[SI-LIST] : Signals crossing plane splits, return currents etc revisited..... ..

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From: Greim, Michael (mgreim@sonusnet.com)
Date: Wed May 16 2001 - 10:14:10 PDT


Hi All,
 
There's been alot of talk about the evils of routing over plane splits,
regarding impedance bumps, impact to return currents etc. I ran
into an interesting structure the other day that I thought might be
some good food for discussion:
 
            --------------------------------- GND
                        2 mil BC
            --------------------------------- Voltage with plane splits
                        Sig 1
                        Sig 2
            --------------------------------- GND
 
In the above structure, Sig 1 was routed with route keep outs across
the split (and it was obeyed). However, Sig 2 did not have route keep
outs and some of the voltage splits are crossed. Spacing between each
sig to plane is 4 mils and to each other is four mils. Given the proximity
of the upper ground to the split plane the case has been made that the
impedance impact to sig 2 is minimal and as the adjacent plane to it is
solid that the current return path is minimally to not impacted. So whaddya
think? and can it be said that if one must deal with plane splits that a
structure like this is viable? Given the density of many of today's
boards,
is adding layers to avoid routing over splits on sig 2. needlessly
increasing
the cost of the fab?
Note: For the purposes of this discussion the moat surrounding these power
islands can be considered 25 mils.

Best Regards,

Michael C. Greim Sonus Networks
mgreim@sonusnet.com 978-589-8336

Making the world safe for digital signals everywhere

And all this science I don't understand
It's just my job six days a week

The time is gone. The email's over
Thought I'd something more to say......

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