[SI-LIST] Re: Disadvantages of wider traces
- From: "Muranyi, Arpad" <arpad.muranyi@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 20 Apr 2007 10:42:03 -0700
Sorry if I repeat what may have been already stated,
but widening the traces will start allowing other
modes of wave propagation also which could complicate
matters. But I will leave it to the electro magnetic=20
experts to explain what I am referring to...
Arpad
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On Behalf Of Barnes, Heidi
Sent: Friday, April 20, 2007 9:58 AM
To: parth.simon@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Disadvantages of wider traces
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 =3D3D (L/C)^0.5. =3D20
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
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