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[SI-LIST] Re: 6-layer vs 7-layer PCB Stackup

From: "Salkow, Steven" <steven.salkow@xxxxxxxx>
To: weirsp@xxxxxxxxxx, kevin98146@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
Date: Tue, 01 Jun 2004 15:55:49 -0700
Kevin:
In practice, there is no such thing as a seven layer board as it would
be pointless to leave a laminate with nothing on one side. The laminator
wants to see copper on both sides to balance the process.
You should check out the cost of a six layers versus eight. There may be
little cost difference but the added ground plane can help with EMI and
reduce noise on power planes. The rise time of your signals and the
number of lines simultaneously switching make a very significant
difference.
Steve's stackup below would likely be quiet for EMI but if the board in
going into an enclosure anyway, the outside Microstrip layers are not
being used well. Keep in mind, Microstrip propagates faster than
stripline. Steve's stackup has only two trace layers which may make it
tough to route signals.
The differential layer, as shown in Steve's  stackup is not symmetrical

I started with your original stackup of seven and added one more GND
layer. It may make sense to swap the +3.3V plane with the +5V plane. I
would guess there is more +3.3V.

        
        1) Signal (Single Ended Signals and High-speed Microstrip)
        2) +5V
        3) GND
        4) Signal (Diffential Signals, high speed)
        5) GND
        6) +12V and +3.3V trace
        7) GND
        8) Signal (Single Ended Signals and High-speed Microstrip)

The power layer dielectrics to GND layers want to be a thin as is
consistent with the company's manufacturing process. (Many require two
pre-preg layers to avoid short causing voids between plane layers.) This
means more capacitance and while this does not contribute significantly
to the total capacitance the effective Q of the PCB material dielectric
spans the geometric area. 

The board stackup is rework friendly as 2/3 of the signals are surface
routed.

Steven Salkow 
Lockheed Martin 
3200 Zanker Road 
San Jose, CA 95134 
(408) 473-4058 (san Jose) 
(408) 742-4162 (Sunnyvale) 
steven.salkow@xxxxxxxx 
salkow@xxxxxxxxxxxx 
(925) 462-1075 Home 
(925-) 487-5946 Cell 
(408) 468-7271 Numeric Pager 



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steve weir
Sent: Tuesday, June 01, 2004 2:10 PM
To: kevin98146@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: 6-layer vs 7-layer PCB Stackup


Kevin,

I would not reference signals with any kind of rise time under 5ns
against
a 12V supply that has nothing to do with your logic.

For a six layer backplane with very directional routing where you need a
whole plane for each +5V and 12V, I would put both of those in the
middle:

1 Ground
2 Signal / 3.3V
big space
3 +12
4 +5
big space
5 High speed differential
6Ground

The routes on layer 2 can be denser than before so hopefully you would
not
need to go to eight layers.

Steve
At 01:41 PM 6/1/2004 -0700, Kevin Khuu wrote:
>Hi All,
>
>I'm working on a backplane design and come up with the following
>7-layer PCB
>stackup:
>
>1) Signal (Single Ended Signals)
>2) +5V
>3) GND
>4) Signal (Diffential Signals, high speed)
>5) GND
>6) +12V
>7) Signal (Single Ended Signals and +3.3V trace)
>
>I don't really need two GND planes, but the reason I added the second
>GND plane (layer 5) was because of symmetry and balancing PCB stackup. 
>How does this stackup compare to the 6-layer stackup (without the
>second GND layer on layer 5)?
>
>Your comments/ideas would be greatly appreciated.
>
>Thanks,
>Kevin
>
>_________________________________________________________________
>Learn to simplify your finances and your life in Streamline Your Life
>from MSN Money. http://special.msn.com/money/0405streamline.armx
>
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[SI-LIST] Re: 6-layer vs 7-layer PCB Stackup

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