Electronic – LAN9512 differential wire routing on PCB

differentialpcb-designpcb-layers

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.

Best Answer

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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