[SI-LIST] Re: EMC
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: <chris.mcgrath@xxxxxxxx>
- Date: Wed, 5 Nov 2003 10:19:34 -0700
Chris,
I agree, too much hassle for too little effect. I would concentrate on
increasing routing density and shrink the board. I've never had that much
room to work with anyway.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris McGrath
Sent: Wednesday, November 05, 2003 8:58 AM
Cc: 'Si-List' (E-mail)
Subject: [SI-LIST] Re: EMC
As long as you keep those fill areas far enough away from the signals
(20H?) on that layer and any other "in the sandwich", I don't think that
you are deluding yourself, but, as Jeff Loyer stated, the SI benefits of
utilizing such fills are minimal compared to the capacitance of the
planes that are already next to the stripline signal. I suppose that
while I agree that there may be some benefit, it is not dominant and not
worth the hassle. (By "hassle", I mean (a) that the thermal profile
must be evaluated to insure that the extended heat cycles during
assembly do not damage any components and (b) the added fills can cloud
up the gerbers and complicate reviewing of the design.)
Thanks for the feedback from everybody!
> -----Original Message-----
> From: Bill Reams [mailto:breams@xxxxxxxxxxxxxx]=20
> Sent: Wednesday, November 05, 2003 10:36 AM
> To: Chris McGrath
> Cc: 'Si-List' (E-mail)
> Subject: RE: [SI-LIST] Re: EMC
>=20
>=20
> One thought I have is that we've already paid for the copper=20
> on all layers. What I have been doing is getting boards=20
> routed properly (well, I hope they're routed correctly).=20
> After routing is all done, checked, verified, etc, I have the=20
> non-used areas filled if there are power/ground vias=20
> available to attach the fill to. But I make certain to keep=20
> back from areas with traces. On inner layers the "keep back=20
> from traces" rule means all signal layers in the sandwich not=20
> just the layer in question (don't want to screw up=20
> impedance). So, in a stack up that has an inner structure like this:
>=20
> .
> .
> PLANE1
> SIGNAL1
> SIGNAL2
> PLANE2
> .
> .
>=20
> the fills on SIGNAL1 and SIGNAL2 are in areas that have no=20
> traces on either SIGNAL1 or SIGNAL2. And the voltage that the=20
> fills attach to would be chosen based on the some logic. For=20
> example, if PLANE1 is 3.3V and PLANE2 is COMMON, then the=20
> fills on SIGNAL1 would attached be COMMON while the fills on=20
> SIGNAL2 would be 3.3V.
>=20
> As I see it, the copper that I've already paid for is being=20
> put to use and by adding some low inductance capacitors=20
> (albeit small capacitance value of 100-200pF/sq.in.)I should=20
> get some (probably) minor improvement at the cost of ~5-20=20
> minutes of layout time. The manufacturing cost is ~zero.
>=20
> So now a question for you: Does that make any sense or am I=20
> deluding myself?
>=20
> -----Original Message-----
> From: Chris McGrath [mailto:chris.mcgrath@xxxxxxxx]
> Sent: Wednesday, 05 November, 2003 09:13 AM
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: EMC
>=20
>=20
> Your point is well taken and in the example I stated the=20
> (theoretical) capacitance that we could expect is around 200=20
> pF/sq.in. which is not negligible given the lower inductance.=20
> However, like most things this list discusses, the whole=20
> system plays a part in determining whether a design method is=20
> appropriate but I am glad to have a better understanding of=20
> the reasoning for having the ground fills. (I must admit,=20
> however, that the cost of passive components- especially on=20
> the order of a penny or less per component- is not a factor=20
> in most of our designs. To me, the greatest benefit to=20
> removing a passive component, especially capacitors, is that=20
> lowering the component count increases MTBF of the system.)
>=20
> While I realize that the theory makes sense for all layers, I=20
> am assuming that if ground fills are actually used that they=20
> are mainly done on the outside (top and bottom) layers and=20
> not on internal layers. From a thermal profiling and=20
> manufacturing perspective, filling up unused regions=20
> everywhere within a PCB can be a serious problem as the layer=20
> count increases (16+ layers). What are your thoughts?
>=20
> -Chris
>=20
>=20
> > -----Original Message-----
> > From: Chris Landrum x311 [mailto:clandrum@xxxxxxxxx]=3D20
> > Sent: Wednesday, November 05, 2003 9:58 AM
> > To: Chris McGrath
> > Cc: si-list@xxxxxxxxxxxxx
> > Subject: RE: [SI-LIST] Re: EMC
> >=3D20
> >=3D20
> > You are correct is noticing the capacitance is small....=20
> But=3D20 don't=20
> >forget, you are getting rid of much of the parasitic=3D20 inductance =
> >that will cause resonance, as compared to an SMT=3D20 or leaded cap. =
=20
> >This parasitic inductance effectively causes=3D20 your bypass=20
> cap to be=20
> >AN INDUCTOR at any freq above 100MHz!!!=3D20
> > In other words it is USELESS to your power sub-system. =20
> You=3D20 might=20
> >as well have not used it at all and saved your company=3D20 the=20
> >$0.001/brd. With the inner planer capacitor, it is much=3D20 more=20
> >effective at high frequency, albeit, it does not provide=3D20 =20
> a lot of=20
> >charge. It can make all the difference in the world=3D20 at higher=20
> >freq. =3D20
> > -----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
> >=3D20
> >=3D20
> > Given, say, a 10"x10" board with 5 mil dielectrics, the=3D20
> > capacitance due to the ground fills would be extremely small,=3D20
> > wouldn't they? =3D3D20
> >=3D20
> > I don't disagree that the fill method has less inductance,=3D20
> > but I think that it would have little effective capacitance=3D20
> > that would have a substantial impact on decoupling the=3D20
> > devices due to the small copper area and the distance to the=3D20
> > dielectric. From your description of the issue, it sounds=3D20
> > like ground filling would be pretty far down the list of the=3D20
> > EMI designer's bag of tricks.
> >=3D20
> > -Chris
> >=3D20
> >=3D20
> > > -----Original Message-----
> > > From: Chris Landrum x311 [mailto:clandrum@xxxxxxxxx]=3D3D20
> > > Sent: Wednesday, November 05, 2003 9:16 AM
> > > To: Chris McGrath
> > > Cc: si-list@xxxxxxxxxxxxx
> > > Subject: RE: [SI-LIST] Re: EMC
> > >=3D3D20
> > >=3D3D20
> > > Ground filling is useful to create inner layer=3D20
> > capacitance=3D3D20 for the=3D20
> > >power sub-system of the PCB. By filling GND on a=3D3D20 =
signal=3D20
> > layer that=3D20
> > >is directly adjacent to a PWR plane a=3D3D20 capacitor will =
be=3D20
> > created. =3D20
> > >This capacitor is often times=3D3D20 MUCH more effective at=20
> > >providing=3D20 energy to components because=3D3D20 it is far less=20
> > >inductive=3D20
> > than a normal=3D20
> > >leaded or chip capacitor. =3D3D20 =3D3D20
> > > EMI can be caused by an improperly designed power =
sub-system.=3D3D20
> > > What can happen here is current gradients can be formed in=3D3D20 =
=20
> > >the PWR/GND planes that can effectively cause radiation=3D3D20 =20
> > >assuming there is an antenna nearby. Also of concern is VCC=3D3D20 =
=20
> > >and GND bounce. The plane capacitors formed can help avoid=3D3D20 =
> > >this problem. =3D3D20
> > > By ensuring that the chips are getting the proper energy =
such=3D3D20
> > > that VCC and GND bounce do not occur, you are thereby=3D3D20
> > > reducing the probability that EMI problems can be caused by=3D3D20
> > > the power sub-system.
> > >=3D3D20
> > > -----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
> > >=3D3D20
> > >=3D3D20
> > > The "ground filling" is a topic that has been discussed at =
my=3D3D20
> > > company recently and I wanted to get the list's feedback on=3D3D20
> > > why this is done. We never do ground filling on any layers=3D3D20
> > > and the only reason that I have ever heard for it was to=3D3D20
> > > reduce EMI, but given the disadvantages (increased thermal=3D3D20
> > > profile, potential for crosstalk, PCB viewer and gerber=3D3D20
> > > viewer complications, etc.) and the fact that I have never=3D3D20
> > > been able to find data or any science to back up the EMI=3D3D20
> > > argument, I don't see any benefit to ground filling on =
signal=3D3D20
> > > layers. (Of note is that by using the term "ground =
filling",=3D3D20
> > > I am not referring to "thieving" to equalize the copper=3D3D20
> > > distribution to facilitate PCB fabrication.)
> > >=3D3D20
> > > I am very interested in hearing feedback from any of you.
> > >=3D3D20
> > > -Chris
> > >=3D3D20
> > >=3D3D20
> > > > -----Original Message-----
> > > > From: Suresh.K [mailto:sureshk@xxxxxxxxxxxxxx]=3D3D3D20
> > > > Sent: Wednesday, November 05, 2003 2:54 AM
> > > > To: subramani
> > > > Cc: si-list@xxxxxxxxxxxxx
> > > > Subject: [SI-LIST] Re: EMC
> > > >=3D3D3D20
> > > >=3D3D3D20
> > > >=3D3D3D20
> > > > Dear Sir,
> > > >=3D3D3D20
> > > > According to your Board Stack you have two strip lines=3D20
> > and=3D3D3D20 one=3D20
> > > >=3D3D
> >=3D20
> > > >assymmetrical dual stripline,but the two strip lines are=3D3D3D20 =
=3D
> =3D3D20
> > > bounded by=3D3D20
> > > >one side power plane and Ground Plane on the other=3D3D3D20 =3D20
> > side......=3D20
> > > >=3D3D3D20 If you have both side ground plane reference for =
the=3D20=20
> > > >strip=3D3D3D20 lines ,you can route all the Impedance=3D20
> > controlled signals=3D20
> > > >on=3D3D3D20 Layer2 & layer8,because strip lines bounded=20
> by ground=3D20=20
> > > >planes=3D3D3D20 will be best layer for better signal=3D3D3D20 =20
> > > >integrity. =3D3D3D20
> > > > But now you have assymmetrical dual stripline bounded =
by=3D3D3D20
> > > > ground planes,so I beleive that layer 5&6 may be better=3D3D3D20
> > > > option for routing clock signals,which will useful for =3D
> return=3D3D3D20
> > > > path, but you need to care of tandem=3D3D3D20
> > > > pair of traces.
> > > >=3D3D3D20
> > > > and regarding board power plane EMI,you can follow 20H=20
> rule=3D3D3D20 =20
> > > >on layer2& layer9 power planes to reduce the power plane =3D
> EMI=3D3D3D20
> > > > and Top & Bottom layers you are doing Ground fill, so I =3D
> think=3D3D3D20
> > > > the device EMI will be controlled by the Ground fills. =3D3D3D20
> > > > Regards,
> > > > Suresh.K,
> > > > Vth EDA Lab,
> > > > C-DOT,
> > > > Bangalore-52.
> > > >=3D3D3D20
> > > >=3D3D3D20
> > > >=3D3D3D20
> > > >=3D3D3D20
> > > >=3D3D3D20
> > > > On Wed, 5 Nov 2003, subramani wrote:
> > > >=3D3D3D20
> > > > > Hello,
> > > > >=3D3D3D20
> > > > > I am doing a board design. It has to pass stringent=20
> EMI tests. =3D
> =3D3D
> > =3D3D3D20
> > > > > Mine is a 10 layer board.
> > > > >=3D3D3D20
> > > > > 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
> > > > >=3D3D3D20
> > > > > The board has SDRAM operating at 100Mhz. Where should I=3D3D20
> > > route the=3D3D3D20
> > > > > clocks. Could anyone tell me about the ways and means=20
> of=3D3D3D20
> > > > reducing EMI.=3D3D3D20
> > > > > The SDRAM is placed that is near to the edge of PCB. Will =3D
> it=3D3D20=3D20
> > > > >cause=3D3D3D20 radiation. Is there a formula for=20
> keepout distance.=20
> > > > >=3D
> =3D3D
> > =3D3D3D20
> > > > > Regards
> > > > > Subramani
> > > > > Mistral
> > > > >=3D3D3D20
> > > > >=3D3D3D20
> > > > >=3D20
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