Re: gEDA-user: PCB suggestion

[Stephen Meier]

I feal like I have to step in here. With a 900 pin BGA (30 rows by 30 
columns) at a 1mm (39 mil) pitch. Four signal layers are just bearly 
able to route all the signals out from under the bga. That was using via 
in pad and squeezing two 4 mil traces between pads/layer. That left 4 
layers for ground and power. The BGA required 4 different power/ground 
levels. 3.3 V  2.5 V 1.8V and ground. Giving each a layer a power level 
and 4 more layers for eight total layers. To get the power from the 
power supplies (various switching and regulated supplies) required using 
traces from the signal layers until I could bring them in at different 
angles finally under the BGA device.

This was done with 8 layers and it felt like I was trying to stuff 10 
acres worth of fall leaves into a 20 gallon trash bin.

Realistically, 12 layers would have made the job one bleep bleep of a 
lot bleeping easier. But I do like challenges.

Thanks,
Steve Meier
harry eaton wrote:
Am 06.01.2005 um 01:10 schrieb harry eaton:
   

The Pentium processor chip has only 7 wiring layers; it must be of
"medium
to low" complexity!
Eight copper layers is not presently a serious limitation to users;
     

Eight copper layers does not mean eight wiring layers, You often have
several power planes.
   

I guess that means there is no power routing in the Pentium processor since
it only has 7 wiring layers. I wonder why they get so hot that they need
fans when there are no power connections.  My point is it's usually possible
to route a board with far fewer layers than are actually used. It's
definitely harder to route that way, but usually possible. Signals integrity
may be lower, but (as in IC design) good enough.
I'm pleased to see that some folks have used the 8 copper layers available,
I hadn't heard of any specific examples before now. Since I now know there
are some fairly high-end-complexity users out there I will extend the copper
layer support to 16 soon; it's really not that hard.
For what it's worth, I never liked the layer grouping concept but I've
preserved it for full backward compatibility for those who have used it. It
was one of the few part of Thomas' original work that I didn't care for.
harry