[SI-LIST] Re: Backdrill under BGA

Hi Sam,

We had implemented backdrill for one of our customers. The ptich of BGA was 
1mm & data rate was 3.75Gbps with 0.2ns rise time. The thickness of board we 
used was 106 mils.

The Vias we used were10mil drill & 12mil drill.

Thanks,
Lingamoorthi


----- Original Message ----- 
From: "Sam Pete" <cygnul@xxxxxxxxx>
To: "Justin''Tabatchnick" <justin.tabatchnick@xxxxxxxxx>; "'Bill Stube'" 
<Bill.Stube@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>; "Aubrey Sparkman" 
<asparkman@xxxxxxxxxxxxx>
Sent: Thursday, July 23, 2009 8:59 AM
Subject: [SI-LIST] Re: Backdrill under BGA


> Thanks to all who shared your experiences.
> My concerns were the thickness of the PCB was more than 82mils and and I 
> need to run traces quite a far. Considering the cost of blind/burried 
> vias, thought of backdrill BGA vias.
> It looks like people have done some experiments with BGA backdrill, still 
> outcome seems not concrete.
> thanks again
> -Sam
>
> --- On Wed, 7/22/09, Aubrey Sparkman <asparkman@xxxxxxxxxxxxx> wrote:
>
> From: Aubrey Sparkman <asparkman@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Re: Backdrill under BGA
> To: "'Tabatchnick, Justin'" <justin.tabatchnick@xxxxxxxxx>, "'Bill Stube'" 
> <Bill.Stube@xxxxxxxxxx>, cygnul@xxxxxxxxx, si-list@xxxxxxxxxxxxx
> Date: Wednesday, July 22, 2009, 10:11 AM
>
> Justin,
>
> You are correct in that an 82 mil board with 6.4GT/s or less data rates
> doesn't need back drill for the usual reasons.
>
>
> Aubrey Sparkman
> Aubrey.K.Sparkman@xxxxxxxx
>
> -----Original Message-----
> From: Tabatchnick, Justin [mailto:justin.tabatchnick@xxxxxxxxx]
> Sent: Wednesday, July 22, 2009 11:54 AM
> To: Aubrey Sparkman; 'Bill Stube'; cygnul@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: RE: [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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