High Density Interconnect (HDI) refers to the high-density interconnect technology using the blind/buried via holes to enhance the density of circuit distributions on a PCB. The advantage is, the HID can significantly increase the availability of PCB space for maximum product miniaturization. However, as circuits are densely distributed, vias cannot be drilled by traditional drilling methods, and some vias (blind) must be made by laser drilling, while buried vias must be made for interconnect.

In general, the inner layers of the HDI PCB are made or laminated in advance by means of building up. After laser drilling and dimpling, the prepreg and copper foil are covered on the outer layers. Then, the manufacture of circuits on the outer layers is repeated or laser drilling continues to build up the layers one after another.

In general, the design diameter of laser drilling is 3–4 mil (about 0.076–0.1 mm), and the prepreg thickness between layers is about 3mil. As laser drilling repeats many times, the quality of an HDI PCB depends on the shape of drilled vias and if the subsequent dimpling can evenly fill the vias.

Examples of the types of HDI PCBs are as follows: In the figure, the pink hole is a blind via hole made by laser drilling with a general diameter of 3–4 mil. The yellow hole is a buried via hole made by mechanical drilling with a minimum diameter of 6 mil (0.15 mm).

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