Forums

BGA hardware question

Started by Andre February 6, 2006
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. -------------------------------------------------------------------- Purveyors of Fine DSP Hardware and other Cool Stuff Available at http://www.danvillesignal.com