High impedance drives your stackup geometries

Anyone that’s ever tried to put 75-ohm video onto a PCB stackup knows how high impedance will drive layer thicknesses.  Most busses operate using 50-60 ohm impedance, which means you would typically use dielectric thicknesses of around 5 mils to achieve those impedances.  But 75-ohm video on a PCB requires dielectric heights more in the 8-10 mil range.  Not a big deal, but those greater heights will require wider traces to meet the 50 and 60-ohm requirements for any other bus signals routed on that same layer.  And also, the signals will require greater spacing between them for crosstalk control.  So it would kill your routing density.  Comparitively, if you had 60 ohms as your highest impedance on that layer, you can assign that a nice skinny trace width of 4-5 mils, and keep everything a lot closer together.  If you ever had to route to the old 150-ohm differential FibreChannel spec and were worried about route density, forget about it!

Speaking of differential, 100-ohm differential pairs often create the highest impedance, and are very common nowadays.  In order to maintain good differential coupling, you typically need a single-ended impedance of around 65 ohms.  In other words, the impedance of a single trace in the 100-ohm differential pair would be 65 ohms.  So that would typically get assigned to the skinniest trace width for a given layer, usually 4-5 mils.  The problem with that is that the 100-ohm differential signals are usually in the multi-GHz range, and are therefore more susceptible to loss isssues.  So using wider traces can help that, but then that drives up the widths of all the other traces on that layer, as well as necessary crosstalk spacing, and hurts your routing density.  That is why you see some differential signals now using 85-ohm differential pairs, to allow for wider traces.

You can read more about stackup design in my recent article: http://pcdandf.com/cms/magazine/171-current-issue/7993-designers-notebook

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This article first appeared on the Siemens Digital Industries Software blog at https://blogs.sw.siemens.com/electronic-systems-design/2011/05/24/high-impedance-drives-your-stackup-geometries/