[SI-LIST] Re: Backdrill under BGA
- From: "Bill Stube" <Bill.Stube@xxxxxxxxxx>
- To: <cygnul@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 22 Jul 2009 11:35:46 -0500
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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