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The main aspects of impedance involved in the PCB board

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The main aspects of impedance involved in the PCB board

The design of PCB single-sided circuit boards requires exposure to a lot of professional data and some physical knowledge, which can be said to be a comprehensive technical project. To make a high-quality circuit board, you need not only a solid foundation, but also sensitivity to impedance. The impedance involved in the PCB board mainly includes the following aspects:
When the direct current of the printed PCB circuit board passes through the wire, it will experience a resistance. This resistance is called resistance, which corresponds to R, and the unit of value is "ohm" (Ω).
The relationship between resistance and current and voltage is: R=V/I
In addition, resistance is also related to the resistivity (β) of the conductor material, the length of the wire (L), and the cross-sectional area of ​​the conductor (S). R = β L/S


1. Resistance
When an AC current flows through a conductor, the resistance it receives is called Impedance, which corresponds to Z and the unit is Ω.
The resistance at this time is different from the resistance encountered by the DC current. In addition to the resistance of the resistance, there are also the resistance of inductive reactance (XL) and capacitive reactance (XC).
In order to distinguish the resistance of direct current, the resistance encountered by alternating current is called impedance (Z).
Z=√ R2 +(XL -XC)2


2. Impedance (Z)
In recent years, with the improvement and application of IC integration, the signal transmission frequency and speed are getting higher and higher. Therefore, in the printed board wire, after the signal transmission (emission) reaches a certain value, it will be affected by the printed board wire itself. The impact of this, resulting in severe distortion or complete loss of the transmitted signal. This shows that the "thing" "circulated" by the PCB wire is not current, but the transmission of square wave signals or pulses in energy.


3. Characteristic impedance control (Z0)
The resistance encountered during transmission of the above-mentioned "signal" is also called "characteristic impedance", and its symbol is Z0.
Therefore, it is not enough to solve the problems of "on", "off" and "short circuit" on the PCB wire alone, but also to control the characteristic impedance of the wire. In other words, transmission lines for high-speed transmission and high-frequency signal transmission are much stricter in quality than transmission wires. It is no longer the “open/short” test that has passed the test, or the gap or burr does not exceed 20% of the line width, it can be received. It must be required to determine the characteristic impedance value, and this impedance must also be controlled within the tolerance, otherwise, it will only be scrapped and not reworked.
The characteristic impedance Z0 of the signal transmission line of the multi-layer circuit board, currently required to control the range is usually: 50Ω ± 10%, 75Ω ± 10%, or 28Ω ± 10%.


To control the scope of change, four factors must be considered:
(1) Signal line width W;
(2) Signal wire thickness T;
(3) The thickness of the dielectric layer H;
(4) Dielectric constant εr.
The biggest influence is the dielectric thickness, followed by the dielectric constant, the wire width, and the smallest is the wire thickness. After the substrate is selected, the change of εr is small, the change of H is also small, T is easier to control, and it is difficult to control the line width W at ±10%, and the line width problems include pinholes, notches, dents, etc. on the wire problem. In a sense, the most effective and important way to control Z0 is to control and adjust the line width.
Mastering the data of these physical properties will be of great help to PCB design. Experienced technicians will consider all factors in order to avoid quality problems in the end. This is not just relying on experience, but also a full understanding of physical properties.

Pub Time : 2020-12-26 09:36:30 >> News list
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