What are the key roles of blind and buried vias in HDI PCBs?
Blind and buried vias in HDI PCBs
Via is one of the important parts of a multi-layer PCB. The cost of drilling is usually 30% ~40% of the cost of PCB board fabrication.
In a simple word, each hole in the printed circuit PCB can be called via. From function, the vias can be divided into two categories: one is used as the electrical connection between the layers; the second is used for fixing or positioning the components. From process, there are three kinds: blind via, buried via, and through via. Blind holes goes a certain depth from top or bottom layer to the neighbor inner layer connecting the circuits; and the depth of the hole is usually not
exceeding a certain ratio (aperture). Buried hole locates in the inner layer of the printed circuit board, which does not extend to the surface layer. These two kinds of holes locate in the inner layer of the circuit board, they will be formed by through-hole process before lamination, and there may be overlap several inner layers if necessary. The third one: through hole, it goes through the entire PCB board to interconnect or can be the mounting holes of the components. This is very common used and at very lower cost relatively.
Through hole: one time lamination, drilling, plating, that’s all.
Blind holes and buried holes require special processes, there are several methods: mechanical deep drilling; sequential lamination; HDI layer adding.
Sequential lamination for example:
Blind vias: There are blind holes between the 1st and 2nd layer. Laminate the etched 1 and 2 layers together, then drilled and plated, and then laminated with 3 layers and 4 layers, we get blind hole.
Buried vias: There is a buried hole between the 2nd layer and the 3rd layer. Laminate the etched 2 layer and 3 layer together, then drilled and plated, and then laminated with 1 layer and 4 layer, we get buried vias.
Try to avoid overlapping buried blind hole design. If you decide to overlap the buried blind hole design, we have to produce as per HDI layer adding method, adding layer by layer and laser drilling.
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