Introduction of PCB via Technology

I. Basic knowledge of PCB vias
Vias are one of the most important components of multi-layer PCBs. The cost of drilling usually accounts for 30% to 40% of PCB manufacturing cost. From the action of via, it can be divided into 2 types, i.e. electrical connections between layers and fixation or positioning of devices. From the point of manufacturing process, vias are generally divided into three categories, namely, blind vias, buried vias, and through vias.

The buried hole refers to the connection hole in the inner layer of a printed circuit board, and it does not extend to the surface of the circuit board. Both of the above types of holes are located in the inner layer of the circuit board. The through hole forming process is used before lamination, and several inner layers may overlap during the formation of the through hole.

The third type is called through holes, which pass through the entire circuit board and can be used to implement internal interconnects or as mounting holes for components. Since through holes are easier and cheaper to be manufactured, most printed circuit boards use it instead of the other two vias.

From a design point of view, a via is mainly composed of a drill hole in the middle and a pad area around the drill hole. The size of these two parts determines the size of the via hole. The smaller the via hole, the smaller value its own parasitic capacitance, which is suitable for high-speed circuits.

However, the reduction in the size of the holes at the same time brings about an increase in cost, which is limited by the technique of drilling and plating.

II. About the parasitic capacitance of the via
The 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 parasitic capacitance on slow rise and delay of single via is not obvious, designers should consider it carefully if vias are used multiple times in the wiring to switch between layers.

III. About the parasitic inductance of the via
There is a parasitic capacitance in the via and a parasitic inductance at the same time. In the design of high speed digital circuits, the parasitic inductance of the through-hole often causes more damage than parasitic capacitance. Its parasitic series inductance weakens the contribution of bypass capacitance and weakens the filtering effect of the whole power supply system. The diameter of the via has little effect on the inductance, but the length of the via is the most important one. The impedance generated by the via cannot be ignored when there is a high frequency current. Especially, the bypass capacitance needs to pass through two vias when connecting the power layer and the ground layer, so the parasitic inductance of the via will increase exponentially.

IV. About the use of vias
1. Considering the cost and signal quality, choose a reasonably sized via size. 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. Using a thinner PCB board helps to reduce the two parasitic parameters of the vias.

3. Trying to wire the signal traces on the same layer as much as possible, that is to say unnecessary vias should not be used.

4. The base pins of power and ground should be near the vias, and the shorter the lead wire between the vias and the pins, the better. Multiple vias can be considered in parallel connection to reduce the equivalent inductance.

5. Some grounding vias are placed near the signal layer-changing vias to provide the nearest loop for the signal. It is even possible to place some extra more grounding vias on the PCB board.

6. For high speed PCBs with high density, micro vias can be considered.