Abstract:
A molding method includes joining first and second molds to define a mold cavity; injecting a plastic into the mold cavity; and projecting the eject pin into the mold cavity with the molds joined together, thereby applying pressure to the plastic in the mold cavity.

Description:
This disclosure is a division of application Ser. No. 08/069,669, filed Jun 1, 1993, now U.S. Pat. No. 5,375,989, which, itself, is a continuation application of application Ser. No. 07/732,178, filed Jul. 18, 1991, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an improvement of a plastic molding device for a semiconductor element and a method for plastic molding a semiconductor element. 
     BACKGROUND OF THE INVENTION 
     FIG. 8 is a view showing a conventional plastic molding device for plastic molding a semiconductor element. 
     In FIG. 8, reference numeral  80  designates a plastic molding means for plastic molding a semiconductor element which comprises separable upper mold  70  and lower mold  71 . The upper and lower molds  70  and  71  are supported by upper and lower platens  65  and  64  of a clamping press (a mold driving mechanism) which is not shown, respectively. 
     The upper mold  70  comprises an upper mold cavity block  1  having a plurality of upper mold cavities la, an upper surface plate  5  with a heater built therein which supports the upper block  1 , a base plate  9  supporting the upper surface plate  5  through a post  15 , and a spacer block  8  interposed between the upper surface plate  5  and the base plate  9 . The upper cavity block  1  has a center block  2  in the center thereof and the center block  2  is equipped with a chamber  3  for housing a plastic tablet  14 . At an upper portion of the chamber  3 , a plunger  12  is provided for pressing the plastic tablet  14 . In addition, an eject pin  4 , one end of which is drawn in or out of the upper cavity  1   a,  is provided in the cavity block  1  and the upper surface plate  5 . A vertically movable pin supporting member  20  comprising an ejector plate  7  and a keep plate  6  which pushes the eject pin  4  toward the side of the ejector plate  7  is provided between the upper surface plate  5  and the base plate  9  in the upper mold  70 . The pin supporting member  20  is urged against the upper surface plate  5  by forcing means  21 . The forcing means  21  comprises a screw bar  10  screwed on the upper surface plate  5  and a spring  11  provided between the screw bar  10  and the pin supporting member  20 . Reference numeral  13  designates a return pin fixed on the pin supporting member  20 . 
     The lower mold  71  has the almost same structure as that of the upper mold  70  and comprises a lower mold cavity block  51  having a plurality of lower mold cavities  51   a,  a lower center block  52  at the center thereof, a lower surface plate  55  with a heater built therein which supports the blocks  51  and  52 , a post  63  supporting the lower surface plate  55 , a spacer block  58 , and a base plate  59 . Eject pins  54 , one end of each of which can be drawn in and out of the lower mold cavity  51   a , are provided in the cavity block  51  and the lower surface plate  55 . A vertically movable pin supporting member  30  comprising an ejector plate  57  and a keep plate  56  is provided between the lower surface plate  55  and the base plate  59 . The pin supporting member  30  is urged away from the lower surface plate  55  by a spring  60 . 
     In the lower center block  52  and the lower mold cavity block  51 , runners  52   a  and  51   b  are respectively formed as passages for the plastic  14 . In addition, in the block  51 , a gate  51   c  connecting the runner  51   b  to the lower mold cavity  51   a  is formed. The runner  52   a  and the gate  51   c  form a plastic injecting mechanism with the plunger  12  and the chamber  3 . In addition, a return stopper  62  is attached to the lower surface plate  55 . This return stopper  62  abuts the return pin  13  at the time of clamping to retract the eject pin  4  of the upper mold  70 . A knockout rod  61  is fixed on the base part of the device. This knockout rod  61  abuts the pin supporting member  30  to eject the eject pin  54  into the lower mold cavity  51   a  when the upper and lower molds  70  and  71  are separated by moving the lower platen  64  downward. Further, a plurality of posts are provided in this device other than the above posts  15  and  63 . 
     Next, the operation of the apparatus of FIG. 8 will be described. 
     A lead frame on which semiconductor chips are bonded is set on the lower cavity block  51 . These chips are connected to the lead frame by metal wires. Then, the lower mold  71  is moved upward by the clamping press and put together with the upper mold  70  and then they are clamped. At this time, since the return stopper  62  pushes the return pin  13 , the pin supporting member  20  slightly moves upward against the force of the spring  11 , whereby the eject pins  4  are retracted upward from the upper mold cavity  1   a.    
     Then, the plastic  14  which is preheated is put in the chamber  3  and the plunger  12  is moved downward to apply pressure to the plastic  14 . Then, the plastic  14  passes through the runners  52   a  and  51   b  and then the gate  51   c  and fills the cavities  1   a  and  51   a.    
     In this state, the plastic is left for 60 to 90 seconds to solidify and then the lower mold  71  is moved down to open the plastic molding means  80 . At this time, the return stopper  62  is separated from the return pin  13 , whereby the pin supporting member  20  is moved downward by the force of the spring  11 . Then, the upper mold eject pins  4  push plastic molded products out from the upper mold cavities  1   a.  Thereafter, the lower platen  64  of the clamping press falls and the knockout rod  61  abuts the bottom of the supporting member  30 . Then, the pin supporting member  30  is pushed by the rod  61  and rises relative to the lower surface plate  55 . Then, the eject pins  54  project into the lower cavities  51   a  and push the products out. The plastic molded products are picked up and then go through the process of cutting and bending the leads, completing plastic molded semiconductor devices. 
     In the conventional plastic molding device constituted as described above, the plastic is injected by the plastic injecting mechanism, but sufficient injection pressure does not reach the cavity because of the pressure loss in the runners  52   a  and  51   b  and the gate Sic or the like. Further, since the sectional area of the gate  51   c  is small as compared with that of other passages, such as the runner, the plastic is likely to thermally harden there. When the plastic solidifies in this portion, the injection pressure does not reach the cavity at all. Accordingly, shrinkage deformation or the like occurs on the surface of the plastic molded product molded in the cavity, causing the quality of the package to degrade. Particularly, in an optical semiconductor element for processing an optical signal, which is molded in a transparent plastic, signal light reflects or refracts because of unevenness of the package surface, causing erroneous operation. 
     In order to solve the problem of shrinkage deformation in general plastic molding, there is proposed a device disclosed in Japanese Utility Model Laid-Open Application No. 63-191013 in which a cylinder and a piston are arranged corresponding to a thick portion of the molded product and pressure is applied to the plastic by actuating the piston after the plastic is injected. However, since there are many small cavities in the plastic molding device for a semiconductor element, it is actually impossible to arrange the cylinder and piston for each cavity, so that the problem of shrinkage deformation in plastic molding for the semiconductor element can not be solved. 
     SUMMARY OF THE INVENTION 
     The present invention was made to solve the above problems and it is an object of the present invention to provide a plastic molding device for a semiconductor element preventing shrinkage deformation or the like on a surface of a plastic molded product and thus producing a plastic molded semiconductor element which has a smooth surface. 
     Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     According to a plastic molding device for a semiconductor element in accordance with the present invention, there is provided driving means for projecting an eject pin into a cavity when the upper and lower molds are put together after plastic is injected into the cavity, whereby the plastic, just before solidification in each cavity, is directly pressurized. Thus, the generation of shrinkage deformation due to molding shrinkage of the plastic is prevented, whereby a plastic package having a smooth surface can be obtained. Particularly, in an optical device an optical characteristic of the plastic package can be improved, so that a high quality package can be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view showing a plastic molding device for encapsulating a semiconductor device in accordance with a first embodiment of the present invention; 
     FIG. 2 is a sectional view showing a plastic molding device in accordance with a second embodiment of the present invention; 
     FIG. 3 is a sectional view showing a plastic molding device in accordance with a third embodiment of the present invention; 
     FIG. 4 is a plan view showing the plastic molding device of FIG. 3; 
     FIG. 5 is a side sectional view showing the plastic molding device of FIG. 3; 
     FIG. 6 is a view showing an eject pin which protrudes into a cavity; 
     FIG. 7 is a view showing the positional relation between a metal wire and the eject pin; 
     FIG. 8 is a sectional view showing a conventional plastic molding device for encapsulating a semiconductor element; and 
     FIG. 9 is a plan view showing the surface of the lower mold of the plastic molding device of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a sectional view showing a plastic molding device for encapsulating a semiconductor device in accordance with a first embodiment of the present invention. 
     In FIG. 1, since the same reference numerals as in FIG. 8 designate the same or corresponding parts, description thereof will be omitted. A cylinder  66   b  for pressurization having a piston  66   a  is provided between the base plate  59  and the ejector plate  57  in the lower mold  71 , and a piston rod  66   c  of the piston  66   a  is connected to the ejector plate  57 , whereby pin driving means (a pressurizing cylinder mechanism) for driving the eject pin  54  is provided. Further, the pressurizing cylinder mechanism works just before the plastic  14  in the cavities  1   a  and  51   a  is solidified and also works when the product is ejected. The amount of protrusion of the eject pin  54  just before the plastic is solidified is set at a value B corresponding to the molding shrinkage as shown in FIG.  6 . 
     Further, the pressurizing cylinder mechanism drives the ejector plate  57  upward (toward the cavity side) when working fluid enters the IN side, i.e., enters into a chamber on the lower side of the piston  66   a,  and drives it toward the base plate  59  when the fluid enters the OUT side, i.e., enters into the chamber on the upper side of the piston  66   a.  Therefore, there is not provided the spring  60  which urges the pin supporting member  30  toward the base plate  59 . Still further, according to this embodiment of the present invention, since the product can be ejected by the pressurizing cylinder mechanism, the knockout rod  61  provided in the conventional device is not provided in order to simplify the structure. 
     The operation of this embodiment is as follows. 
     Clamping is performed and then the plastic  14  is pressurized by the plunger  12  to start the injection of the plastic  14 . The injected plastic  14  passes through the runners  51   b  and  52   a  and the gate  51   c  and is injected into the cavities  1   a  and  51   a.  The working fluid is pressurized at the IN side of the cylinder  66   c  to drive the piston  66   a  upward 2 to 5 seconds after the injection of the plastic  14  is completed, that is, in a state where the plastic  14  in the gate  51   c  is slightly solidified and that in the cavities  1   a  and  51   a  is not yet solidified. Then, the ejector plate  57  moves upward and the plastic in the cavities  1   a  and  51   a  is pressurized by the eject pin  54 . Thus, the plastic  14  is solidified without shrinking. 
     Thereafter, the eject pin  54  is slightly retracted to relieve the pressure on the plastic  14 , and the lower mold  71  is moved downward to open the plastic molding means  80 . Then, the ejector plate  57  is driven by the above driving mechanism to extend the eject pin  54  into the cavity  51   a , whereby the plastic molded product is ejected. 
     As described above, according to the first embodiment of the present invention, there is provided a pressurizing cylinder mechanism between the base plate  59  and the ejector plate  57  in the lower mold  71 , whereby the plastic  14  in each cavity  51   a  is pressurized by the eject pin  54  just before it is solidified. Thus, shrinkage deformation due to molding shrinkage of plastic  14  is prevented and a smooth surface package without shrinkage deformation is obtained. Particularly, in a plastic molded optical device, the optical characteristics of the package are improved. 
     In addition, since the pressurizing cylinder mechanism is provided on the lower mold  71  to drive the eject pin  54  on the lower mold cavity block  51 , the eject pin  54  protrudes toward the lower surface of the lead frame  67  where the metal wires  68  are not present. Thus, the metal wires  68  are not deformed and short circuits between an element surface  69  and the metal wires  68  are prevented. Further, since the pressurizing cylinder mechanism is built in the lower mold  71 , a conventional press can be used. 
     Further, although the pressurizing cylinder mechanism drives the eject pin  54  when the product is ejected and also drives the eject pin  54  when the plastic  14  is pressurized in the above first embodiment, the ejection of the product and the pressurization of the plastic  14  can be performed by separate pressurizing mechanisms. 
     Still further, although the pressurizing cylinder mechanism is built in the lower mold  71  in the above first embodiment, the pressurizing cylinder mechanism can be built in the upper mold  70  or it can be built in both upper and lower molds. Alternatively, the pressurizing cylinder mechanism can be arranged on the lower surface of the lower platen  64  of the clamping press. 
     FIG. 2 is a sectional view showing a plastic molding device in accordance with a second embodiment of the present invention. In this device, the pressurizing cylinder mechanism is provided on a lower surface of the lower platen  64  of the clamping press and the cylinder rod  66   c  for pressurization penetrates the lower platen  64  to abut the lower surface of the pin supporting member  30 . The pin supporting member  30  is urged toward the base plate  59  by the spring  60 . Other aspects are the same as in the first embodiment of the present invention. 
     According to this second embodiment, since the pressurizing cylinder mechanism is arranged on the lower surface of the lower platen  64  of the clamping press, the structure of the lower mold  71  can be used without any change. 
     FIGS. 3 to  5  are views showing a plastic molding device in accordance with a third embodiment of the present invention. In this device, the pressurizing cylinder mechanism as a driving source is not used unlike the above embodiments and the plastic  14  just before solidification is pressurized by using elastic deformation of a part forming the lower mold  71 . 
     More specifically, in this third embodiment, a plurality of posts  63  which penetrate the keep plate  56  and the ejector plate  57  of the pin supporting member  30  are provided between the base plate  59  and the lower surface plate  55  to maintain a space A between the spacer block  58  and the lower surface plate  55 . Thus, the lower surface plate  55  is supported on the base plate  59  by these posts  63  only. Further, a controlling means  91  for controlling the clamping force is provided in a clamping press  90 , whereby the clamping force, i.e., the compressive force between the lower platen  64  and the upper platen  65  varies between two stages, i.e., at the time of clamping and at the time of pressurizing the plastic. 
     In this case, an amount of protrusion B of the eject pin  54  is set as follows. 
     The amounts of compressive deformation δ 1  and δ 2  of the post by the first and second clamping force P 1  and P 2  are represented as follows:            δ   1     =         P   1        l     AE       ,       δ   2     =         P   2        l     AE                              
     where P 1  is the first clamping force at the time of clamping, P 2  (&gt;P 1 ) is the second clamping force at the time of pressurization, A is the cross sectional area of the posts, l is the length of the posts, and E is the Young&#39;s modulus of the posts. Then, the amount of protrusion B of the eject pin is represented by the following equation: 
     
       
           B=δ   2 −δ 1 . 
       
     
     FIG. 4 is a plan view of FIG. 3, which shows the arrangement of the post  63 . FIG. 5 is a side sectional view of FIG.  3 . 
     Next, the operation of this embodiment will be described. 
     First clamping is performed by the first clamping force P 1 . At this time, the eject pin  54  is at the position shown by a solid line in FIG.  7 . Then, the plastic  14  is injected into the cavity and second clamping is performed by the second clamping force P 2  just before the solidification of the plastic, whereby the eject pin projects to the position shown by a dotted line in FIG.  7 . Thus, the plastic  14  is pressurized and solidified without any molding shrinkage. After the plastic  14  is solidified, the lower platen  64  is lowered. When the pin supporting member  30  abuts the knockout rod  61 , the pin supporting member  30  rises relative to the lower surface plate  55  to project the eject pin  54 . Then, the product is ejected. 
     According to the third embodiment of the present invention, since the plastic  14  in the cavity  51   a  is pressurized by using the elastic deformation of the post  63  caused by the clamping force, a pressurizing cylinder mechanism as a driving source is not necessary, providing an inexpensive plastic molding device for a semiconductor element. 
     Further, while in the above third embodiment the pressurizing mechanism using elastic deformation is built in the lower mold  71 , this can be built in the upper mold  70  or built in both the lower and upper molds  71  and  70 . 
     As described above, according to the plastic molding device for a semiconductor element of the present invention, there is provided a driving means for projecting the eject pin into the cavity, and the plastic in each cavity just before solidification is directly pressurized when the upper and lower molds are put together after injection of the plastic into the cavity is completed. Thus, shrinkage deformation or the like on a surface of the plastic molded product is prevented, whereby a plastic molded semiconductor element having a smooth surface is obtained. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.