Troubleshooting issues related to PCB "vias"

Troubleshooting issues related to PCB "vias"

Vias, also known as metallized holes, are one of the important elements of PCB design. In the double-sided and multi-layer boards, in order to connect the printed wires between the layers, a common hole, that is, a via hole, is drilled at the intersection of the wires that need to be connected on each layer. There are three types of vias, namely blind vias, buried vias and through vias. This article collects some classic questions and answers related to PCB "vias", hoping to help everyone.

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

Answer: No. Minimize the use of vias, and when you have to use vias, you must also consider reducing the impact of vias on the circuit.

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

Answer: For low-frequency signals, vias do not matter, and high-frequency signals minimize vias. If there are many lines, multi-layer boards can be considered.

3. How big is the difference between through holes and blind holes on the signal? What are the principles applied?

A: The use of blind or buried vias is an effective way to increase the density of multilayer boards, reduce the number of layers and the size of the board, and greatly reduce the number of plated through holes. However, in comparison, through-holes are easy to implement in terms of technology and have a lower cost, 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?

A: It is difficult to say that there is a simple proportional relationship, because the simulations of the two are different. One is surface transmission and the other is ring transmission. You can find an impedance calculation software for vias on the Internet, and then keep the impedance of the vias consistent with the impedance of the transmission line.

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

A: The copper foil thickness of a general PCB is 1 ounce, and if it is about 1.4 mil, the maximum current allowed by the roughly 1 mil line width is 1A. Vias are more complicated, not only related to the size of the via pads, but also to the thickness of the copper sinking on the hole wall after electroplating during processing.

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

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

7. Is there a corresponding relationship between temperature change and via impedance?

A: The temperature change mainly affects the reliability of the via, and the material selection needs to consider the parameter of the CTE value of the material.

8. How to deal with the avoidance of vias in high-speed PCBs during the wiring process, any good suggestions?

Answer: For high-speed PCBs, it is best to make fewer vias, and increase the signal layer to solve the need for additional vias.

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

Answer: The vias of the PCB board, according to their functions, can be divided into the following categories:

1) Signal vias (via structure requirements have the least impact on the signal)

2) Power and ground vias (via structure requires the smallest distributed inductance of vias)

3) Heat dissipation vias (the via structure requires the smallest thermal resistance of the vias)

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

Note: The via hole of the signal switching layer is a discontinuous point of impedance, and 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 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 aperture of the signal via is relatively small (such as 0.3mm diameter), will the via metallization be insufficient in this case?

A: If the aperture is small and deep (that is, the aperture is relatively large), it may not be fully metallized.