[Milkymist-devel] PCB-Layers and Design Rules for Milkymist hardware platform

[Sébastien Bourdeauducq]

Hi,

On Tuesday 20 October 2009 11:45:19 you wrote:
>     Sébastien> I am planning to use 6 layers, with 6 mil width and spacing,
>     Sébastien> without special features. This is however only a plan, I may
>     Sébastien> change those rules if routing turns out to be too difficult.
> 
> With two supply layers you have 4 routing layers. Roughly you can fan out 2
> rows of balls on top and another row with each layer, as each via takes 600
> um with 300 um standard drill and leave only space to pass one signal with
> space/line/space 450 um. So I guess you can bring out not so many of the
> available Pins, if my estimation has no fundamental flaw.

You are probably right. I did not have time to look for the BGA routing yet - 
I have been busy with choosing parts and the schematics. However, the number 
of required FPGA I/O is small (199/316) and I have already routed a board with 
a BGA320 and these constraints. There were unconnected pins of course, and I 
was using FPGA pin-swapping to solve some problems. It might be harder here, 
mostly because I want to use DDR SDRAM memory with a pinout compatible with 
the Spartan-6 MCB (in case I choose to use it later on) and at high 
frequencies (400Mbps/pin) which mean short and length-matched tracks. What 
would you suggest?

Best regards,
Sébastien