The factors of effecting PCB’ impedance

Impedance is defined as a good laminated structure that can control characteristics of printed circuit board impedance and its circuits are formed easily to controlled and predictable transmission structures. According to the theory of signal transmission, the signal is a function of time and distance variables, so the signal on every part of circuit is likely to change. The ratio of voltage change and current change is characteristic impedance of transmission circuit. Impedance is an important factor for PCB design, particularly for high frequency circuits. The impacts of circuit impedance include width and thickness of copper, dielectric constant, medium thickness, pad thickness etc.

Dielectric constant

Dielectric constant is Inverse to impedance, the smaller figure of Dielectric constant, the bigger impedance. In order to have high transmission speed, must use high impedance, so choose a low dielectric constant material is very important.


Medium thickness

Medium thickness is Proportional to impedance, the thicker the Medium thickness, the bigger impedance, normally, Medium thickness increase 0.025mm, impedance improve 5-8 ohms, but in actual production, stack up thickness in every layers have change have big influence in impedance.

Width and thickness

The smaller of circuit width, the bigger impedance, the thicker of copper, the larger impedance.

 
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