The board’s outermost circuit is connected to the adjacent inner layer by a plating hole, which is called a”Blind through” because the opposite side can not be seen. In order to increase the space utilization of PCB circuit layer, the”Blind hole” process was developed. This method requires special attention to the depth of the hole (Z axis) to be just right, but this method often leads to the hole electroplating difficulties, so almost no manufacturers adopt. It is also possible to drill holes for the circuit layers to be connected in advance when individual circuit layers are used, and then put them together, which requires a more precise positioning and alignment device.
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