What is the significance of PCB impedance for PCB circuit boards? Why do PCB circuit boards need impedance? This article first introduces what impedance and types of impedance are, and then introduces PCB
Why do circuit boards need impedance, and finally explain the significance of impedance for PCB circuit boards, follow the editor to learn more about it.
In a circuit with resistance, inductance and capacitance, the resistance 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. For inductive reactance, the blocking effect caused by capacitance and inductance on alternating current in the circuit is collectively called reactance. The unit of impedance is the ohm.
(1) Characteristic impedance
In electronic information products such as computers and wireless communications, the energy transmitted by the circuit board in the line is a square wave signal (called pulse pulse) composed of voltage and time, and the resistance it encounters is called is the characteristic impedance.
(2) Odd mode impedance
The impedance Zoo between the two lines - the line to the ground, the impedance value of the two lines is the same.
(3) Even mode impedance
The drive end inputs two identical signal waveforms with the same polarity, and the impedance Zcom when the two lines are connected together.
(4) Differential impedance
The driving end inputs two identical signal waveforms with opposite polarities, 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 lines.
(5) Common mode impedance
The impedance Zoe of one line to the ground in the two lines, the impedance value of the two lines is the same, usually larger than the odd-mode impedance.
PCB circuit board impedance refers to the parameters of resistance and reactance, which play a hindering role in alternating current. In the production of PCB circuit boards, impedance processing is essential. The reasons are as follows: 1. The PCB circuit board (board bottom) should consider plugging and installing electronic components, and consider issues such as electrical conductivity and signal transmission performance after plugging, so the lower the impedance, the better, and the resistivity should be lower than per square meter. Centimeter 1 & Tlmes; 10-6 or less.
2. In the production process of PCB circuit boards, it has to go through manufacturing processes such as copper sinking, electroless tin plating (or chemical plating, or thermal spray tin), connector soldering, etc., and the materials used in these links must ensure low resistivity in order to ensure The low overall impedance of the circuit board meets the product quality requirements and can operate normally.
3. The tin plating of the PCB circuit board is the most prone to problems in the entire circuit board production, and it is the key link that affects the impedance. The biggest defect of the electroless tin plating layer is that it is easy to change color (easy to oxidize or deliquescence), poor solderability, which will make the circuit board difficult to solder, high impedance, poor conductivity or unstable performance of the whole board.
4. There will be various signal transmissions in the conductors in the PCB circuit board. In order to increase its transmission rate, its frequency must be increased. If the line itself is different due to factors such as etching, laminate thickness, and wire width, the impedance value will change. , to distort the signal and lead to a decrease in the performance of the circuit board, so it is necessary to control the impedance value within a certain range.
