The Influence of High-Frequency PCB Processing on Impedance Control and Its Solutions (Part 2)

2. The Dielectric Constant of the Material and Its Influence

The dielectric constant of the material is determined by the manufacturer of the material at a frequency of 1Mhz. The same material produced by different manufacturers is different due to the different resin content. This study takes epoxy glass cloth as an example to study the relationship between the dielectric constant and the frequency change. The dielectric constant decreases with the increase of frequency. Therefore, the dielectric constant of the material should be determined according to the operating frequency in practical applications. Generally, the average value can be used to meet the requirements. The signal transmission speed in the dielectric material will decrease with the increase of the dielectric constant. Therefore, to obtain a high signal transmission speed, the dielectric constant of the material must be reduced, and at the same time, a high transmission speed must be obtained. Use high characteristic resistance value, and high characteristic resistance value must choose low dielectric constant material.

3. The Influence of Wire Width and Thickness

The wire width is one of the main parameters that affect the characteristic impedance change. The figure uses the surface microstrip line as an example to illustrate the relationship between the impedance value and the wire width. It can be seen from the figure that when the wire width changes by 0.025mm, the impedance value will change by 5-6 ohms. In actual production, if 18μm copper foil is used to control the impedance of the signal line surface, the allowable wire width variation tolerance It is ±0.015mm. If the tolerance of the control impedance change is 35μm copper foil, the allowable wire width variation tolerance is 0.025mm. It can be seen that the allowable wire width change in production will cause a great change in the impedance value. The width is determined by the designer according to a variety of design requirements. It must not only meet the requirements of 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 plating layer is generally 25μm on average, and the thickness of the wire is equal to the thickness of the copper foil plus the thickness of the plating layer.It should be noted that before electroplating, the surface of the wire must be clean, and there should be no residues 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, you must be careful in the process of brushing, do not change the thickness of the wire and cause the impedance value to change.