BGA Decoupling

[eo1668]

Hi,
I have this BGA-1152, I was asked to keepout 4mm for any component placement surrounding the BGA. The decoupling caps used was 0402 (over 100 of them) and 0603 ( 30 over parts). I was told that I can't place 0402 on the solder side.

I'm about to start plcement but looking at it, the 4 corners will be all pack with 0402 decoupling. What is the best way to connect these decoupling with good filtering. Should I connect individual vcc/gnd to their respective pins then connect from cap to gnd/pwr plane.

Anyone got some routing guide for the BGA routing and decoupling caps.

Thanks for help.

Best regards,
PL

[David Ricketts]

From an EMI perspective, the longer the trace, the more noise you'll generate, so I recommend routing immediately to a via.

I have never seen routing guidelines that didn't require caps to on the bottom, right next to the power pins. I honestly don't think that decoupling caps can do much good on the Top layer, especially if the power/ground of the BGA is on the inner balls. Your best bet then is to have the power and ground planes within 4-6 mils of each other, and effectively become capacitively coupled.

[phillipr]

im assuming the 4mm clearance is for BGA inspection!

I agree with David
Placing decouplers to far away is not good.
We Place them directly underneath the bga on the rear of the card.

We use this structure to help keep the bottom free of via holes so placement of caps is easier and as close to the pins as possible.

L1 - L2 Micro
L2 - L11 Buried
L11 -L12 Micro

L1 - L12 through

[eo1668]

Hi,
Can 0402 part be place on the solder side? Any constraints on the assembly? Is yes, it is possible to place these decoupling cap directly below the BGA pwr/gnd pins using through via, no blind or burried vias.
Anyone got some example of the decoupling placement and routing that can show me. The BGA I am using is 1mm pitch, 1152 pins.

Thanks for help.

Best regards,
PL

[phillipr]

Here is a example on placing caps on the bottom side.

http://www.pcbstandards.com/forums/showthread.php?s=&threadid=750