A complete introduction to PCB via technology

1. Basic knowledge of PCB vias

Via is one of the important components of multi-layer PCB, and the cost of drilling usually accounts for 30% to 40% of the cost of PCB board manufacturing. From the function of via, it can be divided into two categories: electrical connection between layers and fixing or positioning of devices. From the process, vias are generally divided into three categories, namely blind via, buried via and through via. Blind via is located on the top and bottom surfaces of the printed circuit board, has a certain depth, and is used to connect the surface circuit and the inner circuit below. The depth of the hole usually does not exceed a certain ratio (aperture). Buried via refers to the connection hole located in the inner layer of the printed circuit board, which does not extend to the surface of the circuit board. The above two types of holes are located in the inner layer of the circuit board, and are completed by the through-hole forming process before lamination. During the via formation process, several inner layers may be overlapped. The third type is called through-hole, which passes through the entire circuit board and can be used to achieve internal interconnection or as a component installation and positioning hole. Because through-hole is easier to implement in terms of process and has lower cost, most printed circuit boards use it instead of the other two types of vias. From a design perspective, a via is mainly composed of the drill hole in the middle and the pad area around the drill hole. The size of these two parts determines the size of the via. The smaller the via, the smaller its parasitic capacitance, which is suitable for high-speed circuits. However, the reduction in hole size also brings about an increase in cost, which is also limited by process technologies such as drilling and plating.

2. About the parasitic capacitance of vias

The parasitic capacitance of vias The vias themselves have stray capacitance to the parasitic ground. The parasitic capacitance of vias will have the main impact on the circuit by extending the rise time of the signal and reducing the speed of the circuit. Although the effect of slowing down the rise time caused by the parasitic capacitance of a single via is not very obvious, if vias are used multiple times in the routing to switch between layers, designers should still consider it carefully.

3. About the parasitic inductance of vias

Vias have parasitic capacitance as well as parasitic inductance. In the design of high-speed digital circuits, the harm caused by the parasitic inductance of vias is often greater than the impact of parasitic capacitance. Its parasitic series inductance will weaken the contribution of bypass capacitance and reduce the filtering effect of the entire power system. The diameter of the via has little effect on the inductance, while the length of the via has the greatest impact on the inductance. The impedance generated by the via cannot be ignored when high-frequency current passes through it. It is particularly important to note that the bypass capacitor 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.

4. About the use of vias

1. Consider both cost and signal quality and choose a reasonable size of via. If necessary, consider using vias of different sizes. For example, for vias of power or ground wires, consider using larger sizes to reduce impedance, while for signal routing, smaller vias can be used. Of course, as the size of the via decreases, the corresponding cost will also increase.

2. From the two formulas discussed above, it can be concluded that using a thinner PCB board is beneficial to reducing the two parasitic parameters of the via.

3. Try not to change layers for signal routing on the PCB, that is, try not to use unnecessary vias.

4. The power and ground pins should be drilled near vias, and the leads between the vias and the pins should be as short as possible. You can consider drilling multiple vias in parallel to reduce the equivalent inductance.

5. Place some grounded vias near the vias where the signal changes layers to provide the closest loop for the signal. You can even place some extra grounded vias on the PCB board.

6. For high-speed PCB boards with higher density, consider using micro vias.