Flip-chip packaging method that treats an interconnect...

Details Flip-chip packaging method that treats an interconnect... Flip-chip packaging method that treats an interconnect...

2001-12-11

2004-07-13

Zarneke, David A. (Department: 2829)

Active solid-state devices (e.g., transistors, solid-state diode

Encapsulated

C257S789000

Reexamination Certificate

active

06762509

ABSTRACT:

BACKGROUND OF INVENTION
Flip-chip packaging can generally provide a small footprint package with a large number of electric connections to an integrated circuit die.
FIG. 1A
illustrates a packaged device
100
using flip-chip packaging of an integrated circuit die
110
. Die
110
is an integrated circuit chip formed from a semiconductor wafer and having solder bumps
115
on an active surface. Solder bumps
115
are electrically connected to circuit elements formed in and on die
110
. In packaged device
100
, die
110
is flipped so that bumps
115
contact a substrate
130
.
Substrate
130
is typically a printed circuit made of a material such as polyimide, polyimide alloy or compound or non alloy general polymer and metal composites; ceramic, silicon, or glass and metal composites; or similar materials forming a flexible or rigid carrier having conductive traces (not shown), which are generally made of copper or another metal. Solder bumps
115
on die
110
contact the conductive traces on the top surface of substrate
130
, and the conductive traces, which extend through substrate
130
, electrically connect solder bumps
115
to solder balls
135
on the bottom surface of substrate
130
. Solder balls
135
, which can be arranged in a ball grid array, form the terminals of packaged device
100
and can be attached to a printed circuit board or other circuitry in a product containing packaged device
100
.
One concern in flip-chip packages is the difference between the coefficients of thermal expansion of semiconductor die
110
and substrate
130
. This difference creates mechanical displacement stress on the connections between die
110
and substrate
130
. In packaged device
100
, underfill
120
between die
110
and substrate
130
strengthens the attachment of die
110
to substrate
130
to help prevent the thermal stresses from breaking the connections between die
110
and substrate
130
.
FIG. 1B
illustrates an edge of underfill
120
. Underfill
120
contains filler particles
122
suspended in an organic resin
124
. Filler particles
122
generally have a size selected according to a gap between die
110
and substrate
130
, e.g., the filler particles have a diameter about one third the size of the gap. Generally, the composition and concentration of filler particles
122
are selected to control the coefficient of thermal expansion and the shrinkage of underfill
120
.
Organic resin
124
that when initially applied in device
100
is a liquid that flows into the gap between die
110
and substrate
130
. Accordingly, the edge of underfill
120
has a concave shape that depends on the viscosity of liquid organic resin
124
and the adhesion of organic resin
124
to die
110
and substrate
130
. Organic resin
124
subsequently cures, and the presence of filler particles
122
helps control the shrinkage that occurs in underfill
120
during curing.
As shown in
FIG. 1B
, the distribution of filler particles
122
is relatively uniform where underfill
120
is significantly thicker than the diameter of filler particles
122
. However, in fillet regions
126
and
128
where the thickness of underfill
120
approaches or is less than the diameter of a filler particle, the density of filler particles
122
falls or is reduced. The lack of filler particles
122
causes more shrinkage in fillet regions
126
and
128
during curing. This shrinkage can warp substrate
130
and disrupt electrical connections between substrate
130
and die
110
or between substrate
130
and an external circuit. In particular, shrinkage and surface tension in underfill
120
causes stress S on substrate
130
near the edge of die
110
. This stress S is along a direction that depends on the wetting angle &agr; of underfill
120
at the edge of die
110
.
The lack of filler particles
122
in region
126
also makes the coefficient of thermal expansion of in regions
126
and
128
differ from the coefficient of thermal expansion in thicker regions of underfill
120
. Accordingly, temperature changes can induce further stress in fillet regions
126
and
128
if the composition of underfill
120
is selected to minimize stress created by thermal expansion in thick regions of underfill
120
.
To improve reliability and yield of good packages, methods and structures are sought that avoid increased shrinkage, stress that warps the substrate, and/or change in coefficient of thermal expansion that occurs at the edges of the underfill.
SUMMARY OF INVENTION
In accordance with an aspect of the invention, a dam, barrier, or other damming feature or discontinuity changes the shape or accumulation of the underfill material to reduce stress resulting from edge effects. In particular, the dam controls the wetting angle of the underfill material to provide a much smaller stress component perpendicular to the surface of the underlying substrate, and the underfill as shaped by the dam lacks underfill fillet regions that shrink significantly and cause stress on the substrate. The dammed underfill additionally avoids or reduces the size of areas having low filler particle concentration and thus avoids thermal coefficients of expansion that differ from the optimal coefficients. The resulting package has superior performance as defined by co-planarity, reliability, and mechanical improvement when compared to conventional overall flip-chip packages.
One specific embodiment of the invention is a packaged device that includes a substrate, a die, and a dam. The die has contacts placed as in a conventional flip-chip package so that the contacts electrically contact conductive traces of the substrate. The dam attaches to the substrate and surrounds the die to confine the edges of underfill that fills a gap between the die and the substrate. The dam controls the shape of the underfill so that wetting angles at the die and at the dam are less than 45° or so that the underfill lacks fillet regions.
Generally, the device has a ball grid array on a side of the substrate opposite to the die. In an exemplary embodiment, the ball grid array has a pitch that is less than or about equal to one half a separation between the dam and an edge of the die. The width of the dam is typically between one and two times the pitch of the ball grid array, and the height of the dam is chosen to provide a wetting angle for the underfill that avoids stress on the substrate or areas of underfill having a low filler concentration.
Another embodiment of the invention is a method for packaging an integrated circuit die. The method includes: attaching the die to a substrate so that conductive traces on the substrate electrically contact contacts on the die; forming a dam on the substrate around the die; and filling a volume between the die and the substrate and between the die and the dam with an underfill material. The dam can be constructed before applying the underfill by placing, depositing, growing, or otherwise accumulating material on the substrate to form the dam. Alternatively, the dam can be preformed to the desired shape and attached to the substrate. The underfill is applied after the dam is in place so that the dam controls the shape and location of the edge of the underfill. Suitable materials for such dams include but are not limited to a material such as a metal layer or feature and a polymer which is filled with property modifying materials such as spheres, fibers or pieces of quartz, ceramic, or metal.
In an alternative embodiment, removing material from the substrate (e.g., by machining or etching) before a die is attached can leave a dam surrounding a die attachment area on the substrate.
In yet another alternative embodiment, treatment of the substrate increases adhesion or stiction between the underfill and the substrate in specific areas on the substrate. The underfill accumulates and can be shaped and cured to form the dam in the treated area of the substrate.


REFERENCES:
patent: 5120678 (1992-06-01), Moore et al.
patent: 5336931 (1994-08-01), Juskey et al.
patent: 5450283 (1995-09-01), Lin et al.
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