[SI-LIST] Re: SV: Re: Copper Fill --- correction
- From: Ritchey Lee <leeritchey@xxxxxxxxxxxxx>
- To: Anders.Ekholm@xxxxxxxxxxxxxxx
- Date: Fri, 14 Jun 2002 08:03:09 -0700
Not a sginificant issue with most PCBs.
Anders Ekholm (ERA) wrote:
> 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:
> > //www.freelists.org/webpage/si-list
> >
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> > List archives are viewable at:
> > //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:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at: =20
> //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:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at:
> //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: vcard.vcf
-- Desc: Card for leeritchey@xxxxxxxxxxxxx
------------------------------------------------------------------
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:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//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:
