[SI-LIST] Re: EMC
- From: "Chris McGrath" <chris.mcgrath@xxxxxxxx>
- Date: Wed, 5 Nov 2003 07:12:53 -0800
Your point is well taken and in the example I stated the (theoretical)
capacitance that we could expect is around 200 pF/sq.in. which is not
negligible given the lower inductance. However, like most things this
list discusses, the whole system plays a part in determining whether a
design method is appropriate but I am glad to have a better
understanding of the reasoning for having the ground fills. (I must
admit, however, that the cost of passive components- especially on the
order of a penny or less per component- is not a factor in most of our
designs. To me, the greatest benefit to removing a passive component,
especially capacitors, is that lowering the component count increases
MTBF of the system.)
While I realize that the theory makes sense for all layers, I am
assuming that if ground fills are actually used that they are mainly
done on the outside (top and bottom) layers and not on internal layers.
From a thermal profiling and manufacturing perspective, filling up
unused regions everywhere within a PCB can be a serious problem as the
layer count increases (16+ layers). What are your thoughts?
-Chris
> -----Original Message-----
> From: Chris Landrum x311 [mailto:clandrum@xxxxxxxxx]=20
> Sent: Wednesday, November 05, 2003 9:58 AM
> To: Chris McGrath
> Cc: si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Re: EMC
>=20
>=20
> You are correct is noticing the capacitance is small.... But=20
> don't forget, you are getting rid of much of the parasitic=20
> inductance that will cause resonance, as compared to an SMT=20
> or leaded cap. This parasitic inductance effectively causes=20
> your bypass cap to be AN INDUCTOR at any freq above 100MHz!!!=20
> In other words it is USELESS to your power sub-system. You=20
> might as well have not used it at all and saved your company=20
> the $0.001/brd. With the inner planer capacitor, it is much=20
> more effective at high frequency, albeit, it does not provide=20
> a lot of charge. It can make all the difference in the world=20
> at higher freq.
>=20
> -----Original Message-----
> From: Chris McGrath [mailto:chris.mcgrath@xxxxxxxx]
> Sent: Wednesday, November 05, 2003 9:43 AM
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: EMC
>=20
>=20
> Given, say, a 10"x10" board with 5 mil dielectrics, the=20
> capacitance due to the ground fills would be extremely small,=20
> wouldn't they? =3D20
>=20
> I don't disagree that the fill method has less inductance,=20
> but I think that it would have little effective capacitance=20
> that would have a substantial impact on decoupling the=20
> devices due to the small copper area and the distance to the=20
> dielectric. From your description of the issue, it sounds=20
> like ground filling would be pretty far down the list of the=20
> EMI designer's bag of tricks.
>=20
> -Chris
>=20
>=20
> > -----Original Message-----
> > From: Chris Landrum x311 [mailto:clandrum@xxxxxxxxx]=3D20
> > Sent: Wednesday, November 05, 2003 9:16 AM
> > To: Chris McGrath
> > Cc: si-list@xxxxxxxxxxxxx
> > Subject: RE: [SI-LIST] Re: EMC
> >=3D20
> >=3D20
> > Ground filling is useful to create inner layer=20
> capacitance=3D20 for the=20
> >power sub-system of the PCB. By filling GND on a=3D20 signal=20
> layer that=20
> >is directly adjacent to a PWR plane a=3D20 capacitor will be=20
> created. =20
> >This capacitor is often times=3D20 MUCH more effective at providing=20
> >energy to components because=3D20 it is far less inductive=20
> than a normal=20
> >leaded or chip capacitor. =3D20 =3D20
> > EMI can be caused by an improperly designed power sub-system.=3D20
> > What can happen here is current gradients can be formed in=3D20
> > the PWR/GND planes that can effectively cause radiation=3D20
> > assuming there is an antenna nearby. Also of concern is VCC=3D20
> > and GND bounce. The plane capacitors formed can help avoid=3D20
> > this problem.
> >=3D20
> > By ensuring that the chips are getting the proper energy such=3D20
> > that VCC and GND bounce do not occur, you are thereby=3D20
> > reducing the probability that EMI problems can be caused by=3D20
> > the power sub-system.
> >=3D20
> > -----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
> >=3D20
> >=3D20
> > The "ground filling" is a topic that has been discussed at my=3D20
> > company recently and I wanted to get the list's feedback on=3D20
> > why this is done. We never do ground filling on any layers=3D20
> > and the only reason that I have ever heard for it was to=3D20
> > reduce EMI, but given the disadvantages (increased thermal=3D20
> > profile, potential for crosstalk, PCB viewer and gerber=3D20
> > viewer complications, etc.) and the fact that I have never=3D20
> > been able to find data or any science to back up the EMI=3D20
> > argument, I don't see any benefit to ground filling on signal=3D20
> > layers. (Of note is that by using the term "ground filling",=3D20
> > I am not referring to "thieving" to equalize the copper=3D20
> > distribution to facilitate PCB fabrication.)
> >=3D20
> > I am very interested in hearing feedback from any of you.
> >=3D20
> > -Chris
> >=3D20
> >=3D20
> > > -----Original Message-----
> > > From: Suresh.K [mailto:sureshk@xxxxxxxxxxxxxx]=3D3D20
> > > Sent: Wednesday, November 05, 2003 2:54 AM
> > > To: subramani
> > > Cc: si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Re: EMC
> > >=3D3D20
> > >=3D3D20
> > >=3D3D20
> > > Dear Sir,
> > >=3D3D20
> > > According to your Board Stack you have two strip lines=20
> and=3D3D20 one=20
> > >=3D
>=20
> > >assymmetrical dual stripline,but the two strip lines are=3D3D20 =
=3D20
> > bounded by=3D20
> > >one side power plane and Ground Plane on the other=3D3D20 =20
> side......=20
> > >=3D3D20 If you have both side ground plane reference for the=20
> > >strip=3D3D20 lines ,you can route all the Impedance=20
> controlled signals=20
> > >on=3D3D20 Layer2 & layer8,because strip lines bounded by ground=20
> > >planes=3D3D20 will be best layer for better signal=3D3D20
> > > integrity.
> > >=3D3D20
> > > But now you have assymmetrical dual stripline bounded by=3D3D20
> > > ground planes,so I beleive that layer 5&6 may be better=3D3D20
> > > option for routing clock signals,which will useful for =
return=3D3D20
> > > path, but you need to care of tandem=3D3D20
> > > pair of traces.
> > >=3D3D20
> > > and regarding board power plane EMI,you can follow 20H rule=3D3D20
> > > on layer2& layer9 power planes to reduce the power plane =
EMI=3D3D20
> > > and Top & Bottom layers you are doing Ground fill, so I =
think=3D3D20
> > > the device EMI will be controlled by the Ground fills.
> > >=3D3D20
> > > Regards,
> > > Suresh.K,
> > > Vth EDA Lab,
> > > C-DOT,
> > > Bangalore-52.
> > >=3D3D20
> > >=3D3D20
> > >=3D3D20
> > >=3D3D20
> > >=3D3D20
> > > On Wed, 5 Nov 2003, subramani wrote:
> > >=3D3D20
> > > > Hello,
> > > >=3D3D20
> > > > I am doing a board design. It has to pass stringent EMI tests. =
=3D
> =3D3D20
> > > > Mine is a 10 layer board.
> > > >=3D3D20
> > > > 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
> > > >=3D3D20
> > > > The board has SDRAM operating at 100Mhz. Where should I=3D20
> > route the=3D3D20
> > > > clocks. Could anyone tell me about the ways and means of=3D3D20
> > > reducing EMI.=3D3D20
> > > > The SDRAM is placed that is near to the edge of PCB. Will =
it=3D20=20
> > > >cause=3D3D20 radiation. Is there a formula for keepout distance. =
=3D
> =3D3D20
> > > > Regards
> > > > Subramani
> > > > Mistral
> > > >=3D3D20
> > > >=3D3D20
> > > >=20
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