[SI-LIST] Re: 8 layer PCB stackups and microvias
- From: pso@xxxxxxxxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 10 Jul 2008 22:30:40 +0200 (CEST)
Hallo Marc
Yes, we did a board with GND on the top and bottom and microvia 1-2 and 2-3
etc.
We used microvia i caps
and absolute no traces on the outer sides.
We found that this way the GND areal was larger on the top layer than on second
layer if
you use normal trhough via's.
It works fine. Our board also had an FPGA and DDR2. Plus a PPC and a DSP both
wih DDR2.
We
had both sides completely filled with components incl. big BGA's on both sides
and therefore need maximum routability.
Because of extreme
density our buried via went from layer 3 instead of 2. You can save a lot on
cost if you can live with the buried from layer 2.
The
cost is not set by the number of layers as many thinks, but more by the number
of pressings and plating.
You can add layers in the
mittle for small cost.
Consider if you can really make god power with one layer,depends on how many
different powers you have.
We had
14 layers with two signal layers in the mittle.
Be sure to have absolut minimum dielectric between Power and GND to get a large
high
frequency cap.
Also be sure to add enough decoupling caps to be able to change half of them to
a lower value.
I recommend doing
SI simulations especially if you like us skips normal DDR2 termination power.
Best regards
Peter
Sørensen
On Tors, Juli 10, 2008 16:22, Marc Battyani wrote:
> Hello,
>
> We are
designing a small 8L board with the following stackup:
> S/G/S/G/P/S/G/S (S = signal, P = power, G = ground)
>
> The
most notable components are a few DDR2 (533) around an FPGA.
>
> If we use microvias on the layers 1-2, 2-3, 6-7,7-8, we can
route most
> of the fast signals on the layers 1 and 3. With no microvias, we have to
> use all the layers.
> To
keep the 50 Ohm tracks at a reasonable width, we use 100µm (4mils)
> dielectric thickness for the layers 1-2-3 and 6-7-8.
>
> Are we missing a better PCB stackup?
> Anybody tried something like G/S/S/G/P/S/S/G?
>
> Can you
recommend a good PCB shop able to make a few prototypes (+-10)
> at a reasonable cost?
> (We have been given very high prices
for this so far)
>
> Thanks,
>
> Marc
>
>
>
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