← Back to team overview

kicad-developers team mailing list archive

Re: [Question #150142]: Layout of differential line


On Mon, Mar 26, 2012 at 03:55:44PM -0000, Dick Hollenbeck wrote:
> I would say weeks, not days.  What some have done as a workaround, quite successfully is
> do the math on the spacing and trace thickness figuring in copper thickness and layer. 
> Maybe using some calculator, KiCad's pcb_calc may work, haven't used it yet.

Anyway you *have* to do the math before, simply because pcbnew doesn't know the *exact* stackup for your board (and with exact I mean the codes of your prepregs, dielectric coefficients and so on). There are $4000 sw packages *only* for calculating impedences in stackups...

> Then create a footprint with two simple round pads on it, spaced at a distance you
> calculated, with diameters matching the trace thickness.  Then put that at various places
> along the route, and route to those pads with straight segments.  You change the
> orientation of the footprint at various places.

Simpler cases can also be handled using netclasses. I've done an USB high-speed transmission line which is working pretty well.

What netclasses are missing is a *matrix* for clearances: A to A isn't necessarily A to B. Such a feature would also be useful for creepage/clearance in mixed voltage boards: 5V track can be packed pretty tight, 220VAC track should have more distance and the distance between 5V and 220VAC track is... well, huge (you ofted cut slots in the pcb to raise it) XD

Lorenzo Marcantonio
Logos Srl