[SI-LIST] Re: 6-layer vs 7-layer PCB Stackup
- From: steve weir <weirsp@xxxxxxxxxx>
- To: kevin98146@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 01 Jun 2004 16:19:27 -0700
Kevin, no, I would pretty much flip the stack upside down:
1 sig
small space
2 GND
small space
3 sig
big space
4 +12
small space
5 +5
space
6 diff
small space
7 GND
small space
8 sig
I agree with most of Steven's earlier comments, primarily with regard to
the lack of routing layers in the six layer and 3.3V versus 5V. Are you
really using so much 5V that it needs a plane and 3.3V doesn't? My
agreement "wanders" on the subject of capacitance. Capacitors we can
buy. Inductance is the undefeatable enemy. Pushing planes close together
drops the inductance between them. When you use a voltage plane as a
signal return this is what really counts. The capacitance comes along for
the ride. Larry Smith of SUN has been beating the drum on this topic for
some time.
I don't know how big your backplane is, but if it is of any significant
size, then your life will be a whole lot easier if you reference your
signals properly. I assume that the diff signals in your system reference
GND in the IC packages. They may or may not. If they reference something
like 2.5V or 3.3V, that is the layer that belongs on 7, and will affect
what you put on 8. The idea of putting as much space as possible between
3/4, and 5/6 is to avoid exciting the cavities formed by the voltage
planes. Your EMC guy will thank you later.
You will do better with connector stubs the further down the stack you push
the high speed signals, and unless you want to buy a lot of extra
capacitors and grief, you don't want them anywhere near your 12V ( battery
? ) supply.
Steve.
At 03:55 PM 6/1/2004 -0700, Kevin Khuu wrote:
>How about this 8-layer stackup?
>
>1) Signals (Single Ended)
>2) GND
>3) Signals (Differential)
>4) +5V
>5) +12V
>6) Signals (Single Ended)
>7) GND
>8) Signals (Single Ended)
>
>Thanks.
>Kevin
>
>
>
> >From: "Moran, Brian P" <brian.p.moran@xxxxxxxxx>
> >To: <kevin98146@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
> >Subject: RE: [SI-LIST] 6-layer vs 7-layer PCB Stackup
> >Date: Tue, 1 Jun 2004 14:07:28 -0700
> >
> >Kevin,
> >
> >Your 7 layer stackup may look symetrical but if you fill in the pre-preg
> >and core designations
> >it becomes clear that its not actaully symetrical. Not unless you have
> >back to back pre-pregs in
> >the center of the stackup. I'm not a PCB fab guy but it doesn't look
> >kosher to me. You'd be better
> >off going with 6 layers, or if neccessary to a truly symetrical 8 layer.
> >Money spent on backplanes
> >is usually money well spent.
> >
> >
> >Brian P. Moran
> >Calistoga SIE Kit Leader
> >Intel Corporation
> >brian.p.moran@xxxxxxxxx
> >
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> >On Behalf Of Kevin Khuu
> >Sent: Tuesday, June 01, 2004 1:42 PM
> >To: si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] 6-layer vs 7-layer PCB Stackup
> >
> >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
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