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PCB impedance control


Published by: Keyou June 20,2019

The absence of impedance control on the PCB will cause considerable signal reflection and signal distortion, resulting in design failure. Common signals, such as PCI bus, PCI-E bus, USB, Ethernet, DDR memory, LVDS signals, etc., require impedance control. Impedance control ultimately needs to be realized through PCB design, and higher requirements are also imposed on the PCB board process.

  PCB impedance control, different routing methods can be calculated to obtain the corresponding impedance value.


                            PCB impedance control


   1, PCB impedance control - microstrip line
   It consists of a strip conductor and a ground plane with a dielectric in between. If the dielectric constant of the dielectric, the width of the line, and its distance from the ground plane are controllable, its characteristic impedance is also controllable and its accuracy will be within ±5%.


   2, PCB impedance control - strip line

   A stripline is a strip of copper placed between the dielectric layers between two layers of conductive planes. If the thickness and width of the line, the dielectric constant of the dielectric, and the distance between the two ground planes are all controllable, the characteristic impedance of the line is also controllable with an accuracy of 10%.


                                 PCB impedance control


   3, PCB impedance control - PCB parameters

   Surface copper foil: There are three types of surface copper foil materials that can be used: 12um, 18um and 35um. The final thickness after processing is approximately 44um, 50um and 67um.

Dielectric constant: The dielectric constant of the prepreg is related to the thickness.

   The dielectric constant of the sheet is related to the resin material used. The dielectric constant of the FR4 sheet is 4.2-4.7 and decreases with increasing frequency.

   Dielectric loss factor: The energy consumed by a dielectric material under the action of an alternating electric field is called dielectric loss, and is usually expressed by the dielectric loss factor tan δ.

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