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22 Nov 2023 09:46:50 GMTFASTPCB

Why Does PCB Need Impedance?

impedance control pcb1

What is printed circuit board impedance? In a circuit with resistance, inductance and capacitance, the obstruction to alternating current is called impedance. Impedance is often represented by Z, which is a complex number. The real part is called resistance and the imaginary part is called reactance. The impeding effect of capacitor on alternating current in the circuit is called capacitive reactance, and the impeding effect of inductor on alternating current in circuit is called capacitive reactance. The obstruction caused by capacitance and inductance to alternating current in the circuit is collectively called reactance. 

This is an ideal circuit model. Output impedance is 50ohm, impedance of the transmission line is 50 ohms. The input impedance of the Load terminal is also 50 ohms. However it's impossible in fact.
ideal circuit model

Impedance control is very important when designing PCB. If not designed properly, the impedance will change and cause problems, such as signal distortion or electromagnetic interference, affecting the performance of the entire PCB. During PCB production and assembly, there are also some processes that will affect its signal transmission. 

1. The PCB  must consider plugging and installing electronic components. After plugging, consider issues such as conductive performance and signal transmission performance, so the lower the impedance, the better.

2. During the production process, PCB  have to go through manufacturing processes such as copper immersion, electroplating tin (or chemical plating, or thermal spray tin), connector soldering, etc., and the materials used in these processes must ensure low resistivity in order to ensure the overall impedance of the circuit board meets the product quality requirements and can operate normally.

3. The tin plating of PCB  is the most prone to problems in circuit board production, and is a key link that affects impedance. The biggest drawbacks of the chemical tin plating layer are that it is easy to change color (easy to oxidize or deliquesce) and has poor solderability, which will make the circuit board difficult to weld. High impedance will lead to poor conductivity or unstable performance of the entire board.

4. Various signals will be transmitted in the conductors in the PCB. When the frequency must be increased in order to increase the transmission rate, the line itself will cause changes in impedance due to factors such as etching, lamination thickness, wire width, etc., distorting the signal, resulting in a decrease in the performance of the PCB. So it is necessary to control the impedance value within a certain range.

When PCB designing, you can change impedance by increasing copper trace line width or increasing distance between wires, increasing the number of circuit boards'a layers.

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