Hi everyone,
I am currently designing a custom electronic board based on the NXP MCIMX93-SOM. I am trying to understand the stackup (6 layers) and differential pairs width and spacing.
For example, for a 85 Ohms on top layer (for RAM signals), the gerbers detail B says that the trace should be 4.6mils wide and 4.4mils spaced. The top copper size is 0.33oz (11.6µm) and prepreg between first layer and second layer is 2.8mils of TU768 which has an epsilon of 4.3. I verified the line size on the gerbers and it matches.
When I put all those parameters in impedance calculators, the differential impedance is more about 100 Ohms than 85. I tried multiple calculators (kicad, digikey, other PCB online calculator, even chatGPT) and I have often 100 Ohm except for digikey which is more about 92 Ohms.
So I have three questions :
1/ Why is there so much difference between what is written in the gerbers and what the calculators say?
2/ I assume you used Altium to route the PCB, so what could be the difference with other calculators helpers ?
3/ Why is the epsilon is different on each layers, even if it is the same materials ? I assume it is effective epsilon and not material epsilon.
Thanks for your help and suggestions.
Kind regards,
