Abstract:
A transfer molding apparatus, wherein said top-half mold and said bottom-half mold form a plurality of cavities interconnected, and wherein said pressure adjusting means reduces the pressure of the cavities every time a specified amount of resin is supplied into any one of a plurality of cavities.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a continuation application of application Ser. No. 10/890,222, filed Jul. 14, 2004, which is a continuation application of application Ser. No. 09/893,455, filed Jun. 29, 2001, now U.S. Pat. No. 6,767,484, which is a divisional application of Ser. No. 09/265,841 filed Mar. 10, 1999, now U.S. Pat. No. 6,267,577, which are hereby incorporated by reference in their entirety for all purposes. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a transfer molding apparatus and a method of manufacturing semiconductor devices.  
         [0004]     2. Related Art  
         [0005]     In the manufacture of semiconductor devices, transfer molding apparatuses for encapsulating semiconductor devices mounted on lead frames are conventionally used. As shown in  FIG. 7 , the transfer molding apparatus comprises a transfer pot  10  into which solid thermosetting resin (tablets) is loaded, a plunger  12  for transferring the thermosetting resin  28  (hereafter referred to as the resin  28 . See FIGS.  8 ( a )- 8 ( d )) that has been fluidified in the transfer pot, a position sensor  14  for detecting the position of the plunger  12 , a top-half mold  16   a  fixed to a top platen  26   a , a bottom-half mold  16   b  fixed to a bottom-half platen  26   b , heaters  18  for heating the top-half and bottom-half molds  16   a ,  16   b  to a predetermined temperature, and a suction pump  24  for reducing the pressure in the cavities  20  by extracting the air from a chamber  30  where the top-half and bottom-half molds  16   a ,  16   b  are disposed.  
         [0006]     When the top-half and bottom-half molds  16   a ,  16   b  are closed, two cavities  20  each for setting up a molding space for a plastic IC package, and runners  22  and gates  23  as resin supply paths leading to the cavities  20  are formed.  
         [0007]     When the top-half and bottom-half molds  16   a ,  16   b  are closed, air-vent slots  25  are also formed at the outer end positions of the two cavities  20  opposite the gates  23  located at the inner ends thereof. When the chamber  30  is placed at reduced pressure by extracting air by a suction pump  24 , the air is sucked out from the runners  22  and the gates  23  through the air-vent slots  25 , so that the cavities  20  are placed at reduced pressure. The runners  22  guide the resin  28  into the cavities  20  through the gates  23  that are open to the corresponding cavities  20 .  
         [0008]     Referring to FIGS.  8 ( a )- 8 ( d ), description will be made of a method of manufacturing semiconductor devices on a transfer molding apparatus structured as described above. FIGS.  8 ( a )- 8 ( d ) show only the principal portions for convenience of explanation. First of all, a semiconductor-device-mounted lead frame (not shown) is set in the bottom-half mold  16   b , a resin tablet is loaded in the transfer pot  10 , and by lowering the top platen  26   a , the top-half mold  16   a  and the bottom-half mold  16   b  are closed, so that a cavity  20 , for example, is formed as shown in  FIG. 8 ( a ). At this point in time, the semiconductor device has been placed almost in the center of the cavity  20 . In addition, the pressure in the chamber  30  has been reduced to about 30 to 99 Pa by the suction pump  24 .  
         [0009]     While the resin tablet charged in the transfer pot  10  is being melted by heating it to 160° to 190° C. with the heaters  18 , the resin is extruded from the transfer pot  10  by raising the plunger  12 . By this operation, the molten resin  28  is introduced into the runner  22  as shown in  FIG. 8 ( b ).  
         [0010]     By the increasing the forcing pressure from the plunger  12 , the resin  28  in the runner  22  is guided through the gate  23  into the cavity  20  as shown in  FIG. 8 ( c ). As shown in  FIG. 8 ( d ), when the resin  28  has been filled into the cavity  20 , the forcing pressure from the plunger  12  is stopped, and the resin  28  in the cavity  20 , the runner  22 , and the gate  23  is cured. After the resin  28  is cured sufficiently, the top platen  26   a  (see  FIG. 7 ) is raised, and the semiconductor device with a lead frame in a package of resin  20  that hardened around the semiconductor element is ejected. Subsequently, the excess resin is removed and whittled down to shape, and the lead-frame portion is cut off and the outer leads are formed to thereby produce a semiconductor package. Subsequently, the excess resin is removed, the package is whittled down to shape, the frame portion of the lead frame is trimmed, and the outer leads are formed. Thus, a semiconductor device is produced.  
         [0011]     In the transfer molding apparatus constructed as described, there are possibilities of an unfilled region (voids)  29  being formed in the top-cavity portion or the bottom-cavity portion of the mold due to a difference in resin-filling speed between the top-cavity portion and the bottom-cavity portion, which partition is made by the semiconductor element loaded in the cavity  20 . Voids are unwanted because they give rise to a warp or deformation in the package or decreases its strength or humidity resistance.  
         [0012]     There have been countermeasures against the voids. One is to provide a suction port communicating with the cavity, and directly reduce the pressure in the cavity by the use of a suction pump to decrease the remaining air in the top-cavity portion or the bottom-cavity portion to prevent the occurrence of voids. The other is to place the chamber  30  itself in a reduced-pressure atmosphere so that the remaining air in the top-cavity portion or the bottom-cavity portion should be extracted through the air-vent slot  25  and to thereby prevent the occurrence of voids.  
         [0013]     However, in the transfer molding apparatus constructed as described above, because the resin passes through the gate of a smaller diameter than that of the runner when it enters a cavity, the resin is subjected to pressure at the gate, and the resin in compressed state is injected into the cavity. Therefore, if the cavity is at reduced pressure when a specified amount of resin is introduced into the cavity, there is a relatively large pressure difference between the pressure in the cavity and the pressure in the resin. A problem here is that when there is such a large pressure difference, the air bubbles in the resin expand notably, and remain as voids in the package.  
         [0014]     Thermosetting resins have a characteristic that curing does not progress in proportion to the passage of time, but curing occurs after the viscosity decreases once. Therefore, with some kinds of thermosetting resins, the viscosity sometimes drops temporarily while the cavity is being filled with a molding compound. Also in this case, there is a problem that the air bubbles expand remarkably in the resin and remain as voids in the package.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention has been made to solve the above problems, and has as its object to provide a transfer molding apparatus and a method for manufacturing semiconductor devices, which are free of voids remaining in a resin when filled in the cavities.  
         [0016]     To achieve the above object, the transfer molding apparatus according to the present invention comprises:  
         [0017]     a top-half mold and a bottom-half mold for forming a cavity as a molding space for a package and a transfer pot as a loading space, communicating with the cavity, for resin to be injected into the cavity;  
         [0018]     a plunger for forcing the resin out of the transfer pot into the cavity; and  
         [0019]     a pressure adjuster for reducing the pressure in the cavity when a specified amount of resin has been injected into the cavity.  
         [0020]     Because the pressure adjuster reduces the pressure in a cavity after a specified amount of resin has been injected into the cavity, the cavity is at normal pressure at a point in time when the supply of a specified amount of resin is finished and the pressure difference between the pressure in the cavity and the pressure in the resin is relatively small. Therefore, the air bubbles in the resin can be prevented from expanding remarkably.  
         [0021]     Because the pressure adjuster reduces the pressure in a cavity when a specified amount of resin has been injected into the cavity, the remaining air in the unfilled region of the top cavity portion or the bottom cavity portion can be decreased, so that the voids can be reduced, which occur due to a difference in filling rate between the top cavity portion and the bottom cavity portion.  
         [0022]     In the transfer molding apparatus described above, the top-half mold and the bottom-half mold form a plurality of interconnected cavities, and the pressure adjuster reduces the pressure of the cavities every time any one of the plurality of cavities is supplied with a specified amount of resin.  
         [0023]     Even in such a construction that a plurality of cavities are connected to one transfer pot, each cavity is kept at normal pressure until it is supplied with a specified amount of resin. Therefore, the entrapped air in the resin in each cavity when it is filled with the specified amount of resin can be prevented from expanding to a great extent, with the result that it is possible to efficiently obtain semiconductor devices in packages of good quality.  
         [0024]     Further, in the transfer molding apparatus, the pressure adjuster has a position detector for detecting the position of the plunger, and reduces the pressure in each cavity by detecting the plunger position at a point in time when the cavity has been supplied with a specified amount of resin. By using this mechanism, the injected amount of resin can be detected with high accuracy, which makes it possible to suitably control timing of pressure reduction by the pressure adjuster.  
         [0025]     In the transfer molding apparatus described above, the pressure adjuster has a time counter, and reduces the pressure in a cavity when the time counter has counted a set length of time from the start of movement of the plunger until the cavity is supplied with a specified amount of resin.  
         [0026]     More specifically, a length of time from the start of plunger movement until the cavity is supplied with a specified amount of resin is measured, and at the end of a preset time, the pressure adjuster reduces the pressure in the cavity. Therefore, it is possible to detect the injected amount with high accuracy, and suitably control timing of pressure reduction by the pressure adjuster.  
         [0027]     In the method for manufacturing semiconductor devices, a semiconductor-element-mounted lead frame is placed between the top-half mold and the bottom-half mold, and the pressure in a cavity is reduced when a specified amount of resin has been filled in the cavity formed by the top-half mold and the bottom-half mold.  
         [0028]     In other words, according to, the method according to the present invention, because the pressure in a cavity is not reduced until the cavity is supplied with a specified amount of resin, the cavity prior to injection of resin is maintained at normal pressure. For this reason, a pressure difference between the pressure in the resin and the pressure in the cavity is relatively small when the cavity has been supplied with resin. Therefore, the entrapped air in the resin can be prevented from expanding remarkably.  
         [0029]     Needless to say, because the pressure in the cavity is reduced after the cavity has been supplied with a specified amount of resin, the remaining air in the unfilled region of the top-cavity portion or the bottom-cavity portion can be reduced, and it becomes possible to prevent the occurrence of voids due to a difference in filling rate between the top-cavity portion and the bottom-cavity portion. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINS  
       [0030]      FIG. 1  is a sectional view showing a schematic construction of a transfer molding apparatus according to a first embodiment of the present invention;  
         [0031]     FIGS.  2 ( a )- 2 ( d ) are fragmentary diagrams for explaining the motion of the transfer molding apparatus shown in  FIG. 1 ;  
         [0032]      FIG. 3  is a sectional view showing a schematic construction of the transfer molding apparatus according to a second embodiment of the present invention;  
         [0033]     FIGS.  4 ( a )- 4 ( d ) are fragmentary diagrams for explaining the motion of the transfer molding apparatus shown in  FIG. 3 ;  
         [0034]      FIG. 5  is a sectional view showing a schematic construction of the transfer molding apparatus according to a third embodiment of the present invention;  
         [0035]     FIGS.  6 ( a )- 6 ( d ) are fragmentary diagrams for explaining the motion of the transfer molding apparatus shown in  FIG. 5 ;  
         [0036]      FIG. 7  is a sectional view showing a schematic construction of a conventional transfer molding apparatus; and  
         [0037]     FIGS.  8 ( a )- 8 ( d ) are fragmentary diagrams for explaining the motion of the transfer molding apparatus shown in  FIG. 6 .  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]     Embodiments of the present invention will be described with reference to FIGS.  1  to  6 ( a )- 6 ( d ).  
       First Embodiment  
       [0039]     A first embodiment will be described with reference to  FIGS. 1 and 2 ( a )- 2 ( d ). As shown in  FIG. 1 , the transfer molding apparatus according to the first embodiment comprises a top-half mold  16   a  fixed to a top platen  26   a ; a bottom-half mold  16   b  fixed to a bottom platen  26   b ; heaters  18  for heating the top-half and the bottom-half molds  16   a ,  16   b  to a predetermined temperature; a transfer pot  10 , formed by the top-half and the bottom-half molds, for accepting a tablet of a thermosetting resin, an epoxy resin for example, formed by the top-half and the bottom-half molds  16   a ,  16   b ; a plunger  12  for extruding the resin  28  melted in the transfer pot  10 ; a position sensor  14  for detecting the position of the plunger  12 ; a suction pump  24  for reducing the pressure in a chamber  30 , having the top-half mold  16   a  and the bottom-half mold  16   b  installed therein, thereby placing the cavities  20  at reduced pressure; and a pressure controller  40  for controlling the drive of the suction pump  24  according to the amount of movement of the plunger  12 .  
         [0040]     When the top-half mold  1   a  and the bottom-half mold  16   b  are closed, two cavities  20  as molding spaces for packages and two runners  22  for guiding the resin into the cavities  20  through gates  23 , which are open to the cavities  20 , are formed. The runners (distribution paths)  22  are provided on both sides of the transfer pot  10  and communicate with each other through the transfer pot  10 .  
         [0041]     An air-vent slot  25  is formed on the side of each cavity  20  that is opposite the side open to the gate  23 . As will be described later, when the chamber  30  is placed at reduced pressure by using a suction pump  24 , the air in the cavities  20  is extracted through the air-vent slots  25 , so that the cavities are also placed at reduced pressure.  
         [0042]     The pressure controller  40  drives the suction pump  24  to reduce the pressure in the chamber  30  when the position sensor  14  detects that the plunger  12  is at the position indicating that each cavity has been supplied with a specified amount of resin. The pressure controller  40  causes the suction pump  24  to stop when the position sensor  14  detects the position of the plunger  12 , which indicates that the cavities  20  have been filled with resin  28 . Then, the pressure controller  40  brings the chamber  30  to normal pressure by releasing a vent valve of the chamber  30 .  
         [0043]     Referring to FIGS.  2 ( a )- 2 ( d ), description will now be made of a method for manufacturing semiconductor devices on the transfer molding apparatus constructed as stated. Note that FIGS.  2 ( a )- 2 ( d ) show the principal parts only for convenience of explanation. A semiconductor-element-mounted lead frame (not shown) is set in the bottom-half mold  16   b , then a resin tablet is charged in the transfer pot  10 , and by lowering the top platen  26   a , the top-half mold  16   a  and the bottom-half mold  16   b  are closed as shown in  FIG. 2 ( a ). At this time, the semiconductor element, not shown, has been placed almost in the center of the cavity  20 .  
         [0044]     Then, while the tablet charged into the transfer pot  10  is melted by heating it to 160° to 190° C., the plunger  12  is raised to extrude the resin from the transfer pot  10 . Consequently, the molten resin  28  is introduced into the runner  22 .  
         [0045]     The position sensor  14  is detecting the position of the plunger  12  from the start of its movement. As shown in  FIG. 2 ( b ), the position sensor  14  outputs a detection signal to the pressure controller  40  when the position sensor  14  detects that the leading end of the plunger  12  has reached the position B, which indicates that a specified amount of resin has been supplied from the runner  22  into the cavity  20 .  
         [0046]     On receiving a detection signal from the position sensor  14 , the pressure controller  40  transmits a drive start signal to the suction pump  24 . In response to the drive start signal, the suction pump  24  starts to extract the air from the chamber  30 , and gradually reduces the pressure in the chamber  30  to about 30 to 90 Pa. Therefore, even if the resin  28  has been compressed at position of the gate  23  and the pressurized resin  28  is injected into the cavity  20 , because the cavity is gradually changed from normal pressure to reduced pressure, the relative pressure difference between the pressure acting on the air bubbles in the resin  28  and the pressure in the cavity  20  can be limited to a small degree, the air bubbles entrapped in the resin can be prevented from expanding greatly.  
         [0047]     Timing for reducing the pressure in the chamber  30  by using the suction pump  24  may be when the resin decreases in viscosity and starts to harden, for example (in this case, when the resin  28  has been injected to about one half of the cavity  20  as shown in  FIG. 2 ( c )). In other words, the timing for chamber pressure reduction may be when the leading end of the plunger  12  has reached the position C.  
         [0048]     As described above, if the pressure in the chamber  30  is decreased after the viscosity of the resin has decreased, because the resin has started to harden, the air bubbles in the resin are less liable to expand.  
         [0049]     As shown in  FIG. 2 ( d ), when the cavity  20  is completely filled with resin  28 , in other words, when the leading end of the plunger  12  has reached the position D, the position sensor  14  outputs a detection signal to the pressure controller  40 . When receiving the detection signal from the position sensor  14 , the pressure controller  40  transmits a drive stop signal to the suction pump  24  to stop its operation. On the other hand, the chamber  30  is brought back to normal pressure. At the same time, the plunger  12  is stopped. Subsequently, the resin  28  is cured.  
         [0050]     After the resin  28  has been sufficiently cured, the top platen  26   a  (see  FIG. 1 ) is raised, and a semiconductor device with a lead frame in a package of resin  28  that hardened around the semiconductor element is ejected. Subsequently, the excess resin remaining in the runner  22  or the like is removed, the package is whittled down to shape, the frame portion of the lead frame is trimmed, and the outer leads are formed. Thus, a semiconductor device is obtained.  
         [0051]     As has been described, in the first embodiment, when the position sensor  14  detects that a specified amount of resin  28  has been injected into the cavity  20 , the pressure controller  40  causes the suction pump  24  to operate to reduce the pressure in the cavity  20 . Therefore, the relative pressure difference between the pressure acting on the air bubbles in the resin  28  and the pressure in the cavity  20  can be limited to a small degree until the cavity  20  is supplied with a specified amount of resin  28 . For this reason, the air bubbles in the resin  28  can be prevented from expanding remarkably, which makes it possible to eliminate chances of voids remaining in the package.  
         [0052]     If the pressure in the chamber  30  is reduced after a drop occurred in the viscosity of the resin  28  being injected into the cavity  20 , it follows that the curing of the resin has started. In this case, the air bubbles in the resin  28  can be prevented from expanding remarkably, leaving less chances of the voids remaining in the package.  
       Second Embodiment  
       [0053]     Referring to  FIGS. 3 and 4 ( a )- 4 ( d ), a second embodiment of the present invention will be described. As shown in  FIG. 3 , the transfer molding apparatus according to the second embodiment comprises a top-half mold  16   a  fixed to a top platen  26   a ; a bottom-half mold  16   b  fixed to a bottom platen  26   b ; heaters  18  for heating the top-half and the bottom-half molds  16   a ,  16   b  to a predetermined temperature; a transfer pot  10 , for accepting a tablet of a thermosetting resin, an epoxy resin for example, formed by the top-half and the bottom-half molds  16   a  and  16   b ; a plunger  12  for extruding the resin  28  melted in the transfer pot  10 ; a suction pump  24  for reducing the pressure in the chamber  30 , having the top-half and the bottom-half molds  16   a ,  16   b  installed therein, thereby placing the cavities  20  at reduced pressure; and a pressure controller  42  for controlling the drive of the suction pump  24  according to the amount of movement of the plunger  12 .  
         [0054]     When the top-half mold  16   a  and the bottom-half mold  16   b  are closed, two runners (distribution paths)  22  for guiding the resin into the cavities  20  through gates  23 , which are open to the cavities  20  are formed.  
         [0055]     An air-vent slot  25  is formed on the side of each cavity  20  that is opposite the side open to the gate  23 . When the chamber  30  is placed at reduced pressure by using a suction pump  24 , the air in the cavities is extracted through the air-vent slots  25 , so that the cavities  20  are also placed at reduced pressure.  
         [0056]     The pressure controller  42  is connected to a timer  42   a  that counts the elapsed time from the start of plunger movement. The timer  42   a , when it has counted the time until the cavity  20  is supplied with a specified amount of resin, outputs a detection signal to the pressure controller  42 . In response, the pressure controller  42  causes the suction pump  24  to operate to reduce the pressure in the chamber  30 . When the timer  42   a  has counted the time until the supply of resin  28  into the cavity  20   a  is finished, the pressure controller  42  causes the suction pump  24  to stop. On the other hand, the chamber  30  is returned to normal pressure.  
         [0057]     Referring to FIGS.  4 ( a )- 4 ( d ), description will be made of a method of manufacturing semiconductor devices on the transfer molding apparatus constructed as described. FIGS.  4 ( a )- 4 ( d ) show the principal parts only for convenience of explanation, and depicts the chamber  30  with an alternate long and short dash line in a conceptual diagram.  
         [0058]     After a semiconductor-element-mounted lead frame (not shown) is set in the bottom-half mold  16   b , a resin tablet is charged in the transfer pot  10 , and by lowering the top platen  26   a , the top-half mold  16   a  and the bottom-half mold  16   b  are closed as shown in  FIG. 4 ( a ). At this point in time, the semiconductor element, not shown, has been placed almost in the center of the cavity  20 .  
         [0059]     While the resin tablet charged in the transfer pot  10  is being melted by heating it to 160° to 190° C. by the heaters  18 , the resin is extruded from the transfer pot  10  by raising the plunger  12 . By this operation, the molten resin  28  is introduced into the runner  22 .  
         [0060]     The timer  42   a  starts counting time from the start of plunger movement, and as shown in  FIG. 4 ( b ), when the leading end of the plunger  12  has moved from position A to position B and the timer  42   a  counts to time t 1  that indicates a specified amount of resin  28  has been supplied from the runner  22  into the cavity  20 , the timer  42   a  outputs a detection signal to the pressure controller  42 . On receiving a detection signal from the timer  42   a , the pressure controller  42  sends a drive start signal to the suction pump  24 . In response to the drive start signal, the suction pump  24  starts to extract the air from the chamber  30 , thus reducing the pressure in the chamber  30  to about 30 to 99 Pa.  
         [0061]     In other words, when the timer has counted the previously calculated time t 1  till each cavity  20  is supplied with a specified amount of resin  28 , the suction pump  24  gradually reduces the pressure in the cavity  20 . For this reason, even if the resin  28  has been compressed at the gate position  23  and the pressurized resin is injected into the cavity, the relative pressure difference between the pressure acting on the air bubbles in the resin  28  and the pressure in the cavity  20  is limited to a small degree. Therefore, the air bubbles entrapped in the resin can be prevented from expanding remarkably.  
         [0062]     Timing for reducing the pressure in the chamber  30  by using the suction pump  24  may be, for example, when the resin decreases in viscosity and starts to harden (in this case, as shown in  FIG. 4 ( c ), when the resin  28  has been injected up to one half of the cavity  20 ). In other words, the timing for chamber pressure reduction may be when the timer  42   a  has counted time t 2 .  
         [0063]     As described above, if the pressure in the chamber  30  is decreased after the resin has decreased in viscosity and has started to harden, the air bubbles in the resin are less liable to expand.  
         [0064]     As shown in  FIG. 4 ( d ), when the cavity  20  is completely filled with resin  28 , in other words, when the timer  42   a  has counted time t 3  corresponding to the complete filling, the timer  42   a  outputs a detection signal to the pressure controller  42 . When receiving the detection signal from the timer  42   a , the pressure controller  42  transmits a drive stop signal to the suction pump  24  to stop its operation. On the other hand, the chamber  30  is brought back to normal pressure.  
         [0065]     After the resin  28  has been sufficiently cured, the top platen  26   a  (see  FIG. 3 ) is raised, and the semiconductor device with a lead frame in a package of resin  28  that hardened around the semiconductor element is ejected. Subsequently, the excess resin is removed, the package is whittled down to shape, the frame portion of the lead frame is trimmed, and the outer leads are formed. Thus, a semiconductor device is obtained.  
         [0066]     As has been described, in the second embodiment, when the timer  42   a  detects the time when a specified amount of resin  28  has been supplied to the cavity  20 , the pressure controller  42  causes the suction pump  24  to operate to reduce the pressure in the cavity  20 . Therefore, the relative pressure difference between the pressure acting on the air bubbles in the resin  28  and the pressure in the cavity  20  can be limited to a small degree. For this reason, even when the specified amount of resin  28  has been supplied to the cavity  20 , the air bubbles in the resin  28  can be prevented from expanding greatly, which makes it possible to eliminate chances of voids remaining in the package.  
         [0067]     Furthermore, even when the pressure in the chamber  30  is reduced after the resin  28  being injected into the cavity  20  has decreased in viscosity and has started to be cured, the air bubbles in the resin  28  can be prevented from expanding greatly during filling.  
       Third Embodiment  
       [0068]     A third embodiment of the present invention will be described with reference to  FIGS. 5 and 6 ( a )- 6 ( d ). In the transfer molding apparatus according to the third embodiment, two pairs of first and second cavities  20   a ,  20   b , each pair being interconnected through a second runner  22   b , are formed by the top-half mold  16   a  and the bottom-half mold  16   b.    
         [0069]     When the top-half mold  16   a  and the bottom-half mold  16   b  are closed, a transfer pot  10 , a first runner  22   a  communicating with the transfer pot  10 , a first cavity  20   a  for accepting resin  28  from the first runner  22   a  through a first gate  23   a , a second runner  22   b  communicating with the first cavity  20   a , and a second cavity  20   b  for accepting resin  28  from the second runner  22   b  through a second gate  23   b  are formed on either side of the transfer pot  10  as shown in  FIG. 5 .  
         [0070]     An air-vent slot  25  is formed on one side of each second cavity  20   b  that is opposite the side where there is the second gate  23   b . When the chamber  30  is placed at reduced pressure by extracting air by a suction pump  24 , the air is sucked out from the first and second cavities  20   a ,  20   b  through the air-vent slots  25 , so that the cavities  20  are placed at reduced pressure.  
         [0071]     The pressure controller  40  drives the suction pump  24  to reduce the pressure in the chamber  30  when the position sensor  14  detects that the plunger  12  is at the position indicating that each cavity has been supplied with a specified amount of resin. When the position sensor  14  detects that the plunger  12  is at the position indicating that the first cavity  20   a  has been filled with resin  28  and also when the sensor  14  detects that the plunger  12  is at the position indicating that the second cavity  20   b  has been filled with resin  28 , the pressure controller  40  causes the suction pump  24  to stop. On the other hand, the chamber  30  is brought back to normal pressure. Note that the other features of the third embodiment are the same as in the first embodiment, and therefore their descriptions are omitted.  
         [0072]     Referring to FIGS.  6 ( a )- 6 ( d ), description will be made of a method for manufacturing semiconductor devices on the transfer molding apparatus constructed as described. FIGS.  6 ( a )- 6 ( d ) show only the principal parts for convenience of explanation. Semiconductor-device-mounted lead frames (not shown) are set in the bottom-half mold  16   b , then a resin tablet is loaded in the transfer pot  10 , and by lowering the top platen  26   a , the top-half mold  16   a  and the bottom-half mold  16   b  are closed. At this time, the semiconductor devices, not shown, have been placed almost in the center of the first and the second cavities  20   a ,  20   b.    
         [0073]     While the resin tablet charged in the transfer pot  10  is being melted by heating it to 160° to 190° C. by the heaters  18 , the resin is extruded from the transfer pot  10  by raising the plunger  12 . By this operation, the molten resin  28  is introduced into the first runners  22   a.    
         [0074]     The position sensor  14  detects the position of the plunger  12  from the start of its movement. As shown in  FIG. 6 ( a ), the position sensor  14  outputs a detection signal to the pressure controller  40  when the position sensor  14  detects that the leading end of the plunger  12  has reached the position A, which indicates that a specified amount of resin has been supplied from the first runner  22   a  into the first cavity  20   a.    
         [0075]     On receiving a detection signal from the position sensor  14 , the pressure controller  40  transmits a drive start signal to the suction pump  24 . In response to the drive start signal, the suction pump  24  starts to extract the air from the chamber  30 , and gradually reduces the pressure in the chamber  30  to about 30 to 90 Pa.  
         [0076]     Therefore, even if the resin  28  has been compressed at the first gate position  23   a  and the pressurized resin  28  is injected into the first cavity  20   a , because the first cavity  20   a  is gradually changed from normal pressure to reduced pressure, the relative pressure difference between the pressure applied to the air bubbles in the resin  28  and the pressure in the first cavity  20   a  can be limited to a small degree, so that the air bubbles entrapped in the resin can be prevented from expanding greatly.  
         [0077]     Timing for reducing the pressure in the chamber  30  by using the suction pump  24  may be when the resin decreases in viscosity and starts to harden, for example (in this case, when the resin  28  has been supplied to about one half of the first cavity  20   a  as shown in  FIG. 6 ( b )). In other words, the timing for chamber pressure reduction may be when the leading end of the plunger  12  has reached the position B.  
         [0078]     As described above, if the pressure in the chamber  30  is decreased after the viscosity of the resin has decreased, because the resin has already started to harden, the air bubbles are less liable to expand in the resin supplied in the first cavity  20   a.    
         [0079]     Subsequent to as shown in  FIG. 6 ( b ), when the cavity  20   a  has been filled with the resin  28  once the plunger  12  moves past the position B, the position sensor  14  outputs a detection signal to the pressure controller  40 . On receiving the detection signal from the position sensor  14 , the pressure controller  40  transmits a drive stop signal to the suction pump  24 . By this drive stop signal, the suction pump  24  is stopped and the chamber  30  is brought back to normal pressure.  
         [0080]     When a specified amount of resin  28  has been supplied from the second runner  22   b  to the second cavity  20   b , the position sensor  14  detects that the plunger  12  has reached the position C, as shown in  FIG. 6 ( c ). When the position sensor  14  detects that the leading end of the plunger  12  arrived at the position C, the position sensor  14  outputs a detection signal to the pressure controller  40 .  
         [0081]     When receiving the detection signal from the position sensor  14 , the pressure controller  40  transmits a drive start signal to the suction pump  24 , and the suction pump  24  starts to extract the air from the chamber  30  until the chamber  30  is reduced in pressure to about 30 to 90 Pa.  
         [0082]     Therefore, even if the resin  28  is compressed at the position of the gate  23   b  and the pressurized resin  28  is injected into the second cavity  23   b , because the second cavity  20   b  is gradually changed from normal pressure to reduced pressure, the relative pressure difference between the pressure applied to the air bubbles in the resin  28  and the pressure in the second cavity  20   b  can be limited to a small degree. Thus, the air bubbles entrapped in the resin can be prevented from expanding greatly.  
         [0083]     Timing for reducing the pressure in the chamber  30  by using the suction pump  24  may be when the resin decreases in viscosity and starts to harden (in this case, when the resin  28  has been injected to about one half of the cavity  20   b ).  
         [0084]     When the pressure in the chamber  30  is reduced after the viscosity of the resin in the second cavity  20   b  has decreased as described above, because the resin has started to harden, the air bubbles in the second cavity  20   b  are less liable to expand.  
         [0085]     Subsequently, when the leading end of the plunger  12  has moved to the position D as shown in  FIG. 6 ( d ), which indicates that the second cavity  20   b  has been filled completely with the resin  28 , the position sensor  14  outputs a detection, signal to the pressure controller  40 . In response to the detection signal, the pressure controller  40  sends a drive stop signal to the suction pump  24 . By the drive stop signal, the suction pump  24  is stopped. On the other hand, the chamber  30  is brought back to normal pressure. At the same time, the plunger movement is also stopped, and the resin in the cavities  20   a  and  20   b  is cured.  
         [0086]     After the resin  28  has been completely cured, the top platen  26   a  (see  FIG. 5 ) is raised, a semiconductor device with a lead frame is ejected which is encapsulated in the resin package that has been cured around the semiconductor element. Subsequently, the excess resin is removed, the package is whittled to shape, the lead frame is trimmed, and the outer leads are formed. Thus, a semiconductor device is produced.  
         [0087]     As described, according the third embodiment, when a specified amount of resin is successively injected into the first and second cavities  20   a ,  20   b  serially interconnected by the second runner  22   b , the pressure controller  40  causes the suction pump to operate to adjust the pressure of the cavities. Therefore, there is provided an advantage that a plurality of semiconductor devices are formed simultaneously in addition to the advantage described with reference to the first embodiment.  
         [0088]     The shape of the mold is not limited to the one shown in  FIGS. 5 and 6 ( a )- 6 ( d ), but may be a type for producing a package with multiple gates, such as formed in matrix. In this case, by detection of the position of the plunger  12  by the position sensor  14 , it is possible to adjust the pressure of the cavities in a predetermined timing pattern, so that products without voids can be produced.  
         [0089]     According to the third embodiment that has been described, the pressure controller  40  controls the suction pump according to the position of the plunger  12  detected by the position sensor  14 . It is also possible to arrange for the pressure controller  40  to control the suction pump according to time counted by the timer  42   a  shown in FIGS.  4 ( a )- 4 ( d ).  
         [0090]     More specifically, the pressure controller  40  drives the suction pump  24  to reduce the pressure in the chamber  30  when the timer  42   a  has counted time until a specified amount of resin  28  has been injected into the first cavity  20   a , and when the timer  42   a  has counted time until a specified amount of resin has been injected into the second cavity  20   b . In this case, the pressure controller  40  causes the suction pump  24  to stop when the timer  42   a  has counted time when the first cavity  20   a  has been completely filled with the resin  28  and time when the second cavity  20   b  has been completely filled with the resin  28 .  
         [0091]     In all the embodiments described above, the transfer molding apparatus has included one transfer pot for convenience of explanation. However, the present invention can be applied to transfer molding apparatuses having a plurality of transfer pots. Similarly, in all the embodiments described above, for convenience of explanation, the transfer molding apparatus has included two runners communicating with the transfer pot, but the present invention can be applied to transfer molding apparatuses including one or more than three runners communicating with the transfer pot.  
         [0092]     As has been described, according to the present invention, there is provided a transfer molding apparatus which prevents the expansion of air bubbles in the resin resulting from a relative pressure difference between the pressure in the resin and the pressure in the cavity and thereby prevents the voids from remaining the package.  
         [0093]     According to the method of manufacturing semiconductor devices with less chances of voids remaining in the package.