The main aspects of the impedance involved in the PCB board

Today author from qihe smt pick and place machine sharing you with The design of PCB single-sided circuit board needs to contact a lot of professional data and some knowledge of physics.
It can be said to be a comprehensive technical project.
To produce high-quality circuit boards,
Not only do you need a solid foundation,
At the same time, it is necessary to maintain sensitivity to resistance

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, there will be 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, the resistance is also related to the resistivity (β) of the conductor material,
The length of the wire (L),
The cross-sectional area (S) of the conductor is related.
R=βL/S

a. Resistance

When an alternating current flows through a conductor, the resistance encountered is called impedance (Impedance), which corresponds to Z, and the unit is still Ω.
The resistance at this time is different from that encountered by direct current,
In addition to the resistance of the resistor, there is also the resistance of the inductive reactance (XL) and the 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

b. Impedance (Z)
In recent years, with the improvement and application of IC integration,

Its signal transmission frequency and speed are getting higher and higher,
Therefore, in the printed circuit board wire, after the signal transmission (emission) reaches a certain value,
will be affected by the printed board wire itself,
This can result in severe distortion or complete loss of the transmitted signal.
This shows that the “thing” that the PCB wire “flows” is not current,
It is the transmission of square wave signals or pulses in energy.

c. Characteristic impedance control (Z0)
The resistance encountered in the transmission of such "signals" as mentioned above,

Also known as “characteristic impedance”, the representative symbol is Z0.
Therefore, it is not enough to solve the problems of “on”, “off” and “short circuit” on the PCB wire.
It is also necessary to control the characteristic impedance of the wire.
That is to say, transmission lines for high-speed transmission and high-frequency signal transmission,
It is much stricter in quality than the transmission wire.
It is no longer acceptable if the “open circuit/short circuit” test is passed, or if the gap or burr does not exceed 20% of the line width.
It must be required to measure the characteristic impedance value, and this impedance should also be controlled within the tolerance.
Otherwise, it can only be scrapped and not reworked.

The characteristic impedance Z0 of the multilayer board signal transmission line,
The current required control range is usually: 50Ω±10%, 75Ω±10%, or 28Ω±10%.
To control the range of variation, four factors must be considered:
(1) Signal line width W;
(2) Signal line thickness T;
(3) The thickness H of the dielectric layer;
(4) Dielectric constant εr.

The greatest influence is the thickness of the medium,
followed by dielectric constant, wire width,
The minimum is the wire thickness.
After selecting the base material, εr changes little, and H changes little,
T is easier to control, but it is difficult to control the line width W within ±10%.
Moreover, the line width problem has problems such as pinholes, gaps, and depressions on the wire.
In a sense, the most effective and most important way to control Z0 is to control and adjust the line width.

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Mastering the data of these physical properties will be of great help to PCB design.
Experienced technicians will consider all aspects of factors,
In order to avoid quality problems after most, this is not just relying on experience,
More is a full understanding of the physical properties.