Ok ...

> > By the way, high speed differental and impendance 
> controlled busses on 
> > the outer layers is a bad idea, just making the layer stack very 
> > inflexible and expensive.
> 
> Doing it on the inner layer as stripline instead of 
> microstrip on the outer layers will be the same as long will 
> be used only through holes vias and back drilling is a 
> complicated issue. A through vias on a 3Gbps line means an 
> unwanted  stub between the used line and the bottom layer.

Well, first of all, you should make clear to your self, how a 12 layer
pcb is build up.
Typical, a 12 layer is build out of 2x 6 layer packages. A 16 layer is
build out of 2x 8 layers. You can also buil 14 layers like that from a 6
and a 8 layer, but that asymetrical design can cause issue under the
vakuum pressure, the PCB has to go through.

So, at least, you have to packaes out of a 6 layer. Take the inner 2
planes as main power supply, use layer 2 and 5 as impendance controlles
layers and take the layer 1 (top layer) and layer 6 for fanout and easy
routing (means stuff without high requirements). The same for the second
package.

Regarding this, you go through a via with your 3 GPS line from top layer
(assembly layer) to layer 5 (routing layer). Having 0.8 mm as thickness
of the 6 layer stack, is leaving you a stub of ~0.1 mm (from the last
pre-preg). I think, that is very acceptable.

In the final stack with 2x 6 layers, that is looking like a blind via,
but it is produced like a standart via, so extra costs for that should
be minimal to nothing.

If you can not go with the ,1 mm stub, you should have a closer look to
plugged vias or partitial plugged vias. With that, you can have standard
via's (not microvia's) inside landing pads without getting trouble, when
soldering is disappearing into the via's. This is offered f.e. by ILFA
in Hannover/Germany. Costs are small, even more, if you can reduce layer
count at the same time. Plugging is sparing a lot of space on the
toplayer, giving you space for direct routing, if this is prefered, but
then, you have to look very close to impendance. You will need a GND or
VCC plane as 2nd plane. And regard, that copper thickness on outer and
inner planes is different, having influence on impendance and other
issues also. 

> > You should start with a clear stack setup for your impendance, then 
> > fanout all power and GND and then routing only the impendance 
> > controlled lines. After that, you can see, if you need additional 
> > layers for the rest or not.
> 
> Yes, but that starategy implies a very difficult stack-up 
> modification. 

Nobody said, that 1100+ BGA is easy to go with ;-)
But my experience is, that you are not able to calculate everything
before having anything placed and routed. You need to spend time to go
after try&error ...

My typical way is that:

- Place parts by hand with knowledge of function and important lines, a
perfect placement is half of the job
- if needed, think about changing pinout of FPGA
- Fanout for VCC and GND
 -Routing important lines by hand
 -Make Backup of this state
 -Try out of autorouting all "normal" and "static" lines with a good
autorouter, just to get an idea if there is a close chance to get it
routed with the selected stack 
- If the autorouter is coming up to <10% open lines left, a good manual
layouter will do the job
- Delete this autoroute tryout (or store it somewhere else for
reference)
- reload Backup and do layout by hand with interactiv manual routing
- If the autorouter is not able to get closer as 10%, you have to rework
layer stack setup, but at that point, it still is easy, using the right
tool.  
- do not use the autorouter to do the complete job. You have to spend
more time to clean up, as to to do it by hand (interactive) from start.


> I don't know what cad is your favourite, mine's 
> is the one company bought.

Yes, but that must not be the perfect one for that job ;-)

I am working with Mentor Expedition, since 98, when it still was
Veribest ...
I worked also with OrCad, Protel and Eagle, but at least OrCad and Eagle
are looking like playtools against Expedition.
I do not like the handling of Protel, but that is more my personal
experience. 

Best Jens

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