[SI-LIST] Re: Current Return Vias
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: <Chris.Cheng@xxxxxxxxxxxx>, "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 26 Jul 2006 08:32:11 -0600
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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