2022-06-08

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Author：pcb

The annual growth rate of the electronics industry will exceed 20%, and the **PCB board** industry will also increase with the trend of the entire electronics industry. And more than a 20% growth rate. The technological revolution and industrial structure changes in the world electronics industry are bringing new opportunities and challenges to the development of printed circuits. Printed circuits develop with miniaturization, digitization, high frequency, and multi-function of electronic equipment. As the electrical interconnects in electronic devices - metal wires in PCB, it is not just a matter of current flow or not. Instead, it acts as a signal transmission line. That is to say, electrical testing of PCBs for the transmission of high-frequency signals and high-speed digital signals. It is not only necessary to measure whether the on, off, and short circuit of the circuit (or network) meets the requirements, but also whether the characteristic impedance value is within the specified qualified range. Only when these two directions are qualified, is the printed board meets the requirements. The circuit performance provided by the printed circuit board must be able to prevent reflection during signal transmission, keep the signal intact, reduce transmission loss, and play the role of matching impedance so that a complete, reliable, interference-free, noise-free transmission signal can be obtained. This paper discusses the problem of characteristic impedance control of the surface microstrip line structure multilayer board commonly used in practice.

1. Surface microstrip line and characteristic impedance

The characteristic impedance of the surface microstrip line is high and widely used in practice. Its outer layer is the signal line surface of controlled impedance, and it is separated from the adjacent reference surface by insulating materials.

For the surface microstrip line structure, the formula for calculating the characteristic impedance is:

Z0=87/SQRT(εr+1.41)×ln[(5.98h)/(0.8w+t)]

Z0: Characteristic impedance of printed wire:

εr: Dielectric constant of insulating material:

h: The thickness of the medium between the printed wire and the reference plane:

w: width of printed wire:

t: Thickness of printed wire.

2. The dielectric constant of the material and its influence

The dielectric constant of the material is determined by the manufacturer of the material measured at a frequency of 1 MHz. The same material produced by different manufacturers is different due to its different resin content. In this study, the relationship between the dielectric constant and the frequency change was studied by taking epoxy glass cloth as an example. The dielectric constant decreases as the frequency increase, so in practical applications, the dielectric constant of the material should be determined according to the operating frequency. Generally, the average value can be used to meet the requirements, and the transmission speed of the signal in the dielectric material will decrease with the increase of the dielectric constant. Therefore, in order to obtain a high signal transmission speed, the dielectric constant of the material must be reduced, and at the same time, a high characteristic resistance must be used to obtain a high transmission speed, and a low dielectric constant material must be selected for a high characteristic impedance.

3. The influence of wire width and thickness

Wire width is one of the main parameters that affect the variation of characteristic impedance. When the wire width changes by 0.025mm, the corresponding change in impedance value will be 5~6Ω. In actual production, if 18um copper foil is used for the signal line surface of the control impedance, the allowable variation tolerance of the wire width is ±0.015mm. If the control impedance variation tolerance is 35um copper foil, the allowable wire width variation tolerance is ±0.003 mm. It can be seen that the variation of the wire width allowed in the production will cause the impedance value to change greatly. The width of the wire is determined by the designer according to various design requirements. It not only needs to meet the requirements of the current-carrying capacity and temperature rise of the wire but also obtains the desired impedance value. This requires the manufacturer to ensure that the line width meets the design requirements and changes 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 25um on average. The wire thickness is equal to the copper foil thickness plus the plating thickness. It should be noted that the surface of the wire should be clean before electroplating, and there should be no residues and trimming oil black so that the copper is not 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 the board, you must be careful not to change the thickness of the wire and cause the impedance value to change.

4. Influence of dielectric thickness (h)

It can be seen from formula (1) that the characteristic impedance Z0 is proportional to the natural logarithm of the dielectric thickness, so it can be seen that the thicker the dielectric thickness, the greater the Z0, 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 requirements can also be used as a fixed value, so controlling the laminate thickness (dielectric thickness) is the main means to control the characteristic impedance in production. The relationship between the characteristic impedance value and the change in the thickness of the medium is obtained. When the thickness of the medium changes by 0.025mm, it will cause a corresponding change in the impedance value of +5 to 8Ω. In the actual production process, the allowable variation in the thickness of each layer of the laminate will result in a large 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. Take the surface microstrip line as an example, determine the dielectric constant of the insulating material at the corresponding operating frequency, then use the formula to calculate the corresponding Z0, and then find out the corresponding dielectric thickness according to the wire width value and the calculated value Z0 proposed by the user. , and then determine the type and number of prepregs according to the thickness of the selected copper-clad laminate and copper foil.

The effect of dielectric thickness of different structures on Z0

Compared with the stripline design, the design of the microstrip line structure has a higher characteristic impedance value under the same dielectric thickness and material, which is generally 20-40Ω larger. Therefore, the design of the 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 with the increase of the dielectric thickness. Therefore, for high-frequency lines 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 change in the dielectric thickness should not exceed 10%. For multi-layer boards, the thickness of the media is also a processing factor, especially when it is closely related to the multi-layer lamination process, so it should also be closely controlled.

5. Conclusion

In actual production, slight changes 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 value to change, and the characteristic impedance value will also be related to other production factors. Therefore, in order to realize the control of the characteristic impedance, the manufacturer must understand the factors affecting 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 on the **PCB board**.