Differential impedance calculation

Thread Starter


Joined Jul 24, 2022

I am doing my first PCB design on USB 1.0/1.1, thus first time dealing with the differential impedance. I did my research on what the differential impedance is and how should I approach it. I have a 2 layer 1.6 mm board that I want to create a 90 Ohm profile on. I used the Altium`s impedance profile calculator and got the parameters, but then I double checked with a Saturn PCB tool - the results were significantly different (Picture 2). I may be missing something as it is my first time dealing with it, but which program should I really trust? Is there any other solution to get reliable results?

Also I contacted my PCB fabrication house to ensure I am using the right dielectric constant value for my calculations. The manager told me that standard 2 layer 1.6 mm FR-4 board has ,,dielectric constant of 4.5 and core dielectric constant of 3.8" (Picture 1, they provided me with this picture) Does that mean, that I should give my solder mask layers the Dk value of 4.5 and my main dielectric layer the value of 3.8 (Picture 3)??? Sounds really weird and counterintuitive...

Thank you in advance.



Joined Jul 24, 2018
FR-4 dielectric constant can vary between 3.8 and 4.8, averaging around 4.3 or 4.5. I'd use 4.5 for your core dielectric (the FR-4). For the soldermask, it varies a bit as well (3.3 to 3.8), but 3.8 is a good general estimate.