In the high-speed PCB design, the vias design is an important factor, it consists of holes, holes around the solder area and POWER layer isolation area, usually divided into blind hole, buried hole and through-hole three categories. In the PCB board design process through the parasitic capacitance and parasitic inductance analysis of the hole, summed up the high-speed PCB board through-hole design of some of the matters needing attention.
 
At present, the design of high-speed PCB board is widely used in the fields of communication, computer and graphic image processing. All the high-tech value-added electronic products are designed in the pursuit of low power consumption, low electromagnetic radiation, high reliability, miniaturization and light , In order to achieve the above objectives, in the design of high-speed PCB board, through-hole design is an important factor.
 
1, through the hole
 
Through-hole is a multi-layer PCB board design is an important factor, a through the main consists of three parts, one hole; Second, the hole around the pad area; Third, POWER layer isolation area. The process of the through-hole is through the hole in the hole on the cylindrical surface with a chemical deposition method of plating a layer of metal, to connect the middle layer of the need to connect the copper foil, and the upper and lower sides of the hole made of ordinary pad Shape, can be directly connected with the upper and lower lines of the line, but also not even. The vias can function as electrical connections, fixed or positioned devices. Through-hole schematic diagram shown in Figure 1.
 High-speed PCB through-hole design techniques _ hole diagram
Figure 1: Through-hole schematic
 
Through holes are generally divided into three categories: blind hole, buried hole and through hole.
 
Blind holes are located on the top and bottom surfaces of the printed circuit board and have a depth for the connection of the surface lines and the underlying inner lines. The depth and aperture of the holes generally do not exceed a certain ratio.
 
The buried hole, which is the connection hole located in the inner layer of the printed circuit board, does not extend to the surface of the circuit board.
 
Both the blind hole and the buried hole are located in the inner layer of the circuit board, before the use of through-hole forming process, in the process of forming the hole may also overlap several inner layers. Through Hole, this hole through the entire circuit board, can be used to achieve internal interconnection or as a component mounting hole. As the through-hole in the process easier to achieve, the cost is low, so the general use of printed circuit board through-hole. The classification of vias is shown in Fig.
 High Speed ​​PCB Through Hole Design Skill - Through Hole Classification
Figure 2: Classification of vias
 
2, the parasitic capacitance of the hole
 
If the diameter of the vias is D1, the thickness of the PCB is T, the dielectric constant of the substrate is E, then The parasitic capacitance of the vias is approximately the same as:
 
C = 1.41ETD1 / (D2-D1)
 
The main effect of the parasitic capacitance on the vias is that the rise time of the signal is prolonged, the speed of the circuit is reduced, and the smaller the capacitance value is, the smaller the effect is.
 
3, parasitic inductance of the hole
 
There is parasitic inductance in the hole itself, in the design of high-speed digital circuit, the parasitic inductance of the hole is often greater than the parasitic capacitance of the impact. The parasitic series inductance of the via hole weakens the effect of the bypass capacitor and reduces the filtering efficiency of the entire power supply system. If L is the inductance of the hole, h is the length of the hole, d is the diameter of the center hole, the parasitic inductance of the hole is similar to:
 
L = 5.08h [ln (4h / d) +1]
 
It can be seen from the formula, the diameter of the hole on the inductance of the smaller, and the greatest impact on the inductor is the length of the hole.