Why is PCB multi-layer circuit board design generally controlled for a 50 ohm impedance

Why is PCB multi-layer circuit board design generally controlled for a 50 ohm impedance

In the process of designing PCB multi-layer circuit boards (HDI boards), before wiring, we usually stack the multi-layer circuit board project we want to design, calculate the impedance based on thickness, substrate, layer number, and other information, and after calculation, we can generally obtain the following diagram.

From the above figure, it can be seen that the single ended network on PCB multi-layer circuit board (HDI board) design is generally controlled at 50 ohms. Many people may ask why it is required to control at 50 ohms instead of 25 ohms or 80 ohms?

Firstly, the default selection is 50 ohms, and this value is widely accepted in the industry. Generally speaking, a recognized organization must have established a certain standard, and everyone designs PCB multilayer circuit boards (HDI boards) according to the standard.

A large part of electronic technology comes from the military. Firstly, the technology was used for military purposes, gradually transitioning from military to civilian use.

In the early days of microwave applications, during World War II, the choice of impedance was entirely dependent on the needs of use, and there was no standard value. With the advancement of technology, it is necessary to provide impedance standards in order to achieve a balance between economy and convenience.

In the United States, the most commonly used conduit is composed of existing ruler rods and water pipes, with 51.5 ohms being very common, but the adapters and converters used are 50-51.5 ohms; To address these issues in collaboration with the Army and Navy, an organization called JAN was established (later known as DESC organization), specifically developed by MIL. After comprehensive consideration, 50 ohms were ultimately chosen, and related conduits were manufactured and transformed into standards for various cables.

At this time, the European standard was 60 ohms. Soon after, under the influence of companies like Hewlett Packard that dominated the industry, Europeans were also forced to change, so 50 ohms eventually became a standard in the industry and became a convention. PCB multilayer circuit boards (HDI boards) connected to various types of cables were also required to meet the 50 ohm impedance standard for impedance matching.

Secondly, the formulation of general standards is based on a comprehensive consideration of the production process, design performance, and feasibility of PCB multi-layer circuit boards (HDI boards).

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

From the impedance calculation process, it can be seen that a low impedance requires a wide linewidth and a thin or large dielectric constant, which is difficult to meet in space for high-density plates currently available; Excessive impedance requires thinner linewidth and thicker dielectric or smaller dielectric constant, which is not conducive to EMI and crosstalk suppression. At the same time, the reliability of PCB multi-layer circuit boards (HDI boards) and their processing from a mass production perspective will be relatively poor.

Controlling a 50 ohm impedance in environments where commonly used boards (such as FR4) and core boards are used, products with commonly used board thicknesses (such as 1mm, 1.2mm, etc.) can be designed with common line widths (4-10mil). This makes it very convenient for circuit board factories to process, and the equipment requirements for its processing and use are not very high.

From the perspective of PCB multi-layer circuit board (HDI board) design, 50 ohms is also chosen after comprehensive consideration. In terms of the performance of PCB multi-layer circuit board (HDI board) routing, low impedance is generally better. For a given line width of transmission line, the closer the distance between the flat surface and the EMI, the lower the corresponding EMI and crosstalk.

But from the perspective of the entire 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, while higher impedance transmission lines were inconvenient to implement, so a compromise was made to use a 50 ohm impedance.

So generally, 50 ohms is chosen as the default value for controlling the impedance of a single ended signal under normal conditions.