When direct current passes through a wire, it experiences a resistance called resistance, which corresponds to R and is measured in ohms (Ω).

The relationship between resistance, 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 alternating current flows through a conductor, the resistance it experiences is called impedance, which conforms to Z and is measured in Ω.

At this point, there is a difference in resistance compared to the resistance encountered by direct current. In addition to the resistance of resistance, there are also resistance issues of inductive impedance (XL) and capacitive impedance (XC).

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 have been increasing. Therefore, in printed circuit board wires, when the signal transmission (emission) reaches a certain value, it will be affected by the printed circuit board wires themselves, resulting in serious distortion or complete loss of the transmitted signal. This indicates that the "thing" flowing through PCB wires is not current, but the transmission of square wave signals or pulses in energy.

3. Characteristic impedance control (Z0)

The resistance encountered during the transmission of the aforementioned "signal" is also known as "characteristic impedance", represented by the symbol Z0.

So, solving the problems of "on", "off", and "short circuit" on PCB wires alone is not enough, and it is also necessary to control the characteristic impedance of the wires. That is to say, transmission lines for high-speed transmission and high-frequency signal transmission are much more stringent in quality than transmission wires. It is no longer necessary to pass the "open/short circuit" test, or if the gap or burr does not exceed 20% of the line width, it can be accepted. It is necessary to measure the characteristic impedance value, which must also be controlled within the tolerance. Otherwise, it will only be scrapped and cannot be reworked.

Signal propagation and transmission lines

1. Definition of Signal Transmission Line

(1) According to the principle of electromagnetic waves, the shorter the wavelength (λ), the higher the frequency (f). The product of the two is the speed of light. Namely, C=λ. f=3 × 1010 cm/s

(2) Any component, despite having a high signal transmission frequency, will experience a decrease in transmission frequency or time delay after being transmitted through PCB wires.

Therefore, the shorter the wire length, the better.

(3) Improving PCB wiring density or shortening wire size is advantageous. However, as the frequency of the component increases or the pulse period shortens, the length of the wire approaches a certain range of signal wavelength (speed), and at this time, the component will experience significant "distortion" during PCB wire transmission.

(4) 3.4.4 of IPC-2141 proposes that when a signal is transmitted in a wire, if the length of the wire is close to 1/7 of the signal wavelength, the wire is considered a signal transmission line.

(5) For example:

Should characteristic impedance control be considered for a component with a signal transmission frequency (f) of 10MHz and a wire length of 50cm on the PCB?

Solution: C=λ. f=3 × 1010 cm/s

λ＝C/f＝（3 ×1010 cm/s）/（1 ×107 /s ）＝3000cm

Wire length/signal wavelength=50/3000=1/60