[SI-LIST] Re: EMC
- From: "Chris McGrath" <chris.mcgrath@xxxxxxxx>
- Date: Wed, 5 Nov 2003 06:42:41 -0800
Given, say, a 10"x10" board with 5 mil dielectrics, the capacitance due
to the ground fills would be extremely small, wouldn't they? =20
I don't disagree that the fill method has less inductance, but I think
that it would have little effective capacitance that would have a
substantial impact on decoupling the devices due to the small copper
area and the distance to the dielectric. From your description of the
issue, it sounds like ground filling would be pretty far down the list
of the EMI designer's bag of tricks.
-Chris
> -----Original Message-----
> From: Chris Landrum x311 [mailto:clandrum@xxxxxxxxx]=20
> Sent: Wednesday, November 05, 2003 9:16 AM
> To: Chris McGrath
> Cc: si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Re: EMC
>=20
>=20
> Ground filling is useful to create inner layer capacitance=20
> for the power sub-system of the PCB. By filling GND on a=20
> signal layer that is directly adjacent to a PWR plane a=20
> capacitor will be created. This capacitor is often times=20
> MUCH more effective at providing energy to components because=20
> it is far less inductive than a normal leaded or chip capacitor. =20
>=20
> EMI can be caused by an improperly designed power sub-system.=20
> What can happen here is current gradients can be formed in=20
> the PWR/GND planes that can effectively cause radiation=20
> assuming there is an antenna nearby. Also of concern is VCC=20
> and GND bounce. The plane capacitors formed can help avoid=20
> this problem.
>=20
> By ensuring that the chips are getting the proper energy such=20
> that VCC and GND bounce do not occur, you are thereby=20
> reducing the probability that EMI problems can be caused by=20
> the power sub-system.
>=20
> -----Original Message-----
> From: Chris McGrath [mailto:chris.mcgrath@xxxxxxxx]
> Sent: Wednesday, November 05, 2003 8:35 AM
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: EMC
>=20
>=20
> The "ground filling" is a topic that has been discussed at my=20
> company recently and I wanted to get the list's feedback on=20
> why this is done. We never do ground filling on any layers=20
> and the only reason that I have ever heard for it was to=20
> reduce EMI, but given the disadvantages (increased thermal=20
> profile, potential for crosstalk, PCB viewer and gerber=20
> viewer complications, etc.) and the fact that I have never=20
> been able to find data or any science to back up the EMI=20
> argument, I don't see any benefit to ground filling on signal=20
> layers. (Of note is that by using the term "ground filling",=20
> I am not referring to "thieving" to equalize the copper=20
> distribution to facilitate PCB fabrication.)
>=20
> I am very interested in hearing feedback from any of you.
>=20
> -Chris
>=20
>=20
> > -----Original Message-----
> > From: Suresh.K [mailto:sureshk@xxxxxxxxxxxxxx]=3D20
> > Sent: Wednesday, November 05, 2003 2:54 AM
> > To: subramani
> > Cc: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: EMC
> >=3D20
> >=3D20
> >=3D20
> > Dear Sir,
> >=3D20
> > According to your Board Stack you have two strip lines and=3D20 one =
> >assymmetrical dual stripline,but the two strip lines are=3D20 =20
> bounded by=20
> >one side power plane and Ground Plane on the other=3D20 side......
> >=3D20
> > If you have both side ground plane reference for the strip=3D20
> > lines ,you can route all the Impedance controlled signals on=3D20
> > Layer2 & layer8,because strip lines bounded by ground planes=3D20
> > will be best layer for better signal=3D20
> > integrity.
> >=3D20
> > But now you have assymmetrical dual stripline bounded by=3D20
> > ground planes,so I beleive that layer 5&6 may be better=3D20
> > option for routing clock signals,which will useful for return=3D20
> > path, but you need to care of tandem=3D20
> > pair of traces.
> >=3D20
> > and regarding board power plane EMI,you can follow 20H rule=3D20
> > on layer2& layer9 power planes to reduce the power plane EMI=3D20
> > and Top & Bottom layers you are doing Ground fill, so I think=3D20
> > the device EMI will be controlled by the Ground fills.
> >=3D20
> > Regards,
> > Suresh.K,
> > Vth EDA Lab,
> > C-DOT,
> > Bangalore-52.
> >=3D20
> >=3D20
> >=3D20
> >=3D20
> >=3D20
> > On Wed, 5 Nov 2003, subramani wrote:
> >=3D20
> > > Hello,
> > >=3D20
> > > I am doing a board design. It has to pass stringent EMI tests. =
=3D20
> > > Mine is a 10 layer board.
> > >=3D20
> > > The board stack up is
> > > 1 TOP component, GND filling
> > > 2 Power
> > > 3 signal
> > > 4 GND filling
> > > 5 signal
> > > 6 Signal
> > > 7 GND filling
> > > 8 Signal
> > > 9 Power
> > > 10 Bottom Component, GND filling
> > >=3D20
> > > The board has SDRAM operating at 100Mhz. Where should I=20
> route the=3D20
> > > clocks. Could anyone tell me about the ways and means of=3D20
> > reducing EMI.=3D20
> > > The SDRAM is placed that is near to the edge of PCB. Will it=20
> > >cause=3D20 radiation. Is there a formula for keepout distance. =
=3D20
> > > Regards
> > > Subramani
> > > Mistral
> > >=3D20
> > >=3D20
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