4. The Influence of Medium Thickness H
It can be seen from the formula that the characteristic impedance is proportional to the natural logarithm of the dielectric thickness. Therefore, it can be seen that the thicker the dielectric thickness, the greater the impedance value, so the dielectric thickness is another main factor affecting the characteristic resistance value. Because the wire width and the dielectric constant of the material have been determined before production, the wire thickness process requirement 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 production. From the figure, the relationship between the characteristic impedance value and the change in dielectric thickness can be drawn. It can be seen from the figure that when the thickness of the medium changes by 0.025mm, it will cause a corresponding change in the impedance value of +5-8 ohms. In the actual production process, the allowable change in the thickness of each layer will cause the impedance value to change greatly. Big change. 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. Take the surface microstrip line as an example: refer to the picture during the production process. Determine the dielectric constant of the insulating material at the corresponding operating frequency, and then use the formula to calculate the corresponding impedance value, and then according to the wire width value and the impedance value proposed by the user, find the corresponding dielectric thickness through the graph, and then according to the selected The thickness of copper-clad laminate and copper foil determines the type and number of prepregs.
It can be seen from the figure that the design of the microstrip line structure has a higher characteristic impedance value than the stripline design under the same dielectric thickness and material, generally 20Ω-40Ω. Therefore, the microstrip line structure design 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 placed on the error of the dielectric thickness of the copper-clad laminate. Generally speaking, the dielectric thickness of the copper-clad laminate does not change more than 10%. For the multilayer board, the dielectric thickness is still a process. Factors, especially closely related to multi-layer lamination processing, should also be closely controlled.
In actual production, a slight change in the width and thickness of the wire, the 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 will also be related to other production factors, so in order to achieve the characteristic impedance, Control producers must understand the factors that affect the characteristic impedance value change, master the actual production conditions, and adjust each process parameter according to the requirements of the designer to make the change within the allowable tolerance range to obtain the desired impedance value.
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