Appeal No. 2004-0760
Application No. 10/010,392
neutralize themselves. This thermoset epoxy can be an
underfill material itself, or overmoulding compound or
silver filled epoxy with comparable C.T.E. as the
underfill. The balance plate should be made out of
material (can be made out of Copper, Aluminum etc.)
with a comparable CTE and modulus of elasticity to the
flexible substrate material.
The flow in a flip chip attach process according
to a preferred embodiment of the invention begins with
fluxing of the die. Next, the die is placed on the
substrate (presently available in ceramic, epoxy board,
laminate, flex, polyimide, “UPILEX” or “KAPTON” and
which sometimes includes a stiffener layer of copper,
aluminum, or an alloy and preferably having a CTE close
to that of the non-conductive substrate, for those
substrate materials requiring reinforcement, with bond
pads on the die being aligned with bond pads on the
substrate. Solder is reflowed between bond pads on the
die an substrate, resulting in the structure
illustrated in FIG. 3, step 2. Next, the die is
underfilled with a thermoset material (or other non-
conductive material similar to thermoset having a good
flow rate, lack of voiding and good adhesion to solder
and solder masks) until the material has wicked under
the die - as illustrated in FIG. 3, step 3. The
underfill material is then heated to a temperature at
which it gels but does not harden (the gelling
temperature and time are material dependent) thus no
complete curing . . . .
An attach epoxy (overfill material), such as
silver filled thermosetting epoxy, or thermally
conductive epoxies, etc. (overfill material), is
dispensed on the die surface as illustrated in FIG. 3,
step 4, on or top of the balance plate itself (or an
over molding material that balances out the stresses in
the lamination of the structure). The metal sheet (or
over molding material) is positioned on the die or on
the die and stiffener material, as illustrated in FIG.
3, step 5. The entire assembly is then cured in one
single step [column 2, line 36, through column 3, line
20].
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