James Nick Sears wrote:
> Hi,
>
> I'm thinking about making the jump into a 256 pin BGA for my next
> project, but I have a lot of unknowns when it comes to actually doing
> the layout.  Does anyone have a good reference for things like what
> sort of footprint to use (a grid of vias, right?)
Generally no! Via-in-pad is possible but only with very specialist PCB 
processes. If you try to do it with ordinary sized unfilled holes it 
will just suck the solder away. Normal practice is to leave the pad 
diagonally and then via down to an internal layer.

The footprint itself should be a simple grid of (generally round) pads. 
For details on the size and spacing of pads check with the manufacturer 
of the device in question.

You will need a PCB process fine enough that with vias positioned as 
above you can get at least one and preferablly two tracks on each layer 
between a pair of vias. For a 1mm fineline BGA to get two tracks between 
a pair of vias you will want something like .1mm track gap and annular 
ring and .3mm holesize.
> , suggestions on
> routing all of those pins out into the open
For the power pins you probablly want to go straight into an internal plane.

For the signal lines it depends how much space you have, if you have 
loads of space try to route out to the side nearest the pad in question. 
If space is tight you will just have to add layers and/or reduce feature 
sizes until the tracks fit in the direction you want.

Blind vias are also an option but I suspect they will considerablly 
drive up costs. If using them you want to route the pins from the middle 
of the chip on the lower layers.

If working with an fpga it is often usefull to switch pins arround as 
you find which pin is easiest to get to for each track.

If the chip has a load of power/ground pins in the middle then by 
carefull positioning of the vias you may be able to squeeze in some caps 
under the middle of the chip.


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