With the upgrading of customer products, it gradually develops in the direction of intelligence, so the requirements for PCB board impedance are becoming more and more strict, which also promotes the continuous maturity of impedance design technology.
What is characteristic impedance?
1. The resistance generated by alternating current in the components is related to capacitance and inductance. When there is an electronic signal waveform transmission in the conductor, the resistance it receives is called impedance.
2. Resistance is the resistance generated by direct current on components, which is related to voltage, resistivity and current.
Application of Characteristic Impedance
1. The electrical properties provided by the printed board applied to high-speed signal transmission and high-frequency circuits must be such that no reflection occurs during the signal transmission process, the signal remains intact, the transmission loss is reduced, and the matching effect can be achieved. Complete, reliable, accurate, worry-free, noise-free transmission signal.
2. The size of the impedance cannot be simply understood. The bigger the better or the smaller the better, the key is matching.
Control parameters for characteristic impedance
The dielectric constant of the sheet, 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 thickness of the solder mask has little effect on the impedance. When the thickness of the solder mask increases by 10um, the impedance value changes only by 1-2 ohms.
2. In the design, the difference between cover solder mask and non-cover solder mask is large, single-ended 2-3 ohms, and differential 8-10 ohms.
3. In the production of the impedance board, the thickness of the solder mask is normally controlled according to the production requirements.
Impedance test
The basic method is the TDR method (time domain reflectometry). The basic principle is that the instrument emits a pulse signal, which is folded back through the test piece of the circuit board to measure the change in the characteristic impedance value of the emission and foldback. After computer analysis, the characteristic impedance is output.
Impedance problem solving
1. For the control parameters of impedance, the control requirements can be achieved by mutual adjustment in production.
2. After lamination in production, the board is sliced and analyzed. 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 a big difference between the theory and the actual, the biggest possibility is that there is a problem with the engineering design and the design of the test strip.
Post time: Mar-15-2022