Jack,
What about using your bottom layer as a voltage plane for at least 2 of your
powers ?   and the layer next to the bottom as a GND plane (solid).
That way you have a external plane carrying your voltage (higher current
capacity), and shielded by your next inner layer, eliminating the traces
running over a split.
Probably be a good idea to put your bypass caps as near as possible under
that part also.. It definitely
becomes a problem when everything needs to be @ the BGA device.
I would split the minor voltages on the bottom, and put a solid 5v on a
internal layer.
Good luck, yer gonna need it :-)
Karl



>From: "Jack C. Olson" <[log in to unmask]>
>To: "(Designers Council Forum)" <[log in to unmask]>,   Karl
>Bates <[log in to unmask]>
>CC: [log in to unmask]
>Subject: Re: [DC] Reference Planes for Impedance Controlled Signals
>Date: Mon, 14 Jul 2003 15:26:11 -0500
>
>
>8MHz
>
>Here's more info. We have a PowerPC processor BGA
>that has many +5V and +2.6V pins. Instead of trying
>to do a split plane, and having the impedance controlled
>stuff possibly traveling over a split, I thought it was
>better to have both a solid +5v plane and solid +2.6v
>planes on different layers. But as soon as you do that,
>you have some signals closer to one plane that the other,
>and the question is will there be signal integrity issues
>by using the +5v plane as a reference.
>
>On the other hand, how do you guys create complicated
>split planes and keep signals from crossing the split?
>(Veribest/Mentor Expedition/Mentor BoardStationRE
>router)
>
>thanks,
>Jack
>
>
>
>
>
>                       Karl Bates
>                       <[log in to unmask]
>                       >
>                       Sent by:
>                       DesignerCouncil         To:
>[log in to unmask]
>                       <DesignerCouncil@listse cc:
>                       rv.ipc.org>
>
>
>                       07/14/2003 02:10 PM       Subject:      Re: [DC]
>Reference Planes for Impedance Controlled Signals
>                       Please respond to
>                       "(Designers Council
>                       Forum)"; Please respond
>                       to Karl Bates
>
>
>
>
>
>Caterpillar: Confidential Green                 Retain Until: 08/13/2003
>                                                 Retention Category:  G90 -
>                                                 General
>                                                 Matters/Administration
>
>
>
>
>And...What frequency is the circuit operating at ?    Khz or Mhz or Ghz ?
>
>
>
>
>
>
>

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