Explain the formation principle of PCB wire impedance interference?

The formation principle of PCB wire impedance interference

Why do PCB board wires form impedance interference?

Because in the manufacturing of printed circuit boards , the wires are generally copper wires, and the physical properties of copper determine that there must be a certain impedance in the conduction process, and the inductance component in the wires will affect the voltage signal. The resistance component will affect the transmission of the current signal, and the influence of the inductance is particularly prominent in high-frequency lines.

A certain section of wire on the printed circuit board can be regarded as a very regular rectangular copper strip. We take a section of wire with a length of 10cm, a width of 1.5mm and a thickness of 50μm as an example, and the size of its impedance can be seen by calculation. .

Wire resistance can be calculated by the formula:

R=ρL/s (Ω)

In the formula, L is the length of the wire (m), s is the cross-sectional area of the wire (square millimeter), and ρ is the resistivity ρ=0.02. The resistance value of the wire is about 0.026Ω by calculation.

When a length of a wire far from other conductors is much greater than its width, the self-inductance of the wire is 0.8μH/m, then a 10cm-long wire has an inductance of 0.08μH. Then we can calculate the inductive reactance presented by the wire of the copy board by the following formula:

XL=2πfL

where π is a constant, f is the frequency (Hz) of the wire passing through the signal, and L is the self-inductance (H) of the wire per unit length. This way we can calculate the inductive reactance of the wire at low and high frequencies separately:

When f=10KHz, XL=6.28×10×103×0.08×10-6≈0.005Ω;

When f=30MHz, XL=6. 28×30×106×0.08×10-6≈16Ω

Through the above formula, we can calculate that the wire resistance is greater than the wire inductance in the low frequency signal transmission, and the wire inductance is much larger than the wire resistance in the high frequency signal.