With the rapid development of electronic industry, the increase of wiring density of high-speed pcb, the speed of frequency and switch, the corresponding requirements of outsourcing painting of high-speed PCB design are becoming more and more strict. In the outsourcing design of high-speed pcb, multilayer boards are usually used for design, so it is inevitable to use through-holes to realize interconnection in the setup. As one of the interconnection structures, via is equivalent to a discrete structure of signal transmission, which will lead to signal reflection, attenuation and signal integrity problems in high-speed pcb design, an important factor affecting signal transmission quality, and then affect the performance of the whole system.
1. Introduction of via holes
In the design of high-speed PCB, microstrip striplines are widely used in microwave signal transmission lines, and vertical vias are the most common form of interconnecting transmission lines. Via holes are divided into three categories: through holes, blind holes and buried holes. Blind holes are located on the top surface of printed matter and the inner circuit board, and have a certain depth. As connecting lines and internal circuits, the depth is usually no longer a certain proportion (aperture). Buried hole is the connecting hole inside the printed circuit board and does not extend to the surface of the circuit board. Two holes only penetrate a part of the PCB of the layer, which is generally a via formation process completed before lamination. The third one is called through hole, which passes through the whole circuit board and can be used for interconnection or as a component of mounting positioning hole.
2. Influence of grounding via on transmission performance
According to the signal function of the connection, vias can be divided into signal vias, power vias, grounding vias and heat dissipation vias.
This paper mainly discusses a grounding via, because in the process of signal transmission, in order to provide the smallest return signal path, grounding vias around signal vias are usually added. When using via connection, because the point connection layer will cause certain impedance discontinuity, in order to reduce the surrounding area of the signal return path, enough grounding vias have to be added around the signal to provide the shortest signal return path, so as to reduce the emi radiation of the signal.
Under the condition of meeting the design requirements, more grounding vias can be punched on the circuit board, which can make good contact with the ground layer connection, provide electromagnetic shielding and reduce electromagnetic radiation.
There are the following design conclusions for grounding vias:
1) Diameter of grounding via: when the dimensions of via, pad and anti-pad are fixed, the insertion loss is proportional to the signal via, that is, the larger the grounding diameter, the smaller the insertion loss and the better the signal transmission quality; Larger diameter of grounding via, smaller reflection.
2) Distance of grounding via: When the dimensions of via, pad and anti-pad are fixed, the smaller the distance, the smaller the insertion loss, the better the signal transmission quality and the smaller the reflection.
3) Number of grounding vias: the size of through-holes, pads and anti-pads is fixed. The more grounding vias, the smaller the transmission loss and the better the quality of signal transmission.
3. Influence of via spacing on signal transmission
In the outsourcing painting of high-speed pcb design, when a large number of vias need to be drilled in an area, what will be the impact of the spacing between vias? When the spacing between vias is close, the copper skin will be split, resulting in longer signal transmission path and increased delay. On the contrary, increasing the spacing between vias can make the signal transmission path shorter and the delay shorter. Therefore, in the design of high-speed pcb, controlling the hole spacing can effectively improve the signal transmission speed and quality.
Conclusion:
The design of via hole is an important factor in the design of high-speed PCB, and the signal integrity analysis seems that the via hole structure and grounding via hole can not be ignored. Reasonable use of via hole in high-speed PCB can improve its signal transmission performance and transmission quality, and also obtain good electromagnetic shielding effect, which is a very important design for high-speed and stable digital system.
