[SI-LIST] Re: 4layer board stack up for PCI boards
- From: "Brent DeWitt" <bdewitt@xxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 27 Sep 2001 19:22:35 -0600
Just a thought to stimulate conversation:
It seems that four layer boards are almost becoming a lose-lose situation as
edge speeds go up. Using the assumption that the circuit will be packed
with small geometry SMD, there is simply no way to both maintain reasonable
trace Z and get good power plane to ground plane decoupling _if_ you have a
typical 0.063" board thickness.
Why not have a thin board (4-5 mil dielectric layers), with a thick
"stiffener" board (roughly 0.040") laminated to the back? Of course this
kills the possibility of components on the "backside" of the assembly, but
it does get you into a manageable SI/EMC situation.
I don't know of any vendors offering such a stack-up, but it does seem
interesting.
Your thoughts?
Brent DeWitt
Datex-Ohmeda
Louisville, CO
---- 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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