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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?

Thanks.

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    \$\begingroup\$ Could be that the stack up of your manufacturer is already in the ballpark of 0.1mm between top layer and the first inner layer. I regularly use PCBWAY and IIRC the height is 0.11mm between outer layers and closest inner layer. \$\endgroup\$ – Wesley Lee Sep 13 '18 at 12:25
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The smaller in width you want your microstrip lines, the thinner your dielectric between ground and signal must be to obtain the required impedance. There's not a lot of getting around that, aside from changing the dielectric constant of your material (but that's usually expensive).

You say you're using a 4-layer PCB, what you may want to consider shopping around for different material stack-ups, or inquiring about custom stack-ups. The stack-up is the layer-by-layer description of the cross section of your PCB. Some places will divide the insulator layers equally (e.g. 0.4mm dielectric between all layers for a PCB), and some will have a thicker middle insulator (between layers 2-3) and thinner outer insulators (between layers 1-2 and 3-4). OSH Park does an assymmetric stack-up, you can see their documentation here.

Asking people to do non-standard (to them) things will likely incur extra costs, and if you need to get down to 0.1mm, that may be on the "more exotic" side of things.

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