Through hole (via) is one of the most important components of multilayer PCB, drilling costs usually account for 30% to PCB of the cost of board. In short, every hole on the PCB can be called a hole. From the point of view of the role of the hole can be divided into two categories:
It is used as an electrical connection between the layers; two is used as a device for fixing or positioning. If the process from the process, these holes are generally divided into three categories, that is, blind hole (blind via), buried hole (buried via) and through hole (through). The blind hole is located on the top and bottom surface of the printed circuit board, and has a certain depth, which is used for the connection between the surface layer and the inner layer of the inner layer, and the depth of the hole is usually not more than a certain ratio (Kong Jing). The buried hole is the connecting hole in the inner layer of the printed circuit board. The two types of holes are located in the inner layer of the circuit board, and the former is formed by the through hole forming process. The third type is called a through hole, which passes through the whole circuit board, and can be used to realize the internal interconnection or the positioning hole of the component. Because the through hole is easier to be realized in the process, and the cost is low, most of the printed circuit boards are used without any other two kinds of vias.
Two, from the design point of view, a hole is mainly composed of two parts, one is the middle of the drilling (drill hole), and the two is around the hole area of the pad. The size of these two parts determines the size of the hole. Obviously, in the high speed, high density PCB design, designers always hope through holes as small as possible, so the board can have more wiring space, in addition, a hole is small, the parasitic capacitance is smaller, more suitable for high speed circuit. But the decreasing pore size also brings cost increases, and the hole size cannot be reduced without limit, it is drilling (drill) and electroplating (plating) technology limited: the smaller the hole drilling takes longer, more easy to deviate from the center position; and when 6 times the depth of the hole over the hole diameter, hole wall can not guarantee uniform copper plating. For example, now a normal 6 PCB board thickness of about 50Mil, so PCB manufacturers can provide the minimum diameter of drilling can only reach 8Mil.
Through hole design through the above on the through hole parasitic characteristics analysis of three, high speed PCB, we can see that in the high-speed PCB design, seemingly simple holes will often bring a negative effect to the circuit design. In order to reduce the adverse effects caused by the parasitic effects of the hole, the design can be done as much as possible:
1, from the cost and signal quality of the two aspects of consideration, select the reasonable size of the hole size. For example, the 6-10 layer of the memory module PCB design, the use of 10/20Mil (drilling / pad) of the hole is better, for some high-density small size of the board, you can also try to use the 8/18Mil hole. Under the current technical conditions, it is difficult to use the smaller size of the hole. For the power supply or ground wire can be considered to use a larger size to reduce impedance.
2, the above discussion of the two formulas can be obtained, using a thin PCB plate is conducive to reducing the parasitic parameters of the hole over the two.
3, PCB board signal line as far as possible without changing the layer, that is, try not to use unnecessary holes.
4, the power and ground pins to the nearest hole, the hole and the pin between the shorter the better, because they will lead to an increase in inductance. At the same time, the power supply and ground lead should be as thick as possible to reduce the impedance.
5, in the signal layer of the hole near the hole to put some grounding, so as to provide a signal to the nearest circuit. You can even place a lot of redundant ground vias on the PCB board.
Of course, the design needs to be flexible. The hole model discussed above is the case of pads on each floor, and sometimes, we can reduce or even remove some of the pads. Especially in the hole density is very large, may lead to a broken circuit partition groove is formed on a copper layer, to solve this problem in addition to move the position of the hole, we can also consider a decrease in the size of the hole pad copper layer.