Abstract:
In construction of a plastic molded semiconductor package provided with a heat sink and electrically conductive leads attached to the periphery of the heat sink, a conventional electrically insulating, adhesive bonding tape is replaced by a non-adhesive insulating sheet and support bars are locally provided in a lead frame between the leads. For uniting the leads to the heat sink, the support bars are fixed to the heat sink whilst sandwiching the insulating sheet. Elimination of the adhesive bonding tape greatly reduces intrusion of contaminants into the package, simplifies the production process and lowers the production cost.

Description:
This is a Divisional of application Ser. No. 08/884,504 filed Jun. 27, 1997, now U.S. Pat. No. 6,020,649. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for producing a plastic molded semiconductor package, and more particularly relates to improvements in operational reliability and production cost of a plastic molded semiconductor package provided with a heat sink. 
     In the case of a semiconductor package in which a semiconductor die provided with an integrated circuit is encapsulated in a plastic package, a metallic heat sink is incorporated in the plastic package in order to promote effective dissipation of heat from the semiconductor die. For example, Japanese Patent Opening Hei. 6-5746 discloses a semiconductor package provided with a heat sink partially exposed outside a plastic package. The heat sink supports on its first surface a semiconductor die at a location sufficiently distant from the outer surface of the plastic package, and exposes its second surface opposite to the first surface outside the plastic package. A plurality of electrically conductive leads are mounted onto the first surface in an arrangement juxtaposed along the outer periphery of the heat sink, and these leads are united to the heat sink via an electrically insulating bonding tape. During production of the semiconductor package, the heat sink is bonded to the plurality of leads in a lead frame via the electrically insulating bonding tape. 
     In the case of the semiconductor package of this prior art, however, use of the electrically insulating bonding tape for uniting of the leads with the heat sink is accompanied with various problems as set out below. 
     First, relatively high degree of humidity absorption of by bonding agents generally used for the bonding tape tends to allow easy intrusion of outside contaminants into the plastic package, thereby seriously lowering operational reliability of the semiconductor package. 
     Second, thermal hardening of the bonding agent and dry cleaning of the bonding areas of the leads are additionally necessary in production of the semiconductor package. Generally, the thermal hardening is performed after uniting the heat sink to the lead frame. During this thermal hardening, gas is generated from the bonding agent and adheres as a contaminant to the surfaces of the leads for connection with the bonding wires. The contaminant tends to cause defective connection of the bond wires. In order to avoid such troubles, it is necessary to perform the above-described dry cleaning of the bonding areas of the leads. 
     Third, connection of the bonding wires tends to cause thermal softening of the bonding agent. Soft bonding agent allows sinking of the leads into the layer of the bonding agent during the process of connection, which causes defective connection of the bonding wires. 
     Fourth, the electrically insulating bonding tape is relatively expensive and, as a consequence, its use induces rise in production cost. 
     SUMMARY OF THE INVENTION 
     It is thus the basic object of the present invention to avoid use of bonding tapes for connection of a lead frame to a heat sink, thereby enhancing operational reliability and production cost of a semiconductor package. 
     In accordance with the basic aspect of the present invention, a non-adhesive, electrically insulating sheet is interposed between inner end sections of leads and a lead supporting surface of a heat sink. Because the insulating sheet has no function of bonding, several support bars are provided locally between the juxtaposed leads in the construction of the lead frame and such support bars are united with the heat sink whilst clamping the insulating sheet. In production of the semiconductor package in accordance with the present invention, the heat sink is supported by the lead frame via the support bars. Since no bonding tape of high humidity absorption is used, undesirable intrusion of contaminants into the plastic package can be well prevented to enhance operational reliability of the semiconductor package. Because no thermal hardening of bonding agent is necessary, contamination of the bonding area of the leads and resultant defective connection can be well prevented. The additional dry cleaning needs not to be performed. Because no bonding agent is present between the lead and the heat sink, no undesirable sinking of the leads at connection of the bonding wires starts and, as a consequence, defective connection of the bond wires can be well avoided. Further, no use of the expensive bonding tapes significantly lowers the production cost of the semiconductor package. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view, partly cut off for better understanding, of one embodiment of the semiconductor package in accordance with the present invention, 
     FIG. 2 is a perspective view of a lead frame, an electrically insulating sheet and a heat sink before assembly, 
     FIG. 3 is a top plan view of a lead frame assembly partly cut off for better understanding, 
     FIG. 4 is a sectional side view taken along a line IV—IV in FIG. 3, 
     FIG. 5 is a sectional side view taken along a line V—V in FIG. 3, 
     FIG. 6 is a sectional side view of a semiconductor die mounted to a lead frame and connected with bond wires, 
     FIG. 7 is a sectional side view of molding process of encapsulating plastic, 
     FIG. 8 is a sectional side view of another formation of the lead frame, and 
     FIG. 9 is a sectional side view of a lead frame assembly provided with a different type of heat sink. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the semiconductor package in accordance with the present invention is shown in FIG. 1, in which a semiconductor package  1  is provided with a semiconductor die  2  bonded onto a heat sink  4  via a thermally conductive adhesive  3 . A plurality of leads  5  and several support bars  6  are Juxtaposed around the heat sink  4  whilst extending outwards. Although the heat sink  4  is provided with the leads  5  on its four sides in the case of the illustrated arrangement, the leads  5  may be provided on  3  or less sides too. The leads  5  and the support bars  6  are coupled to the heat sink  4  via an electrically insulating sheet  7 . The support bars  6  are arranged on the four corners of the heat sink  4  so that their inner ends are fixed onto the heat sink  4  to clamp the insulating sheet  7 . Electrically conductive bond wires  8  are used to connect the inner ends of the lead  5  to selected group of contact pads  9  arranged on the semiconductor die  2 . Encapsulating plastic  10  covers the heat sink  4 , the semiconductor die  2 , bond wires  8 , the inner end sections of the leads  5  and the support bars  6 . Though not illustrated in the drawing, the surface of the heat sink  4  opposite to the semiconductor die  2  is exposed outside the encapsulating plastic  10 . 
     The semiconductor package  1  shown in FIG. 1 is produced for example in the following manner. FIG. 2 depicts a lead frame  11 , a insulating sheet  7  and a heat sink  4  to be assembled together in the later phase of production. The lead frame  11  includes a plurality of leads  5  and several support bars  6  connected to each other at outer ends by an outer gallery  12 . A through hole  13  is formed in the inner end of each support bar  6 . The lead frame  11  is made of various metals which are generally known in the field of art as materials for making lead frames. The lead frame  11  is flat in configuration and formed by die cutting or etching from a metallic plate. Although a square outer gallery  12  and a square center opening  14  are employed in the case of the illustrated embodiment, any configuration can be adopted in accordance with the design and size of the heat sink to be united to the lead frame. 
     The insulating sheet  7  is formed by die cutting from a synthetic resin sheet of electrically insulating nature. The insulating sheet  7  is given in the form of a square gallery having a center opening  15  somewhat smaller than the center opening  14  in the lead frame  11 . Through holes  16  are formed in the four corners of the insulating sheet  7 . Because the insulating sheet  7  of the present invention is not covered with adhesive, its production cost is relatively low. 
     The heat sink  4  is made of a relatively thin plate of a metallic material of high thermal conductivity such as aluminum via press shaping. The heat sink  4  is substantially square in shape and provided with the first surface  17  and an opposite second surface  18 . The first surface  17  is provided with central die receptive surface  20  and a lead receptive surface  19  surrounding the die receptive surface  20 . In the case of the illustrated example, the die receptive surface  20  is given in the form of a recess parallel to the plane of the lead receptive surface  19 . However, the die receptive surface  20  may be coplanar with the lead receptive surface  19 . The die receptive surface  20  is square in shape somewhat smaller than the center opening  15  in the insulating sheet  7 . A surface  22  of the heat sink  4  opposite to the die receptive surface  20  is exposed outside the encapsulating plastic  10  of FIG. 1 for dissipation of heat generated at the heat sink  4 . 
     The heat sink  4  provided with such an exposed surface can be produced from a thin plate material which can be stocked in the form of a large roll. Such a mode of stock is well suited for continuous processing. Supply of the material in the form of a thin plate assures very easy shaping. The configuration of the heat sink  4  provides a long boundary with the encapsulating plastic  10 , which well resists against undesirable intrusion of outside contaminants along the boundary. Any configuration may be, however, designed quite freely for the heat sink  4 . At each corner of the heat sink  4 , the lead receptive surface  19  is provided with a projection  23 . The projections  23  are arranged to correspond in position to the through holes  13  in the support bars  6  and the through holes  16  in the insulating sheet  7 . 
     In the arrangements shown in FIGS. 3 to  5 , the lead frame  11 , the insulating sheet  7  and the heat sink  4  are combined in one body together to form a lead frame assembly  24 . The insulating sheet  7  is placed on the lead receptive surface  19  of the heat sink  4  and the projections  23  are put into engagement with the through holes  16 . 
     The inner end sections of the leads  5  and the support bars  6  are placed on the insulating sheet  7  and the projections  23  of the heat sink  4  are inserted into the through holes  13  in the support bars  6 . Heads of the projections  23  are crashed to unite the lead frame  11 , the insulating sheet  7  and the heat sink  4  together. In this united position, the heat sink  4  is supported by the lead frame  11  via the support bars  6 . Because the heat sink  4  is made of a thin metallic plate and light in weight, there is little danger of support bar deformation. 
     Now, in the arrangement of the lead frame assembly  24  shown in FIG. 6, the semiconductor die  2  is placed on the die receptive surface  20  of the heat sink  4 . The semiconductor die  2  is provided with the first surface  25  and the second surface  26  opposite to the first surface  25 . A plurality of electrically conductive contact pads  9  are arranged on the first surface  25  of the semiconductor die  2  whereas the semiconductor die  2  is bonded at the second surface  26  to the die receptive surface  20  of the heat sink  4  via conductive adhesive  3 . Each of a plurality of electrically conductive bond wires  8  is connected at one end to one of the contact pads  9  on the semiconductor die  2  and, at the other end, to one of the leads  5  on the lead frame  11 . Because the leads  5  are directly supported on the insulating sheet  7  of low elastic nature without any adhesive, they move very little during connection of the bond wires  8 , thereby assuring reliable and smooth connection. 
     Next, referring to FIG. 7, the lead frame assembly  24  bearing the semiconductor die  2  is placed in position in a mold cavity  30  of a mold unit  29  made up of mold halves  27  and  28 . After closing the mold halves  27  and  28 , the encapsulating plastic  10  is filled into the mold cavity  30 . When the mold halves  27 ,  28  are closed, the exposed surface  22  of the heat sink  4  is put into pressure contact with the wall of the mold cavity  30 . By filling of the encapsulating plastic  10 , the heat sink  4 , the semiconductor die  2 , the inner end sections of the leads  5 , insulating sheet  7  and bond wires  8  are covered with the encapsulating plastic  10  whereas the surface  22  of the heat sink  4 , the outer end sections of the leads  5  and the outer end sections of the support bars  6  are exposed outside the encapsulating plastic  10 . After solidification by cooling, the mold unit  29  is made open and the semiconductor package  1  is taken out of the mold unit  29 . Thereafter, the outer gallery  12  (see FIGS. 2-3) of the lead frame  11  is cut and removed to separate the leads  5  and the support bars  6  (see FIGS. 1-3) from each other. The cut support bars  6  do not project outside the encapsulating plastic  10 . When necessary, the outer ends of the leads  5  may be bent properly outside the encapsulating plastic  10 . 
     Another embodiment of combination of the lead frame  11 , the insulating sheet  7  and the heat sink  4  is shown in FIG. 8, in which through holes  31  are formed in the heat sink  4  at positions corresponding to the through holes  13  in the lead frame  11  and the through holes  16  in the insulating sheet  7  as a substitute for the projections and a rivet  32  is inserted into each group of the through holes  13 ,  16  and  31 . Thereafter the heads of the rivet  32  are crashed to unite the lead frame  11 , the insulating sheet  7  and the heat sink  4  together. 
     In the case of the embodiment shown in FIG. 9, the die receptive surface  20  is formed coplanar with the lead receptive surface  19  in the construction of the heat sink  4 . The above-described depressed configuration of the die receptive surface  20  limits the size of the semiconductor die  2  to be placed thereon and, as a consequence, the heat sink  4  needs to be shaped in accordance with the size of the semiconductor die  2  to be combined therewith. The coplanar configuration of the both surfaces  19  and  20  broadens size range of the semiconductor die  2  to be combined. Stated otherwise, the coplanar configuration of this embodiment can span a wide range of semiconductor die size. The opposite surface  22  in this embodiment is somewhat bulges to provide an elongated boundary with the encapsulating plastic  10 , thereby well resisting against undesirable intrusion of outer contaminants along the boundary. 
     In accordance with the present invention, non-adhesive electrically insulating sheet is interposed between the inner end sections of the leads and the lead receptive surface of the heat sink. The support bars are locally provided in the lead frame between juxtaposed leads, the heat sink and the insulating sheet are united together via the support bars. In the production process in accordance with the present invention, the heat sink is supported by the lead frame via the support bars. Since no bonding tape of relatively high humidity absorption is used for combination of the elements, intrusion of contaminant associated with humidity absorption can be prevented very effectively, thereby raising operational reliability of the semiconductor package. Because no thermal solidification of adhesive is necessary in production, ill lead connection to be otherwise caused by contamination of the lead bonding are can be evaded. It is no longer necessary to employ additional dry cleaning process. Since no adhesive is present between the leads and the heat sink, the leads can be stable held during connection of the bond wires thereto and, as a consequence, defective connection between the two elements can be prevented. The production cost can be much reduced by elimination of expensive bonding tapes.