[SI-LIST] Re: SV: Re: Copper Fill --- correction

This prompts another question --

If a relief pattern is used in the copper land (to avoid solid planes for
thermal expansion, mechanical reasons), has anyone considered optimal mesh
dimensions for various operating frequencies?   Do you have to use a solid
plane at high frequencies to guarantee trace impedances, inter-plane
capacitances?

I've used relief patterns in low frequency / low-level applications with
good results (for board twist / bend) and good electrical performance.   

On the issue of EMI -- I should think that a floating plane could affect EMI
in as much as it presents a metallic surface whereby mutual capacitive
coupling could occur.

Timothy J. Christman
Test Engineer
Tel 651.582.3141  Fax 651.582.7599
timothy.christman@xxxxxxxxxxx
Guidant Corporation 
4100 Hamline Ave. N.  
St. Paul,  MN   55112  USA 
www.guidant.com

-----Original Message-----
From: Anders Ekholm (ERA) [mailto:Anders.Ekholm@xxxxxxxxxxxxxxx]
Sent: Friday, June 14, 2002 8:25 AM
To: 'Jason.d.Leung@xxxxxxxxxxx'; scott@xxxxxxxxxxxxx;
RayCaliendo@xxxxxxxxxx
Cc: silist
Subject: [SI-LIST] SV: Re: Copper Fill --- correction



Hi.
For other reasons than Signal Integrity ones, we want a uniform copper =
distribution
on the PCB, since otherwise it will bend. A uniform copper distribution =
is good
for mechanical reasons.

              Regards /Anders Ekholm


-----Ursprungligt meddelande-----
Fr=E5n: Jason D Leung [mailto:Jason.d.Leung@xxxxxxxxxxx]
Skickat: den 14 juni 2002 15:00
Till: scott@xxxxxxxxxxxxx; RayCaliendo@xxxxxxxxxx
Kopia: silist
=C4mne: [SI-LIST] Re: Copper Fill --- correction


Hi guys,
I understand that if the fill is too close to the signal trace it is =
going
to increase the capacitance and decrease the impedance in turn.
But my question will be why do we need to put a fill on the pcb. ( this =
may
be a basic question , and I always heard my colleagues saying it, but =
it
helps me alot, if there's a kind soul to help me to understand it)

thanks in advance
Regards
Jason

Scott McMorrow wrote:

> Okay, now I really need some coffee ... and it's 5:30 in the evening.
> Here's yet another correction and more elaboration.
>
> A square patch of copper on top of a dielectric and plane forms a =
crude
> cavity resonator which will have a number of natural resonant
> eigenmodes.  The first resonant point is a half-wave resonance.  The
> resonance frequency is equal to the round trip of a wave propagating
> back and forth across the region, much like what would happen in a
> square pool of water between the two walls.  For a one square inch =
patch
> of copper, the resonant frequency on FR-4 is somewhere between 2.7 =
and
> 3.3 GHz
>
> A very crude square patch
>
>          w
> --------------
> |                    |      resonant frequency =3D 1/( 2 * w * Vp)
> |                    |     Vp =3D velocity of propagation of wave in
> dielectric medium =3D c/sqrt(Er)
> |                    |      w =3D width and height of square
> |                    |
> |                    |
> ---------------
>
> Adding vias to the patch effectively pins the structure down and =
raises
> it's resonant frequency.  The spacing of the vias control the =
resonance,
> with a half-wave resonance being the lowest generally supported by =
the
> structure. Stitching the patch at all four corners actually does =
nothing
> to change the resonant frequency of the cavity.  However, placing a
> fifth via at the center will push the resonance up by a factor of 2.
>
>            w
> @------------@
> |                       |      resonant frequency =3D 1/( 4 * w * Vp)
> |                       |      Vp =3D velocity of propagation of wave =
in
> dielectric medium
> |           @        |       w =3D width and height of square
> |                       |     @ =3D Via stitch
> |                       |
> @------------@
>
> In essence, the via spacing has changed from w to w/2. These formulas
> are quite approximate, but will place us in the general playing =
field. A
> full-wave field solver such as PowerSI from Sigrity, or SIwave and =
HFSS
> from Ansoft, will provide much more exact resonance frequency =
analysis.
>
> So, the "rough" formula I gave for via spacing should be:
>
> via spacing =3D1/(2 * (f * Vp)) not 2/(f * Vp) as I had indicated.
>
> In my previous example, this would amount to a via spacing of 0.59
> inches, not 1.18.
>
> Sorry about my confusion.
>
> Scott
>
> --
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 2926 SE Yamhill St.
> Portland, OR 97214
> (503) 239-5536
> http://www.teraspeed.com
>
> Scott McMorrow wrote:
>
> > My equation below has an error.
> >
> > The maximum via spacing should be:
> >
> > 2/(f * Vp)
> >
> > This accounts for the half-wave resonance which occurs between =
vias.
> >
> >
> > regards,
> >
> > Scott
> >
> >
> > Scott McMorrow wrote:
> >
> >> Ray,
> >>
> >> When the separation of the copper fill from any signal is > 20 =
times
> >> the signal-to-plane spacing there is no effect on the signals. So,
> >> for 5 mil dielectric thickness, the fill area to signal trace
> >> clearance should be 100 mils or greater.  I would also suggest
> >> grounding each fill area at multiple points to eliminate possible =
EMI
> >> coupling problems..  At a minimum, use ground stitch vias at four
> >> corners to contain resonances of the fill area.  For extremely =
high
> >> speed multi-gigahertz systems, the vias should be spaced a maximum
> >> distance apart of 1/(f * Vp) apart. Where f =3D the upper =
frequency of
> >> operation and Vp is the velocity of propagation of signals within =
the
> >> dielectric.
> >>
> >> For FR4 Vp is around 180ps/in.  For 3.125 Gbps signallling the =
upper
> >> frequency you are concerned with is the 3rd harmonic of the =
switching
> >> frequency, 4.68 GHz.  In this case, the maximum stitch via spacing
> >> would be
> >> 1/(4.68e9 * 180e-12) =3D  1.18 inches.
> >>
> >> Best regards,
> >>
> >> Scott
> >>
> >>
> >>
> >>
> >
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at:
>                 http://www.freelists.org/archives/si-list
> or at our remote archives:
>                 http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>                 http://www.qsl.net/wb6tpu
>


-- Binary/unsupported file stripped by Ecartis --
-- Type: text/x-vcard
-- File: jason.d.leung.vcf
-- Desc: Card for Jason D Leung


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:    =20
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages=20
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
 =20
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: