I. Basic knowledge of PCB via
Via is one of the important parts of multi-layer PCB. The cost of drilling usually accounts for 30% to 40% of the cost of PCB manufacturing. From the role of vias, it can be divided into two types: electrical connection between layers and fixing or positioning of devices. Generally speaking, vias can be divided into three categories: blind via, buried via and through via. The blind hole is located on the top and bottom surface of the printed circuit board and has a certain depth. It is used for the connection of the surface circuit and the bottom inner circuit. The depth of the hole usually does not exceed a certain ratio (aperture). Buried hole refers to the connection hole in the inner layer of printed circuit board, which will not extend to the surface of printed circuit board. The above two types of holes are located in the inner layer of the circuit board. Before laminating, the through-hole forming process is used. In the process of through-hole formation, several inner layers may be overlapped. The third type is called through-hole, which passes through the whole circuit board and can be used to realize internal interconnection or as the installation positioning hole of components. Because through-hole is easier to realize in technology and lower in cost, most PCB use it instead of two other through-holes. From the design point of view, a through hole is mainly composed of the middle hole and the pad area around the hole. The size of these two parts determines the size of the through hole. The smaller the via is, the smaller its parasitic capacitance is, which is suitable for high-speed circuits. However, the reduction of hole size brings about the increase of cost, which is also limited by the drilling and plating technology.
On the parasitic capacitance of vias
The parasitic capacitance of the via itself has stray capacitance to the parasitic ground. The main effect of the parasitic capacitance of the via on the circuit is to extend the rise time of the signal and reduce the speed of the circuit. Although the effect of slow rise and delay caused by the parasitic capacitance of a single via is not obvious, if the vias are used for layer switching many times in the routing, the designer should consider it carefully.
On the parasitic inductance of via
In the design of high-speed digital circuit, the harm of parasitic inductance of via is often greater than that of parasitic capacitance. Its parasitic series inductance will weaken the contribution of bypass capacitance and the filtering effect of the whole power system. The diameter of the via has little effect on the inductance, while the length of the via has the greatest effect on the inductance. The impedance generated by via can not be ignored when there is high-frequency current passing through. It should be noted that the bypass capacitance needs to pass through two vias when connecting the power supply layer and the stratum, so the parasitic inductance of via will be multiplied.
IV. use of vias
1. Considering the cost and signal quality, choose a reasonable size of vias. If necessary, different sizes of vias can be considered. For example, for vias of power supply or ground wire, larger sizes can be considered to reduce impedance, while for signal wiring, smaller vias can be used. Of course, with the decrease of via size, the corresponding cost will increase.
2. From the two formulas discussed above, it can be concluded that the use of thinner PCB is conducive to reducing the two parasitic parameters of via.
3. As far as possible, the signal wiring on PCB should not be changed, that is to say, unnecessary vias should not be used.
4. The pin of the power supply and the ground shall be punched through the hole nearby. The shorter the lead between the hole and the pin, the better. Multiple vias can be drilled in parallel to reduce the equivalent inductance.
5. Place some grounded vias near the vias of the signal exchange layer to provide the nearest circuit for the signal. You can even place some redundant ground vias on the PCB.
6. For high-speed PCB with high density, micro via can be considered.
Sep 20, 2019
