PCB Tech - Why does PCB design generally control 50 ohm impedance?

Impedance: In a circuit with resistance, inductance and sum, the hindrance to the current in the circuit is called impedance. It is often represented by Z, which is a complex number, the real part is called resistance, and the imaginary part is called reactance.

The unit of impedance is ohm.

In the process of PCB design, before routing, we generally stack the items we want to design, and calculate the impedance based on the thickness, base material, number of layers and other information. After the calculation, the following figure can generally be obtained.

Figure 1 Illustration of overlay information

As can be seen from the above figure, the single-ended network design above is generally controlled by 50 ohms, so many people will ask, why is it required to control according to 50 ohms instead of 25 ohms or 80 ohms?

First of all, 50 ohms is selected by default, and everyone in the industry accepts this value. Generally speaking, a certain standard must be formulated by a recognized organization, and everyone is designing according to the standard. A large part of electronic technology comes from the military. First of all, the technology is used in the military, and it is slowly transferred from military to civilian use. In the early days of microwave application, during the Second World War, the choice of impedance was completely dependent on the needs of use, and there was no standard value. With the advancement of technology, impedance standards need to be given in order to strike a balance between economy and convenience. In the United States, the most commonly used conduits are connected by existing rods and water pipes. 51.5 ohms is very common, but the adapters and converters seen and used are 50-51.5 ohms; this is solved for the joint army and navy. Problem, an organization named JAN was established (later DESC organization), specially developed by MIL, and finally selected 50 ohms after comprehensive consideration, and related catheters were manufactured and transformed into various cables. Standards. At this time, the European standard was 60 ohms. Soon after, under the influence of companies that dominate the industry like Hewlett-Packard, Europeans were also forced to change, so 50 ohms eventually became a standard in the industry. It has become a convention, and the PCB connected to various cables is ultimately required to comply with the 50 ohm impedance standard for impedance matching.

Secondly, the formulation of general standards will be based on comprehensive consideration of PCB production process and design performance and feasibility.

From the perspective of PCB production and processing technology, considering the equipment of most existing PCB manufacturers, it is relatively easy to produce PCBs with 50 ohm impedance. From the impedance calculation process, it can be seen that too low impedance requires a wider line width and a thin medium or a larger dielectric constant, which is difficult to meet the current high-density board space; too high impedance requires a thinner line Wide and thick media or small dielectric constants are not conducive to the suppression of EMI and crosstalk. At the same time, the reliability of processing for multi-layer boards and from the perspective of mass production will be relatively poor. Control 50 ohm impedance. Under the environment of using common boards (FR4, etc.) and common core boards, produce products with common board thicknesses (such as 1mm, 1.2mm, etc.). Common line widths (4~10mil) can be designed. The factory is very convenient to process, and the equipment requirements for its processing are not very high.

From the perspective of PCB design, 50 ohms is also selected after comprehensive consideration. From the performance of PCB traces, low impedance is generally better. For a transmission line of a given line width, the closer the distance to the plane is, the corresponding EMI will be reduced, and the crosstalk will also be reduced. However, from the perspective of the full signal path, one of the most critical factors needs to be considered, that is, the drive 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. Therefore, 50 ohm impedance is used as a compromise.