What is PCB Impedance

In a circuit with resistors, inductors, and capacitors, 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. The resistance of capacitance to alternating current in a circuit is called reactance. Resistance is called reactance. The unit of impedance is the ohm.

What are the types of impedance?

(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, and the resistance it encounters is called characteristic impedance.

(2) Differential impedance
The driver inputs two identical signal waveforms with opposite polarities, which are transmitted through two differential lines respectively, and the two differential signals are subtracted at the receiving end. Differential impedance is the impedance Zdiff between the two wires.

(3) Odd mode impedance
The first of the two lines has the same impedance ZOO to ground.

(4) Even mode impedance
The driver inputs two identical signal waveforms with the same polarity, and when the two lines are connected together, the impedance ZCOM.

(5) Common mode impedance
Impedance ZOE is one of the two lines to ground. Both lines have the same impedance value, usually greater than the odd-mode impedance.

Why does the PCB board need impedance?
PCB circuit board impedance refers to the parameters of resistance and reactance, and the blocking effect on alternating current. Impedance processing is essential in PCB circuit board production. The reasons are as follows:

1. The PCB line (bottom of the board) should consider the installation of electronic components, electrical conductivity and signal transmission performance when plugging in, so the lower the impedance, the better, the resistivity should be lower than 1× per square centimeter; 10 - 6.

2. In the production process of PCB circuit board, it has to go through manufacturing processes such as copper sinking, electroless tin plating (or chemical plating, or thermal spray tin), connector welding, etc., and the materials used in these links must ensure the resistivity of the bottom to ensure The overall impedance of the circuit board is low enough to meet the quality requirements of the product to work properly.

3. PCB circuit board tin plating is the most likely problem in the entire circuit board production, and it is a key link that affects impedance. The biggest disadvantage of the electroless tin plating layer is that it is easy to change color (both easy to oxidize and dissolve), poor brazing, which will cause difficulty in soldering the circuit board, high impedance, resulting in poor conductivity or unstable performance of the entire board.

4. There will be various signals in the conductor of the PCB circuit board. When the transmission rate is increased, its frequency must be increased. If the line itself is different due to factors such as etching, lamination thickness, and line width, then It will cause changes in the resistance, 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.

Beton is committed to providing customers with multi-layer board special process customization services and solving the problems of enterprise multi-layer board production. Users are welcome to come to consult.

Find us for custom craft

1. Support 1-14 layers of small and medium batch orders, and consult customer service for more layers;
2. Can do HDI blind buried hole process;
3. The board uses Kingboard A-grade board (KB-6164), which can be used as a halogen-free board;
4. The maximum size can be one meter;
5. Pink, transparent, purple and other ink colors;
6. Support copper thickness 10oz, hole copper 35μm;
7. Lamination structure can be customized for multi-layer boards;
8. Support multi-level impedance matching;
9. Copper-clad board edge, two-color ink, socket hole, countersunk hole, square groove, stepped hole, stepped groove, nickel-palladium-gold and other special processes;
10. PCBA can be pasted in one set, and can be pasted on both sides;