Plastic and nonmetallic article shaping or treating: processes – With step of cleaning – polishing – or preconditioning...
Reexamination Certificate
2001-10-02
2003-12-23
Ortiz, Angela (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With step of cleaning, polishing, or preconditioning...
C264S161000, C264S272110, C264S272170, C264S276000
Reexamination Certificate
active
06666997
ABSTRACT:
TECHNICAL FIELD
This invention related to injection molding of integrated circuit packages, and, more particularly, to a device and method for facilitating the removal of cleaning compound from integrated circuit injection molds.
BACKGROUND OF THE INVENTION
As shown in
FIG. 1
, integrated circuits are typically encapsulated in a package
10
of a suitable material, such as epoxy, from which conductive leads
12
project. Although the leads
12
are shown in
FIG. 1
as being conductors that project laterally and then downwardly, other lead configurations are in common use.
The encapsulation of the integrated circuit is typically performed by placing the integrated circuit in a mold then injecting a molding compound into the mold. A typical integrated circuit injection mold
20
is shown in FIG.
2
. The mold
20
includes a rectangular upper mold section
26
and a matching lower mold section
28
, each of which have a series of mold cavities
36
,
38
, respectively, formed in respective adjoining mold surfaces
30
,
32
. Each of the adjoining pairs of mold cavities
36
,
38
generally encapsulate a single integrated circuit, although it is possible to encapsulate several interconnected integrated circuits in a single mold cavity pair. Eight mold cavities
36
,
38
are shown in
FIG. 2
, but a greater or lesser number of mold cavities may be formed in conventional mold sections. The mold cavities
36
,
38
are typically rectangular to match the desired shape of the integrated circuit package
10
(FIG.
1
), but other shapes are also possible.
In practice, before the integrated circuits are placed in respective pairs of mold cavities
36
,
38
, they are attached to a leadframe, and the integrated circuit and leadframe are placed between the mold sections
26
,
28
. A typical leadframe
40
is shown in FIG.
3
A. The leadframe
40
includes several leadframe sections
44
corresponding in number to the number of pairs of mold cavities
36
,
38
, and corresponding in size and shape to the size and shape of the mold cavities
36
,
38
. A single leadframe section
44
is shown in FIG.
3
B. With reference to
FIG. 3B
, each leadframe section
44
includes a central mounting plate
46
to which an integrated circuit
48
is mounted by suitable means, such as adhesive tape (not shown). Each leadframe section
44
also includes a plurality of inner leads
50
projecting from each side of the central mounting plate
46
to a respective gasket strip
52
, and a plurality of outer leads
56
aligned with respective inner leads
50
extending from the gasket strips
52
. The outer leads
56
are what eventually form the leads
12
shown in FIG.
1
. The leadframe
40
also includes leadframe rails
60
,
62
extending along the longitudinal edges of the leadframe
40
. Although not shown in the Figures, after the package
10
has been formed, the portions of the gasket strips
52
between the leads
50
,
56
are removed to electrically isolate the leads
50
,
56
from each other, and the outer leads
56
are bent downwardly as shown in
FIG. 1
to form the leads
12
.
The leadframe
40
to which the integrated circuit
48
is attached is placed in the mold
20
between the mold sections
26
,
28
, with each leadframe section
44
aligned with a respective pair of mold cavities
36
,
38
. The leadframe
40
is also placed in the mold
20
so that the gasket strips
52
and leadframe rails
60
extend around the cavities
36
,
38
in contact with the mold surfaces
30
,
32
so that the leadframe
40
acts as a gasket to retain material within the cavities
36
,
38
.
After the leadframe and integrated circuit
48
have been placed in the mold
20
, a molding compound is injected into each pair of the cavities
36
,
38
through a respective injection inlet
70
(
FIG. 2
) provided for each pair of mold cavities
36
,
38
at one edge thereof. The injection inlets are formed in either or both of the mold sections
26
,
28
. The injection inlets
70
provide a path for the molding compound, generally an epoxy compound, to be injected into the mold cavities
26
,
28
. The molding compound attempts to displace air in the mold cavities
36
,
38
, and this air must therefore be vented from the cavities
36
,
38
. For this purpose, mold vents
74
are formed in either or both of the mold sections
26
,
28
through an edge of each pair of mold cavities
36
,
38
opposite the injection inlets
70
.
In practice, before the mold
20
can be used, it must be prepared by injecting a conditioning compound into the mold cavities
36
,
38
. The conditioning compound contains release agents to make the surfaces of the mold cavities
36
,
38
slippery so that the integrated circuit packages
10
(
FIG. 1
) can be removed from the mold cavities
36
,
38
after being molded. Although the molding compound also contains release compounds, the molding compound does not contain a sufficient quantity for initial use of the mold. The conditioning compound is normally injected into the mold cavities
36
,
38
after a leadframe has been inserted between the mold sections
26
,
28
. The leadframe
40
serves as a gasket to retain the conditioning compound in the cavities
36
,
38
. Also, since the conditioning compound surrounds the leadframe
40
within the mold cavities
36
,
38
, removal of the leadframe
40
effectively removes the conditioning compound from the mold cavities
36
,
38
.
After the mold
20
has been used to mold a large number of integrated circuit packages
10
, typically on the order of 500-2,500 packages, the release compound in the molding compound and possibly other components in the molding compound build up as deposits on the surfaces of the mold cavities
36
,
38
. These deposits must be removed to prevent the molding compound from sticking to the surfaces of the cavities
36
,
38
and thereby damaging the integrated circuit packages
10
(FIG.
1
). These deposits are removed by injecting a cleaning compound into the mold cavities
36
,
38
. The cleaning compound is injected through the injection inlets
70
to displace all of the air in the mold cavities
36
,
38
until some cleaning compound starts to exit the vents
74
. The cleaning compound is somewhat “sticky” so that the cleaning compound adheres well to the deposits, thus allowing the deposits to be removal by simply removing the cleaning compound from the mold cavities
36
,
38
. The cleaning compound is also typically relatively viscous, contains cleaning chemicals and abrasives, and shrinks after cooling to draw the deposits from the surfaces of the mold cavities
36
,
38
.
The leadframe performs the same two functions during the cleaning process that it performs in the conditioning process. First, as previously explained, it forms a gasket between the mold sections
26
,
28
. Without a leadframe between the mold sections
26
,
28
, the cleaning compound could leak from between the mold sections
26
,
28
. Second, the cleaning compound adheres to the leadframe so that the cleaning compound is removed from the mold
20
along with the leadframe. The leadframe thus facilitates the removal of cleaning compound from the mold cavities
36
,
38
.
Although removal of the leadframe adequately removes the cleaning compound from the mold cavities
36
,
38
, some cleaning compound residue, known as “flash,” tends to remain in the vents
74
. Yet substantially all of the cleaning compound flash must be removed from the vents
74
before the mold
20
can be used to mold integrated circuit packages
10
. If the flash is not removed from the vents
74
, molding compound injected through the injection inlet
70
will be unable to displace air in the mold cavities
36
,
38
. Cleaning compound flash in the vents
74
is conventionally removed by a laborious and time-consuming process of manually scraping shreds of flash from the vents
74
using a pointed tool (not shown). During the time the flash is being removed from the vents
74
, the injection molding machine containing the mold
20
cannot be use
Gifford Michael D.
Williams Vernon M.
Dorsey & Whitney LLP
Micro)n Technology, Inc.
Ortiz Angela
LandOfFree
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