To paraphrase Paul Reid's comment: given you have a well filled through via
The stacked structure that you describe is very common, but perhaps less so
in North America. For example, Intel has been sourcing their Pentium CPU
substrates (using similar stacked structures) from Japan for more than a
decade.
Regarding reliability, there are numerous factors that come into play. The
obvious ones are the Tg and z-axis CTE of the fill material. In addition,
one must also consider the adhesion of the fill material to the barrel wall,
dimensional stress induced during planarization, method of surface prep
cleaning used prior to Cu cap plating, adhesion between fill material and Cu
cap, etc. Regarding the level of competency among fabricators that do via
fill processing, I have found those in North America to be primarily
cost-driven, whereas the technology leaders in Japan and Taiwan are
performance-driven. The point I am trying to make here is that the
reliability of a via fill material in your application should not be judged
exclusively by what a fabricator, particularly in North America, can deliver
to you.
There are via fill materials readily available that meet your Tg and CTE
targets; however, this is just one piece of the puzzle.
Stephen Pierce
SGP Ventures, Inc.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mark F. Focht
Sent: Wednesday, December 12, 2007 4:37 AM
To: [log in to unmask]
Subject: [TN] Stacked Via Help
I have a design that,due to space constraints, requires stacking microvias
(L1-
L2) directly on top of (in line with) drilled buried vias (L2-L7, L2-L11).
I am
receiving conflicting information about this strategy being reliable
(storage
temp. range -50C to 110C, operating temp. range -40C to 85C) because of
the different CTE's of the via fill material used in the L2-L7/L2-L11 vias
and
the board material (FR406, 170Tg). While the board shops that will fab this
admit they prefer the vias be staggered (not very pratical in this design),
they
claim they have produced boards of this type without negative feedback or
issues. The CM that will be assembling this is extremely hesitant about
using
this via strategy. Does anyone have experience using a stack-up similar to
this? Would this be reliable? Is there a plugging paste material that very
closely matches the CTE of FR406, 170Tg? Any and all comments are
welcome.
Thanks,
Mark Focht C.I.D.+
Pole/Zero Corporation
5530 Union Centre Dr.
West Chester, Oh 45069-4821
Voice: 513.870.9060 X173
FAX: 513.870.9064
Email: [log in to unmask]
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