Like has been said throughout this thread, it all depends on your
design. Maybe 90% of the people laying out boards cannot do what I
am able to...if they wanted to that is.
Hope that helps
>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?
>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.
>> Chris Padilla
>> EMC Engineer
>> Cisco Systems
>> >I agree with Larry on this and would add that at the frequencies of
>> >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
>> >board becomes a slot radiator such that stitching becomes necessary.
>> >you become lazy, stitching left to the layout designer and not defined
>> >the SI engineer has been known to increase the slot radiation.
>> >of stitching is not good. My point on all of this is to analyze on a
>> >and technology basis and don't over generalize to a blindly followed
>> >-----Original Message-----
>> >From: Larry Smith [mailto:firstname.lastname@example.org]
>> >Sent: Tuesday, May 25, 1999 2:45 PM
>> >To: email@example.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
>> >Larry Smith
>> >Sun Microsystems
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>>-- End of excerpt from Chris Padilla
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