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

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From: Larry Smith ([email protected])
Date: Fri Jun 02 2000 - 14:25:18 PDT

Doug - With Hadco (Zycon) buried capacitance technology, power planes
can be as close as 2 mils. If you put a single transmission line layer
between the two power planes, the planes will be separated by 8 to 10
mils. Capacitance will be 1/4 or 1/5 of what it could have been.
Power plane spreading inductance will be 4 to 5 times what it could
have been.

There is no hard and fast rule about placing power and ground planes
adjacent to each other in a PCB stackup. But the rules of physics tell
you what you will get when you make your choice. If low impedance
power at high frequency is important to your product, you will want to
make the right choice.

Products with signal speeds of 40 to 80 MHz will probably not have much
energy at 1 GHz or be affected by the high frequency energy. But
products with 200 MHz signals will have edge rates in the neighborhood
of 0.5 nSec (1/10 period). Using this week's discussion, the frequency
content is .35/tR = .35/.5 which is pretty close to 1 GHz. Discrete
capacitors are not very effective at 1 GHz and you will want to lean on
the interplane capacitance to keep the noise down. Low spreading
inductance makes the discrete capacitors more effective.

Larry Smith
Sun Microsystems

> Date: Fri, 02 Jun 2000 11:24:58 -0700
> 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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