A rule of thumb for 50 Ohm Microstrip
January 27th, 2009
CB Designers often have to get some quick answers to quick questions. In designing PCB, they need to get the characteristic impedance of common structures in a quick way.
One quick thumb rule for the microstrip characteristic impedance that gives 50 Ohm is that the width of the trace has to be twice the trace to ground plane thickness.
This rule of thumb give impedance at slightly lower than 50 Ohm. In other words the width has to slightly less than twice to get closer to 50 Ohm.
As an example with
H = 5 mils
W = 10 mils
er = 4
t = 1 mils
Gives,
Z = 45 Ohms. A pretty close figure. Now make W = 8.5 mils and you get 50 Ohms.
You can find the microstrip impedance calculator using IPC formula at