[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 =
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 =
entirely done on the top half of the package and reference to the planes =
on top. The power and ground planes are connected through blind and =
buried vias through the package to the PCB. As such a package design can =
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 =
peripheral (not directly underneath the die) to the bottom half of the =
package only and not to the top half where the signals are reference to. =
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 =
required
to provide a path for return currents, it needs to be accompanied with =
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 =
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, =
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 =
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 =
layer
> that has a differrent reference ground plane.  For example, on an 8 =
layer
> PCB stackup where the two ground planes are layers 2 and 7,  the =
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, =
where
> there are only two planes, one Vdd and one ground, where would such =
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 =
collection
> of decoupling capacitors and interplane capacitance that you had to
> engineer into the power delivery system in order to make it stable. =
Focus
> on this and the return currents take care of themselves. EMI is =
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 =
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 =
(133Mhz
> or slower)?
> > If this is a requirement, what would be a good signal via to ground =
via
> ratio? For example, there are five signal vias within a 1 inch area, =
how
> many ground vias do I need?
> >
> > Thank you
> > Kenny
> >
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