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