In the design process of PCB multilayer circuit board (HDI board), before routing, we generally stack the multilayer circuit board project we want to design, and calculate the impedance according to the thickness, substrate, number of layers and other information. , after the calculation, the following figure can generally be obtained.

　　As can be seen from the above figure, the single-ended network on the PCB multilayer circuit board (HDI board) design is generally controlled by 50 ohms, so many people will ask why it is required to be controlled according to 50 ohms instead of 25 ohms or 80 ohms ohm?

　　First of all, the default choice is 50 ohms, and everyone in the industry accepts this value. Generally speaking, a certain standard must be formulated by a recognized organization, and everyone designs the PCB multilayer circuit board (HDI board) according to the standard. .

　　A large part of electronic technology comes from the military. First of all, the technology is used for military use, and slowly it is converted from military use to civilian use.

　　In the early days of microwave applications, during World War II, the choice of impedance completely depended on the needs of use, and there was no standard value. As technology advances, impedance standards need to be given in order to strike a balance between economy and convenience.

　　In the United States, the most used conduits are connected by existing rods and water pipes. 51.5 ohms are very common, but the adapters and converters seen and used are 50-51.5 ohms; for the joint army and navy to solve these problems Problem, an organization called JAN was established (later DESC organization), which was specially developed by MIL. After comprehensive consideration, 50 ohms was finally selected, and the related conduits were manufactured and transformed into various cables. standard.

　　At this time, the European standard was 60 ohms. Soon after, under the influence of companies such as Hewlett-Packard that dominated the industry, Europeans were also forced to change, so 50 ohms eventually became a standard in the industry. It has become a convention, and the PCB multi-layer circuit board (HDI board) connected with various cables is finally required in accordance with the 50 ohm impedance standard for impedance matching.

　　Secondly, the formulation of general standards will be based on a comprehensive consideration of the production process and design performance and feasibility of PCB multilayer circuit boards (HDI boards).

　　From the perspective of PCB multilayer circuit board production and processing technology, considering most of the existing PCB multilayer circuit board (HDI board) equipment, it is relatively easy to produce a 50-ohm impedance PCB.

　　It can be seen from the impedance calculation process that too low impedance requires a wider line width and a thin medium or a larger dielectric constant, which is difficult to meet in space for the current high-density boards; too high impedance requires thinner lines. Wide and thicker dielectrics or smaller dielectric constants are not conducive to the suppression of EMI and crosstalk. At the same time, for PCB multilayer circuit boards (HDI boards) and from the point of view of mass production, the processing reliability will be relatively poor.

　　Control 50 ohm impedance In the environment of using common plates (FR4, etc.) and common core boards, to produce products with common plate thicknesses (such as 1mm, 1.2mm, etc.), common line widths (4~10mil) can be designed, so that the lines The board factory is very convenient to process, and the equipment requirements for its processing are not very high.

　　Considering the design of PCB multilayer circuit board (HDI board), 50 ohm is also selected after comprehensive consideration. From the performance of PCB multi-layer circuit board (HDI board) wiring, generally low impedance is better. For a transmission line with a given line width, the closer the distance to the plane, the corresponding EMI will be reduced, and the crosstalk will also be reduced accordingly. Small.

　　However, from the perspective of the full signal path, the most critical factor to consider is the driving capability of the chip. In the early days, most chips could not drive transmission lines with impedance less than 50 ohms, and transmission lines with higher impedance were inconvenient to implement. So the compromise is to use 50 ohm impedance.

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