The circuit performance provided by the printed circuit board must be able to prevent reflections during signal transmission, keep the signal intact, reduce transmission loss, and play the role of matching impedance, so that a complete, reliable, accurate, interference-free, and noise-free transmission signal can be obtained. . The characteristic impedance has a very close relationship with the substrate material (copper clad sheet), so the choice of substrate material is very important in PCB design.
The main factors affecting characteristic impedance are:
1.The dielectric constant of the material and its influence
Generally, the average value can be used to meet the requirements. The signal transmission speed in the dielectric material will decrease as the dielectric constant increases. Therefore, to obtain a high signal transmission speed, the dielectric constant of the material must be reduced. At the same time, a high characteristic resistance value must be used to obtain a high transmission speed, and a low dielectric constant material must be used for a high characteristic resistance value.
2.Influence of wire width and thickness
Changes in the width of the wires allowed in production will result in large changes in the impedance value. The width of the wire is determined by the designer according to a variety of design requirements. It must not only meet the requirements of the wire carrying capacity and temperature rise, but also obtain the desired impedance value.
This requires the manufacturer to ensure that the line width meets the design requirements during production and changes it within the tolerance range to meet the impedance requirements. The thickness of the wire is also determined according to the required current carrying capacity of the conductor and the allowable temperature rise. In order to meet the requirements of use in production, the thickness of the coating is generally 25μm on average. The thickness of the wire is equal to the thickness of the copper foil plus the thickness of the plating. It should be noted that the surface of the wire must be clean before electroplating, and there should be no residue and repairing oil, otherwise it will affect the characteristic impedance value. In addition, in the process of brushing, you must be careful not to change the thickness of the wire and cause the impedance value to change.
3.Influence of dielectric thickness
The thicker the dielectric, the greater the impedance, so dielectric thickness is another main factor that affects the characteristic resistance. Because the wire width and the dielectric constant of the material have been determined before production, and the wire thickness process requirements can also be used as a fixed value, controlling the laminate thickness (dielectric thickness) is the main means to control the characteristic impedance in production. In the actual production process, the allowable change in the thickness of each layer will result in a great change in the impedance value. For multi-layer PCB boards, the thickness of the dielectric is still a processing factor, especially closely related to the multi-layer lamination process. Therefore, it should also be strictly controlled.
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