With the improvement of signal transmission speed and the widespread application of high-frequency circuits, impedance control of circuit boards has become a key factor in ensuring signal integrity, reducing signal interference, and improving system stability. Today, the editor will introduce the basic concept of circuit board impedance and the specific requirements for impedance control in the PCB making process.

What does circuit board impedance mean?

Circuit board impedance, in short, refers to the resistance encountered when current is transmitted through wires or signals on a transmission line on a circuit board. It is a complex physical quantity that combines resistance, inductance, and capacitance effects, usually measured in ohms (Ω). In high-frequency signal transmission, we are more concerned with characteristic impedance, which is the impedance value exhibited by a signal when it stably propagates on an infinitely long lossless transmission line. This concept is crucial for ensuring efficient and non reflective transmission of the signal.

Why is it necessary to control the impedance of PCB circuit boards?

Ensure signal integrity: Improper impedance matching can cause signal reflection, weaken signal strength, and even cause signal distortion, affecting the accuracy of data transmission.

Reducing Electromagnetic Interference (EMI): Good impedance control can reduce radiation at signal edges, reduce interference with external devices, and improve the circuit’s anti-interference ability.

Improve system stability and reliability: Ensure that the impedance of all signal lines is consistent, which can avoid signal transmission problems caused by impedance discontinuity and improve the stability and reliability of the entire system.

Requirements for PCB board impedance:

Determine impedance standards: Based on the requirements of circuit design, first determine the required impedance value. Common standard impedances include 50 Ω, 75 Ω, 100 Ω, etc., which are suitable for different signal transmission standards.

Material selection: The dielectric constant (Dk) and loss factor (Df) of PCB boards directly affect the impedance value. Low Dk and low Df materials should be selected, and the thickness of the materials should also be strictly controlled to meet the impedance design requirements.

Circuit design: The width, spacing, copper foil thickness, and distance from the reference plane of the circuit all affect the impedance value. Professional impedance calculation software should be used during design to accurately calculate specific parameters and ensure that the design meets impedance requirements.

Manufacturing process: In the PCB manufacturing process, precise control of lamination thickness, copper thickness, drilling accuracy, etc. is the key to achieving design impedance. The use of advanced manufacturing equipment and technologies, such as laser direct imaging (LDI), chemical copper plating, etc., can effectively improve the accuracy of impedance control.

Test verification: After completing the board making, the actual impedance of the PCB is measured through methods such as flying needle testing and time-domain reflectometer (TDR) testing to ensure that the impedance is controlled within the design range, and adjustments and optimizations are made if necessary.

In short, The impedance control of PCB circuit boards is a comprehensive process involving design, materials, manufacturing, and testing, aimed at ensuring the high performance, high stability, and low interference of electronic products. With the continuous progress of electronic technology, the importance of impedance control has become increasingly prominent, becoming one of the important indicators for measuring PCB quality.