Drilling hole (via) is one of the important components of multilayer PCB, and the cost of drilling hole usually accounts for 30 to 40 percent of the cost of PCB board making.
Simply put, each hole on a PCB can be called a through-hole. From the perspective of function, through-hole can be divided into three categories:
One is for electrical connections between layers;
Second, it is used for fixing or positioning devices. As for the process, these holes are generally divided into three categories: blind via, buried via and through via). Blind hole located at the top of the printed circuit board and the underlying surface, which has certain depth, used for surface line and below inner line connection, the hole depth is usually no more than a certain ratio (diameter). Buried hole refers to the inside connection hole located in printed circuit board, it does not extend to the surface of circuit board. The above two kinds of holes are located in inner layer of circuit board, the through hole forming process is used before lamination, and several inner layers may be overlapped in the process of through hole forming.
The third one is called through-hole, which is through the whole circuit board, can be used to implement internal interconnection or as a component installation location hole. Because the hole is more easy to produce in the process and with lower cost, so it should be used in most of printed circuit board, rather than the other two via. For following via if there is no special instructions, both as a through-hole considered. From the view of design point, a through hole is mainly composed of two parts, one is in the middle of the hole (drill hole), the second is the bonding pad zone around the hole. The two parts size determines the via size.
Obviously, at a high speed and high density PCB design, designers always want a hole as small as possible, and this model can leave more space for line laying, in addition, a hole is smaller, its parasitic capacitance is smaller, and it's more suitable for high speed circuit. But the hole size decrease with the cost increase, and the via hole size is impossible to unlimited decreases, and it is limited by drilling and plating technology: the smaller the hole, the longer drilling cost, also the easier it is off center location; and when the hole depth exceeds 6 times hole diameter, there is no guarantee that the hole wall can be uniformly copper-plated. For example, now the thickness (through hole depth) of a normal 6-layer PCB board is about 50mil, so the minimum diameter of the hole provided by the PCB manufacturer can only reach 8mil.