[SI-LIST] Re: Backdrill under BGA

What is the data rate , rise time , maximum stub length, and dielectric 
constant . For an 82 mil board and reasonable data rates , you may not have to 
worry about back drilling at all because stub length may not be significant as 
compared to rise time wave length.

Justin Tabatchnick
SI , Intel

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Aubrey Sparkman
Sent: Wednesday, July 22, 2009 9:41 AM
To: 'Bill Stube'; cygnul@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Backdrill under BGA

We looked at backdrilling an 82 mil thick pcb under a 1mm (40 mils) pitch
BGA once.  Almost all of the low cost PCB suppliers said no. 


Aubrey Sparkman
Aubrey.K.Sparkman@xxxxxxxx

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Bill Stube
Sent: Wednesday, July 22, 2009 11:36 AM
To: cygnul@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [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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