[SI-LIST] Re: differential pair routing topology

I haven't seen a PCB fabricator in years who has trouble etching sharp
corners.  The advocated of this rule commonly have hundreds or thousands of
sharp corners (both inside and outside corneres) on their surface layers
where their surface mount parts are and don't worry about that.  Avoiding
right angles is another form of Elephant Repellant.  


> [Original Message]
> From: Andrew Ingraham <a.ingraham@xxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 4/24/2006 6:43:07 AM
> Subject: [SI-LIST] Re: differential pair routing topology
>
> > 1) I'm considering the routing topology. Density is a factor, space for
> > routing channels is limited. For width / clearance rules I could use,
> > say, 4/8/4 (mils). I think this could be considered "loosely coupled".
> > This could be routed using a 12 (or divisor of) mil grid. But this
> > spacing can not be maintained the entire length, it must vary slightly
> > at the IC and connector fanouts / vias. So the question is, would it be
> > better to increase the spacing to maintain a more consistent impedance
> > profile, or does it not make a significant difference?
>
> Consider the length of the impedance discontinuity with respect to the
> shortest wavelength.  Maybe it doesn't make enough difference.
>
> Decreasing the spacing is similar to allowing traces of other signals
nearby
> in that region.  Some people get anal trying to maintain precisely some
> particular diff. pair spacing rule, while allowing other traces to impinge
> on the area.
>
> > 2) I understand the primary reason for not using 90 degree corners is
> > the impedance discontinuity due to the capacitance of the extra corner
> > area, so it is better to use 45 degree corners.
>
> Actually, that's not it.  For most of us who work with frequencies below
> 10 GHz or more, the reason for avoiding 90 degree corners is not an
> electrical one.  It is to improve yield, since sharp corners tend to be
> somewhat problematic for good etching.
>
> The electrical discontinuity is there but it's very slight and hardly
> matters unless you're way up into the microwaves.  But there's a lot of
> folklore that ignores this fact.  Some also claim corners become antennas,
> which may be true but only if you go really high in frequency.
>
> Check with your board vendor about using non-chamfered (90 degree)
corners.
> 45 degree bends are generally better but not always necessary.  Consider
> also whether you're running significant lengths of traces at 45 degrees,
or
> only tiny segments.
>
> Regards,
> Andy
>
>
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