PCB printed circuit board PCB by hole type classification method

Via It is an important part of the multi-layer PCB circuit board, and the drilling cost usually accounts for 30%~40% of the PCB production cost. Therefore, via design has also become one of the important parts of PCB design. Simply put, every hole on a PCB can be called a via. In terms of function, vias can be divided into two categories: one is for electrical connection between layers; the other is for device fixation or positioning. In terms of process, these vias are generally divided into three categories, namely Blind Via, Buried Via and Through Via.

Blind vias are located on the top and bottom surfaces of the printed circuit board, with a certain depth, for the connection of the surface line and the underlying inner line, the hole's The depth usually does not exceed a certain ratio (aperture). Buried vias refer to connection holes located on the inner layer of the printed circuit board, which do not extend to the surface of the circuit board. The buried via is located in the inner layer of the circuit board, which is completed by the through-hole forming process before lamination, and several inner layers may be overlapped during the formation of the hole. The third type of hole is called a through hole, which passes through the entire circuit board and can be used to realize internal interconnection or installation and positioning holes of components. Because the through hole is easier to realize in the process and the cost is lower, most of the printed circuit boards use it instead of the other two kinds of through holes. From a design point of view, a via is mainly composed of two parts, one is the drill hole in the middle, and the other is the pad area around the drill hole. The size of these two parts determines the size of the via.

Obviously, when designing high-speed and high-density PCBs, circuit board designers always want the holes to be as small as possible, so that more can be left on the PCB In addition, the smaller the via, the smaller its own parasitic capacitance, which is more suitable for high-speed circuits. However, the reduction of the hole size also brings about an increase in cost, and the size of the via hole cannot be reduced infinitely, which is limited by process technologies such as drilling and electroplating. The smaller the hole, the longer it will take to drill and the more likely it will be off-center. As far as the current level of PCB manufacturing technology is concerned, when the ratio of the thickness of the PCB substrate to the aperture (that is, the thickness-to-diameter ratio) exceeds 10, the uniform copper plating of the hole wall cannot be guaranteed, and the thickness of the copper layer is not uniform, especially in the middle of the coating. Loose and too thin coating will seriously affect the fatigue life of the hole.