Why Does the PCB Need Impedance?

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PCB circuit board impedance refers to the parameters of resistance and reactance, which hinders the alternating current.

In PCB circuit board production, impedance processing is indispensable. Why should PCBA factory do impedance to the circuit board?

The bottom of the PCB circuit board should be plugged and installed with electronic components. After the plugging, the problems of conductivity and signal transmission performance must be planned. Therefore, the lower the impedance, the better, and the resistivity should be less than 0.000001 per square centimeter. In the production process of PCB circuit boards, there are also process steps such as sinking copper, electroplated tin, and connector solder. The materials used in these links must ensure low resistivity, so that the overall impedance of the circuit board can meet the quality requirements of the product. . The tin plating is the most prone to problems in the entire PCB circuit board manufacturing process, this link has a great impact on impedance. The chemical tin plating layer has a big defect-easy to oxidize or deliquesce, and poor brazing, which makes the PCB circuit board difficult to solder, the impedance is too high, which leads to poor conductivity and unstable performance of the entire board.

There are various signal transmissions in the conductors in the PCB circuit board. When the transmission rate must be increased to increase its frequency, if the circuit itself is different due to factors such as etching, stack thickness, and wire width, it will cause the impedance to change. The signal distortion causes the performance of the circuit board to decrease, so it is necessary to control the impedance value within a certain range. There is no problem for Kingsheng PCBA to support such service.

Producing PCBs with a 50 ohm PCB impedance is simple from the standpoint of PCB production and processing technologies, taking into account the machinery used by the majority of PCB producers today.

The PCB impedance calculation technique reveals that a low impedance necessitates a big dielectric constant, a thin medium, and a wide linewidth, all of which are challenging to achieve in space for the present high-density PCB board. Conversely, a high impedance makes it necessary to have a thinner linewidth, a thicker medium, or a lower dielectric constant, all of which are detrimental to the reduction of crosstalk and EMI. At the same time, processing reliability is also very good for multilayer PCBs when looking at mass production. It will deteriorate.

Control PCB 50 ohm impedance can be designed with a common line width of 4 ~ 10 ml, which is very convenient for the plate factory to process and does not require a lot of processing equipment in an environment where common PCB boards (FR4, etc.) and common core boards are used to produce products with common PCB thickness (such as 1 mm, 1.2 mm, etc.).

After careful analysis, 50 ohm PCB is also the preferred option when it comes to PCB design. Low impedance is preferable in terms of PCB routing performance. The closer a transmission line is to the plane, the lower the corresponding EMI and crosstalk will be for a given transmission line width.

But, we must take into account a crucial component when viewing the entire signal route, and that is the chip's driving capability. Early chips were unable to drive transmission lines with a PCB impedance of less than 50 ohm, and as it was difficult to build transmission lines with a higher impedance, they switched to a 50 ohm PCB impedance.

As a result, the default value of the single-ended signal control PCB impedance is often set at 50 ohm.