Impedance control is essential for high-speed circuit design to achieve the best transmission efficiency. Impedance calculation is related to factors such as plane, line width, dielectric thickness, dielectric constant, etc. In the calculation process, the values of each factor should be adjusted according to impedance requirements to find the best solution. This paper introduces the trend relationship between these factors and impedance results for readers’ reference in the calculation process. In this paper, the microstrip single-ended line shown in the following figure is taken as an example, and the default frequency is 1GHz.
Relationship between impedance and dielectric thickness
Prerequisites: copper thickness 1oz, line width 8mil, dielectric constant 4, solder resist thickness 1mil.
Conclusion: The impedance is proportional to the thickness of the medium. The thicker the medium is, the higher the impedance is, and the larger the change rate is. For every 1 5mil thickened within 5 mils, the impedance increases by about 10 ohms on average.
Relationship with impedance linewidth
Prerequisites: copper thickness 1oz, dielectric thickness 4mil, dielectric constant 4 and solder resist thickness 1mil.
Conclusion: The impedance is inversely proportional to the line width. The wider the line width, the lower the impedance, and the larger the change rate. For every 1 5mil within 5 mils, the impedance decreases by about 10 ohms on average.
Relationship between impedance and dielectric constant
Prerequisites: copper thickness 1oz, line width 8mil, dielectric thickness 4mil and solder resist thickness 1mil.
Conclusion: The impedance is inversely proportional to the dielectric constant. The larger the dielectric constant is, the lower the impedance is, but the rate of change is low. With the increase of 0.2, the impedance decreases by about 1 ohm.
Relationship between impedance and copper thickness
Prerequisites: line width 8mil, dielectric thickness 4mil, dielectric constant 4, solder resist thickness 1mil.
Conclusion: The impedance is inversely proportional to the thickness of copper. The thicker the copper, the lower the impedance, but the lower the change rate. The impedance from 0.5oz(0.7mil) to 1.5oz(2.1mil) decreases about 2Ohm on average, and the impedance above 1.5oz decreases less than 1 ohm.
Relationship between impedance and solder resistance
Prerequisites: copper thickness 1oz, line width 8mil, dielectric thickness 4mil and dielectric constant 4.
Conclusion: The impedance is inversely proportional to solder resistance. The thicker the solder resistance is, the lower the impedance is, but the change rate is low. The impedance from 1mil to 2mil decreases by less than 1 ohm.
Relationship between impedance and frequency
Precondition: copper thickness 1 oz, line width 8 mil, dielectric thickness 4 mil, dielectric constant 4, solder mask thickness 1 mil
The higher the impedance is, the lower the impedance is
Summary
From the above conclusion, the conventional impedance control factor adjustment priority is given to line width and medium thickness, and under the condition of meeting the processing capacity, it should be increased as much as possible to reduce the production difficulty.When the frequency reaches a certain level, we should not only pay attention to the impedance, but also consider the influence of material properties such as loss and dielectric constant on the signal.
