[SI-LIST] Re: Disadvantages of wider traces
- From: "Barnes, Heidi" <heidi.barnes@xxxxxxxxxx>
- To: <parth.simon@xxxxxxxxx>
- Date: Fri, 20 Apr 2007 11:58:20 -0500
Dear Simon,
Yes, you are correct in that just widening the trace width increases the
capacitance by the simple increase in area between parallel plates....
the problem is that for proper Signal Integrity design one is always
concerned about maintaining the characteristic impedance of the design
to minimize reflections at high frequencies. Typically when one widens
the trace, the dielectric thickness is increased to maintain the correct
characteristic impedance, and in turn the L and C per unit length stays
the same as the trace width is increased since one is keeping Zo fixed
Zo =3D (L/C)^0.5. =20
The other thing to look at is that for stripline where the dielectric
material is uniform around the conductor the dielectric constant does
not change with trace width. This is not true for microstrip,
microstrip has a combination of air and PCB dielectric and Eric
Bogatin's book on Signal Integrity Simplified on page 144 has a nice
plot showing how the "effective" dielectric constant changes with line
width. So for microstrip you may see a slight increase in loss with an
increase in trace width but not for stripline.
Regards,
Heidi Barnes
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Partha Simon
Sent: Friday, April 20, 2007 9:15 AM
To: Zhenggang Cheng
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Disadvantages of wider traces
Hello Cheng,
At higher frequencies we model transmission lines as Lossy Lines, along
with
defuault transmission line modeling, we consider Skin Effect as series
component and Dielectric loss in parallel, which is dependent on the
capacitance per length and which is highly influencied by loss tangent
of
the material, as we increase the trace with capacitance per length
increases
which causes Dielectric loss to increase.
Dielectric loss causes high frequency components attenuation which
casues
rise and fall time degradation, which leads to Inter Symbol
Interference.
Regards,
Simon
On 20/04/07, Zhenggang Cheng <zcheng@xxxxxxxxxx> wrote:
>
> Hi Simon,
>
> What do you mean that wider trace will reduce skin-effect? And how is
> that related to dielectric loss? To me, if skin-effect is reduced, it
> means R is deceased, and then copper loss is decreased.
>
> Look forward to your reply.
>
> Thanks,
>
> Zhenggang
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Partha Simon
> Sent: Thursday, April 19, 2007 6:02 AM
> To: babid_a@xxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Disadvantages of wider traces
>
> Hello,
> Wider traces will lower the Z0 and may cause anti -reflection and it
> depends
> on what stack you are using, for typical 6-layer and nomial
freqenciese
> they
> work well, and at higher higer frequencies they reduce skin-effect and
> may
> cause more dielectric loss.
>
> To work between these tradeoffs we need simulator and we cant
generalise
> the
> case.
>
> Regards,
> Simon
>
>
> On 19/04/07, Babid A <babid_a@xxxxxxxxx> wrote:
> >
> > Hello,
> > Assuming that there is a chance for routing traces as wide as
> possible,
> > what are the disadvantages of having wider traces on the boards
> (>12mil) in
> > respect to noise, jitter etc?
> >
> > -Thanks,
> >
> >
> >
> >
> >
> > ---------------------------------
> > Ahhh...imagining that irresistible "new car" smell?
> > Check outnew cars at Yahoo! Autos.
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