There will be various signal transmissions in the conductors of the circuit board. When the transmission rate increases, the frequency must increase. If the line itself is different due to etching, stack thickness, wire width, and other factors, the impedance will be worth the change, and the signal will be distorted. Therefore, the impedance value of the conductor on the high-speed circuit board should be controlled within a certain range, which is called “impedance control”. The main factors affecting the impedance of the circuit board are the width of the copper wire, the thickness of the copper wire, the dielectric constant of the medium, the thickness of the medium, the thickness of the pad, the path and winding of the ground wire. Therefore, when designing the PCB circuit board, the impedance of the circuit on the circuit board must be controlled to avoid electromagnetic interference and signal integrity problems such as signal reflection as much as possible, and to ensure the stability of the actual use of the PCB board.
1. The characteristics of the circuit board with impedance control:
According to the theory of signal transmission, the signal is a function of time and distance variables, so the signal may change in each part of the connection. Therefore, determine the AC impedance of the transmission line, that is, the ratio of the voltage change to the current change, as the characteristic impedance of the transmission line: the characteristic impedance of the transmission line is only related to the characteristics of the signal connection itself. In actual circuits, the resistance of the conductor itself is smaller than the distributed impedance of the system, especially in high-frequency circuit boards. The characteristic impedance mainly depends on the distributed impedance caused by the connected unit distributed capacitance and unit distributed inductance.
2. Impedance control of the circuit board:
The characteristic impedance of the conductor on the circuit board is an important indicator of circuit design. Especially in the PCB circuit board design of high-frequency circuit boards, it is necessary to consider whether the characteristic impedance of the conductor is consistent with the characteristic impedance of the device or signal, and whether it matches. Therefore, in the reliability design of PCB circuit board design, two concepts must be paid attention to.
