Jay-

With 1 mil min annular ring, you are assured that you have minimum
probability of interconnect failure due to plating faults and barrel
cracks.

If you accept breakout and still need the circuit to be reliable, then
you need to do further analysis.  You might think that at as little as
50% of the circumference still in the pad means it should be reliable,
but what if the breakout is in the direction of the trace leading to the
via--breakout on that side could take you down to below 25%
circumferential contact, which would certainly give me pause, anyway. 
To combat that, many shops still add "teardrops" to internal pads.

You note you have blind vias as well.  If they are laser drilled blind
vias, my opinion is that any breakout is a big risk.  The reason is that
while trying to clean the target pad off real well with the laser to
assure good contact, breakout will cause damage to the dielectric at the
side of the pad, potentially even going through to conductors on the
next layer.  This doesn't apply for mechanically drilled blinds.

Good Luck,

Wayne

>>> [log in to unmask]  >>>
Need help in quantifying difference in reliability prediction for a
specific board 
design meeting 6012 Class 2 vs. Class 3 requirements, given all
parameters 
equal except internal annular rings.  My concern is breakout allowed by
class 2 
vs. 1 mil min. internal annular ring per class 3.

Parameters:

high quality FR4 material
15" x 15" size
20 layer high density
layers 1 -5 contain blind vias
finished board thickness appx. 0.110"
1mm dot pitch bga's 
benign operating environment

Jay Otts
Raytheon

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