Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame
Reexamination Certificate
2000-06-28
2003-07-01
Williams, Alexander O. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
C257S676000, C257S696000, C257S698000, C257S670000, C257S671000, C257S672000
Reexamination Certificate
active
06586821
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to integrated circuit device packages, and more specifically to a plastic integrated circuit device package with a lead frame having improved thermal dissipation characteristics.
BACKGROUND OF THE INVENTION
The trend in the electronics industry towards higher density integrated circuit devices has required that packages which house such higher density integrated circuit devices be able to dissipate more power. Because high density integrated circuit devices require more power and thus generate more heat, the manner in which the integrated circuit device package dissipates heat is critical. Generally speaking, it is quite advantageous to remove the heat from the integrated circuit device to the outside environment as quickly as possible.
The critical thermal path for removing heat from the integrated circuit device is defined as the path: (1) from the integrated circuit die to the die pad on which the integrated circuit die is seated, (2) from the die pad, by way of the horizontal air gap between the lead fingers and die pad, to the lead fingers of the lead frame, and (3) from the lead fingers to the printed circuit board on which the integrated circuit device is seated. Shortening this thermal path improves the thermal dissipation characteristics of the integrated circuit device package. As integrated circuit devices become more dense and thus must dissipate higher power, there is a continual need in the art to improve the heat dissipation characteristics of integrated circuit devices by shortening the thermal path of integrated circuit device packages.
A lead frame is the backbone of a molded plastic package. Lead frames are described in Chapter 8 of the 1989 edition of the Microelectronics Packaging Handbook (available from Van Nostrand Reinhold, 115 Fifth Avenue, New York, N.Y. 10003). In general, a lead frame is fabricated from a strip of sheet metal by stamping or chemical milling. The lead frame serves first as a holding fixture during the assembly process, then, after molding, becomes an integral part of the package. A lead frame includes a plurality of finger-like connections that extend from the periphery of the lead frame toward a center die pad. A semiconductor or chip is mounted on the center die pad.
Lead frames are either chemically milled or mechanically stamped from rolled strip stock. Typical strip thickness is approximately 0.25 mm, with thinner material (of approximately 0.20 mm) used for high lead-count packages such as 84-pin PLCC and quad flat pacs. Chemical milling is a process that uses photolithography and metal-dissolving chemicals to etch a pattern from a metal strip.
Stamped lead frames are fabricated by mechanically removing metal from strip stock with tools called progressive dies. The energy required to shear metal is directly proportional to the length of shear. Lead frames have large shear lengths per unit area. Therefore, a large amount of energy is required to stamp a full frame with one press stroke. Progressive dies are usually made of tungsten carbide and are arranged in stations. Each station punches a small area of metal from the strip as it moves through the die set.
To allow for the cutting tool, also known as a punch die, to be strong enough to operatively cut the lead frame, the prior art uses a cutting tool that has a narrow width of approximately 0.2 mil at the end increasing in width to a maximum width of approximately 30 to 40 mil at the base.
Referring to
FIGS. 1
a
to
7
b
, the manufacturing process for fabricating a conventional lead frame
10
of a plastic integrated circuit package according to the prior art is illustrated. Fabrication begins with a rectangular sheet of metal from which the plurality lead fingers
12
of the lead frame are formed. Referring to
FIG. 1
a
, the top view of a quadrant of a lead frame
10
after the lead fingers
12
have been defined, including a quadrant of die pad
14
, is shown. The plurality of lead fingers
12
are formed from a rectangular sheet of metal as is well known in the art.
FIG. 1
b
illustrates the cross-sectional view of the quadrant of the lead frame
10
at this stage of the process. The next step in the prior art process, as shown in the top view of
FIG. 2
a
, is to clamp the lead frame
10
into a fixed position prior to being cut with a punch die
22
.
FIG. 2
b
illustrates the cross-sectional view of the quadrant of the lead frame
10
, the upper clamp
18
and lower clamp
20
, and the punch die
22
. The next step in the prior art process, as shown in the top view of
FIG. 3
a
, is to separate the lead fingers
12
from the die pad
14
of the lead frame. At a substantially central portion of the lead frame
10
, a square die pad
14
, configured for mounting a semiconductor or chip thereon, supported by a plurality of suspension tie bars
16
is formed by cutting the lead frame
10
with the punch die
22
. The punch die
22
having a plurality of recesses along the cutting surface forming the plurality of tie bars
16
as the lead frame
10
is cut.
FIG. 3
b
illustrates the cross-sectional view of the quadrant of the lead frame, the upper clamp
18
and lower clamp
20
, and the punch die
22
after the lead frame
10
is cut with the punch die
22
. Tie bars
16
connect lead fingers
12
to die pad
14
.
FIG. 4
a
is a top view of a quadrant of the lead frame after the lead frame has been cut with a punch die
22
showing the physical separation aim between the lead fingers and the die pad
14
.
FIG. 4
b
illustrates the cross-sectional view of the quadrant of the lead frame at this stage of the process.
FIG. 5
is a top view of the lead frame after the top and bottom portions of the lead frame have been cut with a punch die
22
.
FIGS. 6
a
and
6
b
illustrate the lead frame showing the physical separation between the lead fingers
12
and the die pad
14
. The plurality of lead fingers
12
extend from the periphery of the lead frame
10
to a position spaced apart from the die pad
14
with a predetermined distance represented as &Dgr;>0, where &Dgr; is defined as the horizontal gap between the lead fingers
12
and the die pad
14
. It is also clear that the lead fingers
12
and the die pad
14
are co-planar at this stage of the fabrication process. Referring to
FIGS. 7
a
and
7
b
, the last step of the process is to downset the die pad
14
in relation to the lead fingers
12
. In performing the downset, it is noted that the physical separation between the lead fingers
12
and the die pad
14
, represented as &Dgr;>0, is maintained. Additionally, the downset of die pad
14
results in a vertical separation between lead fingers
12
and die pad
14
.
Referring to
FIG. 8
, the fabrication of the lead frame of a plastic integrated circuit package, according to the prior art, is illustrated in process flow
30
. First, the lead frame begins as a flat metal sheet as shown in step
32
. Next, at step
34
, the lead fingers
12
are defined. Step
34
corresponds to
FIGS. 1
a
and
1
b
. After the lead fingers
12
are defined, they are separated from the die pad
14
in step
36
, Step
36
corresponds to
FIGS. 3
a
and
3
b
. Finally, at step
38
, the die pad
14
is downset with respect to the lead fingers
12
as illustrated in
FIGS. 7
a
and
7
b.
According to the lead frame formed in
FIGS. 1-8
, the critical thermal path by which heat must be dissipated is defined as the distance from the integrated circuit die to the downset die pad
14
on which the integrated circuit die is placed; from the die pad
14
, by way of the horizontal air gap &Dgr; between the lead fingers
12
and die pad
14
, to lead fingers
12
; and from lead fingers
12
to the printed circuit board on which the integrated circuit device is placed. Shortening this thermal path would improve the thermal dissipation characteristics of the integrated circuit device. There is therefore an unmet need in the art to shorten the critical thermal path of the prior art lead frame used in plastic integrated cir
Hundt Michael J.
Zhou Tiao
Jorgenson Lisa K.
Larson Reneé Michelle
STMicroelectronics Inc.
Williams Alexander O.
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