- Jun 14, 2018 -
Basic knowledge of PCB circuit board vias
First, on the parasitic capacitance of the via
The via's parasitic capacitance via itself has stray capacitance to the parasitic ground. The main effect of via's parasitic capacitance on the circuit is to extend the signal's rise time and reduce the circuit's speed. Although the effect of the slowing of the rise delay caused by the parasitic capacitance of a single via is not very obvious, if the vias are used multiple times in the trace to switch between layers, the designer must carefully consider.
Second, on the use of vias
1. Consider the reasonable dimensions of the via size in terms of both cost and signal quality. If necessary, consider using different sizes of vias. For example, for power or ground vias, consider using larger dimensions to reduce the impedance. For signal traces, use smaller vias. Of course, as the size of the via decreases, the corresponding cost increases.
2. For high density PCB circuit boards with high density, consider using micro vias.
3. The signal traces on the PCB should not be changed as much as possible, which means that unnecessary vias should not be used.
4. The two equations discussed above can be concluded that the use of a thinner PCB board facilitates the reduction of the two parasitic parameters of the via hole.
5. The power supply and ground pins should be punched in the nearest hole. The lead between the via and the pin should be as short as possible. Multiple vias can be considered in parallel to reduce the equivalent inductance.
6. Place some grounded vias near the signal layer vias to provide the signal with the closest loop. It is even possible to place some extra ground vias on the PCB board.
Third, on the parasitic inductance of the via
There are parasitic capacitances in the through-holes of PCB circuit boards and parasitic inductances. In the design of high-speed digital circuits, the parasitic inductance caused by via holes is often greater than the parasitic capacitance. Its parasitic series inductance weakens the contribution of the bypass capacitor and attenuates the filtering effect of the entire power system. The diameter of the via has less influence on the inductor, and the greatest influence on the inductor is the length of the via. The impedance generated by the vias cannot be neglected by the passage of high-frequency currents. It is important to note that the bypass capacitors need to pass through two vias when connecting the power plane and the ground plane, so that the parasitic inductance of the vias will be doubled. increase.