The production process has a great influence on the impedance. In theory, continuously adjusting the thickness of the medium can obtain continuously changing impedance control, but this is difficult for PCB manufacturers to achieve, because currently domestic manufacturers generally use laminated boards. The production method, so the medium thickness of each layer is divided into many specifications, rather than continuously changing. The PCB of most PCB manufacturers uses two kinds of media: core material and prepreg. The alternate superposition of core material and prepreg forms a PCB board. The typical structure of an eight-layer PCB board is shown in the figure below.
The core material is a dielectric with copper foil on both sides, a simple double-sided board. The core material has the following dozen specifications: 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1.0mm, 1.2mm, 1.5mm, 1.6mm, 2.0mm , 2.4mm. Note: When performing impedance control, it is necessary to consider whether the thickness of the core material includes the thickness of the copper foil.
The prepreg is mainly composed of resin and reinforcing material, and the reinforcing material is divided into several types such as glass fiber cloth, paper base, composite material, etc., and the prepreg (adhesive sheet) used in multi-layer boards is mostly made of glass fiber cloth. . There are three specifications of prepregs: 1080, 2116, and 7628. At least two or more prepregs should be selected for combination. Since the thickness of the medium becomes thinner due to the excessive pressure of the prepreg during lamination, attention should be paid to the copper foil of the wiring layer when calculating the impedance. Embedded in the medium when laminated, planar layers are unaffected.
From the physical meaning of the above impedance, the impedance is determined by the self-inductance, self-capacitance, mutual inductance, and mutual capacitance of the PCB traces, and the parasitic parameters of these PCBs are closely related to the processing technology of the board and PCB manufacturers, so manufacturers The processing technology directly affects the control accuracy of impedance. According to theoretical analysis, the impedance on the same PCB trace should be consistent, but due to the deviation of the width of the line, the thickness of the medium, and the influence of the processing technology, the impedance of each point of the line is inconsistent.
Compared with the stripline, the microstrip line is more likely to radiate outward and be interfered. Therefore, for key signals such as clocks, low-bit addresses and other periodic signal lines, the form of stripline should be used, and impedance continuity should be maintained. . In addition, the heavy load will also affect the characteristic impedance. Generally, an excessive capacitive load will reduce the characteristic impedance of the object.