PCB drill hole is one of the important part for multilayer PCB, usually drilling cost account for 30% to 40% of the total cost in PCB fabrication.
In terms of function, the holes can be divided into two categories, one is used for electrical connection between each layers; The other is used to fix or position the components.
In terms of technology in PCB manufacturing, the holes are generally divided into three categories, which are blind vias, buried vias and through holes. Blind via located at the top layer and bottom layer surface of the printed circuit board, which used for the connection between surface layer and inner layer, the depth of the hole is usually not exceed a certain ratio. Buried hole refers to connection hole that is located in inner layer of the printed circuit board, buried hole does not extend to the surface of the circuit board. Through holes are drilled through the whole printed circuit board, which can be used for internal electrical interconnection or used as a positioning hole to assemble components. Because the through holes are easier to achieve in the pcb manufacturing process with lower cost, so most of printed circuit boards choose to use it rather than the other two holes (blind via and buried via).
In terms of pcb design, the hole is mainly consist of two parts, one is the drill hole in the center area, the other is the pad around the drill hole. The size of these two parts determine the hole size. Obviously, in the design of a printed circuit board with high speed and high density, the designers always hope that the holes the smaller the better, so that it can leave more space for circuit layout. But the hole size decrease will bring the increase of the pcb fabrication cost, moreover, the hole size is impossible to decrease unlimitedly, because it's limited by the process of drilling and plating. The smaller hole size, the longer drill time required, and the easier off center position. When the depth of hole is more than 6 times of hole diameter, the copper plating process won't guarantee that the hole wall could plate copper smoothly. For example, a standard 6 layer PCB with thickness (depth of the hole) about 50 mil, so the PCB manufacturer can provide the minimum hole diameter to 8 mil.