Method of plastic molding

Details Method of plastic molding Method of plastic molding

1994-12-14

2001-05-01

Ortiz, Angela (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

To produce composite, plural part or multilayered article

C264S272170, C264S328700

Reexamination Certificate

active

06224810

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an improvement of a plastic molding device for a semiconductor element and a method for plastic molding a semiconductor element.
BACKGROUND OF THE INVENTION
FIG. 8
is a view showing a conventional plastic molding device for plastic molding a semiconductor element.
In
FIG. 8
, reference numeral
80
designates a plastic molding means for plastic molding a semiconductor element which comprises separable upper mold
70
and lower mold
71
. The upper and lower molds
70
and
71
are supported by upper and lower platens
65
and
64
of a clamping press (a mold driving mechanism) which is not shown, respectively.
The upper mold
70
comprises an upper mold cavity block
1
having a plurality of upper mold cavities la, an upper surface plate
5
with a heater built therein which supports the upper block
1
, a base plate
9
supporting the upper surface plate
5
through a post
15
, and a spacer block
8
interposed between the upper surface plate
5
and the base plate
9
. The upper cavity block
1
has a center block
2
in the center thereof and the center block
2
is equipped with a chamber
3
for housing a plastic tablet
14
. At an upper portion of the chamber
3
, a plunger
12
is provided for pressing the plastic tablet
14
. In addition, an eject pin
4
, one end of which is drawn in or out of the upper cavity
1
a,
is provided in the cavity block
1
and the upper surface plate
5
. A vertically movable pin supporting member
20
comprising an ejector plate
7
and a keep plate
6
which pushes the eject pin
4
toward the side of the ejector plate
7
is provided between the upper surface plate
5
and the base plate
9
in the upper mold
70
. The pin supporting member
20
is urged against the upper surface plate
5
by forcing means
21
. The forcing means
21
comprises a screw bar
10
screwed on the upper surface plate
5
and a spring
11
provided between the screw bar
10
and the pin supporting member
20
. Reference numeral
13
designates a return pin fixed on the pin supporting member
20
.
The lower mold
71
has the almost same structure as that of the upper mold
70
and comprises a lower mold cavity block
51
having a plurality of lower mold cavities
51
a,
a lower center block
52
at the center thereof, a lower surface plate
55
with a heater built therein which supports the blocks
51
and
52
, a post
63
supporting the lower surface plate
55
, a spacer block
58
, and a base plate
59
. Eject pins
54
, one end of each of which can be drawn in and out of the lower mold cavity
51
a
, are provided in the cavity block
51
and the lower surface plate
55
. A vertically movable pin supporting member
30
comprising an ejector plate
57
and a keep plate
56
is provided between the lower surface plate
55
and the base plate
59
. The pin supporting member
30
is urged away from the lower surface plate
55
by a spring
60
.
In the lower center block
52
and the lower mold cavity block
51
, runners
52
a
and
51
b
are respectively formed as passages for the plastic
14
. In addition, in the block
51
, a gate
51
c
connecting the runner
51
b
to the lower mold cavity
51
a
is formed. The runner
52
a
and the gate
51
c
form a plastic injecting mechanism with the plunger
12
and the chamber
3
. In addition, a return stopper
62
is attached to the lower surface plate
55
. This return stopper
62
abuts the return pin
13
at the time of clamping to retract the eject pin
4
of the upper mold
70
. A knockout rod
61
is fixed on the base part of the device. This knockout rod
61
abuts the pin supporting member
30
to eject the eject pin
54
into the lower mold cavity
51
a
when the upper and lower molds
70
and
71
are separated by moving the lower platen
64
downward. Further, a plurality of posts are provided in this device other than the above posts
15
and
63
.
Next, the operation of the apparatus of
FIG. 8
will be described.
A lead frame on which semiconductor chips are bonded is set on the lower cavity block
51
. These chips are connected to the lead frame by metal wires. Then, the lower mold
71
is moved upward by the clamping press and put together with the upper mold
70
and then they are clamped. At this time, since the return stopper
62
pushes the return pin
13
, the pin supporting member
20
slightly moves upward against the force of the spring
11
, whereby the eject pins
4
are retracted upward from the upper mold cavity
1
a.
Then, the plastic
14
which is preheated is put in the chamber
3
and the plunger
12
is moved downward to apply pressure to the plastic
14
. Then, the plastic
14
passes through the runners
52
a
and
51
b
and then the gate
51
c
and fills the cavities
1
a
and
51
a.
In this state, the plastic is left for 60 to 90 seconds to solidify and then the lower mold
71
is moved down to open the plastic molding means
80
. At this time, the return stopper
62
is separated from the return pin
13
, whereby the pin supporting member
20
is moved downward by the force of the spring
11
. Then, the upper mold eject pins
4
push plastic molded products out from the upper mold cavities
1
a.
Thereafter, the lower platen
64
of the clamping press falls and the knockout rod
61
abuts the bottom of the supporting member
30
. Then, the pin supporting member
30
is pushed by the rod
61
and rises relative to the lower surface plate
55
. Then, the eject pins
54
project into the lower cavities
51
a
and push the products out. The plastic molded products are picked up and then go through the process of cutting and bending the leads, completing plastic molded semiconductor devices.
In the conventional plastic molding device constituted as described above, the plastic is injected by the plastic injecting mechanism, but sufficient injection pressure does not reach the cavity because of the pressure loss in the runners
52
a
and
51
b
and the gate Sic or the like. Further, since the sectional area of the gate
51
c
is small as compared with that of other passages, such as the runner, the plastic is likely to thermally harden there. When the plastic solidifies in this portion, the injection pressure does not reach the cavity at all. Accordingly, shrinkage deformation or the like occurs on the surface of the plastic molded product molded in the cavity, causing the quality of the package to degrade. Particularly, in an optical semiconductor element for processing an optical signal, which is molded in a transparent plastic, signal light reflects or refracts because of unevenness of the package surface, causing erroneous operation.
In order to solve the problem of shrinkage deformation in general plastic molding, there is proposed a device disclosed in Japanese Utility Model Laid-Open Application No. 63-191013 in which a cylinder and a piston are arranged corresponding to a thick portion of the molded product and pressure is applied to the plastic by actuating the piston after the plastic is injected. However, since there are many small cavities in the plastic molding device for a semiconductor element, it is actually impossible to arrange the cylinder and piston for each cavity, so that the problem of shrinkage deformation in plastic molding for the semiconductor element can not be solved.
SUMMARY OF THE INVENTION
The present invention was made to solve the above problems and it is an object of the present invention to provide a plastic molding device for a semiconductor element preventing shrinkage deformation or the like on a surface of a plastic molded product and thus producing a plastic molded semiconductor element which has a smooth surface.
Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in

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