[SI-LIST] Re: Current Return Vias

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Ken.Cantrell@xxxxxxxxxxxxxxxx,<Ken.Cantrell@xxxxxxxxxxxxxxxx>, <Chris.Cheng@xxxxxxxxxxxx>, "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
  • Date: Wed, 26 Jul 2006 09:30:13 -0700

Ken, I agree you did not.  But rightly or wrongly it is easily 
inferred from some statements of others.  Since we all agree that is 
a bad extreme, I think it is reasonable that Chris objects to it.

Regards,


Steve.
At 09:09 AM 7/26/2006, Ken Cantrell wrote:
>Steve -
>I never said or implied that arbitrarily traversing any cavity with any
>combination of signals without concern for the Z-axis return path was
>acceptable.  I do agree about the silly extreme.
>
>Ken
>
>-----Original Message-----
>From: steve weir [mailto:weirsi@xxxxxxxxxx]
>Sent: Wednesday, July 26, 2006 9:47 AM
>To: Ken.Cantrell@xxxxxxxxxxxxxxxx; Chris.Cheng@xxxxxxxxxxxx; Ken
>Cantrell; si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Re: Current Return Vias
>
>
>Ken, I disagree.  I think that Chris' examples graphically
>demonstrate the fact that we cannot arbitrarily traverse any cavity
>with any combination of signals without concern for the Z axis return
>path and yield desired results.  I find it very much in context with
>the original discussion of the what is for many cases at current
>signaling edge rates an equally silly extreme in the form of an
>individual return stitch via per signal via.
>
>Regards,
>
>
>Steve.
>
>At 07:32 AM 7/26/2006, Ken Cantrell wrote:
> >Chris -
> >That was my point, the example was extreme and didn't address the issue.
> >Ken
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx
> >[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Cheng
> >Sent: Tuesday, July 25, 2006 6:05 PM
> >To: Ken Cantrell; si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: Current Return Vias
> >
> >
> >How stupid it is for the design below vs. statements like "all you need =
> >is the power/ground plane and decoupling capacitance for the return =
> >current path, no vias needed". Wait, aren't they the same stupid concept =
> >?
> >While the example is extremely, I have been called to fix package =
> >designs (not my own) that conveniently forgot to provide return vias =
> >between upper and lower power/gnd planes because package designers want =
> >to steal those large core via locations for their signal escapes to =
> >bottom layers. If you read Larry's reply carefully, you will see similar =
> >experiences he refers to. Some highspeed signals experience a huge =
> >glitch everytime some slow JTAG signals toggles hundreds of mils away on =
> >a package. As it turns out, the package designer forgot to provide the =
> >through vias for the reference planes and the return current decided to =
> >take the nearest return via as return path which is on the highspeed =
> >signal area hundreds of mils away.
> >At the end, talk is cheap. If you truly believe return via is useless =
> >and plane and decoupling capacitance is good enough, why border to drill =
> >those vias on packages. Put your money where your mouth is. And if you =
> >are too chicken to do so, you are not practicing what you preach.
> >
> >-----Original Message-----
> >From: Ken Cantrell [mailto:Ken.Cantrell@xxxxxxxxxxxxxxxx]
> >Sent: Tuesday, July 25, 2006 6:49 AM
> >To: Chris Cheng; si-list@xxxxxxxxxxxxx
> >Subject: RE: [SI-LIST] Re: Current Return Vias
> >
> >
> >Chris,
> >Nobody said it was OK to do stupid stuff.
> >Ken
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx
> >[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Cheng
> >Sent: Monday, July 24, 2006 5:15 PM
> >To: si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: Current Return Vias
> >
> >
> >Ok, I'll give you an evidence if you dare to take it up and try on your =
> >=3D
> >clients design and see if you can keep your job after that.
> >
> >In a typical multi-layer BGA design, the signal fan out is almost =3D
> >entirely done on the top half of the package and reference to the planes =
> >=3D
> >on top. The power and ground planes are connected through blind and =3D
> >buried vias through the package to the PCB. As such a package design can =
> >=3D
> >therefore stop the via connection at any plane he/she desire.
> >
> >My challenge to you is just connnect all those power/ground pin on the =
> >=3D
> >peripheral (not directly underneath the die) to the bottom half of the =
> >=3D
> >package only and not to the top half where the signals are reference to. =
> >=3D
> >Let's see how much ground/power bounce you will observe.
> >
> >BTDT, have you ?
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx
> >[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Lee Ritchey
> >Sent: Monday, July 24, 2006 3:46 PM
> >To: Ken Cantrell; kenny_frohlich@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: Current Return Vias
> >
> >
> >Ken,
> >
> >Well put.  When someone makes the statement that ground vias are =3D
> >required
> >to provide a path for return currents, it needs to be accompanied with =
> >=3D
> >some
> >evidence.
> >
> >
> > > [Original Message]
> > > From: Ken Cantrell <Ken.Cantrell@xxxxxxxxxxxxxxxx>
> > > To: <kenny_frohlich@xxxxxxxxx>; <leeritchey@xxxxxxxxxxxxx>;
> ><si-list@xxxxxxxxxxxxx>
> > > Date: 7/24/2006 6:51:24 AM
> > > Subject: [SI-LIST] Re: Current Return Vias
> > >
> > > Kenny,
> > > At 133MHz, and assuming your edge rates are > 500ps, you aren't going =
> >=3D
> >to
> > > need them, even if you have twice as many layers.  It's an important
> >effect,
> > > but at 133MHz it's not relevant.  What is important, as Lee indicated, =
> >=3D
> >is
> > > your PDS design.
> > >
> > > Ken
> > >
> > > -----Original Message-----
> > > From: si-list-bounce@xxxxxxxxxxxxx
> > > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Kenny Frohlich
> > > Sent: Saturday, July 22, 2006 1:56 PM
> > > To: leeritchey@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Re: Current Return Vias
> > >
> > >
> > > Lee,
> > >   I'm not talking about jumping from one layer to the next adjacent =
> >=3D
> >layer
> > > which shares the same referrence plane.  In this case, I do not need a
> > > ground via.  But I'm asking about jumping from one layer to another =
> >=3D
> >layer
> > > that has a differrent reference ground plane.  For example, on an 8 =
> >=3D
> >layer
> > > PCB stackup where the two ground planes are layers 2 and 7,  the =3D
> >signal
> > > jumps from the top layer (layer 1) to the bottom layer (layer 8).
> > >
> > >   Thank you
> > >   Kenny
> > > Lee Ritchey <leeritchey@xxxxxxxxxxxxx> wrote:
> > >   Kenny,
> > >
> > > It is not true that you need a "return current" via next to each layer
> > > changing signal via. I continue to be amazed that engineers who are
> > > looked upon as SI experts say such things.
> > >
> > > Imagine you have a 4 layer PCB, such as the mother board in a PC, =3D
> >where
> > > there are only two planes, one Vdd and one ground, where would such =
> >=3D
> >vias
> > > connect? There have been billions of these made to date that work just
> > > fine and have very fast signals on them. The return currents you are
> > > concerned about find their way from plane to plane through the =3D
> >collection
> > > of decoupling capacitors and interplane capacitance that you had to
> > > engineer into the power delivery system in order to make it stable. =
> >=3D
> >Focus
> > > on this and the return currents take care of themselves. EMI is =3D
> >minimized
> > > he same way..
> > >
> > >
> > >
> > >
> > > > [Original Message]
> > > > From: Kenny Frohlich
> > > > To:
> > > > Date: 7/22/2006 6:45:56 AM
> > > > Subject: [SI-LIST] Current Return Vias
> > > >
> > > > Dear Experts,
> > > > I understand that I need to provide ground vias next to via =3D
> >explictly
> > > for the purpose of letting return currents jump between layers. I know
> > > it's a requirement for high speed signals, especially differrential
> > > signals. Is this also required for low speed single-ended signals =3D
> >(133Mhz
> > > or slower)?
> > > > If this is a requirement, what would be a good signal via to ground =
> >=3D
> >via
> > > ratio? For example, there are five signal vias within a 1 inch area, =
> >=3D
> >how
> > > many ground vias do I need?
> > > >
> > > > Thank you
> > > > Kenny
> >
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