Re: [SI-LIST] : Board Stackup - again ...

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From: Ritchey Lee (leeritchey@earthlink.net)
Date: Mon Jun 05 2000 - 16:29:36 PDT


Like Brian, I add layers for the express purpose of increasing the
capacitance between planes. However, I don't use BC. Instead, I look at
where I can add two new planes next to two planes already in the stackup. If
you do this right, you can separate the planes with 3 mils of prepreg and get
almost twice the capacitance you would otherwise get. In addition, it is not
necessary to use a patented material such as BC.

Lee

Moran, Brian P wrote:

> Doug,
>
> Most high layer count designs I have done use dual stripline pairs between
> power and ground layers. Its nice if you happen to get power and ground
> adjacent but its hard to guarantee, es[ecially when you have multiple
> voltages. One option I have used is to have a standard 12 layer with no
> adjacent planes and then build a version using BC (buried capacitance)
> pwr/gnd pairs in place of the normal pwr plane positions. This makes it a
> 14 layer stack but the BC spacing is only 2 mils so you get good high
> frequency decoupling without blowing out the stack dimensions.
>
> Brian P. Moran
>
> Intel Corporation
> Platform Design Engineering
> brian.p.moran@intel.com
> (916) 356-1912
>
> -----Original Message-----
> From: Doug McKean [mailto:dmckean@corp.auspex.com]
> Sent: Friday, June 02, 2000 11:25 AM
> To: SI - List Discussion Group
> Subject: [SI-LIST] : Board Stackup - again ...
>
> What are the opinions of separating power and
> ground planes with a signal layer?
>
> Is it a *hard fast rule* that never shall they be separated?
> Are there some caveats conditions that allow separation?
>
> Assuming that everything except stackup remains the same,
> is there some significant measurable difference with say
> a digital board handling clocks around 40 MHz to 80 MHz
> and bus speeds of say PCI, and fsb of 133 (maybe even 200)
> and/or Ethernet with power and ground planes separated with
> a signal layer as opposed to one that has them adjacent?
>
> I've been of the opinion that power and ground planes should
> NEVER be separated, but lately, I'm not so sure about that.
>
> Comments?
>
> - Doug McKean
>
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