Re: [SI-LIST] : 20-H Rule for Power Planes

Ron Matthews (ronm@cadence.com)
Wed, 26 May 1999 16:20:37 -0400

Hi Chris,

I have a couple of questions about how you are doing your board design
with reference to the description you gave. If I understand correctly,
you have voltage and ground planes on the same layer. I'm assuming that
you have signal traces that traverse the inter-plane moats. If this is
the case, how do you provide for your return path, given that the trace
will reference tow different planes?

Regards,

Ron

On May 26, 12:00pm, Chris Padilla wrote:
> Subject: RE: [SI-LIST] : 20-H Rule for Power Planes
> Consider a multi-layer board of 16 layers (4 GND, 2 PWR, 8 internal
> routing, 2 external routing). My goal with this board is to make it
> a Faraday cage as much as possible.
>
> In one direction, I've almost got this due to the ground and power
> planes. But it's the sides or edges of the board where I could see
> a lot of leakage. (Yes, the populated top and bottom layers radiate
> and are "holes" in the cage. Some of these vendor's chips make my
> job a challenge as well.)
>
> I then do a ground stitch around the perimeter of the board along with
> a metal trace on all layers as space permits. This automatically
> forces a cut-back of the power plane of anywhere from 50 - 100 mils.
> And why is a non-uniform stitching better than a uniform stitich? The
> point is to close up the edges of the board as much as possible and
> get a Faraday cage effect.
>
> Also, why flood a (typically noisy) power plane into areas that might
> not need the particular voltage? I only do this if I have a lot of
> traces referencing the power plane and ripping it out would cause
> image currents to take big loops. Otherwise, I see if I can fill in
> the rest of the power plane with a ground plane.
>
> I don't have a shred of data to suggest that this design works better
> than equally sized ground and power planes but, it makes sense to me
> to try and go for the Faraday cage effect--at least a Faraday cage
> for your inner layer traces.
>
> Thanks,
>
> Chris Padilla
> EMC Engineer
> Cisco Systems
>
> >I agree with Larry on this and would add that at the frequencies of
> >interest
> >a server board with a setback of one plane vs another MAY constitute a
> >reasonably good slot antenna. In fact, at some conditions, the edge of
> >the
> >board becomes a slot radiator such that stitching becomes necessary.
> >Lest
> >you become lazy, stitching left to the layout designer and not defined
> >by
> >the SI engineer has been known to increase the slot radiation.
> >Uniformity
> >of stitching is not good. My point on all of this is to analyze on a
> >use
> >and technology basis and don't over generalize to a blindly followed
> >rule.
> >
> >Tomm
> >
> >-----Original Message-----
> >From: Larry Smith [mailto:ldsmith@lisboa.eng.sun.com]
> >Sent: Tuesday, May 25, 1999 2:45 PM
> >To: si-list@silab.eng.sun.com
> >Subject: Re: [SI-LIST] : 20-H Rule for Power Planes
> >
> >
> >I don't believe in the "20-H Rule". Suppose the power plane was at
> >3.3V and the ground plane was at 0V. It would be easy to reconfigure
> >the system so that the "power" plane is at 0 volts and the"ground"
> >plane is at -3.3V. Does this mean that the power plane should now be
> >bigger than the ground plane?
> >
> >The only difference between the power and ground plane is that one is a
> >0V and the other 3.3V WRT (...thats with respect to, lest I start
> >another discussion...) earth ground. But even this is not true in a
> >battery operated system. In any modern digital system, the impedance
> >between the power and ground plane is much less than 1 ohm well into
> >the EMI frequencies.
> >
> >The ground plane probably has a path out to frame ground and eventually
> >earth ground somewhere. But if that path is more than an inch long, it
> >is going to be well over 10 nH. Ten nH is 1 Ohm at 15 MHz (Z=jwL) and
> >higher impedance at higher frequencies. So, above 15 MHz, the voltage
> >between the power and ground planes is insignificant compared to the
> >voltage across the earth ground connection.
> >
> >The power and ground planes should be exactly the same size. To make
> >one larger than the other will simply have the effect of turning nice
> >diffential currents into common mode current and common mode
> >radiation.
> >
> >regards,
> >Larry Smith
> >Sun Microsystems
>
>
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>
>-- End of excerpt from Chris Padilla

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