10 FAQs about Vias

Vias are one of the important elements of PCB design. In double-sided and multi-layer boards, to connect the printed wires between each layer, a common hole, that is, a via hole, is drilled at the intersection of the wires that need to be connected on each layer. Vias are divided into three categories, namely blind vias, buried vias and through vias.

Through-hole: A through-hole is a hole that passes from one side of a circuit board to the other. They can run completely through the board and can be soldered through pins, sockets, or other components. Vias are often used to connect different levels of circuitry, providing electrical connections and mechanical support.

Blind via: A blind via is a hole that only enters the circuit board from one side and does not penetrate the entire board thickness. They are used to connect circuits between surface layers and inner layers. Blind vias allow for component soldering using surface mount technology, but cannot be connected to the other side of the board via through holes.

Buried via: A buried via is a hole that is completely inside the circuit board. It neither enters or exits from one side nor penetrates the entire board thickness. Buried vias are used to connect circuits between internal layers without affecting external surfaces. Buried vias are invisible and can only be observed in circuit board cross-sections or using special inspection methods such as X-rays.

1. It is often seen that there are many holes on the PCB board. Is the more through-holes the better? Are there any rules?

no. The use of vias should be minimized as much as possible. When vias have to be used, the impact of vias on the circuit should also be considered.

2. When laying out the board, if the lines are dense, there may be more vias, which will of course affect the electrical performance of the board. How can I improve the electrical performance of the board?

For low-frequency signals, via holes do not matter, and for high-frequency signals, minimize via holes. If there are many lines, consider multi-layer boards.

3. How much difference do through holes and blind holes have on the signal? What principles apply?

The use of blind holes or buried holes is an effective method to increase the density of multilayer boards, reduce the number of layers and board size, and greatly reduce the number of plated through holes. But in comparison, through holes are easier to implement in terms of technology and have lower costs, so through holes are generally used in designs.

4. Can you explain the relationship between the line width and the size of the matching via hole?

It is difficult to say that there is a limit to the proportional relationship between PCB array vias. After all, their simulations are different. One side is often transferred and the other side is ring-shaped. Therefore, it is necessary to calculate the vias specifically when applying. The impedance of the via is consistent with the impedance of the transmission line.

5. What is the relationship between the line width and via hole size on the PCB board and the amount of current passing through it?

The thickness of the copper foil of a general PCB is 1 ounce, which is about 1.4 mil. The maximum current allowed for a line width of 1 mil is approximately 1 A. Via holes are relatively complex. In addition to the size of the via pad, it is also related to the thickness of the copper deposited on the hole wall after electroplating during the processing.

6. Should the via hole match Sqrt(L/C) according to the requirements?

Yes, to put it simply, it is impedance matching, adjusting the parameters of the vias to achieve a better smooth transition of impedance.

7. Is there a corresponding relationship between temperature changes and the impedance of the vias?

Temperature changes mainly affect the reliability of vias. Material selection needs to consider the CTE value of the material.

8. High-speed PCB, how to avoid via holes during wiring, any good suggestions?

For high-speed PCBs, it is best to drill fewer vias and add signal layers to solve the need for more vias.

9. What is the function and principle of adding ground vias near the wiring vias?

Vias on PCB boards can be divided into the following types according to their functions:

1) Signal vias (via structure is required to have the least impact on the signal)

2) Power supply and ground vias (the via hole structure requires the minimum distributed inductance of the via hole)

3) Heat dissipation vias (the via-hole structure requires the minimum thermal resistance of the via-hole)

The vias mentioned above are ground type vias. The purpose of adding ground vias near the wiring vias is to provide the shortest return path for the signal.

Note: The via hole for signal layer change is a discontinuous point of impedance. The return path of the signal will be disconnected from here. In order to reduce the area surrounded by the return path of the signal, some ground passes must be made around the signal via hole. The hole provides the shortest signal return path and reduces the EMI radiation of the signal. This radiation increases significantly as the frequency of the signal increases.

10. When the diameter of the signal via hole is relatively small (such as 0.3mm diameter), will the metallization of the via hole be insufficient in this case?

If the hole diameter is small and deep (i.e. the hole diameter is relatively large), it may not be fully metalized.