[SI-LIST] Re: Reference layers for high speed diff pairs
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: shchifwork@xxxxxxxxx
- Date: Thu, 10 Jan 2013 08:45:23 -0500
Ilan
Having modeled, analyzed, designed, measured, and correlated measurements
to modeling for 16 and 25G packages, you most definitely want to use
GND/GND referencing. In fact, the entire stackup should be encapsulated by
Gnd layers above and below any power layers, so that the first and last
thing that a signal via sees is a ground layer. Otherwise, noise injection
into the power supplies and crosstalk peaking will occur at very
inconvenient places that are not necessarily localized.
Unless you want to do the 3D package analysis necessary to convince
yourself that you might be able to use GND/Power referencing, don't do it.
When it comes to the signal path in packages, ground layers, ground vias,
and ground balls, are good ... and power layers, power vias, and power
balls are bad.
best regards,
Scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
16 Stormy Brook Road
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
On Thu, Jan 10, 2013 at 5:56 AM, Ilan Wolff <shchifwork@xxxxxxxxx> wrote:
> Hi experts,
> I'm workingon a package design. This chip will have multiple 10G (and up)
> differential pairs running between the PCB balls & the silicon bumps.
> We are able (in terms of ball-out, bump-out & package layer count) to
> accommodate the following 2 configurations:
> 1.Sandwiching the diff pairs between 2 GND (Analog Vss) layers.
> 2. Sandwiching the diff pairs between1 GND (Analog Vss) layer& 1 SERDES
> supply layer (Tx supply for Tx pairs & Rx supply for Rx pairs).
>
>
> Additional information:
> Both non-GND supplies will have AC decoupling caps underneath the chip, on
> the PCB, near the supply's vias into the package.
> If using option 2, on the PCB end of the package, each diff pair will have
> two reference viasof the relevant non-GND supply & (at least) two reference
> viasof GND.
>
> looking at our past designs we have packages using both options.All of
> them seem to work well. But now that we're moving on in data rates I'd like
> to make an informative decision.
> I'm trying to figure out if there is any preference in terms of SI.
>
> Care to voice your opinion?
>
> Many thanks,
> Shchif
>
>
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