What is the Characteristic Impedance and Problem Solving Method in PCB?

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With the upgrading of customers' products, they are gradually developing towards intelligence. As a result, the requirements for Standard PCB impedance have become more stringent, which has also promoted the continuous maturation of impedance design technology. Now the editor summarizes impedance applications and control methods for everyone to communicate. share it.

What is characteristic impedance?

1. The resistance generated by the alternating current at the components is related to the capacitance and inductance. When the electronic signal waveform is transmitted in the conductor, the resistance it receives is called impedance.

2. Resistance It is the resistance generated by the direct current on the components and it is related to the voltage, resistivity and current.

Application of characteristic impedance

1. The electrical properties provided by printed boards used in high-speed signal transmission and high-frequency circuits must be such that they do not reflect during signal transmission, the signal remains intact, reduces transmission loss, and plays a matching role. Complete, reliable, accurate, worry-free, noise-free transmission signal.

2. The size of the impedance cannot be simply understood. The larger the better, or the smaller the better. The key is matching.

Control Parameters of Characteristic Impedance

The dielectric constant of the plate, the thickness of the dielectric layer, the line width, the copper thickness, and the thickness of the solder mask.

Influence and control of solder mask

1. The effect of solder mask thickness on impedance is small. The thickness of solder mask increases by 10um, and its resistance value changes only 1-2 ohms.

2. There is a big difference between cover and non-cover soldering resistance in the design. The single end is 2-3 ohms, and the difference is 8-10 ohms.

3. In the production of the impedance plate, the thickness of the solder resist is normally controlled according to the production requirements.

Test of impedance

The basic method is the TDR method (time domain reflection method). The basic principle is that the instrument emits a pulse signal, which is folded back after the test piece of the circuit board, and the changes in the characteristic impedance values of the emission and folded back are measured. After the computer analysis, the characteristic impedance is output.

Dealing with Impedance

1. For the control parameters of impedance, the control requirements can be achieved by adjusting each other in production.

2. After lamination in production, slice the board for analysis. If the thickness of the medium is reduced, the line width can be reduced to meet the requirements; if it is too thick, the copper can be thickened to reduce the impedance value.

3. In the test, if there is much difference between the theory and the actual situation, it may be that the engineering design and test strip design have problems.

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