Take a look at the impedance solution of PCB single side circuit board design

The demand of PCB single sided circuit board contacts many professional data and some physical knowledge, which can be said to be a comprehensive skill project. To make high-quality circuit boards, we need not only a solid foundation, but also sensitivity to impedance. The impedance involved mainly includes the following aspects:

When DC current of printed PCB circuit board passes through the wire, it will encounter a resistance, which is called resistance. The resistance is R, and the numerical unit 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 of the conductor material（ β)， The length (L) of the conductor is related to the cross-sectional area (S) of the conductor. R=  β  L/S

1. Resistance

When the AC 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 the resistance encountered by DC current. In addition to the resistance of resistance, there are resistance problems of inductive reactance (XL) and capacitive reactance (XC).

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, its signal transmission frequency and speed are getting higher and higher. Therefore, when the signal transmission (transmission) reaches a certain value in the PCB wires, it will be affected by the PCB wires themselves, resulting in severe distortion or complete loss of transmission signals. This shows that the "things" that PCB wires "flow" are not currents, but the transmission of square wave signals or pulses in energy.

The resistance encountered during the transmission of such "signal" is also called "characteristic impedance", and the symbol is Z0.

Therefore, it is not enough to solve the problems of "on", "off" and "short circuit" on the wire, but also to control the characteristic impedance of the wire. In other words, the quality of transmission lines for high-speed transmission and high-frequency signal transmission is much stricter than that of transmission lines. It can be accepted when the "open circuit/short circuit" test is no longer passed, or the notch and burr do not exceed 20% of the line width. The characteristic impedance value must be measured, and the impedance must also be controlled within the service life, otherwise, it is only invalid and cannot be reworked.

The characteristic impedance Z0 of the multilayer board signal transmission line is now required to be controlled at 50 Ω± 10%, 75 Ω± 10%, or 28 Ω± 10%.

Four factors must be considered for the scale of control change:

(1) Signal line width W;

(2) Signal wire thickness T;

(3) Thickness of medium layer H;

(4) Dielectric constant ε r 。

The biggest influence is the dielectric thickness, followed by the dielectric constant, wire width, and the smallest is the wire thickness. After selecting the substrate, ε The change of r is small, the change of H is small, T is easy to control, while the control of wire width W is difficult at ± 10%, and the problem of wire width also includes pinhole, notch, depression, etc. In a sense, the most effective and important way to control Z0 is to control and adjust the line width.

Pcb board design

Grasping these data of physical properties will be very helpful for PCB design. Experienced skilled personnel will consider all aspects of factors to avoid quality problems finally. This is not only relying on experience, but also full understanding of physical properties.