Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame – Small lead frame for connecting a large lead frame to a...
Reexamination Certificate
1999-10-12
2001-06-12
Williams, Alexander O. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
Small lead frame for connecting a large lead frame to a...
C257S676000, C257S671000, C257S670000, C257S666000
Reexamination Certificate
active
06246110
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor packaging, and specifically to an improved downset lead frame for semiconductor packages.
BACKGROUND OF THE INVENTION
Plastic semiconductor packages are fabricated using lead frames. A conventional plastic package includes a semiconductor die attached to a lead frame, and encapsulated in a plastic body. The lead frame supports the die during a molding process, and provides the internal traces and terminal leads of the completed package. Typically, the lead frame comprises a stamped or etched metal, such as a nickel-iron alloy, or a copper based alloy. Lead frames are provided in strips adapted to form multiple packages, which are subsequently singulated into individual packages.
One type of lead frame includes mounting paddles for the dice, and patterns of lead fingers spaced around the peripheries of the mounting paddles. Prior to the molding process, the backsides of the dice can be attached to the mounting paddles using an adhesive. In addition, metal wires can be wire bonded to the lead fingers, and to bond pads on the faces of the dice. Using a transfer molding process, a plastic material, such as an epoxy resin, is molded to either side of the dice and lead frame, and over the wire bonded wires. The lead frame is then singulated into multiple packages, and the lead fingers are trimmed and shaped to form the terminal leads of the packages.
One approach to fabricating a plastic semiconductor package involves providing the lead frame with a mounting paddle that is recessed from a center line of the package. This type of semiconductor package
10
is illustrated in
FIGS. 1A-1C
. In
FIG. 1A
, the package
10
includes a semiconductor die
12
and a lead frame
14
. A backside of the die
12
is attached to a mounting paddle
18
of the lead frame
14
. In addition, wires
16
are bonded to bond pads on the die
12
, and to lead fingers of the lead frame
14
. For simplicity the lead fingers are not shown in
FIGS. 1A-1C
. However, the lead fingers are spaced around a periphery of the mounting paddle
18
, in a plane coincident to a major plane of the lead frame
14
. The package
10
also includes a plastic body
20
molded to either side of the die
12
and lead frame
14
.
To minimize thermal stresses resulting from a TCE mismatch between the plastic body
20
, and the lead frame
14
, it is preferable to locate the lead frame
14
along a center line
22
, or neutral bend axis, of the package
10
. The center line
22
is also coincident to a major plane of the lead frame
14
. To improve the wire bonding process, it is also desirable to locate the die
12
below this center line
14
. This reduces drag forces on the wires
16
during wire bonding, and allows the wires to be more easily encapsulated by the plastic body. In order to satisfy these requirements, and properly locate the die
12
with respect to the lead frame
14
, the mounting paddle
18
can be offset from the center line
22
of the package
10
.
A downset
24
can be provided on the lead frame
14
to recess the mounting paddle
18
below the center line
22
of the package
10
. As shown in
FIG. 1B
, the downset
24
has a length “L” and a depth “D”. The depth “D” is equal to an offset of the die
12
below the center line
22
of the package
10
. As used herein the term “downset” refers to a stepped portion of a lead frame, that provides an offset for a first portion of the lead frame relative to a second portion of the lead frame. Typically, the downset provides an offset for a mounting paddle, or other die mounting portion of the lead frame relative to lead fingers on the lead frame.
Some recent package designs require a relatively large offset for the die
12
and thus relatively large downsets
24
. For example, a depth “D” of the downsets
24
can be from 7 mils to 14 mils. One problem with this construction is that the planarity of the mounting paddles
18
can be effected. As shown in
FIG. 1C
, the mounting paddle
18
is in effect suspended from the downsets
24
. Any differences in the depth “D” of the downsets
24
, or in the parallelism or locations of the downsets
24
, can cause the mounting paddle
18
to be angled with respect to the lead frame
14
.
Another problem with large downsets
24
is that the lead frames
14
are often stacked for handling. The large downsets
24
can cause the lead frames
14
to interlock and become difficult to separate. Also, lead frames
14
that are fabricated by stamping can include downsets
24
which require overworking and work hardening of the metal which forms the downsets. This can cause cracks and weak spots in the downsets
24
.
In view of the foregoing deficiencies in lead frames, the present invention is directed to an improved downset lead frame and to an improved method for fabricating semiconductor packages using the downset lead frame.
SUMMARY OF THE INVENTION
In accordance with the present invention, an improved downset lead frame, and a method for fabricating semiconductor packages using the lead frame, are provided. The lead frame includes spaced side rails and multiple patterns of lead fingers. The lead frame also includes multiple die mounting paddles attached to the side rails by support members. The support members include multiple downset segments configured to offset the die mounting paddles from the lead fingers.
In a first lead frame embodiment, each support member includes at least one mating pair of mating downset segments oriented at opposing angles relative to a longitudinal axis of an associated die mounting paddle. In a second lead frame embodiment, the support members include at least one pair of mating downset segments oriented along axes that are parallel to a longitudinal axis of an associated die mounting paddle.
In either embodiment the construction and orientation of the downset segments provide improved x, y and rotational orientation of the die mounting paddles. In addition, the construction and orientation of the downset segments improves the stacking of multiple lead frames, and the alignment of adjacent lead frames in a stack. Still further, the multiple downset segments can be formed without excessively work hardening of the lead frame metal, as can occur with large offset downset segments.
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Kinsman Larry D.
Wolfe Mark
Gratton Stephen A.
Micro)n Technology, Inc.
Williams Alexander O.
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