Plunger for a multi-plunger type resin mold device

A plunger is used for a multi-plunger type resin mold device which is adapted to be slidably inserted into a pot to permit a resin in the pot into a mold. The plunger includes a forward end member whose forward end edge portion is formed of silicon nitride, silicon carbide or zirconia, the forward end edge portion of said forward end pot member being in contact with the resin in the pot, and a plunger body to which the forward end member is fixed. The forward end member is fixed to the plunger body by threadably inserting an insertion section of the forward end member into a recess formed in the plunger body.

BACKGROUND OF THE INVENTION 
This invention relates to a plunger for a multi-plunger type resin mold 
device slidably insertible into a pot to supply a resin into a mold. 
As a transfer mold device for use in the resin encapsulation of electronic 
components such as semiconductor elements, a single plunger type was 
usually used, but a multi-plunger type capable of a high-speed molding has 
recently been adopted in which there is no runner between a pot and a mold 
cavity. 
A conventional resin mold device of a multi-plunger type will be explained 
below with reference to FIGS. 1 to 3. FIG. 1 is a perspective view showing 
upper mold half 1; FIG. 2 is a plan view showing lower mold half 2 and 
FIG. 3 is a cross-sectional view showing the upper and lower mold halves. 
At the mating surfaces of the upper and lower mold halves 1 and 2 are 
provided a plurality of culls 3, gates 4 communicating with the culls and 
mold cavities 5 communicating with the gates 4. A plurality of cylindrical 
pots 6 are each provided in upper mold half 1 to communicate with the 
culls. A vertically movable plunger 7 is adapted to be inserted into the 
pot. The mold cavity has a configuration corresponding to a package of the 
resin-encapsulation type semiconductor device. 
In the molding of the package the upper and lower mold halves are preheated 
to a temperature (for example, about 180.degree. C.) at which the resin 
for molding is cured. A lead frame F with a semiconductor chip placed 
thereon is sandwiched between the upper and lower mold halves such that 
the side edge portions of the frame extend from the mold cavity as shown 
in FIG. 3. Then, resin tablet T is inserted through the upper open end of 
the pot into the inner space thereof as shown in FIG. 1 and, while the 
resin tablet is molten there, the plunger is lowered into the pot. By the 
pressure of the plunger 7 the molten resin on the cull is forced past the 
gates into the corresponding mold cavities 5. After the molten resin has 
been filled into the mold cavities, the molding device is maintained at 
that pressure for a predetermined time period and is cured. Thereafter, 
the plunger pressure is released and the plunger is lifted, followed by 
opening the mold and taking a molded article integral with the lead frame 
F out of the mold. 
In the aforementioned multi-plunger type resin mold device, since the resin 
is molded at four and five times as fast a cycle (about 60 seconds/cycle) 
as that of the single plunger type resin mold device, the plunger is 
prominently worn out due to a filler contained in the resin. In the 
aforementioned multi-plunger type resin mold device, therefore, use is 
made, as the plunger, of a type which is made of a material, such as a 
quenched die steel, high-speed steel or powdered high-speed steel, whose 
surface is covered with a hard chromium plating film of about 10 .mu.m in 
thickness. The use of such material causes a greater wear to occur on the 
plunger, presenting the following drawbacks as will be set forth below: 
(1) A greater clearance is produced between the inner wall of the pot and 
the outer wall of the plunger due to a greater wear, causing the resin to 
penetrate into the clearance to increase the frictional resistance of the 
plunger. As a result, a resin charging pressure in the mold cavity is 
lowered and a molding defect may occur due to the insufficient filling of 
the resin and the occurrence of cavities, resulting in a poor yield in the 
semiconductor devices. 
(2) The resin entrapped between the pot and the plunger is peeled as a 
larger resin scum and deposited onto a sensor etc., within the device. Due 
to the presence of the resin scum the normal operation of the sensor etc., 
is prevented, producing a device malfunction and a consequent lower rate 
of device operation. 
(3) It is necessary to frequently exchange plungers in a shorter period of 
time, for example, seven to fourteen days. This necessitates preparing a 
greater number of spare plungers. Furthermore, the operation of the 
plungers must be stopped for each plunger exchange, involving a lower rate 
of device operation and a high running cost. 
In the conventional multi-plunger type resin mold device a lowered rate of 
production and a resultant lower article yield are involved due to an 
excessive wear of the plungers, providing a far to a higher production of 
the semiconductor devices. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a plunger for a multi-plunger 
type resin mold device which is free from the aforementioned drawbacks and 
higher in wear resistance than a conventional plunger. 
According to this invention there is provided a plunger for a multi-plunger 
type resin mold device, comprising a forward end member made of at least 
one kind of material selected from the group consisting of silicon 
nitride, silicon carbide and zirconia, and a plunger body to which the 
forward end member is fixed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The embodiments of this invention will be explained below in more detail 
with reference to the accompanying drawings. 
EMBODIMENT 1 
A plunger according to this embodiment comprises, as shown in FIG. 4, 
forward end member 8 having a configuration of stepped cylinder and formed 
of silicon nitride, silicon carbide or zirconia and cylindrical plunger 
body 9 made of a metal. The forward end member 8 comprises large-diameter 
head 8a defining a plunger head and small-diameter insertion section 8b 
formed on a side opposite that on which the head is provided. The 
insertion section of the forward end member is inserted into a hole 9a 
formed at the forward end portion of the plunger body. The forward end 
member is attached as one unit to the plunger body, noting that the 
forward end member 8 is attached to the plunger body by a method, such as 
a thermal shrinking or press fitting method. The head of member 8a has a 
smoothed side wall and front face. The forward end edge portion of the 
head, i.e. an angular area of the head where the end face of the head 
intersects the inner wall of the pot, is formed of silicon nitride, 
silicon carbide or zirconia. As shown in FIG. 5, locking pin through hole 
9b is formed in a small-diameter section of plunger 9a such that it 
extends into hole 9a of the plunger in a direction of the diameter of the 
small-diameter section of the plunger. Locking pin groove 8c is formed in 
an outer periphery of insertion section 8b of forward end member 8. In 
this embodiment, the plunger member is firmly attached to the forward end 
member by inserting locking pin 18 from the outer periphery of the 
small-diameter section of the plunger through the locking pin hole into 
the locking pin groove. Since this embodiment uses the locking pin, it can 
obviate the necessity of using the aforementioned method, such as the 
thermal shrinking or press fitting method. Table 1 shows the results of a 
comparison between the plunger of FIG. 5 and the conventional plunger. 
TABLE 1 
__________________________________________________________________________ 
Plunger of this Invention 
Material 
Silicon 
Silicon 
Nitride 
Carbide 
Zirconia 
Conventional 
Comparison Item (Si.sub.3 N.sub.4) 
(SiC) 
(ZrO) 
Plunger 
__________________________________________________________________________ 
Amount 
After Ten Thousand Shots 
1 0.5 1.5 .sup. 10.about.40*.sup.1 
of Wear 
After Twenty Thousand Shots 
3 1 5 .sup. 20.about.70*.sup.2 
(.mu.m) 
After One-hundred and 
20 13 30 -- 
Twenty Thousand Shots 
Plunger Exchange Cycle (month) 
6 8 4 0.25.about.0.5 
Yield % 99.6 99.4 99.5 92.about.96 
Device Operation Rate % 
81 79 80 32.about.49 
Running Cost (yen/piece) 
0.1 below 
0.1 below 
0.1 below 
0.8.about.0.5 
__________________________________________________________________________ 
*.sup.1 the number of plungers left unusable after ten thousand shots. 
*.sup.2 the number of plungers all unusable after twenty thousand shots. 
In Table, the item "running cost" represents a running cost per piece of IC 
and the item "device operation rate" a ratio of an actual operation time 
to a whole operation time (not including an initial check time and the 
other dwell time). Note that, as the conventional plunger, use was made of 
a type made of a die steel covered with an about 10 .mu.m-thick hard 
chromium plating film. According to this embodiment use was made of a 
plunger whose forward end edge portion is formed with a silicon nitride, 
silicon carbide or zirconia. As appreciated from the above, the plunger of 
this embodiment can assure a reduced amount of wear, a much longer 
exchange cycle, a higher yield and a much lower running cost. 
EMBODIMENT 2 
A plunger according to this embodiment has forward end member 10 made of 
the same material as embodiment 1 and plunger body 11 made of a metal. The 
forward end member 10 comprises large-diameter head section 10a serving as 
a plunger head and small-diameter, externally-threaded section 10b which 
is provided on the side opposite that on which the plunger head is 
situated. The plunger body has internally threaded hole 11a formed at the 
forward end portion thereof. The forward end member is attached to the 
plunger body 11 by threadably inserting the externally threaded section of 
the forward end member into the internally threaded hole of the plunger. 
In this case, the forward end member is detachably mounted on the plunger 
body. The forward end member may be nondetachably attached to the plunger 
body by initially coating the internally threaded hole of the plunger body 
with a thermosetting resin, inserting the externally threaded section of 
the forward end member into the internally threaded hole of the plunger 
body and heating the resin there to permit it to be cured. 
EMBODIMENT 3 
A plunger according to this embodiment comprises forward end member 12 
formed of the same material as embodiments 1 and 2 and plunger body 13 
made of a metal. The forward end member 12 comprises large-diameter head 
section 12a acting as a plunger head and small-diameter shaft section 12b 
projecting on the side opposite that on which the head section is 
situated. Over that end portion of the shaft section situated remote from 
the head section of the member a flange 12c is fitted which has a diameter 
greater than shaft section 12b of the member. Plunger body 13 has a 
spacing within a forward end portion thereof where the flange is fitted 
over the shaft section of the member. The plunger body is bisected, as 
semicircular sections, along a plane including a center line l. Hole 13d 
is formed in the plunger body such that it is located perpendicular to 
that plane. Bolt 13a is inserted into hole 13d with an end thereof 
projected from the hole 13d, and nut 13b is fitted over that end of the 
bolt. By tightening the nut relative to the bolt, the semicircular 
sections of the plunger are connected together whereby the forward end 
member is attached as one unit to the plunger body 13. The head of the 
bolt and nut are buried in recesses which are formed at the open opposite 
end portions of hole 13d. This arrangement prevents the head section of 
bolt 13a and nut 13b from externally projecting beyond the outer periphery 
of the plunger body. In this embodiment the forward end member can be 
attached to the plunger body 13 by merely tightening the nut. It is 
therefore unnecessary to use the method such as the thermal shrinking 
method. Thus, the forward end member can be detached from the plunger body 
as required. 
EMBODIMENT 4 
A plunger of this embodiment comprises, as shown in FIG. 8, forward end 
member 14 made of the same material as in embodiment 1 to 3 and plunger 
body made of a metal. The forward end member 14 is cylindrical in 
configuration and has a stepped center hole having a forward open end 
section 14a and a rear open end section 14b with narrower intermediate 
hole section 14c defined between the forward and rear open end sections. 
Step 14d is defined between hole sections 14a and 14c and step 14e is 
defined between hole sections 14d and 14c. The plunger body 15 has a 
large-diameter flange 15b larger than the diameter of hole section 14b and 
small-diameter insertion section 15a ahead of flange 15b. An internally 
threaded hole is formed at the free end portion of insertion section 15a. 
Insertion section 15a is inserted into hole section 14b and the forward 
end face of flange 15b abuts against the rear end face of forward end 
member 14. Bolt 16 is inserted from the side of hole section 14a into the 
internally threaded hole of insertion section 15a. The tightening of bolt 
16 causes the head of the bolt to be brought into pressure contact with 
step 14d and the forward end face of insertion hole 15a to be brought into 
pressure contact with step 14e of the forward end member. In this way, the 
forward end member is attached to the plunger body. Filler 17 made of a 
mixture of a ceramic filler and thermosetting resin is filled in a spacing 
which is defined around the head of bolt 16 and between the head of the 
bolt and the forward hole section of the forward end member. Filler 17, 
after being thermally cured, has its forward end face polished flat such 
that the end face is flush with the forward end face of the forward end 
member. 
A preferable pot will be explained below in connection with the plunger so 
formed. 
In FIG. 9, cylindrical pot 20 is wholly made of silicon nitride and has 
flange 20a on the outer periphery thereof. The pot is fixed to upper mold 
40 by fitting the flange of the pot into a recess which is formed in the 
upper mold. 
The pot has tapering section 20b at the upper inner wall section, through 
which the plunger is guided. 
In FIG. 10, pot 21 has inner cylinder 22 made of silicon nitride and outer 
cylinder 23 made of a metal, noting that the outer cylinder is thermally 
shrunk on the inner cylinder to provide a unitary assembly. The outer 
cylinder has through hole 23b across the wall of the other cylinder which 
corresponds to recess 22a formed in the outer periphery of the inner 
cylinder. Locking pin 24 is inserted through the through hole into the 
recess, whereby the inner and outer cylinders are fixed against a vertical 
movement. Flange 23a is formed on the outer periphery of the outer 
cylinder and pot 21 is fixed to upper mold 40 with flange 23a fitted in a 
recess formed in the upper mold. Taper section 22b is formed at the upper 
inner wall of the inner cylinder and the plunger is guided through the 
taper section. Generally, a ceramics material is very hard, but readily 
breakable if it is thinner, and difficult to machine if it is intricate in 
configuration. The ceramic material has a smaller in linear expansion 
coefficient than the metal (for example, 3.times.10.sup.- 6 for silicon 
nitride) and, when used together with a metal at high temperature, cannot 
be positively fixed to the metal due to a clearance produced between the 
metal and the ceramic. This specific arrangement of the pot may eliminate 
the aforementioned drawbacks. 
In FIG. 11, a pot is comprised of, as shown in FIG. 10, inner cylinder 26 
made of silicon nitride and outer cylinder 27 made of a metal. The outer 
cylinder is thermally shrunk on the inner cylinder as in the case of the 
pot shown in FIG. 11 to provide an integral assembly. In the arrangement 
shown in FIG. 11, the inner cylinder is located in a position where the 
plunger is slidably moved and taper section 27b is formed on the outer 
cylinders to permit the plunger to be guided. By this specific arrangement 
the inner cylinder is simpler in configuration and easier to machine. In 
FIG. 11, like reference numerals are employed to designate parts or 
elements corresponding to those shown in FIG. 10. 
In FIG. 12, a pot comprises inner cylinder 28 made of silicon nitride and 
outer cylinder 29 made of a metal. As in the case of the arrangement of 
FIG. 11, the inner cylinder is provided in a location where the plunger is 
slidably moved and taper section 29b is formed at the outer cylinder to 
permit the plunger to be guided. Unlike the arrangement shown in FIG. 11, 
externally threaded section 28a is formed on the outer periphery of the 
inner cylinder and internally threaded section 29a is formed on the inner 
wall of the outer cylinder 29. The outer cylinder is fixed to the inner 
cylinder by threadably inserting the externally threaded section of the 
inner cylinder into the internally threaded section of the outer cylinder. 
In this case, a thermosetting resin is coated on the threaded sections of 
the inner and outer cylinders. The externally threaded section of the 
inner cylinder is firmly fixed to the internally threaded section of the 
outer cylinder by thermally curing the thermosetting resin there. In FIG. 
11, like reference numerals are employed to designate parts or elements 
corresponding to those shown in FIG. 10. Further explanation is therefore 
omitted for brevity. 
Although, in the aforementioned pot, silicon carbide has been used, silicon 
nitride or zirconia may be used instead. Although, in these embodiments, 
the upper plunger type resin mold device has been used in connection with 
the pot incorporated in the upper metal, use can be made of the other 
multi-plunger type resin mold device, such as a lower plunger type resin 
mold device. FIG. 13 shows a state in which the pot of FIG. 9 is 
incorporated into the lower plunger type resin mold device. In FIG. 13, 
the pot of FIG. 9 is attached to lower mold 41 and the plunger as shown in 
connection with embodiment 1 is inserted from below the pot. Pocket 30 is 
formed above the forward end member and resin tablet 42 is inserted into 
pocket 30. Gate 43 is formed in the upper mold half. The upper and lower 
mold halves 40 and 41 are closed with lead frame 44 sandwiched 
therebetween and the plunger is moved upward to permit the resin to be 
transfer molded under compression. 
According to the embodiments the following advantages are obtained since 
the forward end edge portion of the plunger which is brought into sliding 
contact with the inner wall surface of the pot is formed of at least one 
kind of material selected from the group consisting of silicon nitride, 
silicon carbide and zirconia. 
(1) The use of the plunger according to the embodiments markedly lowers an 
amount of wear thereon. For this reason, the plunger exchange cycle 
becomes longer. 
(2) There is no risk that a wider clearance will be produced between the 
plunger and the pot due to the filler present in the resin. This assures a 
high rate of operation and a consequent high yield, as well as a marked 
decrease in the running cost of the plunger. 
(3) The sliding contact area of the plunger with respect to the pot better 
improves a wear-resistant characteristics and a clearance is difficult to 
produce between the pot and the plunger, resulting in a high rate of 
operation and thus a higher yield. 
Although, in the aforementioned embodiment, the metal has been used as a 
material for the plunger body, any other material, such as ceramics, may 
be used instead.