In this context, HDI stands for high density interconnector. This means that an HDI PCB is a printed circuit board that can take a very high density of connections and components within a small area. The lines and spaces for the connections and the electronic parts are very fine and packed closely together.

Looked at with the naked eye, it might not even be possible to make out all of the individual traces because they are so small and close together. This is one technology that has reduced the size of many circuit boards, and in turn, it reduces the size needed for the cases of the finished products.
Advantages of High Density Interconnector Printed Circuit Boards
The advantage of this solution is that it lets engineers design much smaller circuit boards for a variety of electronic devices, and this miniaturization lets companies produce smaller and smaller devices. Examples of products that keep getting smaller are the newer small smart phones, tablets, and personal computers that pack the same power as much larger models. These HDI printed boards are also used in game consoles, music players, and other products.
How Are HDI Printed Circuit Boards Produced?
Because the connections and components are so small, it is impossible for workers to put these circuit boards together by hand on an assembly line. They much be produced by machines that are capable of working at a much finer level of detail than any human worker could.
Every HDI board must get designed first, usually with PCB software from a manufacturer. Engineers create the plans for these boards according to their design specs with this software. After the design gets modeled in the software, prototypes can be produced and tested.
The process of modifying design specs and testing prototypes may need a few iterations to get it right. However, this process can produce a workable design that could end up getting producing millions of these high density circuit boards for use in devices distributed all over the world. Once the process has been completed, automated manufacture of the production PCBs is very quick and efficient.