[SI-LIST] Re: Backdrill under BGA

Sam,

I do not have experience with backdrilling under BGAs specifically.  

My suggestion would be to avoid putting vias on the signal at BGA in the first 
place, or if you must have vias to transition from layer 1 to layer N and avoid 
the stub.  I realize this doesn't exactly address your question, but if you 
avoid having a via/stub altogether, you will avoid the added complexity and 
cost of having to backdrill and account for the items David mentions below.

Just thought I would throw it out there.

Bill Stube
Hardware Engineer

Plexus - http://www.plexus.com
The Product Realization Company


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of David Mullenex
Sent: Wednesday, July 22, 2009 4:13 AM
To: cygnul@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Backdrill under BGA


Sam,

The critical factors missing here are your BGA pitch and location of the 
proposed back-drills relative to the BGA grid and each other. This as 
Back-drilled vias require larger anti-pads to accomodate the larger diameter of 
the back-drill head that assures removal of the matal stub, as well as the 
minor mis-alignment tollerace for the back-drill. Note, that you will also lose 
routing channels on the back drilled layers due to the larger antipad and 
back-drill diameter.

Looking at the drill size, anti-pad, routing channels, board thickness, and 
ground/power plane coverage under the BGA, may indicate that back-drilling is 
not the preffered solution for fine pitch BGAs. Also, selection of the right 
routing layers and pad stack may allow you to get by with negligable stubs... 
assuming you do not have a boat load of multi-gig signals on a poorly 
pinned-out BGA. 

Porvide more details here or work with your fab house to arrive at 
the manufacturable solution. 

cheers, 
- David.


----- Original Message ----
From: "cygnul@xxxxxxxxx" <cygnul@xxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Sent: Tuesday, July 21, 2009 5:46:21 PM
Subject: [SI-LIST] Backdrill under BGA

SI Gurus,
Has anybody done back drilling on BGA vias for multi gig traces?
is that achievable? If so, what was the via structure? 

-Sam
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