[SI-LIST] Re: 4layer board stack up for PCI boards

Hi Everyone,

I like #1 also as long as the required trace impedances can be made, because in 
addition to 
pwr-gnd decoupling, it allows one to transfer a signal between signal layers 
with less of a problem. 
If the pwr and gnd layers are tens of mils apart, the inter plane impedance can 
cause a significant 
problem for the return part of the signal loop (source, signal path, load, 
return path to source).

I have made measurements that show (at least on one of the test boards I use 
for seminar 
demonstrations) over 30 dB higher emissions for a live path going from to top 
to the bottom layer. 
Also noted was a similar reduction in immunity to ESD events. Both effects were 
on a board similar 
to #2 compared to staying on the same layer of the same board. This effect gets 
much less of a 
problem when the two planes are very close to each other.

Doug

---- Matthias Mansfeld <m.mansfeld@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> 
> On 27 Sep 01 at 12:52, GOPALCHETI,MANOJ (HP-Cupertin wrote:
> 
> [....] 
> > 1. With Power and Ground plane closer and far from signal layers
> > Signal Layer 1.5Oz, 8 mil trace
> > Dielectric 26mils
> > Plane Layer 1.00oz
> > Dielectric 8mils
> > Plane Layer 1.00oz
> > Dielectric 26mils
> > Signal Layer 1.5Oz, 8 mil trace
> with FR4 er=4.2 about 110 Ohm
> 
> > 2. With Power and Ground plane separated far but closer to signal layers.
> > Signal Layer 1.5Oz, 8 mil trace
> > Dielectric 20mils
> > Plane Layer 1.00oz
> > Dielectric 16mils
> > Plane Layer 1.00oz
> > Dielectric 20mils
> > Signal Layer 1.5Oz, 8 mil trace
> about 100 Ohm.
> 
> Nevertheless, I'd prefer Stackup 1. Better broadband decoupling of 
> VCC/GND. Perhaps consider decreasing inner dielectricum down to 4 
> mit. That's much better. If necessary, fatten these traces where the 
> impedance value ist important (mostly the stuff which goes out to the 
> bus).  
> 
> Regards
> Matthias Mansfeld
> -----------------------------------------------
> Matthias Mansfeld Elektronik
> * Printed Circuit Board Design and Assembly
> Am Langhoelzl 11, D-85540 Haar, GERMANY
> Phone: +49-89-4620 0937, Fax: +49-89-4620 0938
> E-Mail: m.mansfeld@xxxxxxxxxxxxxxxxxxxxxx
> Internet: http://www.mansfeld-elektronik.de
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    ___          _       Doug Smith
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