The impedance characteristics and impedance control of PCB circuit board
In direct current, an object's action against current is called resistance.However, in the field of alternating current, the capacitor and inductance can impede the flow of current except for resistance, which is called reactance.Capacitance and inductance are called reactive reactance and inductive reactance.Their units is the ohm same with resistance, while its value is depended on the frequency of the alternating current, the higher the frequency of the more big, the smaller the capacitive reactance impedance frequency is low, the less the greater the capacitive reactance and impedance;The resistance to ac is called impedance, and the impedance is usually expressed in Z, but the impedance value is not simply the sum of the resistance, reactance and tolerance. Because of the problem of the electrical reactance and the electrical reactance and the aspect of the phase Angle, the relationship between the vectors and the vectors, the impedance is the sum of the resistance and the reactance on the vector.For a specific circuit, impedance is not constant but varies with frequency.In the resistance, inductance and capacitance series circuit, the impedance of the circuit is generally larger than the resistance, and the impedance decreases to its minimum when the resonance is resonating.In the inductance and capacitance parallel circuit, the impedance increases to its maximum when the resonance is in the circuit, which is opposite to series circuit.
In the circuit board there are all kinds of signals delivered through the conductor, when the frequency of signal transmission and speed is higher and higher and to a certain value, will cause the change of the impedance value,because of the factors of the line itself such as etching, laminated thickness and wire width, make the signal distortion, there will be a Reflection, Dissipation, Attenuation or Delay,or fault signal when serious,which reduces the transmission performance. Therefore, the conductor on the high-speed circuit board should be controlled in a certain range, called "impedance control".In the circuit board, when signals sent from the power, we hope they can be delivered smoothly to the receiver, and receiver to be absorbed completely without any reflection,which is under the lowest energy loss. To achieve this transmission, the impedance in the circuit must be equal to the impedance of the outgoing end, which is called impedance matching.
Impedance matching is one of the most important elements when designing high-speed PCB circuit board. The factors that affect the impedance PCB are mainly the width of copper wire, the thickness of copper wire, the dielectric constant of the medium, the thickness of the medium, the thickness of the welding disk, the path of the ground wire and the density of the walking line. And impedance values have absolute relationship with linear way, to avoid possible signal reflection and other electromagnetic interference and signal integrity problems,and guarantee the stability of PCB's actual use, we must control board walk line impedance when designing PCB. In other words, the impedance value can be determined after setting circuit lines, and there are slight differences in the characteristic impedance produced by different PCB manufacturers. The real fundamental solution to the problem is to avoid the impedance discontinuity as much as possible.
On the other hand, PCB circuit boards experience even sink copper, electroplating tin (or chemical plating, or thermal spray tin), connector soldering in the process of production, must ensure that all the resistivity of materials are lower, which can guarantee the overall circuit board with low impedance that conform to the requirements of the product quality, otherwise the PCB circuit board will not be able to run normally.