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Our country is in a favorable situation with economic construction as the center and reform and opening up. The annual growth rate of the electronic industry will exceed 20%. The printed circuit board industry depends on the entire electronics industry and will rise with the trend, and the growth rate will exceed 20%. The technological revolution and industrial structural changes in the world’s electronics industry are bringing new opportunities and challenges to the development of printed circuits. With the development of miniaturization, digitization, high frequency, and multi-functionalization of electronic equipment printed circuits, as the metal wires in the electrical interconnections of electronic equipment, are not only a question of whether current flows or not, but also serve as signal transmission lines. effect. That is to say, for the electrical test of the PCB used for the transmission of high-frequency signals and high-speed digital signals, it is necessary to measure whether the circuit continuity and short-circuit meet the requirements, but also whether the characteristic impedance value is within the specified qualified range. Only when these two directions are qualified, the circuit board meets the requirements.

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. This article discusses the characteristic impedance control of the surface microstrip structure multilayer board commonly used in practice.

**1. Surface Microstrip Line and Characteristic Impedance **

The characteristic impedance of the surface microstrip line is relatively high and is widely used in practice. Its outer layer is the signal line surface that controls the impedance. It is separated from the adjacent reference plane by insulating materials. The calculation of the characteristic impedance The formula is:

**a. Microstrip** Z={87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)] where W is the line width, T is the copper thickness of the trace, and H is the trace to the reference plane Distance, Er is the dielectric constant of the PCB material. This formula must be applied when 0.1<(W/H)<2.0 and 1<(Er)<15.

**b. Stripline** Z=[60/sqrt(Er)]ln{4H/[0.67π(0.8W+T)]} where H is the distance between the two reference planes and the trace is located in the middle of the two reference planes. This formula must be applied when W/H<0.35 and T/H<0.25 It can be seen from the formula that the main factors affecting the characteristic impedance are (1) dielectric constant Er, (2) dielectric thickness H, (3) wire width W, and (4) wire copper thickness T.

Therefore, the characteristic impedance and the substrate material ( The relationship between copper clad board) are very close, so the choice of substrate material is very important in PCB design.

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