PCB Circuit Board Processing Which Factors Will Affect the Transmission Line Impedance

Whether it is high-speed circuits, high-frequency circuits or millimeter wave, as long as the electronic products basically need to use PCB circuit boards, PCB circuit board processing is a very complex system engineering, involving various aspects of the problem, such as PCB materials, potions, processing technology and so on. In this process there will be many factors on the transmission line impedance impact, such as PCB materials involved in the thickness of copper foil, dielectric thickness, dielectric constant, dielectric loss angle of the impact of the processing of the etching factor involved (Etch), etching the characteristics of the potion, processing stability and so on.

a, The Line Width of The Transmission Line

If the PCB in the production process is not stable, resulting in changes in line width. Based on the experience of cooperation with most manufacturers, it seems that the transmission line width changes will be about 10%; if it is in a very long section of the transmission line, there is an extreme state is going to exist, then this will lead to return loss is relatively large, the same insertion loss has increased.

b, Copper Foil / Copper Thickness

In the PCB products, copper thickness is divided into base copper thickness and copper-plated thickness, the base copper will generally be more uniform (which is relative, in fact, is not completely uniform), and copper-plated uniformity will be different with the stability of different factories, some of the difference is also relatively large. Different thicknesses of copper plating can also lead to changes in transmission line impedance and loss. Relative to the line width, the change interval will be quite small.

c, The Thickness of The Medium

In PCB production, the main source of change in the thickness of the medium is the raw materials and production process of compression and glue filling. If the media thickness changes, it will cause changes in impedance, as well as changes in loss, serious cases will lead to a large loss of transmission lines.

d, Etching Factor

Because the conductor is a certain thickness, so in the production of etched wire is not a standard "rectangular" structure, but a close to the "trapezoidal" structure (in fact, the real situation is not completely trapezoidal structure). The angle of the trapezoid varies with the thickness of the copper (and also with the copper plating), the thinner the thickness, the closer the angle is to 90°. The size of this angle affects the size of the impedance.

And in the whole PCB circuit board production process there are more than these parameters will lead to changes in impedance. So for high-speed high-frequency circuit products, or high-end products, the entire PCB design and production process should be strictly controlled every kind of material as well as every link, otherwise it will lead to some unexpected problems in the product.

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