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Whether it is high-speed circuits, high-frequency circuits, or millimeter waves, almost all electronic products require the use of PCBs. The processing of PCB boards is a very complex system engineering, involving various issues such as PCB materials, chemicals, processing technology, and so on. There are many factors that can affect the impedance of the transmission line during this process, such as the thickness of copper foil, dielectric thickness, dielectric constant, and dielectric loss angle involved in PCB materials, the etching factor (Etch) involved in processing, the characteristics of etching solution, processing stability, and so on. This article will analyze the impact of several influencing factors on SI impedance from a simulation perspective. When analyzing impedance problems, we can use multiple approaches.

1. Line width of transmission lines

In previous articles, we have introduced a lot about the impact of line width on signal integrity. We know that line width directly affects the impedance and loss of transmission lines. Most excellent engineers will specify the range of line width adjustment when producing Gerber for PCB manufacturers, such as when the line width is designed to be 6.2, the impedance is 50ohm:

If the process of PCB production is unstable, resulting in changes in line width. Based on the experience of collaborating with a large number of manufacturers, it is believed that the variation in transmission line width will be around 10%. Therefore, the type of line width variation is set to Gaussian distribution, and the std is set to 10% for statistical analysis. The simulation results in ADS CILD are as follows:

From the analysis of the results, the lowest impedance will reach 46ohm, while the highest will reach 58ohm; If it is on a long transmission line, extreme states will exist, which will lead to significant return loss and increased insertion loss.

2. Copper foil/copper plating thickness

In PCB products, copper thickness is divided into base copper thickness and copper plating thickness. The base copper is generally relatively uniform (this is relative, but it is not completely uniform), and the uniformity of copper plating will vary depending on the stability of the factory, with some differences being quite significant. Different thicknesses of copper plating can also lead to changes in transmission line impedance and losses. Assuming a variation range of 10% for copper plating, statistical analysis was conducted in ADS CILD, and the results are as follows:

From the analysis of the results, the impedance mainly varies between 49.5 and 51ohm. Compared to line width, the variation range will be much smaller.

3. The thickness of the medium

In PCB production, the main sources of variation in medium thickness are raw materials and the pressing and gluing during the production process. If the thickness of the medium changes, it will cause changes in impedance and losses, and in severe cases, it can lead to significant losses in the transmission line.

From the analysis of the results, the impedance variation is distributed between 44ohm and 54ohm. The range of impedance variation has reached up to 10 ohms.

4. Etching factor

Due to the fact that conductors have a certain thickness, the etched wires produced in production are not a standard “rectangular” structure, but rather a structure close to a “trapezoid” (in fact, the actual situation is not a completely trapezoidal structure), as shown in a schematic diagram of the conductor in the following figure:

The angle of this trapezoid will change with the thickness of copper (and the same applies to copper plating), and the thinner the thickness, the closer the angle is to 90 °. The magnitude of this angle will affect the magnitude of the impedance. The comparison between the 90 ° and 70 ° results is shown in the following figure:

The above experiments were conducted under a single factor change, and in the production process, it is not a single variable change that may occur simultaneously. If it occurs simultaneously, the statistical results are shown in the following figure:

From the results, it can be seen that the impedance mainly varies between 40ohm and 56ohm, which far exceeds the general requirement of 50 ± 10%. And throughout the entire production process, changes in these parameters not only lead to changes in impedance. So for high-speed and high-frequency circuit products, or high-end products, the entire PCB design and production process must strictly control every material and every link, otherwise it will lead to some unexpected problems in the product.