Which Impedance Calculator is Right ?

Regarding controlled impedance calculators. What I learned in the IPC CID+ Class is that most impedance calculators only get you in the ballpark and that the only calculation that counts is the one that your fabricator uses to meet the design requirements.

Every fabricator has a slightly different process.  Some use more subtractive (Etching) processes while others will use more additive (plating) processes.  

Polar Instruments impedance calculator seems to be very popular with many fabricators. However, in addition to the calculator's results the fabricators apply their own correction factors based on experience.

You can send the same design with the same stack up and material call outs to a 2 or 3 fabricators and they will come back with different results and recommendations.

I typically use the Saturn PCB Toolkit to estimate trace widths, then consult with the fabricator when precise (+/-10%) controlled impedance lines are required.

There are plenty of free impedance calculators on the the internet, however you should consult with your fabricator if you need controlled impedance traces.

For FR4 materials the dielectric constant (also known as Er or Dk) can vary greatly from ~3.8 to 4.7.  And the Dk number in the datasheet for the material is the neat resin number. The neat resin number is the epoxy only Dk, before factoring in the fiberglass woven material.   

The final Dk and is a function of the resin type, glass type, and the ratio of resin to glass.  Also the glass weave can have a significant influence on controlled impedance of FR4 traces.

Here are three of the most popular glass weaves used in FR4 laminates.

 







That's it !