[SI-LIST] Re: Stack up for EMI reduction

Nima,
I would think that one of the source of you problem is a single ended 70 
Mhz signals (5 harmonic in particle).

Looking on you stack up I could say that the layer 8 GND/Power (mixed) 
could create some (or may be all) of you emission problems if the signal on 
layer 7 and 9 are crossing the gap between power and/or ground layers. 
Usually in my design I am trying to keep ground layers solid.

I would move a mixed power islands from layer 8 to layer 5 (if you have 
enough real estate on the board to keep all power islands on the same 
layer). In this case mixed power islands will be between two ground layers 
(continues).
The other alternative solution might be to put a power traces on one of 
signal layer/s (top and bottom is OK too).

Let see if this will improve you design.  Good luck.


  At 06:11 PM 1/30/2004 -0500, Nima Lotfi wrote:
>Hi,
>I'm looking at re-layout of a 10 layer board because of EMI problems.  The
>board contains 4 differential serial signals a ~700Mhz, and about 20 single
>ended signals at 70 MHz.  The main problem frequencies are around 350 Mhz.
>There are 4 large BGA components on the board.
>
>I would ideally like to move into a 12 layer board, however there is quite a
>bit of (non technical) resistance to doing that.
>
>I like to eliminate 2 signal layers, by making my other 2 signal layer
>denser.  Then I'll use the eliminated signal layers as ground/power layers,
>but I will need to use these new gnd/power layers to assist with breakout of
>4 high density (500pins) BGA.
>
>My question is can any one comment on whether I should in theory get an EMI
>improvement?  What should I look out for?
>
>
>My current stack up is:
>
>1 Mainly GND with some breakout
>2 Signal
>3 Gnd
>4 Signal
>5 Power
>6 Gnd
>7 Signal
>8 GND/Power (mixed)
>9 Signal
>10 mainly GND with some breakout
>
>
>
>New proposed stack up
>
>1 GND with some breakout
>2 power with some breakout
>3 Gnd
>4 Signal
>5 Power
>6 Gnd
>7 Signal
>8 GND/Power (mixed)
>9 Power with some breakout
>10 mainly GND with some breakout
>
>
>Thanks,
>Nima
>
>
>
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Val Mandrusov
Sr. Compliance Engineer

CISCO Systems, Inc
375 East Tasman Drive
San Jose, CA 95134
M/S SJ-06/04
408-853-7415 Phone
408-853-3872 FAX

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