Mitch,

*talk to your vendors*

.25 mm drill (we always spec drill sizes, not FHS) (.0098")
.55 mm pad external
.50 mm pad internal (adjust anti-pad based on drill for planes)
.1 mm trace / space

We have designed a few boards where the trace to trace spacing is .075
thus allowing some increase in pad/drill diameter. (.0875 trace to via)
.35mm drill. (.0138")

Or one might look at the trace vs. space as a .2mm pitch and let the
vendor adjust based on impedance requirements (core thickness).
Of course you are hoping that the clk environment is synchronous,
otherwise you may have issues with cross talk. Otherwise you might create
a minimum spacing region for the BGA's to simply get out.

Our designs are high layer count, sequential lam, 50 ohm stripline. So
the anti-pads are critical.
We keep non-functionals on the signal layers, and add thieving, both for
stability and equal etch pressure.
We remove non-fuctionals on planes and "cheat" the anti pad to clear the
drill, but maintain mfg tol for the stripline. We do not allow anything
less than tangent. (trace edge to relative anti pads)

Jack's comments with tenting and tear dropping are good.

You might suggest pulse plating vs. electrolytic plating depending on
thickness, aspect ratio and hole wall quality.
We have found Sn/Pb to work well as an etch mask in holes this small.
But don't let the Cu tarnish prior to Sn/Pb etch mask plating.

We use OSP and hard gold as our typical BGA surface finishes. Planarity
is important. Be aware when you tent a thru via you may be causing a
reliability problem for an OSP finish. The microetch may be entrapped
which can etch away or thin out Cu barrels to the point where a thermal
event (reflow) may cause stresses and crack the barrel.

What surface finishes do you require?

Hope this helps,

-Shawn





Mitch Morey wrote:

> Hi all,
>
> I'm looking for some help. I've got some 1mm BGAs on my board, and I
> need to get two between routing on the inner layers without going to
> uVias. I'm currently trying with .012" FHS vias, but they don't do two
> between. The board is .070" thick and I can go to an 8/1 aspect ratio.
> Give me your best shots...
>
> TIA.
>
> Mitch
> _____________________________________________________________
> Tired of limited space on Yahoo and Hotmail?
> Free 100 Meg email account available at http://www.dacafe.com

--
Shawn R. Utz                [log in to unmask] -or- [log in to unmask]
Cray Inc.                   http://www.cray.com