[SI-LIST] Re: Backdrill under BGA

Sam,


It depends on how many multi gig traces/via are needed in BGA design.

You could try eliminating stub altogether by transition from top to last layer; 
and design a transparent via to account for capacitance using a 3D field solver.



Other options - use coreless substrates or build-up substrates with 
blind/buried vias.







-Vijaya



-----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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