The design of single-sided PCB requires a lot of professional data and some physical knowledge, which can be said to be a comprehensive skill project. If you want to make a high-quality PCB, you need not only a solid foundation, but also sensitivity to impedance. The impedance involved in PCB mainly includes the following aspects:
When the direct current of the printed circuit board passes through the wire, it will encounter a resistance, which is called resistance, and the compliance is R, and the numerical unit is “ohm” (ω).
The relationship between resistance and voltage is: R = V/I.
In addition, the resistance is also related to the resistivity (β) of the conductor material, the length (L) of the conductor, and the cross-sectional area (S) of the conductor. R= β L/S
1. resistance
When an alternating current flows through a conductor, the resistance it encounters is called Impedance, which is z and the unit is still ω.
At this time, the resistance is different from that encountered by DC current. Besides the resistance of resistance, there are resistance problems of inductive reactance (XL) and capacitive reactance (XC).
Is the resistance of differential direct current, and 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 frequency and speed of signal transmission are getting higher and higher. Therefore, when the signal transmission (emission) reaches a certain value in PCB wires, it will be affected by the PCB wires themselves, resulting in severe distortion or complete loss of transmitted signals. This shows that the “thing” that PCB wires “circulate” is not the current, but the transmission of square wave signals or pulses in energy.
The resistance encountered when the above-mentioned “signal” is transmitted is also called “characteristic impedance”, and its representative symbol is Z0.
Therefore, it is not enough to solve the problems of “on”, “off” and “short circuit” in PCB design wires, but also to control the characteristic impedance of the wires. That is to say, the transmission line for high-speed transmission and high-frequency signal transmission is much stricter in quality than the transmission line. It can be accepted if it no longer passes the “open/short circuit” test, or if the gap and burr do not exceed 20% of the line width. It is required to measure the characteristic impedance value, and this impedance should also be controlled within the public service, otherwise, it will only be invalid and cannot be reworked.
At present, the characteristic impedance Z0 of multilayer signal transmission line requires that the control scale is usually 50Ω 10%, 75Ω 10%, or 28Ω 10%.
To change the scale of control, four factors must be considered:
(1) signal line width w;
(2) signal line thickness t;
(3) thickness h of dielectric layer;
(4) Dielectric constant ε r.
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, εr changes little, so does H, and T is easy to control, but it is difficult to control the line width W at 10%. Moreover, the line width problem has pinholes, gaps, dents and other problems on the wire. In a sense, the most effective and important way to control Z0 is to control the line width.
Grasping the data of these physical properties will be of great help to PCB design. Experienced technicians will consider all factors and avoid the final quality problems. This is not only based on experience, but also a full understanding of physical properties.
The above is the PCB single-sided circuit board design impedance. I hope I can help you with the introduction of impedance involved in PCB.
