Method of producing epoxy resin-encapsulated semiconductor device

Abstract: In a method of producing a resin-encapsulated semiconductor device, a lead frame having an semiconductor element bonded thereto and wire-bonded thereto is set as an insert into an injection mold. An epoxy resin molding compound is injected into the mold by an injection molding. In this method, an injection pressure of the injection molding machine is gradually increased in such a manner that a maximum pressure of 30 kg/cm 2 to 300 kg/cm 2 is achieved at the time when 80% to 95% of a total amount of the epoxy resin molding compound to be injected is injected into the mold. Subsequently, the remaining epoxy resin molding compound is injected into the mold at an injection pressure of 20 kg/cm 2 to 100 kg/cm 2 . A heating cylinder of the injection molding machine is divided into a plurality of zones which are controlled in the temperature independently. The zone nearest to a nozzle of the heating cylinder is controlled to 65°C - 110°C while that zone nearest to a hopper of the heating cylinder is controlled to the ordinary temperature to 50°C.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method of producing a resin-encapsulated semiconductor device. More particularly, it relates to a method of producing a resin-encapsulated semiconductor device by injecting an epoxy resin compound into an injection mold, in which a lead frame, to which a semiconductor has been bonded and wire-bonded, is fixedly set as an insert in an injection mold, and by curing such epoxy resin compound in the injection mold. Description of the Related Art In the encapsulation of semiconductor devices such as an IC and a LSI, a transfer molding method, using an epoxy resin molding compound, has heretofore been used because of the low cost, high reliability and efficient productivity thereof. Generally, in the transfer molding, the epoxy resin molding compound is formed into a tablet-like shape, and then this tablet is charged into a pot in a mold, and the tablet, while melted by heat in the mold, is pressurized by a plunger, so that the molten molding compound is caused to flow, and then is cured. With this molding method, however, it is necessary to first shape the epoxy resin molding compound into the required configuration (that is, tablet-like shape), and therefore the shaping step is needed. Depending on the shape and size of semiconductor devices, the shape of the tablets varies widely, and therefore there are required many mold devices for shaping the epoxy resin molding compound. Since the charging of the tablet and the melting of the tablet by heat are required for each molding operation, the time period of the molding cycle can not be reduced to less than a predetermined time period, and this limits the reduction of the cost and the mass productivity. Further, there is a possibility that impurities are intruded into the molding compound in the preprocessing steps such as the shaping step. In the transfer molding, residual culls in runners through which the molding compound, charged into the pot, flows co a mold cavity, and in the pot are completely cured, and therefore can not be reused. This results in a problem that a large amount of resin other than the resin used to form the semiconductor packages is wasted. US-A-4 554 126 discloses a method for producing a resin-encapsulated semiconductor device by injecting an epoxy resin molding compound into an injection mold at a pressure of 40 kg/cm 2 to 120 kg/cm 2 , in which a lead frame, to which a semiconductor has been bonded and wire-bonded, is fixedly set as an insert in the injection mold, and by curing such epoxy resin molding compound in said injection mold. On the other hand, there has heretofore been made a study of injection molding using a thermosetting resin molding compound, such as an epoxy resin. In the injection molding, an epoxy resin molding compound in the form of powder or granules is supplied into an injection molding machine, and is kept molten in a cylinder, and is injected into a mold by a screw. Therefore, the step of shaping the epoxy resin molding compound into a tablet is not necessary, and besides the continuous production is possible, and the device for this shaping purposes, as well as the time for this shaping, is omitted. Further, since the molding material in the molten state is injected into the mold, the curing time is shorter as compared with the transfer molding, and therefore this method is suitable for mass production. However, at present, the injection molding for encapsulating with epoxy resin molding compound has not yet been put into practical. The reason for this is that in the molten state of the conventional epoxy resin molding compound in the cylinder heated to 70°C - 110°C, the viscosity of this molding compound increases as the curing reaction of the resin in the molding compound proceeds. As a result, its fluidity is lost in 5 minutes - 10 minutes, and the thermal stability of the molten molding compound is extremely low. Therefore, it is impossible to effect the injection molding at a low pressure, and then a high pressure injection is required As a result, the binding wires on the semiconductor device may be deformed or cut off, so that the reliability of the produced semiconductor package is greatly degraded. There has been pointed out another problem that when interrupting the molding operation for a predetermined time period, for example, for cleaning the mold, the epoxy resin molding compound is cured in the cylinder, and then can not be injected thereafter, so that the continuous production is adversely affected. SUMMARY OF THE INVENTION It is an object of this invention to provide a method of producing a semiconductor device encapsulated by injection molding using an epoxy resin compound, by which a long-time continuous molding and mass production can be effected efficiently. To this end, according to one aspect of the present invention, there is provided a method of producing a resin-encapsulated semiconductor device by injecting an epoxy resin molding compound into an injection mold, in which a lead frame, to which a semiconductor has been bonded and wire-bonded, is fixedly set as an insert in the injection mold, and by curing such epoxy resin molding compound in the injection mold, the method comprising the steps of: injecting epoxy resin molding compound into the injection mold; gradually increasing an injection pressure of said injection molding machine in such a manner that a maximum pressure of 30 kg/cm 2 to 300 kg/cm 2 is achieved at the time when 80% to 95% of a total amount of the epoxy resin molding compound to be injected is injected into the injection mold; and injecting the remaining epoxy resin molding compound at an injection pressure of 20 kg/cm 2 to 100 kg/cm 2 . The epoxy resin molding compound used in the present invention usually contains, as essential components, an epoxy resin, a curing agent, a curing accelerator and an inorganic filler. It is in the form of powder or granules. It is not needed to be shaped into a tablet as in the transfer molding. The epoxy resin molding compound is preferably required to exhibit a good thermal stability in a cylinder of an injection molding machine and an excellent fluidity in a cavity, and be cured rapidly. In view of these, the epoxy resin should be one having a low melt viscosity, such as a novolak-type epoxy resin and a biphenolic resin, and particularly having a softening point of 50°C - 80°C. As the curing agent, a phenolic resin, such as a novolak-type phenolic resin, a paraxylylene modified phenolic resin and dicyclopentadiene modified phenolic resin, is used, and particularly a phenolic resin, having a softening point of 60°C - 120°C and containing a low monocaryon content and a low dicaryon content, is preferred. As the curing accelerator, for example, diazabicycloundecenes (DBU) or organic phosphine, such as a triphenylphosphine, is used, and preferably the curing accelerator is one of high latency exhibiting low activity at low temperature. In the present invention, a sprueless mold or a sprue-runnerless mold can be used. More specifically, the temperature of the sprue portion of the mold or the temperature of the sprue portion and the runner portion is controlled to a temperature at which the molding compound is hardly cured, that is, a temperature generally equal to the temperature within the cylinder, and by doing so, at the next cycle after the molded product (i.e., the resin-encapsulated semiconductor device) is removed or taken out, the uncured molding compound, residing in the sprue portion or the sprue-runner portion, can be filled in the cavity, thereby effecting the molding. Accordingly, any cured molding compound other than the molded product is hardly formed, and therefore there is achieved an advantage that the amount of resin waste is greatly reduced. In the encapsulation by the ordinary transfer molding, the ratio of the cured resin waste in the cull-runner portion to the total molding compound is 40% - 60%, but in the encapsulation by the injection molding of the present invention, the ratio of the cured resin waste is slightly reduced to 30% - 50%, and in the sprueless molding, this ratio is much reduced to 25% - 35%, and in the sprue-runnerless molding, this ratio is much reduced to 10% - 30%. The above objects, construction and effects of the present invention will become manifest from the following description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relation between an injection pressure of an injection molding machine and an injection time and the relation between the injection pressure and the position of an injection screw; Fig. 2 is a cross-sectional view showing a closed mold and its associated parts in hoop molding; Fig. 3 is a cross-sectional view taken along the line IV-IV of Fig. 2; Fig. 4 is a cross-sectional view of the mold, showing a condition in which molded products are ejected and discharged; Fig. 5 is a cross-sectional view of the mold, showing the transfer of a lead frame; Fig. 6 is an enlarged, plan view showing the arrangement of a runner, gates and cavities in the mold, and showing a condition immediately after a molding compound is filled in the cavities; Fig. 7 is a cross-sectional view taken along the line VIII-VIII of Fig. 6; Fig. 8 is a schematic plan view of a rotary injection molding apparatus; Fig. 9 is a schematic side-elevational view of the rotary injection molding apparatus; and Fig. 10 is a plan view of a lower mold of a mold device, showing a condition in which lead frames are set in the mold device, and a molding compound is filled in the cavity of the mold device. DESCRIPTION OF THE PREFERRED EMBODIMENTS An injection molding machine used in the present invention is not limited to a particular type, and any suitable injection molding machine, such as a screw inline type, a plunger type and a screw plunger type, can be used; however, in view of easy control and uniform melting, the screw inline type is most preferred. One embodiment of the present invention is characterized in that an injection pressure of the injection molding machine is increased to a maximum pressure which is between 30 kg/cm 2 and 300 kg/cm 2 when 80% - 95% of a total injection amount of an epoxy resin molding compound has been injected into the mold. The remaining molding compound is injected at an injection pressure of 20 kg/cm 2 to 100 kg/cm 2 . In this case, the ratio of the latter pressure to the former maximum pressure is preferably 1/4 to 1/2. If the above two pressures are higher than the above-mentioned values, respectively, there is a possibility that bonding wires on a semiconductor device may be deformed or cut off. In contrast, if the above two pressures are lower than the above-mentioned values, respectively, there is a possibility that the molding compound may not be properly filled in a mold cavity. Preferably, the maximum injection pressure is in the range of 100 kg/cm 2 - 250 kg/cm 2 , and the latter pressure is in the range of 40 kg/cm 2 - 120 kg/cm 2 . These injection pressures are lower as compared with ordinary injection molding of a thermosetting resin. Since the injection pressure is low and the temperature control can be effected easily, a long-time, continuous operation can be effected at a low pressure. Accordingly the molding is effected at the low pressure, the degree of deformation of the bonding wires is low. Therefore a lead frame, having the semiconductor mounted thereon, can be formed into a finer pattern. The lead frame, having the semiconductor bonded thereto, is fixedly set as an insert in the mold, and then the mold is closed or clamped. The molding compound is injected into the mold. The molding compound is filled at a generally constant rate from the start of the filling to the time when the maximum injeccion pressure is achieved. If the filling of the molding compound is so continued that the maximum pressure is achieved at the time when the filling is effected 100%, the pressure acts directly on the molded portion at a final stage of the filling operation even if the value of the maximum pressure is small, the deformation or cutting-off of the bonding wires occurs. Therefore, according to the present invention, the injection pressure is reduced slightly before the filling is completed, and thereafter the filling is continued at the lower pressure. Accordingly, the deformation or cutting-off of the bonding wires can be prevented. The set temperature of a cylinder is usually 65°C, to 110°C, and the temperature control thereof is easy in this range. However, in view of a melt viscosity and a thermal stability, it is preferred that this set temperature is between 70°C to 90°C. If the cylinder temperature is low, the thermal stability is good although the melt viscosity is high. In the epoxy resin molding compound in the present invention, particularly when the above preferred composition is used, the melt viscosity is low, and therefore the cylinder temperature can be set to a considerably low level. However, if the cylinder temperature is set to less than 65°C, the temperature control of the molding compound becomes difficult. In contrast, if the cylinder temperature is higher than 110°C, the melt viscosity is too low, so that it is sometimes difficult to effect the proper injection, and besides the satisfactory thermal stability is often not achieved. In this embodiment of the invention, in view of the curing time of the molding compound and so on, the set temperature of the mold is 150°C to 190°C, and preferably 165°C to 185°C. With this set temperature, the molding cycle can be not more than 150 seconds, and in the case of the molding compound of the preferred composition, the molding cycle can be not more than 80 seconds. A further embodiment of the present invention is characterized in that hoop molding is used. Referring to Figs. 2 to 7, a long lead frame 1, having portions on which the semiconductors are to be mounted, is unwound or supplied from a reel (not shown), and semiconductors 12 are bonded to the lead frame 1, and are wire-bonded to the lead frame 1. The lead frame 1 is guided by a feed guide 10 into a mold 4 so that a predetermined length of the lead frame 1 is set in the mold 4. The feed guide 10 is supported by return pins 11. Lower ends of the return pins 11 are supported by a support plate 18 which is moved upward and downward by a drive cylinder 16. The mold 4 comprises an upper mold 2 and a lower mold 3. The upper mold 2 can be moved upward away from the lower mold 3 by drive means (not shown) so that the mold 4 can be opened. Ejector pins 6, well known in the art, extend slidably through the lower mold 3 to cavities 34 in the mold 4. Lower ends of the ejector pins 6 are supported by an ejector pin operating plate 17. The plate 17 is connected to the support plate 18, and is moved upward and downward by the drive cylinder 16. Referring to Figs. 6 and 7, a molding compound from an injection molding machine is filled in each cavity 34 in the mold 4 through a runner 14 and a gate 15. Gate cut pins 7 are provided for opening and closing the respective gates 15, lower ends of which pins are supported by a pin support plate 19. The pin support plate 19 is moved upward and downward with a distance t (see Fig. 3) by drive means (not shown) so that the gate cut pins 7 can open and close the respective gates 15. A feed mechanism 9 for moving another feed guide 8 in a right-hand direction (Fig. 2) is provided between the lower mold 3 and the feed guide 8. In the condition shown in Fig. 2, the molding compound is injected into the cavities 34 in the mold 4, and the gate cut pins 7 are operated to shut off the gates 15, so that each molded product (comprising the semiconductor 12 encapsulated by the resin molding compound 13) is isolated from the runner 14. After the injected material is cured, the upper mold 2 is moved upward to open the mold 4 as shown in Fig. 5. Then, as shown in Fig. 4, the drive cylinder 16 is driven to move the support plate 18 upward. As a result, the feed guides 10 and 8, supported by the return pins 11, are moved upward away from the lower mold 3. The ejector pin operating plate 17 is also moved upward, so that the ejector pins 6 eject the respective molded products from the respective cavities 34 in the lower mold 3. They are discharged from the lower mold 3. Then, the feed mechanism 9 is operated to move the feed guide 8 in the right-hand direction, thereby moving a molded lead frame portion 5 by a predetermined distance in the right-hand direction. Usually, the separation of the molded lead frame portion 5 and the bending of leads of the frame portion 5 are effected at the same time. Flashes formed on the periphery of each molded product are easily removed since such flashes are usually very thin. As shown in Fig. 5, after the feed guide 8 is returned to its initial position by the feed mechanism 9, the drive cylinder 16 is operated to move the return pins 11 and the ejector pins 6 downward. The lead frame 1 is again set in the lower mold 3, and then the upper mold 2 is moved downward to close the mold 4. The above-mentioned molding is repeated on a non-molded portion of the lead frame 1 subsequent to the precedingly-molded lead frame portion 5. Figs. 6 and 7 are enlarged views showing the runner, the gate and the cavity in the mold, and show a condition immediately after the molding compound 13 is filled in the cavity 34. The injected molding compound is filled in the respective cavities 34 through the sprue (not shown), the runner 14 and the gate 15. The lead frame 1, each having the semiconductor 12 bonded thereto, have been set again in the cavities 34, respectively. Then, each gate cut pin 7 is moved upward by a distance corresponding to the height t of the gate, thereby shutting off the gate 15. After the molding compound is cured, the mold is opened, and the molded products are ejected. The molding compound cured in the runner 14 is isolated from the molded lead frame portion 5. Therefore the molded lead frame portion 5 is smoothly transferred by the feed mechanism 9, so that the subsequent molding operation can be carried out. In case of no gate cut pins 7, it is necessary to cut off the gate by some means after the product is ejected, or it is necessary that the gate be formed into such a shape as that of a pin point gate or a film gate so that it can be spontaneously cut off easily. In this embodiment of the invention, the hoop molding is used, and therefore the sequential molding process can be automatically effected in an unmanned manner, which process includes the steps of unwinding of the lead frame, bonding of the semiconductors, wire bonding, seeding of the lead frame in the mold, molding, removing the molded produces, separating the molded produces from the lead frame, and bending of the leads of the molded products Therefore, the sequential steps from the unwinding of the lead frame to the post-processing of the molded product can be carried out in one system. A further embodiment of the present invention is characterized in that a rotary molding method is used. Referring to Figs. 8 to 10, a rotary injection molding apparatus 35 comprises an injection molding machine 20 having a hopper 23, and a rotary table 24 provided in generally juxtaposed relation co the injection molding machine 20. Four mold devices 25, 26, 27 and 28 are mounted on the rotary table 24, and are circumferentially spaced by 90° from one another. In Fig. 8, a nozzle 36 of the injection molding machine 20 communicates with a sprue-runner 32 (see Fig. 10) of the mold device 25, and a molding compound is filled in the mold device 25. Thus, the molding compound is filled in the mold device 25, and is held at a predetermined pressure, and at the same time the curing of the molding compound proceeds in the mold devices 26 and 27 where the mold is closed or clamped. In the mold device 28, the mold is opened to remove the molded products (each comprising a semiconductor element 31 encapsulated by the molding compound 30), and lead frames 29 are set in the mold and the mold is closed. When each of the four mold devices is return back to the original position, one molding cycle for such mold device is completed. Therefore, if the molding cycle of each mold device is 120 seconds, the molding is effected at a cycle of 30 seconds since this rotary injection molding apparatus 35 has the four stages, and therefore the overall molding cycle can be greatly reduced. Particularly, a certain time period is necessary for the epoxy resin molding compound to be cured by heating, and it is difficult to reduce the time period from the injecting start to the mold opening. In this embodiment of the invention, the rotary molding is used, and therefore this time period can be greatly reduced. Therefore, the molding cycle can be greatly reduced. Further, gate cut pins 33 are provided in each mold, and after the molding compound is injected and filled in the mold, the gate cut pins 33 are operated to shut off the gates so as to isolate the molded products from the sprue-runner 32, thereby facilitating the removal of the molded product and the post-processing. Usually, the separation of the molded products from the lead frame and the bending of leads of the molded products are effected at the same time. At this time, flashes formed on the periphery of each molded product are easily removed since such burrs are usually very thin. It is possible to encapsulate a plurality of kinds of semiconductors simultaneously by means of varying the cavity shape of the mold devices and/or the configuration of the lead frames. If the removal of the molded products and the setting of the lead frames are effected automatically by a robot, the continuous molding can be effected without manual operations, and different kinds of molded products can be easily assorted without mistake. In this embodiment of the invention, if the separation of the molded products from the lead frame and post-processing of the molded products after the removal of the molded product from the mold are effected in continuous relation to the molding operation, the sequential steps from the setting of the lead frame in the mold to the post-processing of the molded product can be carried out in one system. According to the invention, the encapsulation of the semiconductors by the epoxy resin molding compound can be effected continuously for a long time period by the injection molding of the rotary molding type without any problem. By effecting the setting of the lead frames, the removal of the molded products, the post-processing of the molded products and so on automatically or by a robot, the molding operation can be completely automated. The molding compound in the form of powder or granules can be supplied to the injection molding machine, and therefore does not need to be preformed into a tablet-like shape as in the transfer molding. Therefore, vase apparatus and time required for the tableting are omitted. Since the pre-processing such as the tableting is unnecessary, there is little possibility that impurities are included into the molding compound. Sprueless molding and sprue-runnerless molding can be adopted, and therefore the ratio of the cured material (waste) other than the molded products is greatly reduced.

Claims: A method of producing a resin-encapsulated semiconductor device by injecting an epoxy resin molding compound into an injection mold, in which a lead frame, (1), to which a semiconductor (12) has been bonded and wire-bonded, is fixedly set as an insert in the injection mold (4), and by curing such epoxy resin molding compound in said injection mold, characterized by following steps: gradually increasing an injection pressure of said injection molding machine in such a manner that a maximum pressure of 30 kg/cm 2 to 300 kg/cm 2 is achieved at the time when 80% to 95% of a total amount of the epoxy resin molding compound to be injected is injected into the injection mold; and injecting the remaining epoxy resin molding compound at an injection pressure of 20 kg/cm 2 to 100 kg/cm 2 . A method according to Claim 1, wherein . filling said molding compound in said mold is stopped by means of shutting off gates (15) through which said molding compound flows in said mold, (4) after said molding compound is filled in said mold (4) and before said mold (4) is opened. A method according to Claim 1, further comprising the step of heating a cylinder of said injection molding machine to 65°C - 110°C. A method according to Claim 1, further comprising the steps of: locating s plurality of said semicorductor elements (12) to said lead frame (1) in series, intermittently; wire-bonding said semiconductor (12) elements to said lead frame (1); setting said lead frame (1) as an insert in said mold (4); injecting said epoxy resin molding compound into said mold (4) by said injection molding machine; ejecting the molded lead frame (1) from said mold (4) after curing of said molding compound; moving said lead frame (1) in a lengthwise direction by a predetermined length, and again setting said lead frame as an insert in said mold; and injecting said epoxy resin molding compound into said mold (4) with respect to a subsequent portion of said lead frame. A method according to Claim 1, further comprising the steps: providing a rotary injection molding apparatus (25) in which a plurality of mold devices are (25,26,27,28) mounted on a rotary table (24), and are spaced by a predetermined angle from one another; setting said lead frame, having the semiconductor element (31) bonded thereto and wire-bonded thereto, in each of said mold devices (25,26,27,28); injecting said epoxy resin molding compound into one of said mold devices at (25,26,27,28) an injection position by said injecting molding machine (35); intermittently rotating said rotary table (24) by said predetermined angle to bring the next mold device to said injection; and removing the resin-encapsulated semiconductor device from said mold device (25,26,27,28). A method according to Claim 1, wherein said epoxy resin molding compound comprises an epoxy resin, a phenolic resin as a cure agent, a curing accelerator, and an inorganic filler. A method according to Claim 1, in which said mold is a sprueless mold. A method according to Claim 1, in which said mold is sprue-runnerless mold.

Inventors: ITO HIDEO, MINAMI KATSUNORI, ITO, HIDEO, MINAMI, KATSUNORI
Applicants: SUMITOMO BAKELITE CO, SUMITOMO BAKELITE COMPANY LIMITED
Application year: 1996
Main classification: B29C45
IPC codes: B29C45, H01L21