PCB Technical - Method to reduce the impedance factor of the circuit board

Method to reduce the impedance factor of the circuit board

How to reduce the impedance factor of the circuit board

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 the circuit board design.

The main factors affecting characteristic impedance are:

1, the dielectric constant of the PCB circuit board material and its influence on impedance circuit board diagram impedance circuit board diagram

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, the influence of wire width and thickness

Changes in the width of wires allowed in the production of circuit boards can cause great changes in impedance values. 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 that the circuit board manufacturer should 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 black, which will cause the copper to not be plated during electroplating, which will change the thickness of the local wire and 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, the influence of medium thickness

The characteristic impedance of the circuit board is proportional to the natural logarithm of the dielectric thickness, so it can be seen that the thicker the dielectric thickness, the greater the impedance, so the dielectric thickness is another major factor affecting the characteristic resistance. Because the wire width and the dielectric constant of the PCB circuit board material have been determined before production, the wire thickness process requirements can also be used as a fixed value, so controlling the laminate thickness (dielectric thickness) is the main method to control the characteristic impedance in the production. In the actual production process, the allowable change in the laminate thickness of each layer of the circuit board will cause a great change in the impedance value. In actual production, different types of prepregs are selected as the insulating medium, and the thickness of the insulating medium is determined according to the number of prepregs.

Under the same dielectric thickness and material, it has a higher characteristic impedance value, which is generally 20-40Ψ larger. Therefore, the design of microstrip line structure is mostly used for high-frequency and high-speed digital signal transmission. At the same time, the characteristic impedance value will increase as the thickness of the medium increases. Therefore, for high-frequency circuits with strictly controlled characteristic impedance values, strict requirements should be imposed on the error of the dielectric thickness of the copper clad laminate. Generally speaking, the dielectric thickness change does not exceed 10%. For multi-layer boards, the thickness of the media is still a processing factor, especially closely related to the multi-layer lamination process, so it should also be strictly controlled.

in conclusion

In actual circuit board production, slight changes in the width, thickness, dielectric constant of the insulating material and the thickness of the insulating medium will cause the characteristic impedance to change. In addition, the characteristic impedance value is also related to other production factors. Therefore, in order to achieve the control of the characteristic impedance, the producer must understand the factors that affect the change of the characteristic impedance value, master the actual production conditions, and adjust the various process parameters according to the requirements of the designer to make the change within the allowable tolerance range. To get the desired impedance value.