BGA hardware question

Hi all!

I am currently designing a prototype board using the blackfin 537. This 
chip has an array of ground pins in the center, which is only accessible 
to my ground layer via two small gaps. I would like to avoid placing 
vias between the BGA grid.

Question: must all of these balls be contacted, or may I leave 2 or 4 
uncontacted and place a via instead to connect the other ones to the 
ground layer? Any experience here?

Best regards,

Andre

On Mon, 06 Feb 2006 16:27:41 +0100, Andre <no_spam@fischer-zoth.de>
wrote:

>Hi all!
>
>I am currently designing a prototype board using the blackfin 537. This 
>chip has an array of ground pins in the center, which is only accessible 
>to my ground layer via two small gaps. I would like to avoid placing 
>vias between the BGA grid.
>
>Question: must all of these balls be contacted, or may I leave 2 or 4 
>uncontacted and place a via instead to connect the other ones to the 
>ground layer? Any experience here?

You probably want to connect them all, with a via per ball to the
ground plane.
"Two small gaps" sounds like it might be a problem.

There's an interesting discussion in comp.arch.fpga about a similar
matter:
http://groups.google.com/group/comp.arch.fpga/browse_thread/thread/c548c274b5f557c4

Regards,
Allan

Allan Herriman <allanherriman@hotmail.com> wrote in
news:icefu1plsc6o4vu153lvgsqjcs2vbbgpac@4ax.com: 

> On Mon, 06 Feb 2006 16:27:41 +0100, Andre <no_spam@fischer-zoth.de>
> wrote:
> 
>>Hi all!
>>
>>I am currently designing a prototype board using the blackfin 537.
>>This chip has an array of ground pins in the center, which is only
>>accessible to my ground layer via two small gaps. I would like to
>>avoid placing vias between the BGA grid.
>>
>>Question: must all of these balls be contacted, or may I leave 2 or 4 
>>uncontacted and place a via instead to connect the other ones to the 
>>ground layer? Any experience here?
> 
> You probably want to connect them all, with a via per ball to the
> ground plane.
> "Two small gaps" sounds like it might be a problem.

You might be able to share a via with two balls, as long as they are 
connected together.



> 
> There's an interesting discussion in comp.arch.fpga about a similar
> matter:
> http://groups.google.com/group/comp.arch.fpga/browse_thread/thread/c548
> c274b5f557c4 
> 
> Regards,
> Allan
> 

Don't leave any of the ground pins unconnected! I haven't used this part, 
but I can tell you that power supply planes, routing and decoupling are 
always critical issues for high speed parts. I usually place my 
decoupling capacitors on the bottom of the board when using BGAs to 
create close low Z connections. Use lots of caps with a mixture of 
values. I like 10nF, 100nF & a few large ones (2.2uF or 10uF).

If the BGA pitch is .8mm or smaller, you might consider microvias. I 
would check with the pcb manuafacturer to see what their capabilities are 
if you are not using microvias. 

You might look at ADI's EZ Kit for this DSP as well, since you know their 
layout works.




-- 
Al Clark
Danville Signal Processing, Inc.
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