Using the Saturn PCB Tool (V7.05 - calculation method IPC-2152 - Etch Factor 1:1), in order to achieve a 90 ohm differential line with a single line impedance equal to 50 ohm for a design based on the LAN9512 chip, I've found these parameters:
conductor width : 0.15 mm conductor spacing: 0.25 mm conductor height : 0.10 mm
This set gives me 91.2 ohm (differential) and 47.7 (Zo) with a FR-4 STD substrate permittivity equal to 4.6 (changing between 4.3 up to 4.9 don't cause trouble and the value stay around the prescribed nominal value in a ±5%).
This track values are compatible with the LAN9512 land pattern width equal to 0.28 mm and also give some free into the board routing process.
As a consequence, this will give a very thin PCB. Does anyone know if there is a way to achieve the goal (to work with a LAN9512 chip) with track width around 0.15-0.2 mm and a final PCB (4 layers) thickness of 1.6 mm nominal?