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January 2004

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Subject:
From:
Brian Toleno <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 31 Jan 2004 16:03:07 -0500
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Certainly underfills were first developed to help increase the reliability
of devices where there is a significant TCE (aka CTE) mismatch between the
package (a flip chip) and the substrate.  These types of systems are
filled premixed and frozen (-40 deg C) epoxies.  The filler drives the CTE
downward to closer match the midpoint between the silicon, FR4, and solder
bumps.  Since then things have gotten a bit more complicated.  We now have
silicon dies that are quite large and the requirements are a bit
different, driving the CTE as low as possible may stop solder joint
fatigue cracking, but then you may run into other issues (e.g. fillet
cracking).  In those cases a blend of properties is needed (a blend
between the low CTE and low modulus - the filler that lowers the CTE
typically raises the modulus).

Now you add the CSP and BGA devices that need some enhanced reliability,
but they have flexible interposers (substrates in the package), now CTE is
definitely not the driver. These tend to need more increased reliability
with respect to shock, drop, and vibrations, plus some extended thermal
cycling.  On top of this you add some "features", such as: fluxing
(no-flow), reworkable, snap-cure, refrigerator storage.  There are quite a
number of choices.

Knowing the size of the die or package, the gap between the package and
the substrate, the pitch between the bumps, the bump and paste metallurgy
(Sn63 or Pb-free), final reliability requirements (does it need to meet
JEDEC, etc.), and knowing what "features" are needed or nice to have, will
help in making the best product selection.

Brian J. Toleno, Ph.D.
Sr. Applications Chemist
Loctite Electronics
15051 East Don Julian Rd
Industry, CA 91746
Ph: 626-968-6511 x304
Cell: 626-215-0879

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