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
Reexamination Certificate
1995-05-02
2003-03-11
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
C264S272140, C264S272150, C264S272170, C264S328500
Reexamination Certificate
active
06531083
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of integrated circuits, and more particularly to the encapsulation packaging of integrated circuits using transfer molding techniques.
BACKGROUND OF THE INVENTION
In producing integrated circuits, it is desirable to provide packaged integrated circuits having plastic or resin packages which encapsulate the die and a portion of the lead frame and leads. These packages have been produced a variety of ways, a few of which will be described here.
Conventional molding techniques take advantage of the physical characteristics of the mold compounds. For integrated circuit package molding applications, these compounds are typically thermoset compounds. These compounds consist of an epoxy novolac resin or similar material combined with a filler, such as alumina, and other materials to make the compound suitable for molding, such as accelerators, curing agents, filters, and mold release agents.
The transfer molding process as known in the prior art takes advantage of the viscosity characteristics of the molding compound to fill cavity molds containing the die and leadframe assemblies with the mold compound, which then cures around the die and leadframe assemblies to form a hermetic package which is relatively inexpensive and durable, and a good protective package for the integrated circuit.
FIG. 1
depicts the viscosity characteristic curve of a typical mold compound. The Y axis depicts the viscosity of the compound. The X axis represents the time elapsed from a starting point where heat is applied. The mold compound transitions from a high viscosity or hard state to a state where it has very low viscosity after an initial time lapse. The low viscosity stage lasts only a limited time period, typically 20 to 30 seconds, then the compound becomes higher in viscosity and begins to set, or cure. For the entire period the mold compound is heated. The mold compound is thermoset material, so that after being heated for a time period longer than the low viscosity time period it will cure or set.
Transfer molding operations have three stages which correspond to the three phases of viscosity shown in FIG.
1
. First there is a preheat stage required to move the mold compound from its hard initial state to the low viscosity state. Second is a transfer stage, where the compound is low in viscosity and easily transported and directed into cavities and runners. This transfer process should be rapid and be completed before the mold compound begins to set. Finally there is a cure stage that occurs following the transfer stage.
FIG. 2
depicts a conventional single plunger transfer mold press
11
. The press consists of a plunger or ram
13
that is operated under hydraulic pressure, a top platen
15
, a top mold chase
17
, a bottom platen
19
, and a bottom mold chase
21
. A fixed head
23
supports the plunger and a movable head
18
support the top platen, and allows the top platen to be removed for loading and unloading the mold from the top. Mold heaters
25
provide heat to the mold in both the top and bottom platens. An automated mold controller, although not shown, is usually coupled to the press. The top and bottom platens are steel and receive the stresses of the pressing operation, both are heated to provide the temperature needed to perform the transfer molding operation.
FIG. 3
depicts a typical bottom mold chase. In
FIG. 3
, a top view of bottom mold chase
21
is shown. There are six primary runners
31
, each will support a pair of leadframe strips holding wire bonded dies and lead assemblies over each cavity
33
. The cavities are formed along the runners
31
, which are cylindrical shaped paths that extend from the mold pot
32
and into the rows of cavities. Each cavity is coupled to the runners by a secondary runner
35
which ends in a gate
37
, a small opening that lets the mold compound into the cavity. The size and shape of the gate is critical to the speed and control of the transfer and filling stages of the molding process.
FIG. 4
is a detailed drawing of a single runner
31
with a single die cavity
33
shown. The secondary runner
35
is shown coupling the primary runner to the gate
37
and to the die cavity
33
. Runner
31
is coupled to the pot
32
.
FIG. 5
depicts a cross section BB from FIG.
4
. This cross section is taken across the primary runner
31
and along secondary runner
35
, and depicts the sloped shape of secondary runner
35
up to the gate
37
. The lead frame
51
of a typical bonded part is shown over the bottom mold chase cavity and under the top mold chase cavity
34
. Die
53
is shown with the bond wires
55
coupling it to leadframe
51
.
The operation of the conventional single pot transfer mold will now be described with reference to
FIGS. 2-5
. To begin a new molding operation, the mold press is opened and the top and bottom mold chases
17
and
21
are separated. The leadframe and die assemblies are loaded into the bottom mold chases. The mold compound is preheated using an R/F heater or other heater before being placed into the heated mold.
The top and bottom platens are closed, bringing the top and bottom mold chases together. The top and bottom mold chases
17
and
21
are patterned to define a cavity around each die, with the lead frames extending outside the cavity and a space formed around each die. Several leadframe strips each having a row of dies
53
which are bonded to their respective lead frames
51
are placed over the cavities
33
in the bottom mold chase
21
. A pellet of resin or similar material mold compound is placed in the mold pot within the top mold chase
17
. After an initial heating stage to put the mold compound into its low viscosity state, the plunger or ram
13
is used to begin the transfer phase of the operation. The plunger
13
is brought down through the top mold chase
17
onto the mold compound pellet at a predetermined rate, forcing the mold compound into the primary runners
31
. As the runners fill with mold compound the compound will begin filling the secondary runners
35
, entering the gates
37
beneath the leadframe and die assemblies
51
and filling the cavities
33
.
At the end of the transfer stage the mold compound should fill each cavity
33
, preferably at the same time and before the mold compound begins to cure. The rate of the downward force brought by the plunger
13
is varied during the transfer phase to help control the transfer process. Experimental use of the press
11
with a particular mold and compound combination will provide the best combination of pressure and transfer speed which can then be programmed into the automatic press controls to uniformly repeat the process.
After the transfer stage, the packaged parts are cured. Curing the molded parts typically takes 1 to 3 minutes of sitting in the heated mold without disturbance. The compound cure is fairly rapid and may be enhanced by adding curing agents to the compound. At the end of the curing cycle the press is opened and the molded parts and the mold compound sprue or flash in the runners and pot are ejected. This is done by having ejection pins extending through the bottom mold chase
21
and bottom platen
19
push upward under pressure at the same instant, popping the molded parts and sprue out of the bottom mold chase
21
. The packaged parts are then removed to other areas where they are separated and trim and form operations performed on the parts.
There are several critical requirements that are to be met in a commercially successful package molding operation. The cavities should be completely and uniformly filled. Using the single plunger mold of
FIGS. 2-5
the cavity fill stage is difficult to perform uniformly across such a large mold using the single pot and the long primary runners to transport the mold compound. A problem commonly observed in a single plunger single pot mold operation using a mold such as shown in
FIG. 2
is an unacceptable void rate. Voids are areas within the mold cavity that are not filled with compound. Thes
Bednarz George A.
Bolanos Mario A.
LiangChee Tay
Libres Jeremias L.
Lim Julius
Brady W. James
Courtney Mark
Ortiz Angela
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