Abstract:
A resin-sealed semiconductor device with built-in heat sink prevents internal bulging and cracking caused by exfoliation of a semiconductor element from the heat sink when the vapor pressure of moisture absorbed into a gap between the semiconductor element and the heat sink rises during mounting of the semiconductor device to a printed circuit board using lead-free solder. By providing a plurality of separated die pads ( 502 ) in a mounting area for a semiconductor element ( 301 ) and adhering the semiconductor element ( 301 ) to the heat sink ( 105 ) via the die pads ( 502 ), space is opened up between the semiconductor element ( 301 ) and the heat sink ( 105 ) for sealing resin ( 304 ) to run into.

Description:
RELATED APPLICATIONS 
     This application is a Divisional of U.S. application Ser. No. 11/194,691, filed Aug. 2, 2005, now abandoned claiming priority of Japanese Application No. 2004-326513, filed Nov. 10, 2004, the entire contents of each of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to a leadframe for use in a semiconductor device with built-in heat sink, and to a resin-sealed semiconductor device that uses the leadframe. 
     2. Related Art 
     In recent years, the miniaturization, high integration and high-density mounting of semiconductor components such as semiconductor devices has been sought in response to the miniaturization of electronic devices. This has resulted in the release of heat generated by semiconductor devices becoming a crucial issue. 
     Semiconductor devices with built-in heat sinks or exposed die pads have been proposed as a countermeasure to the problem of heat release in semiconductor devices. For example, Japanese Patent Application Publication No. 2001-15669 discloses a semiconductor device with built-in heat sink. 
     On the other hand, there is growing concern worldwide for the environment, which has lead to increasingly persistent calls for lead-free manufacturing also in relation to semiconductor devices. 
     However, with a conventional semiconductor device having a built-in heat sink (i.e. two-tiered frame), it is necessary to limit the amount of adhesive used when adhering a semiconductor element to the heat sink, possibly creating a gap between the heat sink and the semiconductor element. Even in the case of the semiconductor device being sealed using sealing resin, the resin does not run into the gap, leaving space for absorbed moisture to gather. 
     The mounting temperature when mounting a resin-sealed semiconductor device with built-in heat sink to a printed circuit board using lead-free solder is higher than for normal lead solder. This increases the temperature within the semiconductor device, which in turn raises the vapor pressure of absorbed moisture and promotes exfoliation of the semiconductor element from the heat sink, giving rise to the danger of internal bulging and cracking. 
     SUMMARY OF INVENTION 
     The present invention aims to provide a resin-sealed semiconductor device in which internal bulging and cracking is prevented even when mounted on a printed circuit board using lead-free solder, and a leadframe for use in the semiconductor device. 
     To resolve the above problems, the present invention is a resin-sealed semiconductor device having a semiconductor element mounted on a heat sink. The semiconductor device includes: a plurality of outer leads for external electrical connection extending at right angles to sides of a rectangle; a plurality of inner leads in series at one end with the outer leads; the heat sink adhered to an underside of an opposite end of the inner leads; a plurality of substantially square openings provided in the heat sink so as to lie partially outside a mounting area for the semiconductor element and with sides thereof positioned at an angle to a direction in which the outer leads extend; the semiconductor element adhered by adhesive to an upper surface of the heat sink in an area sandwiched by the openings; a plurality of metallic wires electrically connecting the inner leads with corresponding electrode pads of the semiconductor element; and a sealing resin that seals the inner leads, the semiconductor element and the metallic wires, with the outer leads left exposed. 
     According to the above structure, even if there is a gap between the semiconductor element and the heat sink, sealing resin runs into the gap via the openings. Exfoliation of the semiconductor element is thus prevented even in the case of lead-free solder being used to mount the semiconductor device to a printed circuit board, as is bulging or cracking of the sealing resin. 
     Note that due to the improved leadframe, semiconductor manufacturing processes using conventional lead-solder may be directly applied as manufacturing processes using lead-free solder. 
     Here, the sides of the openings may be positioned at an approximately 45° angle, and the semiconductor element may be adhered to the heat sink via a plurality of die pads. 
     According to this structure, the formation of a gap is prevented because of the semiconductor element being securely adhered via a plurality of die pads. 
     Here, the resin-sealed semiconductor device may further include a loop-shaped body encircled by the ends of the inner leads and surrounding the mounting area, the loop-shaped body being adhered to the upper surface of the heat sink via an underside thereof and having an inward protrusion positioned centrally on each side thereof. 
     According to this structure, warping of the heat sink when adhering the heat sink to the underside of the ends of the inner leads during the manufacture of the leadframe is prevented, enabling the shape of the leadframe to be stabilized. 
     The present invention is also a leadframe for use in a resin-sealed semiconductor device having a semiconductor element mounted on a heat sink. The leadframe includes: a plurality of outer leads for external electrical connection extending at right angles to sides of a rectangle; a plurality of inner leads in series at one end with the outer leads and electrically connected to electrode pads of the semiconductor element via connecting members; the heat sink adhered to an underside of an opposite end of the inner leads; a plurality of substantially square die pads adhered to the heat sink in a mounting area for the semiconductor element so that sides thereof are positioned at an angle to a direction in which the outer leads extend; and a plurality of openings provided in the heat sink so as to form a checkered pattern with the die pads. 
     According to this structure, the semiconductor element can be securely adhered to the die pads of the leadframe with a small amount of adhesive. Moreover, the formation of a gap between the semiconductor element and the heat sink when manufacturing a resin-sealed semiconductor device that uses the leadframe is prevented because of sufficient sealing resin running between the semiconductor element and the heat sink via the openings. Thus, even in the case of lead-free solder (i.e. requires higher temperature than for normal lead solder) being used to mount the resin-sealed semiconductor device, bulging and cracking of the sealing resin due to the semiconductor element exfoliating can be prevented. 
     Here, the sides of the die pads may be positioned at an approximately 45° angle, the openings may have rounded vertices, and each opening positioned circumferentially may lie partially outside the mounting area. 
     According to this structure, sealing resin runs between the semiconductor element and the heat sink via openings positioned partially on the outside of the mounting area for the semiconductor element, enabling the formation of a gap to be securely prevented. Moreover, rounding the vertices of the openings allows the concentration of local stress to be alleviated. 
     The present invention is also a method of manufacturing a leadframe for use in a resin-sealed semiconductor device having a semiconductor element mounted on a heat sink. The method includes the steps of: etching or stamping a piece of sheet metal to manufacture a metallic member that includes outer leads for external electrical connection extending at right angles to sides of a rectangle, inner leads in series at one end with the outer leads, substantially square die pads positioned with sides thereof at an angle to a direction in which the outer leads extend, a coupling ring coupling together the die pads, a dambar coupling the inner leads to the outer leads in side directions of the rectangle, and hanging leads holding the die pads from vertices of the dambar; adhering an upper surface of the heat sink to an underside of an opposite end of the inner leads and an underside of the die pads; and providing a plurality of openings in the heat sink to form a checkered pattern with the die pads and at the same time sectioning the coupling ring and the hanging leads. 
     According to this structure, the die pads can be separated at the same time as the openings are formed, which avoids complicating the processes and allows for an improved leadframe. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, advantages, and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings, which illustrate specific embodiments of the present invention. 
       In the drawings: 
         FIG. 1  is a plan view of an embodiment 1 of a leadframe pertaining to the present invention; 
         FIG. 2  is a bottom view of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view cutting  FIG. 1  at S-S′ of a resin-sealed semiconductor device that uses the leadframe of embodiment 1; 
         FIGS. 4A-4F  are cross-sectional views of processes for manufacturing the resin-sealed semiconductor device of embodiment 1; 
         FIG. 5  is a plan view of an embodiment 2 of a leadframe with die pads pertaining to the present invention; 
         FIG. 6  is a bottom view of  FIG. 5 ; 
         FIG. 7  is a plan view of the leadframe shown in  FIG. 5  prior to openings being formed; 
         FIG. 8  is a bottom view of  FIG. 7 ; 
         FIG. 9  is a cross-sectional view cutting  FIG. 5  at S-S′ of a resin-sealed semiconductor device that uses the leadframe with die pads of embodiment 2; 
         FIGS. 10A-10F  are cross-sectional views of processes for manufacturing the resin-sealed semiconductor device of embodiment 2; 
         FIG. 11  is a plan view of an embodiment 3 of a leadframe with loop-shaped body pertaining to the present invention; 
         FIG. 12  is a bottom view of  FIG. 11 ; 
         FIG. 13  is a cross-sectional view cutting  FIG. 11  at S-S′ of a resin-sealed semiconductor device that uses the leadframe with loop-shaped body of embodiment 3; 
         FIGS. 14A-14F  are cross-sectional views of processes for manufacturing the resin-sealed semiconductor device of embodiment 3; 
         FIG. 15  is a cross-sectional view schematically showing the flow of sealing resin in embodiment 2; 
         FIG. 16  is a cross-sectional view schematically showing the flow of sealing resin in embodiment 1; 
         FIG. 17  is a plan view of a conventional leadframe used in a comparative example for verifying the occurrence of cracking etc. in the resin-sealed semiconductor device of embodiment 2; 
         FIG. 18  is a bottom view of  FIG. 17 ; and 
         FIG. 19  is a cross-sectional view of a resin-sealed semiconductor device that uses the leadframe shown in  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of a leadframe and a resin-sealed semiconductor device that uses the leadframe pertaining to the present invention are described below with reference to the drawings. 
     Embodiment 1 
       FIG. 1  is a plan view of an embodiment 1 of a leadframe pertaining to the present invention, while  FIG. 2  shows a bottom view of the same. 
     A leadframe  101  includes a rectangular frame  102 , a plurality of outer leads  103  extending at right angles to the four sides of frame  102 , a plurality of inner leads  104  that are in series at one end with the outer leads and extend toward the inside of frame  102 , and a heat sink  105  adhered to the underside of the opposite ends of inner leads  104 . A plurality of substantially square openings  106  with rounded vertices are formed at an approximately 45° angle to the direction in which the outer leads extend. The openings are positioned partially outside a mounting area  107  for the semiconductor element marked by the chain line. The border area between outer leads  103  and inner leads  104  is coupled in the four side directions of frame  102  by a dambar  108 . An area  109  marked by the chain line on the inside of dambar  108  indicates the area to be covered by sealing resin in a resin-sealed semiconductor device that uses leadframe  101 . 
     The components of leadframe  101  apart from heat sink  105  are obtained by processing sheet metal made from copper alloy having a thickness of 0.15 mm and a hardness of 150 to 185 Hv, for example, using an etching or stamping technique. Heat sink  105  is a copper alloy sheet having a thickness of 0.13 mm, for example. Heat sink  105  is thermally adhered to the underside of the ends of inner leads  104  with adhesive.  FIG. 3  is a cross-sectional view cutting  FIG. 1  at S-S′ of a resin-sealed semiconductor device that uses leadframe  101 . 
     A semiconductor element  301  is adhered to heat sink  105  in mounting area  107  thereof using a die bond  302  such as silver paste, for example. Electrode pads of semiconductor element  301  are connected to corresponding inner leads  104  by wires (e.g. metallic wires). The semiconductor device is resin sealed using a sealing resin  304  made from an epoxy resin, for example, with outer leads  103  left exposed. 
     The processes for manufacturing a resin-sealed semiconductor device that uses leadframe  101  of the present embodiment are described next with reference to  FIGS. 4A to 4F .  FIGS. 4A to 4F  are cross-sectional views cutting  FIG. 1  at S-S′. 
       FIGS. 4A and 4B  show the manufacturing processes for leadframe  101 . 
       FIG. 4A  shows a process of using adhesive to adhere a metallic member (i.e. processed copper alloy sheet metal) that includes outer leads  103  and inner leads  104  etc. coupled together by frame  102  to the upper surface of heat sink  105  via the underside of the ends of inner leads  104 . Note that the cross-sectional views omit extraneous items to simplify the diagrams. The same approach is taken below. 
       FIG. 4B  shows a process of forming openings  106  by punch processing heat sink  105  adhered to the metallic member. This completes the manufacture of leadframe  101 . 
       FIG. 4C  shows a process of adhering semiconductor element  301  to leadframe  101 . Die bond  302  is applied at points  110  on heat sink  105  marked by the dotted lines in  FIG. 1 , and semiconductor element  301  is placed over these points and adhered thereto. 
       FIG. 4D  shows a wire bonding process. The electrode pads of semiconductor element  301  are connected to the tip of corresponding inners lead  104  by wires  303 . 
       FIG. 4E  shows a resin sealing process. A mold is disposed so as to cover leadframe  101  and wires  303  while leaving outer leads  103  exposed, and a sealing resin made from epoxy resin is injected at a mold temperature of 180° C. The injection time is set to 8 seconds, for example. The mold is removed after cooling. 
       FIG. 4F  shows a process of bending outer leads  103 . Frame  102  of leadframe  101  is separated using a tie bar cut and outer leads  103  are bent to complete the manufacture of the resin-sealed semiconductor device. 
     In the present embodiment, sealing resin injected via openings  106  which extend beyond mounting area  107  runs into the space between semiconductor element  301  and heat sink  105 , enhancing the adhesion of semiconductor element  301  with heat sink  105 . The formation of a gap between semiconductor element  301  and heat sink  105  can thus be prevented. 
     Embodiment 2 
     An embodiment 2 of a leadframe with die pads and a resin-sealed semiconductor device that uses the leadframe pertaining to the present invention is described next. The following description relates only to the features of the present embodiment, with description of those parts similar to embodiment 1 having been omitted. 
       FIG. 5  is a plan view of a leadframe with die pads, while  FIG. 6  is a bottom view of the same. A plurality of die pads  502  is provided in mounting area  107  of a leadframe  501  so as to form an oblique checkered-pattern with openings  106 . Die pads  502  are substantially square in shape, and as with openings  106  the sides of the die pads are angled at approximately 45 degrees to the direction in which outer leads  103  extend. Those openings  106  positioned circumferentially lie partially outside mounting area  107 . 
     Since openings  106  and die pads  502  surrounded by openings  106  form a checkered pattern and are angled at approximately 45 degrees to the direction in which outer leads  103  extend, the area of die pads  502  occupying mounting area  107  is ideal for applying the adhesive to adhere semiconductor element  301 . 
     Hanging leads  503  protrude toward a central point from the vertices of dambar  108  of leadframe  501 . 
     Heat sink  105  is adhered to the underside of both die pads  502  and the ends of inner leads  104  using adhesive. 
       FIG. 7  is a plan view of the leadframe with die pads shown in  FIGS. 5 and 6  prior to openings  106  being formed, while  FIG. 8  is a bottom view of the same. 
     Die pads  502  are coupled together by a coupling ring  701 , which is connected to dambar  108  via hanging leads  503 . The underside of die pads  502 , coupling ring  701  and hanging leads  503  are adhered to the upper surface of heat sink  105  using adhesive. 
     Note that leadframe  501  excluding heat sink  105  is manufactured, similar to embodiment 1, by one-piece molding copper alloy sheet metal using a stamping technique, for example. 
       FIG. 9  is a cross-sectional view cutting  FIG. 5  at S-S′ of a resin-sealed semiconductor device that uses the leadframe with die pads. 
     With this resin-sealed semiconductor device, die pads  502  are interposed between and adhered with adhesive to both semiconductor element  301  and the upper surface of heat sink  105 . The formation of a gap between semiconductor element  301  and heat sink  105  can thus be minimized since this expands the space between semiconductor element  301  and heat sink  105  and facilitates the flow of sealing resin  304  via openings  106 . 
     The processes for manufacturing a resin-sealed semiconductor device that uses leadframe  501  of the present embodiment are described next with reference to  FIGS. 10A to 10F . 
     Note that  FIGS. 10A to 10F  are cross-sectional views cutting  FIG. 5  at S-S′, and that extraneous items have been omitted to simplify the diagrams. Note also that description of the processes shown in  FIGS. 10D and 10F  have been omitted given the substantial similarities with processes shown in  FIG. 4  of embodiment 1. 
       FIG. 10A  shows a process of using adhesive to adhere a metallic member (i.e. processed copper alloy sheet metal) consisting of outer leads  103 , inner leads  104  and die pads  502  etc. coupled together by frame  102  to the upper surface of heat sink  105  via the underside of both die pads  502  and the ends of inner leads  104 . 
       FIG. 10B  shows a process of forming openings  106  by punch processing heat sink  105  adhered to the metallic member and at the same time removing coupling ring  701  and part of hanging leads  503  to separate die pads  502 . Openings  106 , disposed so as to form a checkered pattern with die pads  502 , are substantially square in shape and positioned at an approximately 45° angle to the sides of the rectangular form of leadframe  501 . Also, those openings  106  positioned circumferentially lie partially outside mounting area  107 . 
       FIG. 10C  shows a process of adhering semiconductor element  301  to leadframe  501 . Die bond  302  is applied to the upper surface of die pads  502 , and semiconductor element  301  is placed over the die pads and adhered thereto. 
       FIG. 10D to 10F  are similar to embodiment 1. 
     Note that the injection of sealing resin shown in  FIG. 10E  is made easier than in embodiment 1 as a result of the space between semiconductor element  301  and heat sink  105 . 
     Embodiment 3 
     An embodiment 3 of a leadframe with loop-shaped body and a resin-sealed semiconductor device that uses the leadframe pertaining to the present invention is described next. The following description relates only to the features of the present embodiment, with description of those parts similar to embodiment 1 having been omitted. 
       FIG. 11  is a plan view of a leadframe with loop-shaped body, while  FIG. 12  is a bottom view of the same. Leadframe  1100  consists of the addition of a loop-shaped body  1101  that surrounds mounting area  107  for semiconductor element  301  to leadframe  101  of embodiment 1. Cap-shaped protrusions  1103  that protrude toward semiconductor element  301  are formed centrally on sides of loop-shaped body  1101 . Hanging leads  1102  that connect loop-shaped body  1101  to dambar  108  are formed at the corners of loop-shaped body  1101 . Note that apart from heat sink  105 , leadframe  1100  is integrally formed from sheet metal. 
       FIG. 13  is a cross-sectional view of a resin-sealed semiconductor device that uses leadframe  1100 . This cross sectional view cuts  FIG. 11  at S-S′. 
     Loop-shaped body  1101  is disposed on the inside of inner leads  104  so as to surround semiconductor element  301 . A cross-section of protrusions  1103  is shown in  FIG. 13 . 
       FIGS. 14A to 14F  are cross-sectional views illustrating processes for manufacturing leadframe  1100  and a resin-sealed semiconductor device that uses leadframe  1100 . Note that these cross-sectional views cut  FIG. 11  at S-S′ with extraneous items having been omitted. 
       FIG. 14A  shows a process of adhering heat sink  105  to the metallic member. 
     Adhesive is applied to the underside of both loop-shaped body  1101  and the ends of inner leads  104 , and the upper surface of heat sink  105  is adhered thereto by the adhesive. 
     The provision of loop-shaped body  1101  connected to hanging leads  1102  on the inside of inner leads  104  in this process prevents the adhesion of heat sink  105  in a warped state, with any heat-related deformation being absorbed by protrusions  1103  provided centrally on the sides of loop-shaped body  1101 . 
     Description of the processes in  FIGS. 14B to 14D  is omitted given the substantial similarities with processes in embodiment 1. 
     The presence of loop-shaped body  1101  in the resin sealing process in  FIG. 14E  prevents sealing resin  304  from impacting on semiconductor element  301  when injected. 
     The bending process in  FIG. 14F  is substantially similar to embodiment 1. 
     Resin-sealed semiconductor devices that use the leadframes of the preferred embodiments have been described above, although the present invention can naturally be implemented through combining the features of these leadframes. 
     An exemplary leadframe incorporates the die pads described in embodiment 2 with the loop-shaped body described in embodiment 3. 
     The flow of the sealing resin in the resin sealing process of embodiments 1 and 2 is described next using  FIGS. 15 and 16 . 
       FIG. 15  shows semiconductor element  301  adhered to heat sink  105  via die pads  502 , while  FIG. 16  shows semiconductor element  301  adhered directly to heat sink  105 . Semiconductor element  301  is adhered using die bond  302  in both cases, although when adhered to heat sink  105  via die pads  502  as in embodiment 2, a space equivalent to the thickness of die pads  502  is opened up between semiconductor element  301  and heat sink  105 . As shown by arrow A 501 , this enables the sealing resin to flow sufficiently into the space between semiconductor element  301  and heat sink  105  via openings  106 . Even with embodiment 1, the flow of sealing resin between semiconductor element  301  and heat sink  105  via openings  106  is as shown by arrow  1601 , preventing the formation of a gap. 
     COMPARATIVE EXAMPLE 
     The following illustrates the results of comparative tests that assumed the use of lead-free solder in mounting a resin-sealed semiconductor device (“present device”) of the present invention manufactured according to embodiment 2 and a conventional resin-sealed semiconductor device (“conventional device”). 
     A plan view of a conventional leadframe is shown in  FIG. 17 , while a bottom view of the same is shown in  FIG. 18 . With this leadframe, a single opening  1701  is provided in heat sink  105  below mounting area  107 .  FIG. 19  is a cross-sectional view cutting  FIG. 17  at S-S′ of a conventional resin-sealed semiconductor device that uses this leadframe. 
     The present and conventional devices were firstly baked for 12 hours at 125° C. and dried, before being placed in an atmosphere having a temperature of 30° C. and a relative humidity of 70% for 72 hours to absorb moisture. Then, after having been subjected to a temperature of 265° C. for five minutes, the devices were again placed in an atmosphere having a temperature of 30° C. and a relative humidity of 70% for 96 hours. After again being subjected to a temperature of 265° C. for five minutes, the devices were cooled to room temperature and then supersonic waves were used to investigate for exfoliation and cracking. 
     15 samples of each device were used. 
     The number of samples in which exfoliation or cracking occurred is shown in the following table. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Comparative Results 
                   
               
               
                   
                   
                 Exfoliation 
                 Cracking 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Present Device 
                 0 
                 0 
               
               
                   
                 Conventional Device 
                 15 
                 7 
               
               
                   
                   
               
             
          
         
       
     
     These results confirm that even when lead-free solder is used to mount the resin-sealed semiconductor device described in embodiment 2 on a substrate, exfoliation of the semiconductor element and cracking of the sealing resin is completely prevented. 
     Note that while only a single leadframe is illustrated in the preferred embodiments, it is naturally possible to manufacture resin-sealed semiconductor devices by adhering and wire bonding semiconductor elements to a plurality of leadframes formed in series (above/below or to the left/right etc. of one another), and covering the leadframes with a mold and injecting sealing resin, before finally separating the individual semiconductor devices. 
     Resin-sealed semiconductor devices using leadframes pertaining to the present invention are for use as environmentally friendly semiconductor devices in the field of semiconductor manufacturing. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.