Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame – With structure for mounting semiconductor chip to lead frame
Reexamination Certificate
1999-02-26
2001-12-18
Everhart, Caridad (Department: 2825)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
With structure for mounting semiconductor chip to lead frame
C257S666000, C257S674000
Reexamination Certificate
active
06331728
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to high-density lead frames, packaging technology containing such lead frames and methods of attaching a silicon die onto a lead frame. More particularly, the present invention relates to high-reliability lead frames and packaging methods that provide excellent resistance to moisture and moisture-related problems, such as delamination and warping.
2. Description of the Related Art
Semiconductor integrated circuits are usually mounted on lead frames, each circuit thereafter being encapsulated in a plastic package formed by an injection molding process.
FIG. 1
shows an example of a first generation copper lead frame
100
. The lead frame
100
includes a plurality of solid copper die-attach platforms
110
. Each die-attach platform
110
is adapted to support a single semiconductor die (not shown). The die attach platforms
110
of the lead frame
100
have about the same dimensions as the semiconductor die they are designed to support. On either side of each die-attach platform
110
are leads
115
. Each die-attach platform
110
is attached to one of the two lead frame side rails
105
as well as to the lead frame center rail
135
. To attach the semiconductor dies onto the solid copper die-attach platforms
110
, an adhesive (such as silver epoxy paste) is applied to each die-attach platform
110
, and the semiconductor die is adhered thereon. A wire bonding process is then typically carried out to attach fine gold wires between the semiconductor die and the leads
115
. The assembly is then enclosed in a mold and molding compound is injected therein to encapsulate the silicon die into the packages. The molding compound is injected at high temperature, most often at or around 175-185 degrees C, and thereafter cooled to room temperature. The resulting packages are then trimmed from the lead frames
100
, the leads
112
appropriately bent and shaped, and the packages separated from one another in a singulation process.
The heating of the lead frame assembly to a temperature suitable for plastic injection molding and subsequent cooling to room temperature, however, causes problems that have been traced to the mismatch in the Coefficient of Thermal Expansion (hereafter “CTE”) of the constituent materials of the semiconductor package. Indeed, the CTE of the silicon die is about 2.2 Parts Per Million per degree C (hereafter “PPM”/degree C), while the CTE of the plastic molding compound is about 8-12 PPM/degree C and that of the copper lead frame
100
is about 17 PPM/degree C. Due at least in part to these rather larger CTE mismatches, packages manufactured utilizing this or similar lead frame technologies tend to warp as the constituent materials thereof expand and shrink to differing extents and rates.
FIG. 6
depicts a side view of a semiconductor package
600
and the leads
115
protruding therefrom. After the high temperature plastic injection molding and subsequent cooling processes, the mismatch in the CTE between the semiconductor die, the plastic molding compound and the copper lead frame
100
causes the package
600
to warp as it cools to room temperature. The extent of this warpage is illustrated in
FIG. 6
by the dimensional arrow labeled W, which represents a measure of the deformation undergone by the package
600
during the heating and/or cooling processes or during a subsequent high temperature process, such as a solder reflow process. The lead frame architecture shown in
FIG. 1
, for example, typically leads to a warpage measure W of about 4-5 mils for a Thin Outline Small Semiconductor Package (hereafter “TSOP”) having a thickness of about 40 mils, or about 1 millimeter.
In an effort to mitigate the warping effects of the solid copper die attach platforms
110
shown in
FIG. 1
, the semiconductor industry turned to the lead frames having an architecture similar to that shown at
200
in FIG.
2
. As shown in
FIG. 2
, the lead frame
200
includes die attach platforms
210
comprising platform cutouts
220
defining an “XX”-like shape, lead frame side rails
205
and a lead frame center rail
235
. The cutouts
220
reduce the amount of copper platform material supporting the semiconductor die (not shown) and thus somewhat reduce the warpage W experienced by the resulting semiconductor package as it undergoes the plastic injection molding and cooling processes and/or other thermal cycling processes. As shown in
FIG. 2
, each die attach platform
210
is connected to one of the side rails
205
as well as to the center rail
235
. Although an improvement over the lead frame architecture of
FIG. 1
, the lead frame of
FIG. 2
still results in an unacceptable degree of warpage W, due to the large area interfaces and CTE mismatches between the copper lead frame
200
, the molding compound and the silicon die.
In a further effort to control this warping phenomenon and the delamination and package cracking (internal and/or external) problems associated therewith, lead frame designers were drawn to design alternative lead frame configurations and die attach platforms, such as shown in FIG.
3
.
FIG. 3
shows a lead frame
300
, including side rails
305
, a center rail
335
, leads
315
and die attach platforms
310
comprising platform interior supports
330
attached to platform frames
325
. The platform frames
325
have about the same footprint as the die attach platforms
110
of
FIG. 1
, but with the interior portion thereof cut out to define the platform interior supports
330
. The platform interior supports
330
, as shown in
FIG. 3
, include a meshwork of paddles interconnected by thin wire-like runners. Each semiconductor die (not shown) is supported, therefore, by a platform frame
325
and by the platform interior supports
330
associated with the platform frame
325
. These die attach platforms
310
, although an improvement over the designs shown in
FIGS. 1 and 2
, nevertheless exhibit a significant degree of warpage W as a result of thermal cycling and the ingress and egress of moisture into and out of the resulting package. Indeed, the surface area of the die attach platforms
310
is still about 69% of the total surface area of the semiconductor die to be attached thereon. Therefore, a significant area of contact remains between the thermally mismatched copper die attach platform
310
and the silicon die, as well as between the copper die attach platform and the molding compound. Typically, the die attach platforms
310
of
FIG. 3
leads to a warpage W of about 2-3 mils for a TSOP package having a thickness of about 40 mils and to significant delamination problems as the constituent materials of the resultant package expand and contract to differing degrees as the device cools.
Still further efforts to reduce the warpage W have led to the lead frame architecture shown in FIG.
4
. As shown therein, the lead frame
400
includes lead frame side rails
405
, a center rail
435
, leads
415
and die attach platforms
410
. Each die attach platform
410
has a generally “II”-like shape, configured as a pair of facing and spaced-apart platform runners (perpendicular to the side rails
405
and to the center rail
435
) to support the silicon die (not shown). The facing and spaced apart runners of the die attach platform
410
define an empty space
420
therebetween. Whereas two of the sides of the silicon die are supported by the facing and spaced apart runners of the die attach platform
410
, the middle portion of the die is suspended over the space
420
. As with the die attach platforms of the lead frames of
FIGS. 1
,
2
and
3
, the die attach platform
410
of
FIG. 4
is attached to one of the side rails
405
as well as to the center rail
435
. Despite a lower contact area between the copper die attach platform
410
and the silicon die and a lower contact area between the die attach platform
410
and the molding compound, the lead frame architecture of
FIG. 4
still results in a warpage factor W of abut 2-4 mils for a 40 mils thick TSOP package,
Chang Bo Soon
Odejar Anthony
Verma Vani
Cypress Semiconductor Corporation
Everhart Caridad
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