WO2001058657A1

WO2001058657A1 - A method and apparatus for producing injection molded tablets

Info

Publication number: WO2001058657A1
Application number: PCT/SG2000/000025
Authority: WO
Inventors: M. S. Karunaratne Palitha; Hing Chin Neep; King Ben Toh; Koon Hong Koh; Kyu Maeng Han
Original assignee: Cookson Semiconductor Packaging Materials
Priority date: 2000-02-11
Filing date: 2000-02-11
Publication date: 2001-08-16
Also published as: JP2003522048A; AU2000228418A1

Abstract

A rotary injection molding apparatus including a mold support block (10) rotatably mounted on a drive shaft (15). The drive shaft (15) is coupled with a motor for rotating the shaft (15) about a central axis of rotation. The mold support block (10) includes a plurality of bores (17) uniformly spaced about a circumferential edge region thereof. A cavity plate (50) is mounted on the mold support block (10) so that a plurality of cavities (55) formed thereon are both in alignment with and communicate with the bores (17) defined on the mold support block (10). An ejector pin (18) having a first end (18a) and second end (18b) is slidably mounted in each bore so that actuation of the first end (18a) of the pin forces the second end (18b) of the pin to extend outwardly and into an associated cavity (55). While the mold support block (10) and cavity plate (55) mounted thereon are controlled to rotate, a plasticizing screw (80) is positioned in close proximity to the cavities (55) for continuously ejecting a reactive epoxy compound into each cavity (55) as it rotates past the screw. The compound contained in each cavity (55) is thereafter compressed by a clamping plate (90) and permitted to cool to solidify the compound. After the compound solidifies, an ejector pin actuation device (100) cooperates with each ejector pin (18) to eject the solidified compound or tablet from each cavity (55).

Description

A METHOD AND APPARATUS FOR PRODUCING INJECTION MOLDED TABLETS

FIELD OF THE INVENTION

This invention relates to injection molding machines and; more particularly, to injection

molding machines that can continuously produce epoxy resin tablets for subsequent use in

encapsulating electronic devices.

BACKGROUND OF THE INVENTION

Injection molding machines are typically employed for producing tablets, which tablets

can, for example, be comprised of a thermosetting resin. The tablets are ordinarily used for

encapsulating electronic components such as semiconductor devices. Specifically, a

semiconductor device is positioned in a mold cavity. Meanwhile, the tablet is separately

plasticized by heating. Thereafter, the plasticized tablet is introduced to the cavity containing the semiconductor device so that the device can be encapsulated by the plasticized tablet. The thermo resin encapsulated device is further processed for shaping and curing the encapsulated

device into a final molded product.

It is important to provide tablets with a hardness that does not allow device chipping or

cracking. Nccordingly, the thermosetting resin is conventionally formed into tablets at a pressure

of about 0.5 to 1.0 ton/cm.sup.2. The compressibility of the obtained thermosetting resin tablets

is about 86to 94%, where the compressibility is given by the following equation:

Compressibility (%)=tablet density/density of molded product. Since the density of the molded product remains the same for each type of thermosetting resin, the tablet density increases with an increase in the compressibility. In other words, the

higher the compressibility, the greater the force necessary to press the resin together. However,

when the cross-section of a molded product obtained by conventional molding is examined, inner

voids of about 0.1 mm size can be detected. Some have several voids of 0.5 to 1.0 mm. Such

voids not only degrade the outer appearance of the device, but also the reliability of the resin-

encapsulated semiconductor device; especially its resistance to thermal shock and to humidity.

Other problems with conventionally produced resin tablets include moisture or water content

which can be introduced to the components of the resin melt injected into the mold cavity during

initial production of the resin tablet. This water content contained in the resin tablet can cause ionization or corrosion of electronic components encapsulated therein. Collectively, tablet

density and compressibility, tablet voids, and tablet water content directly affect the reliability of

an electronic device encapsulated therein.

One conventional method and apparatus for increasing the density and thus the

compressibility of resin tablets is shown and described in U.S. Patent No. 5,645, 787, entitled:

"Process for Producing Semiconductor Devices Using Resin Tablet" to Taruno et al. In Turano

et al., an apparatus for producing resin tablets having compressibility of not less than 98% is set

forth. Generally in Turano et al., high density resin tablets are produced by injecting a resin melt

into a mold cavity, which resin is thereafter compressed between an upper plunger and a lower

plunger. After the resin cools and solidifies, the tablet is ejected from the cavity. One problem

with Turano et al. is that the apparatus structure is relatively complex and requires relatively expensive process control equipme t. Furthermore, each mold cavity is filled with melt one at a

time which consumes significant piocessing time.

Therefore, an unsolved need remains for a method and apparatus for producing injection

molded tablets with reduced processing time. A further unsolved need exists for a method and

apparatus for producing injection molded tablets that have a relatively high tablet density and

compressibility, as well as containing minimal tablet voids, and minimal tablet water content.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and apparatus for producing

injection molded tablets for encapsulating electronic devices which overcome deficiencies and

limitations of the prior art.

In accordance with principles of the present invention, a rotary injection molding

apparatus is set forth comprising a cylindrical mold support block rotatably mounted on a drive shaft. The drive shaft is coupled with a motor for rotating the shaft about a central axis of rotation. The mold support block includes a plurality of bores uniformly spaced about a

circumferential edge region thereof. An ejector pin is slidably mounted in each bore defined on

the mold support block. Each ejector pin includes a first end and a second end. The first end of

each ejector pin is dimensioned to engage an actuator so that actuation thereof forces the second

end of the ejector pin to extend outwardly from its associated bore which bore is defined on the

mold support block. A cylindrical cavity plate is mounted on the mold support block such that a plurality of cavities defined on the cavity plate substantially align with the bores defined on the mold support block.

An injection head segment is mounted adjacent to and in close proximity with the cavity

plate. The injection head segment includes an orifice that is dimensioned to accept a first end of

a plasticizing screw. A second end of the plasticizing screw is coupled with a hopper or

compound supply (not shown). Compound is moved from the compound supply, which supply

is defined at the second end of the plasticizing screw, to the first end of the plasticizing via a hollow interior threaded cavity defined on the plasticizing screw. Thereafter, the compound is ejected out of the first end of the plasticizing screw and into the cavities defined on the cavity

plate. At the same time the cavity plate is rotated so that each cavity is sequentially filled with the compound.

Each cavity containing compound is initially compressed between a portion of the

injection head segment and the cavity. A clamping segment is mounted immediately adjacent to the injection head segment for providing a predetermined compression or clamping force on the

compound contained in each cavity.

The mold support block and cavity plate mounted thereon continues to rotate about the

central axis of rotation while the compound contained in each cavity is permitted to cool and solidify. After solidification, each ejector pin associated with each cavity is individually actuated

to eject the solidified compound or tablet from the associated cavity. Therefore, what is provided is a rotary injection molding apparatus including a drive shaft

which rotates the mold support block and cavity plate mounted thereon about the central axis of

rotation. The mold support block and cavity plate are arranged so that the bores defined on the

mold support block and the central bores defined on the base of each cavity are substantially

aligned. The ejector pin actuator engages the first end of each ejector pin to actuate each ejector

pin to force the second end thereof to extend outwardly from each bore and into each cavity via

the central bore included on the base of each cavity.

The orifice defined on the injection head segment receives the first end of the plasticizing

screw. Axial rotation of the plasticizing screw translates to linear motion of a compound

introduced to the compound supply defined at the second end of the screw, which compound is

moved from the second end of the plasticizing screw to the first end of the plasticizing screw via

the hollow interior portion of the plasticizing screw for ejecting the compound from the first end

thereof. A continuous stream of compound is ejected from the first end of the plasticizing screw

while each cavity is controlled to move through the compound stream to fill each cavity with

compound. The compound contained in each cavity is thereafter compressed; permitted to cool;

and subsequently ejected from the cavity to provide a tablet. The sequential operation of the

rotary injection molding apparatus enables sequential production of tablets which minimizes

tablet processing time.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects of this invention, the various features thereof, as well as

the invention itself, may be more fully understood from the following description when read

together with the accompanying drawings in which:

Fig 1(a) is a partial cross-sectional view of the rotary injection molding apparatus having

principles of the present invention;

Fig 1(b) is a front view of the cavity plate shown in Fig. 1(a);

Fig 2(a) is an elevated side view of the mold support block shown in Fig.1(a);

Fig 2(b) is a front view of the cavity plate, injection head segment, and clamping plate

segment shown in Fig. 2(a); and

Fig 3 is a linearized side view of an ejector pin action device.

DETAILED DESCRI *TION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a method and apparatus for producing injection

molded tablets for encapsulating electronic devices.

Referring to Fig. 1(a), in accordance with the present invention, rotary injection molding

apparatus 5 is set forth comprising mold support block 10 which is rotatably mounted on drive

shaft 15. Mold support block 10 includes a plurality of cylindrical bores 17, which bores 17 are

formed along a circumferential edge region of mold support block 10. Ejector pin 18 is slidably

mounted in each bore 17 defined on mold support block 10. Drive shaft 15 is journaled to

mounting table 20. Actuator 25, which can be an electric motor or hydraulic motor, is coupled to

drive shaft 10 for rotating shaft 10 about a central axis of rotation 30. Preferably, drive shaft 10

is controlled to rotate from approximately 2-revolutions-per-minute to approximately 8-

revolutions-per-minute.

Ejector pin guide plate 35 which includes a plurality of cylindrical bores 40 defined

thereon is mounted on mold support block 10 so that each bore 40 defined thereon substantially aligns with one of the ejector pins 18 mounted in each of the bores 17 defined on mold support

block 10. Bores 40 defined on ejector pin guide plate 35 are each dimensioned to closely

conform to each associated ejector pin 18. Cavity plate 50 which includes a plurality of

cylindrical cavities 55 formed thereon is mounted on ejector pin guide plate 35 so that each of

cavities 55 defined on cavity plate 50 substantially aligns with each of the ejector pins 18.

Referring further to Fig. 1(b), base 55a of each cavity 55 includes central bore 60 dimensioned to

slidably accept ejector pin 18 therethrough to permit ejector pin 18 to enter cavity 55. Referring to Fig. 2(a), central portion 10a of mold support block 10 includes cooling chamber 65 dimensioned to permit a cooling fluid (not shown) to circulate through cooling

chamber 65 for cooling mold support block 10. Cooling chamber 65 further includes cooling

fluid inlet 65a and cooling fluid outlet 65b. Cooling fluid inlet 65a and cooling fluid outlet 65b

enable cooling chamber 65 to be coupled with a cooling fluid supply (not shown).

Injection head segment 70 is mounted in close proximity to cavity plate 50 so that several

of the plurality of cavities 55 defined on cavity plate 50 are positioned between injection head

segment 70 and cavities 55 defined on cavity plate 50. Injection head segment 70 includes an orifice 75 which is dimensioned to accept first end 80a of plasticizing screw 80. Second end 80b of plasticizing screw 80 is coupled to a hopper, or compound supply (not shown). Axial rotation of plasticizing screw 80 in the direction of arrow-90 translates to linear motion of a compound (not shown) introduced to the compound supply, which compound is moved from second end 80b of plasticizing screw 80 to first end 80a of plasticizing screw 80 via hollow threaded interior portion 80c defined in plasticizing screw 80. While the compound traverses from second end

80b to first end 80a of plasticizing screw 80, frictional heating generated within thread interior portion 80c thereof causes the compound to become plasticized prior to ejection from first end

80a of plasticizing screw 80.

Clamping plate segment 90 is mounted in close proximity to cavity plate 50 immediately

adjacent to and conformingly with injection head segment 70. Clamping plate segment 90 is

comprised of a relatively rigid material such as steel or a metal alloy for example. Clamping

plate segment 90 provides a compression force on the plasticized compound contained in each cavity 55 of cavity plate 50 until the compound solidifies. One preferred compression force of clamping plate segment 90 ranges from approximately 20 tons to approximately 50 tons.

Fig, 2(b) illustrates a front view of cavity plate which includes the plurality of

cylindrical ly shaped cavities as well as injection head segment 70 with orifice 75 defined thereon

and clamping plate segment 90.

Referring to Fig. 3, each ejector pin 18 is spring biased so that first end 18a of each

ejector pin 18 can be momentarily actuated by ejector pin actuation device 100 to slide ejector

pin 18 within bore 17 (Fig. 2) until second end 18b of ejector pin 18 extends outwardly from

bore 17 and into cavity 55 defined on cavity plate 50. When ejector pin actuation device 100 is

removed from first end 18a of ejector pin 18, ejector pin 18 slides back to its original position

defined within bore 17.

In an embodiment of the present invention, ejector pin actuation device 100 comprises first ramped segment 100a and second ramped segment 100b coupled to ejector track 100c. Ejector pin actuation device 100 is secured to the mounting table 20 (Fig. 1) in close proximity to

the rotating mold support block 10 (Fig. 1) and ejector pins associated therewith. Ejector track

100c is dimensioned to slidably engage with first end 18a of each ejector pin 18 so that first end

18a thereof can ride along ejector track 100c during rotation of mold support block 10 (Fig. 1).

When first end 18a of ejector pin 18 engages first ramped segment 100a of ejector pin actuation

device 100, first end 18a of ejector pin 18 rides upwardly along the first ramped segment 100a to

force second end 18b of ejector pin 18 to extend outwardly. The outwardly extending second

end 18b of ejector pin 18 is guided into an adjacently aligned cavity 55 defined on cavity plate 50 via ejector pin guide plate 35 (Fig. 1). Thereafter, first end 18a of ejector pin 18 rides

downwardly along second ramped segment 100b defined on ejector pin actuation device 100 to

retract second end 18b of ejector pin 18 from cavity 55 and return ejector pin 18 to an original

position defined in bore 17.

In an embodiment, ejector pin actuation device 100 including first ramped segment 100a

and second ramped segment 100b can have a substantially arcuate shape.

First 100a ramped segment and second ramped segment 100b respectively incorporated

in ejector pin actuation device 100 are positioned a predetermined radial distance from

plasticizing screw 80 to permit compound previously ejected into cavity 55 to cool and solidify.

The size of the ramps may be adjusted accordingly to take into account the number of cavities

55, and materials temperatures. In an embodiment, first 100a ramped segment and second

ramped segment 100b of ejector pin actuation device 100 are oriented approximately 180- degrees from first end 80a of plasticizing screw 80. The position of ramped segments 100a and

100b with respect to plasticizing screw 80 can be adjusted to compensate for the cooling and

solidification rates of compound contained in each cavity 55.

Referring to Figs. 2 and 3, during operation first end 80a of plasticizing screw 80 is

inserted into the orifice 75 defined on injection head segment 70, whereby first end 80a of

plasticizing screw 80 is positioned in close proximity to cavities 55 defined on cavity plate 50. A

reactive epoxy molding compound is introduced to the compound supply defined at second end

80b of plasticizing screw 80, which compound is forced from second end 80b of screw 80 to first

end 80a of screw 80 via hollow threaded interior portion 80c defined in plasticizing screw 80. The plasticized reactive epoxy moiding compound is continuously ejected out of first end 80a of

plasticizing screw 80 and into the plurality of cavities 55 defined on cavity plate 50. At the same

time, mold support block 10, ejector pins 18, ejector pin guide plate, and cavity plate 50 are

controlled to rotate about central axis of rotation 30.

While the reactive epoxy molding compound is ejected into cavities 55 defined on cavity

plate 50, cooling fluid is circulated through mold support block 10. The cooling fluid thermally

conducts with cavity plate 50 via ejector guide pin plate 35 to cool the epoxy molding compound

contained in the plurality of cavities 55. Therefore, as each cavity 55 rotates about central axis

of rotation 30, the epoxy contained therein is compressed by clamping plate 90 for a

predeteπnined radial distance of travel until the epoxy solidifies with the assistance of the

cooling fluid circulating through mold support block 10.

As cavity plate 50 continues to rotate about central axis of rotation 30, first ends 18a of

ejector pins 18 slide along ejector track 100c and sequentially ride up and ride down first 100a

ramped segment and second 100b ramped segment respectively defined on ejector pin actuation

device 100. Consequently, second ends 18b of ejector pins 18 are also sequentially forced to

extend outwardly and into each cavity 55 associated therewith. As a result, solidified compound,

or tablets 120, are sequentially ejected from cavities 55. After tablet 120 is ejected from an

associated cavity 55, second end 18b of ejector pin 18 is retracted therefrom as first end 18a of

ejector pin 18 rides downwardly along second ramped segment 100b of ejector pin actuation device 100. Thereafter, the ejected solidifed tablets 120 can be subjected to further processing for encapsulating electronic devices.

Advantages of the present invention include a method and apparatus for producing

injection molded tablets at an increased processing rate. The tablets include a relatively high

tablet density and compressibility. Furthermore, the tablets contain minimal voids and minimal

tablet water content.

The invention may be embodied in other specific forms without departing from the spirit

or essential characteristics thereof. The present embodiments are, therefore, to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the

appended claims rather than by the foregoing description, and all changes which come within the

meaning and range of the equivalency of the claims are therefore intended to be embraced

therein.

Claims

What is claimed is:

1. A rotary injection molding apparatus, comprising:

a drive shaft, the drive shaft being coupled with a motor for rotating the shaft about a central axis of rotation;

a mold support block rotatably mounted on the drive shaft;

a plurality of bores spaced about a circumferential edge region of the mold support block;

a cavity plate having a plurality of cavities formed thereon, each cavity including a central bore, the cavity plate being mounted on the mold support block so that the plurality of cavities defined on the cavity plate substantially align with the bores defined on the mold support block;

an ejector pin slidably mounted in each of the plurality of bores defined on the mold support block, each ejector pin having a first end and a second end;

an ejector pin actuator adapted to engage the first end of each ejector pin,

wherein the ejector pin actuator cooperates with the first end of each ejector pin to force the second end of each ejector pin to extend outwardly from each bore and into each cavity via the central bore included on each cavity.

2. The rotary injection molding apparatus of claim 1, further comprising an ejector pin guide plate having a plurality of bores formed thereon, the ejector pin guide plate being mounted intermediate the mold support block and the cavity plate so that the plurality of bores formed thereon substantially align with the ejector pins.

3. The rotary injection molding apparatus of claim 1 , wherein the mold support block further includes a cooling chamber, the cooling chamber enabling a cooling fluid to circulate therethrough for cooling the mold support block.

4. The rotary injection molding apparatus of claim 1, further comprising:

an injection head segment, the injection head segment being mounted adjacent to the cavity plate;

an orifice formed on the injection head segment;

a plasticizing screw having a first end, a second end and a hollow interior portion, the first end being dimensioned to conform to the orifice, the second end being coupled to a compound supply,

wherein axial rotation of the plasticizing screw translates to linear motion of a compound introduced to the compound supply, which compound is moved from the second end of the plasticizing screw to the first end of the plasticizing screw via the hollow interior portion of the plasticizing screw for ejecting the compound from the first end thereof.

5. The rotary injection molding apparatus of claim 4, further comprising a clamping plate segment mounted adjacent to the injection head segment.

6. The rotary injection molding apparatus of claim 4, wherein the ejector pin actuator is dimensioned to slidably engage the first end of each ejector pin, the ejector pin actuator further comprising:

an ejector track;

a first ramped segment coupled to the ejector track

a second ramped segment coupled to the ejector track,

wherein the first ramped segment cooperates with the first end of each ejector pin to force the second end of each ejector pin to extend outwardly from each bore and into each cavity via the central bore defined on each cavity; and

wherein the second ramped segment cooperates with the first end of each ejector pin to

enable the second end of each ejector pin to retract from each cavity for returning each ejector

pin to a position defined in each bore.

7. The rotary injection molding apparatus of claim 6, wherein each ejector pin is spring biased.

8. The rotary injection molding apparatus of claim 6, wherein the first ramped segment and

the second ramped segment are defined on the ejector pin actuator at a predeteπnined radial

distance from the plasticizing screw for enabling compound contained in each cavity to cool prior to ejection.

9. The rotary injection molding apparatus of claim 8, wherein the first ramped segment and

the second ramped segment are arcuate shaped.

10. The rotary injection molding apparatus of claim 6, wherein the first ramped segment and

the second ramped segment are defined on the ejector pin actuator at approximately 180-degrees

from the first end of the plasticizing screw for enabling compound contained in each cavity to

cool prior to ejection.

11. A method for producing injection molded tablets with a rotary injection molding apparatus, the rotary injection molding apparatus comprising a mold support block rotatably mounted on a drive shaft, the mold support block having a plurality of bores uniformly spaced about a circumferential edge region thereof, each bore containing an ejector pin having first and second ends, a cavity plate having a plurality of cavities formed thereon, each cavity including a central bore, the cavity plate being mounted on the mold support block so that the plurality of cavities defined thereon substantially align with the bores defined on the mold support block, the apparatus further including an injection head segment mounted adjacent to the cavity plate, the injection head segment having an orifice, the method for producing injection molded tablets comprising the steps of:

positioning a first end of a plasticizing screw in the orifice;

introducing a molding compound to a compound supply coupled to a second end of the plasticizing screw;

rotating the plasticizing screw to force the molding compound to travel from the second end of the screw to the first end of the screw via a hollow interior portion defined on the screw for ejecting the compound from the first end of the screw;

rotating the mold support block and cavity plate mounted thereon to sequentially move each cavity past the first end of the plasticizing screw for enabling each cavity to be sequentially filled with compound;

compressing the compound contained in each cavity with a pressure plate, the pressure plate being mounted adjacent to the injection head segment;

engaging the first end of each ejector pin with an ejector pin actuation device to force the second end of each pin to extend outwardly and into the cavity via the central bore formed on the cavity, the second end of the pin contacting and ejecting the molding compound contained in the cavity.

12. The method for producing injection molded tablets of claim 11, wherein the step of engaging the first end of each ejector pin further comprises the steps of:

sliding the first ends of the ejector pins along an ejector track; sliding the first ends of the ejector pins upwardly along a first ramped segment defined on the ejector track for forcing the second ends of the ejector pins to extend outwardly; and

sliding the first ends of the ejector pins downwardly along a second ramped segment defined on the ejector track for enabling the second ends of the ejector pins to retract to a position defined in the bore.

13. The method for producing injection molded tablets of claim 11 , wherein after the step of rotating the mold support block, the method further comprising the steps of:

circulating a cooling fluid through a cooling chamber defined in the mold support block, the cooling fluid thermally conducting with the cavity plate for cooling the molding compound contained in the plurality of cavities.