Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame
Reexamination Certificate
1998-09-28
2001-11-13
Potter, Roy (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
C257S787000
Reexamination Certificate
active
06316821
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to high density lead frames and methods for plastic injection molding. More particularly, the present invention relates to high density lead frames and methods for plastic injection molding that minimize waste of both the lead frame material and the plastic injection molding material, while increasing manufacturing productivity.
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 of a lead frame assembly
100
. The lead frame assembly
100
includes a first lead frame
110
and a second lead frame
140
. The first and the second lead frames
110
,
140
are connected by an injection molding assembly
105
. The injection molding assembly
105
includes a plurality of culls
115
through which the molding compound (not shown) travels. The molding compound is injected into each of the culls
115
and travels through pairs of subrunners
120
formed during the molding process to the first and second lead frames
110
,
140
. The subrunners
120
are shaped like tunnels through which the molding compound flows. The molding compound, initially in powder form, is heated to about 175° C., at which temperature the powder compound becomes a liquid. The hot liquid molding compound is then injected through an opening in the culls
115
and forced to flow through the subrunners
120
. The molding compound flows through the subrunners
120
until the gates
125
, also generally tunnel-shaped, are reached, the gates
125
guiding the molding compound to the mold location where the individual packages
130
are to be formed. The molding compound then forms the individual packages
130
, encapsulating the integrated circuits therein in individual packages, leaving the leads
132
protruding therefrom. In
FIG. 1
, the packages
130
can be seen to be oriented with their leads
132
oriented perpendicularly to the long axes
150
of the first and second lead frames
110
,
140
.
As shown in
FIG. 1
, the first and the second lead frames
110
,
140
each support five pairs of packages
130
, each pair of packages
130
having been supplied with molding compound by a single subrunner
120
and a gate
125
. The lead frame assembly
100
of
FIG. 1
thus creates twenty such packages
130
, ten packages
130
in each of the first and second lead frames
110
,
140
. The packages
130
are then trimmed from the first and second lead frames
110
,
140
, the leads
132
appropriately bent and shaped, and the packages
130
separated from one another in a simulation process.
The first and second lead frames
110
,
140
are formed of copper or of a copper alloy. After the simulation process, all but the packages
130
themselves are discarded. Indeed, the remaining portions of the first and second lead frames
110
,
140
are discarded, as is the injection molding assembly
105
, including the culls
115
, the subrunners
120
and the gates
125
. Because of the design and layout of the lead frame assembly
100
, a great deal of lead frame material and injection molding assembly
105
is thrown out, thus significantly adding to the cost of the end product. Moreover, because of the low density of packages
130
on each of the first and second lead frames
110
,
140
, labor costs are high and manufacturing yield is low, both also contributing to increased unit cost of the packages
130
.
In an effort to reduce material waste and labor costs and to increase manufacturing yield, the lead frame assembly
200
of
FIG. 2A
has been developed.
FIG. 2A
shows the lead frame assembly
200
with a plurality of packages
230
molded thereon. The lead frame assembly
100
of
FIG. 2A
includes a first and a second lead frame
210
,
240
joined by an injection molding assembly
205
. The structure of the injection molding assembly
205
is similar to that of its counterpart
105
in
FIG. 1
, and includes a plurality of centrally located culls
215
through which the molding compound (not shown) is injected. The molding compound is injected into each of the culls
215
and travels through pairs of subrunners
220
formed by the mold to the first and second lead frames
210
,
240
. The subrunners
220
are generally tunnel-shaped structures through which the molding compound flows. The molding compound flows through the subrunners
220
until it reaches the gates
225
(also generally tunnel-shaped), the gates
225
guiding the molding compound to the location where the individual packages
230
are to be formed. The molding compound then fills these locations to encapsulate the integrated circuits therein to form the individual packages
230
, leaving the leads
232
protruding therefrom. In
FIG. 2A
, the individual packages
230
can be seen to be again oriented with their respective leads
232
oriented generally perpendicularly to the long axes
250
of the first and second lead frames
210
,
240
.
The lead frame assembly
200
shown in
FIG. 2A
is designed to support a higher number of packages
230
than its predecessor lead frame assembly
100
of FIG.
1
. Indeed, each subrunner
220
of
FIG. 2A
is connected to three gates
225
, each gate
225
supplying a pair of packages
230
. Therefore, each subrunner
220
supplies molding compound to
6
separate packages
230
. The lead frame assembly
200
depends upon a network of subrunners disposed squarely within the confines of the lead frames
210
,
240
to form pairs of packages
230
.
The first and second lead frames
210
,
240
are formed of copper or of a copper alloy. After the simulation process referred to above, all but the packages
230
are discarded. Indeed, the remaining portions of the first and second lead frames
210
,
240
are discarded, as is the injection molding assembly
205
, including the cuffs
215
, the subrunners
220
and the gates
225
joining pairs of packages
230
. Because of the architecture of the lead frame assembly
200
, much lead frame material and injection molding assembly
205
is discarded, thus again significantly adding to the cost of the end product. Moreover, because of the low density of packages
230
on each of the first and second lead frames
210
,
240
, labor costs are high and manufacturing yields low, albeit not as high nor as low, respectively, as with the lead frame assembly
100
discussed relative to FIG.
1
.
FIG. 2B
shows a lead frame
210
, before the packages
230
(shown in
FIG. 2A
) are molded thereon. As shown in
FIG. 2B
, the die attach pads
231
, the pads onto which the semiconductor die or dice is to be attached, are oriented in such a manner that the leads
232
extend generally perpendicularly relative to the long axis
250
of the lead frame
210
. As shown in both
FIGS. 2A and 2B
, the die attach pads
231
and their corresponding packages
230
shown in
FIG. 2A
are organized in rows and columns. Indeed, the rows of die attach pads
231
are parallel to the long axis
250
of the lead frame
210
(and
240
in FIG.
2
A), whereas the columns thereof are perpendicular thereto. Each column of die attach pads
231
is separated from its next adjacent column by a space called a street, shown in
FIG. 2B
at reference numeral
260
. The streets
260
may be perforated, as shown in
FIG. 2B
, to reduce the amount of lead frame material (e.g., copper) needed. The streets
260
include notches
265
for the gates
225
through which the liquid molding compound emerges to form the packages
230
.
The streets
260
provide the space and necessary support for the subrunners
220
and the gates
225
. To accommodate the network of subrunners
220
and gates
225
shown in
FIG. 2A
, the die attach pads
231
and their corresponding packages
230
must necessarily be separated by a substantial space. Such space inherently precludes a greater unit density on the lead frames
210
,
240
and leads to manufa
Beyerlein Dagmar
Chang Bo Soon
Cypress Semiconductor Corporation
Potter Roy
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