Blind via, the outermost circuit layer is connected with the adjacent inner layer with an electroplated hole, which is called "blind via" because the opposite side cannot be seen. In order to increase the space utilization of PCB circuit layer, the manufacture procedure of "blind via" emerges correspondingly. Special attention is required to be paid to a proper drilling depth (Z axis) for this production method, but this method often results in difficult electroplating in the holes so almost no manufacturers adopt it; You can also drill holes in advance in the circuit layer which needed to be connected at the time of fabricating individual circuit layer before lamination, but relative precise positioning and alignment devices are required.
Blind via holes are very useful in SMT circuits since the solder cannot be drained away from the solder pads during soldering. Blind via holes can be placed within the footprint pads and not outside. With this method, great real estate savings can be achieved.
a) All blind via holes will be filled with epoxy resin from the prepreg sheets. Usually we cannot see it just by eyes. It's closed with a virtually flat surface of the copper lid. The via hole should be 100% filled with resin, and the thickness of the copper wall should not be less than 13μm (0.5 mil).
b) Blind via holes are often specified with a finished diameter of 0.20 or 0.25 mm (8 or 10 mils), which means that the holes can be drilled by means of 0.3 mm (12 mils) drill bits.
Cross section of a 6 layer PCB with blind via