PCB News - The meaning of impedance to PCB board

What is the significance of impedance to PCB boards, and why do they need impedance when making PCB boards? This article first introduces what impedance and its types are, then it introduces why PCB circuit boards need impedance, and finally explains the meaning of impedance to PCB boards. Follow the editor to learn more about it.

In a circuit with resistance, inductance, and capacitance, the obstacle to alternating current is called impedance. Impedance is often represented by Z, which is a complex number. The real part is called resistance, and the imaginary part is called reactance. The hindering effect of capacitance on alternating current in the circuit is called capacitive reactance, and the hindering effect of inductance on alternating current in the circuit is called For inductive reactance, the obstructive effect of capacitance and inductance on alternating current in the circuit is collectively called reactance. The unit of impedance is ohms.
(1) Characteristic impedance
In electronic information products such as computers and wireless communications, the energy transmitted in the PCB circuit is a square wave signal (called pulse) composed of voltage and time. The resistance it encounters is It is called characteristic impedance.
(2) Differential impedance
Two identical signal waveforms with opposite polarities are input at the driving end, which are respectively transmitted by two differential lines, and the two differential signals are subtracted at the receiving end. The differential impedance is the impedance Zdiff between the two wires.
(3) Odd mode impedance
The impedance value of one line to ground of the two lines is the same as the impedance value of the two lines.
(4) Even mode impedance
Two identical signal waveforms with the same polarity are input to the drive end, and the impedance Zcom when the two wires are connected together.
(5) Common mode impedance
The impedance Zoe of one of the two lines to the ground is the same as the impedance value of the two lines, which is usually greater than the odd mode impedance.
The impedance of PCB circuit board refers to the parameters of resistance and reactance, which hinder the alternating current. When making PCB boards, impedance processing is essential. The reasons are as follows:
1. The PCB circuit (bottom of the board) should consider plugging and installing electronic components. After plugging, the conductivity and signal transmission performance should be considered. Therefore, the lower the impedance, the better, and the resistivity should be less than 1 per square centimeter. -6 or less.
2. During the production process,PCB circuit boards have to go through process steps such as copper sinking, tin electroplating (or chemical plating, or thermal spray tin), connector soldering, etc., and the materials used in these links must ensure that the resistivity is low, in order to ensure The overall impedance of the circuit board is low to meet the product quality requirements and can operate normally.
3. The tinning of PCB circuit boards is the most prone to problems in the entire circuit board production, and it is a key link that affects impedance. The biggest defect of the electroless tin coating is easy discoloration (easy to be oxidized or deliquescent) and poor solderability, which will lead to difficult soldering of the circuit board, high impedance, poor electrical conductivity, or instability of the overall board performance.
4. There are various signal transmissions in the conductors in the PCB circuit board. When it is necessary to increase its frequency in order to increase its transmission rate, if the circuit itself is different due to factors such as etching, stack thickness, wire width, etc., the impedance value will change., So that its signal is distorted and the performance of the circuit board is degraded, so it is necessary to control the impedance value within a certain range.
For the electronics industry, according to industry surveys, the most fatal weakness of the electroless tin coating is its easy discoloration (easy to be oxidized or deliquescent), poor brazing properties leading to difficult soldering, and high impedance leading to poor electrical conductivity or instability of the overall board performance., Easy long tin whiskers cause short circuit of PCB circuit and even burn out or catch fire.

The above is an introduction to the meaning of impedance for PCB boards. ipcb also provides PCB board manufacturers, PCB manufacturing technology, etc.