high trace density. HDI is a compact product designed for low-volume users.

Blind vias: These are vias that connect traces on inner layers of a PCB to traces on the surface without penetrating the entire board. (Generally used on PCB with four or more layers.)

Buried vias: Buried vias connect traces only between inner layers and are invisible from the PCB surface. (Generally used on PCB with six or more layers.) 

Advantages: They increase trace spacing. 

Disadvantages: The process is costly and is not commonly used in electronic products, except for very advanced products.