HDI PCB is defined as a printed circuit board with a higher wiring density per unit area than a conventional PCB. They have much finer lines and spaces, smaller vias and capture pads, and higher connection pad density than employed in conventional PCB technology.
Generally speaking, HDI circuit boards are built up by using the Build Up method, where the internal layers are first made or pressed together, and then the outer layer is covered with insulation layer (prepreg) and copper foil after laser drilling and electroplating of the outer layer is completed, and then the outer layer is repeatedly wired, or laser drilling is continued to stack up the layers one at a time.
Since laser drilling is used many times, the key to the quality of HDI boards is the hole shape after laser drilling and whether the holes can be filled evenly after subsequent electroplating.
The following is an example of HDI board type. The pink hole in the picture is a blind hole, which is made by laser drilling, and the hole diameter is usually 3~4 mil; the yellow hole is a buried hole, which is made by mechanical drilling, and the hole diameter is at least 6 mil (0.15 mm).
Material：FR-4 High Tg . Board Thickness：1.2mm
Layer：10 Layers . Surface Finished：ENIG：2μ" / 150μ"
Material：FR-4 High Tg/ Board Thickness：1.6mm/Layers：6 Layers/ Surface Finished：ENIG