Gary,

I've seen this kind of request on data sheets as well.  We did it here a few years back - before we went overseas with our production assemblies - using 0603's under a BGA. It was a through hole & blind via design (L1-L8 & L9-L16).  There were some problems with doing this. 

You'll probably get some strong objections from your assembly house.  Mostly due to "rework" issues.  When you heat up one side of the board, the other side heats up too.  That causes parts to come loose that shouldn't and it just creates a nightmare for them.  Also, reflow soldering can be difficult.  Sometimes a special fixture has to be made to make sure the parts stay in place.  Talk to them about what you want to do and see if you can get them to find a solution.  Or be willing to take the lower yield rate.

To prevent DRCs, we created a part outline that fit between the via pads.  We kept the cap pads round and put a via-in-pad on the cap.

It can help with the signals, but plays havoc with assembly.  We don't do them this way anymore.

Good luck,
Cherie


-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]]On Behalf Of Gary
Boccoleri
Sent: Wednesday, December 08, 2004 5:04 PM
To: [log in to unmask]
Subject: [DC] FW: Bypass caps under BGAs using via in pad


pad

Hi All,

I'm looking at a spec for a new processor we may be using. They
recommend using via in pad and show a diagram of how to place 0402
bypass caps right on the vias under the chip. Has anybody done this?
This brings up a few questions. Since 2 adjacent vias will form the
footprint for the caps, should I make the bottom pad of the via a square
that approximates the pads I normally use on these caps? How do I place
the caps without creating DRCs? Will our assembly house be completely
dumbfounded when they see this? Is this whole idea just a figment of
some engineers imagination with no basis in reality?

Thanks for all help 

Gary Boccoleri
Bivio Networks
925-924-8621
www.bivio.net
 


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