2018-06-04 17:22:08 by Tim

# Microstrip Impedance

A microstrip trace is defined as a conductive trace on a dielectric material over a reference plane. The top and sides of the trace are exposed to air called a surface microstrip trace. In actual manufacturing the top is usually covered with solder mask and is called a coated microstrip trace. The microstrip trace can also be buried or embedded in a dielectric rather than exposed to air and this is called an embedded microstrip trace.

The value of the effective dielectric constant Ereff is somewhere between 1 (Air) and 4.5 (FR-4 material). Coated and embedded microstrip traces use the dielectric constant of the solder mask and the dielectric material.

The impedance of a surface microstrip trace can be calculated using the formula below:

This formula is only valid over certain ranges of the actual microstrip trace geometry and a field solver should be used to calculate the final desired impedance for the best accuracy.

There are several factors that can change the microstrip trace impedance. For a coated microstrip trace the thickness is usually 1mm thick and this will change the impedance by a few Ohms. Often times the PCB manufacturer will plate up the surface copper to meet the desired trace impedance or change the overall board thickness causing the trace thickness to go from half ounce copper to two or even three ounces. These are all good reasons to use a field solver to calculate the trace impedance.

However by looking at the formula for the microstrip 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 microstrip 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 microstrip trace you can get the desired trace impedance.