2018-06-04 17:24:28 by Tim
A stripline trace is defined as a conductive trace in a dielectric material between two reference planes. The dielectric constant of the dielectric material is homogenous meaning it has the same dielectric constant throughout. This is unlike a microstrip trace that has an effective dielectric constant that is a mix of air and FR-4 dielectric material. This leads to the propagation velocity being dependent on Er and is constant as well.
The impedance of a stripline trace can be calculated using the formula below:
This formula is only valid over certain ranges of the actual stripline trace geometry and a field solver should be used to calculate the final desired impedance for the best accuracy. In manufacturing there will be variations in Er of the same FR-4 dielectric material since a stripline trace is often constructed with a pre-preg and core layer. However with a controlled PCB impedance stackup the impedance difference is not usually an issue. This is a good case for when to use a field solver to calculate the trace impedance.
By looking at the formula for the stripline trace and doing some calculations you can see that the trace width W and the dielectric height H (or thickness) of the FR-4 really determines the impedance.
Below are graphs from the above formula for a stripline trace showing the impact of trace width W and dielectric height H on the characteristic impedance:
You can also see by plotting the dielectric constant Er of the FR-4 dielectric material that it can have a small impact on the trace impedance if you consider FR-4 Er to only vary from 3.5 to 4.5.
By working with the trace width W and the dielectric height H of a stripline trace you can get the desired trace impedance.