[SI-LIST] Re: Copper Pours
- From: "Geoff Stokes" <GStokes@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 24 Apr 2006 11:37:12 +0100
Hi Robert
This is an interesting question.=20=20
I have used copper pour, (or split/mixed plane in Mentor PADS for
example) to create an infill around surface components for RF and high
speed designs, mainly analog. I have found that with a continuous
ground plane on the next layer, the RF performance is not affected
greatly, except perhaps in trying to get the best RF connection to
surface components like decoupling capacitors. Both practical testing
and RF simulations using Sonnet Lite show little difference in RF
effects including stray coupling. This has also been seen in CST
Microwave Studio. The gap between the in-fill and the signal trace does
affect the characteristic impedance (Zo) as you say, but is a fairly
slow-varying relationship. The extra top ground creates a structure
called Coplanar Waveguide with Ground (CPWG). Without the infill you
just have Microstrip.
To calculate the transmission line parameters for microstrip or CPWG, I
use AWR TxLine. I have checked the results with Sonnet Lite and
obtained fairly good agreement, but it does depend on the distance of
the sidewalls and lid, and there is some small effect from the launch
connection. However you can get close enough for practical purposes,
which involves the effects of construction tolerances.
You raise the question, what are the advantages of copper pour? I use
it for the following main reasons.
1. To provide better thermal balance between the layers and try to
prevent warping of the board.
2. To shield signal traces on the next layer, in areas where there are
no top signal traces.
3. To provide useful ground connections for probes should it be
necessary to go hunting after solutions to problems.
4. To provide a small amount of extra isolation between traces to a
package with small pin spacings.
One disadvantage is that you need plenty of ground stitching vias
between the top infill and the ground plane to be sure that the top
ground is not "live" which would otherwise create problems. Many RF
microstrip designs work quite well with no infill. But often for me the
advantages won the case.
I try to avoid rules of thumb but have found that if the gap is similar
to or greater than about 1.5 times the dielectric thickness, the infill
does not have a great effect on Zo. A factor of 1.8 or 2.0 is often
good choice.
Regarding control of the spacing, Mentor PADS provides the ability to
set up rules on groups of named connections. That is without paying
extra for the "classes" option which I think might help on large
designs. When the split/mixed plane is connected, the gap is then set
automatically. (I think for practical purposes, "Copper Pour" in PADS
has now been superseded by "Split/Mixed Plane" which does the same job
more effectively for me.)
Now, hatched pours are another thing - maybe a waste of ferric chloride?
Geoff Stokes
Systems Engineer
Zetex Semiconductors plc
Zetex Technology Park
Chadderton
Oldham
OL9 9LL
UK
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+44-161-622-4857
www.zetex.com
www.zetex.cn
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Robert.Havlik@xxxxxxxxxxxx
Sent: 22 April 2006 21:24
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Copper Pours
I understand copper pours are commonly used on PCB's for EMC
shielding,
power supplies, and in analog designs, but I have not found any
resources with
good advice on when a copper pour should or should not be used.
I can see that placing a copper pour connected to a ground plane
around high
speed signal traces would reduce EMI and also possibly signal coupling,
but it
also seems that having a ground in close proximity to a signal trace it
could
also change the characteristic impedance of the transmission line. If
the
copper pour is not a uniform distance around the trace, it seems it
could
potentially cause impedance discontinuities.
Also, I have seen both hatched and solid pours, and I have not seen
anything
on the advantages and disadvantages of either approach or in what
circumstances one should be used over the other.
Any advice on copper pour usage or good rules of thumb would be
greatly
appreciated. Thanks.
-Robert Havlik
University of Colorado, Boulder
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