Abstract:
A method of making a stacked semiconductor package having at least a leadframe, a first die mounted above and soldered to the lead frame and a first clip mounted above and soldered to the first die. The method includes positioning the leadframe, first die and first clip in a vertically stacked relationship and nonsolderingly locking the first clip in laterally nondisplaceble relationship with the leadframe. A stacked semiconductor package and an intermediate product produced in making a stacked semiconductor package are also disclosed.

Description:
BACKGROUND 
       [0001]    Multi-chip modules (MCM&#39;s) are integrated circuit packages in which multiple semiconductor dies are packaged on the same substrate. MCM&#39;s traditionally have dies mounted side by side on a substrate. However, more recently MCM&#39;s have been developed in which semiconductor dies are stacked vertically. Such vertically stacked dies have a smaller footprint than conventional MCM&#39;s and are often used in applications such as cell phones and tablet computers in which chip space is at a premium. The stacked dies are typically encapsulated in protective epoxy, such as by transfer molding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]      FIG. 1  is a perspective view of a stacked semiconductor package subassembly. 
           [0003]      FIG. 2  is a cross sectional elevation view of the stacked semiconductor package subassembly of  FIG. 1 . 
           [0004]      FIG. 3  is a cross sectional view of a portion of the stacked semiconductor package subassembly of  FIG. 1 . 
           [0005]      FIG. 4  is a perspective view of the stacked semiconductor package subassembly of  FIG. 1  after reflow heating, wire bond connection, encapsulation and singulation thereof (shown partially in phantom). 
           [0006]      FIG. 5  is a perspective view of a stacked semiconductor package, fully encapsulated and singulated, but showing only a portion of the encapsulation layer in dashed lines. 
           [0007]      FIG. 6  is a perspective view of a stacked semiconductor package, fully encapsulated and singulated, which shows the entire encapsulation layer. 
           [0008]      FIG. 7  is a flow chart of a method of making a stacked semiconductor package. 
           [0009]      FIG. 8  is a perspective view of a semiconductor die that forms a part of the stacked semiconductor package of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    In conventional flat MCM&#39;s, dies are initially positioned side by side and held in position on a substrate by an underlying layer of solder paste. The solder paste is sufficiently adhesive to prevent the dies from moving when the assembly is transferred to a reflow oven. The leadframe and dies are then heated in the reflow oven which causes the solder paste to liquefy and bond to the surfaces of the substrate and dies. When the molten solder cools, the dies and substrate are firmly attached to each other. 
         [0011]    Applicants have discovered a problem in the production of stacked semiconductor packages in which multiple stacked dies are connected to a leadframe by clips and in which the dies and clips are connected with solder. To produce such stacked die packages, the dies and clips are initially stacked on top one another on a leadframe with a layer of solder paste positioned between the first die and leadframe and between each die and clip. Probably due to the height of the die/clip stack, the adhesive property of the solder paste is not sufficient to prevent lateral displacement of the dies and clips relative to the leadframe and to one another. This relative lateral displacement typically occurs when the leadframe/die/clip assembly is moved to a reflow oven. Such relative lateral movement between leadframe dies and clips often causes defects in the resulting stacked die package. Applicants have also discovered that such lateral displacement and associated defects can be prevented by a mechanical locking feature. 
         [0012]      FIGS. 1-6  and  8 , in general, show various features of a stacked semiconductor package  110  and show how it is produced. As best shown in  FIG. 5 , the stacked semiconductor package  110  may include a leadframe  12  and a first die  30  that is stacked on the leadframe  12  and attached to the leadframe  12  by a first solder layer  81 A. The package  110  may also include a first clip  40  that is stacked on the first die  30  and attached to the first die with a second layer of solder  82 A. The first clip  40  is mechanically locked against lateral displacement with respect to the leadframe  12  by structure other than solder. The semiconductor package  110  may include other die(s) and clip(s) stacked on top the first die  30  and clip  40  in which the clip(s) is/are each locked in place by a nonsolder structure as well as by a layer of solder. The stacked semiconductor package  110  may also include an encapsulation layer  100 . 
         [0013]    In describing the various features of a stacked semiconductor package  110 , applicants have used terms of positional/directional reference such as up, down, bottom, above, and below, which are sometimes used in reference to an orientation with respect to the surface of the earth. Such terms are not used in that sense in this application. Rather, terms such as up, down, etc. are used only in a relative sense to indicate the position of an object or surface with respect to other objects or surfaces in a structure which initially is oriented as shown in the drawings. As used in this sense the “top” of a car would still be referred to as the “top” of the car, even when the car is subsequently positioned upside down in a ditch. 
         [0014]    Having described the stacked semiconductor package  110  generally above, further details of its construction and methods of production will now be described.  FIG. 1  illustrates a stacked semiconductor package subassembly  10  which includes a leadframe  12  that is a portion of a larger leadframe sheet  11 . For illustrative purposes, the only portion of the leadframe sheet  11  shown in  FIG. 1  is the portion attached about the periphery of leadframe  12 . During later singulation of the stacked semiconductor package subassembly  10 , the portion of the leadframe sheet  11  positioned outwardly of saw streets AA, BB, CC, and DD is separated from subassembly  10 . The leadframe  12  includes a front portion  13  , a top surface  14  and a bottom surface  16  ( FIG. 2 ). The leadframe  12  includes a centrally positioned die pad  18  and a plurality of peripherally positioned leads  20 . A first die  30  is mounted on the die pad  18  of the leadframe. The first die may be relatively flat and rectangular in shape and includes a top surface  32  and bottom surface  34  ( FIG. 2 ). The first die  30  has a number of contact pads  36  on the top surface  32  thereof including a large contact pad  37  beneath first clip  40 . 
         [0015]    The first clip  40  has a top surface  42  and a bottom surface  44  ( FIG. 2 ). The first clip has a body portion  45  which is adapted to be positioned on top of the first die  30  and further includes a first leg portion  46  which extends downwardly from the body portion  45  and engages a plurality of leads  20 . In some embodiments, first clip  40  includes a tie bar  48  which projects outwardly from one lateral side of the first clip  40 . 
         [0016]    The second die  50  ( FIGS. 1 ,  2  and  8 ), which may be relatively flat and rectangular, is mounted on the top surface  42  of the first clip  40 . The second die  50  includes a top surface  52 , bottom surface  54  and a plurality of contact pads  56  located in the top surface  52 , including a large contact pad  57  positioned below a second clip  60 . 
         [0017]    A second clip  60  having a top surface  62  and a bottom surface  64  ( FIG. 2 ) has a body portion  65  positioned on top second die  50 . The second clip  60  also includes a leg portion  66  integrally formed with the body portion  65  and projecting downwardly therefrom that engages a plurality of peripherally positioned leads  20 . In some embodiments, a tie bar portion  68  projects laterally outwardly from a side portion of the second clip  60 . 
         [0018]    A third die  70  may be mounted on the leadframe die pad  18  at a position thereon immediately forward of first die  30 . The third die  70  has a top surface  72 , a bottom surface  74  ( FIG. 1 ), and a plurality of contact pads  76 . 
         [0019]    The leadframe  12 , first die  30 , first clip  40 , second die  50 , second clip  60  and third die  70  are held in place in the final stacked die semiconductor package  110 ,  FIG. 5 , by a plurality of solder layers. In order to provide the solder layers, a number of layers of solder paste are applied to the various components as will now be described. 
         [0020]    Prior to mounting first die  30  on die pad  18 , a first layer of solder paste  81  is applied to the top surface of the die pad  18 . The solder paste has a mildly adhesive property and thus tends to hold the first die  30  in position on the die pad  18 . Rather than placing the first layer of solder paste  81  on the die pad  18 , it will, of course, be understood that the layer of solder paste could be applied to the bottom surface  34  of the first die  30  instead. This will also be understood with respect to the description of the remaining layers of solder paste, dies and clips. Next a second layer of solder paste  82  is applied to the top surface  32  of the first die  30  and the first clip  40  is mounted thereon. Next a third layer of solder paste  83  is applied to the top surface  42  of first clip  40  and the second die  50  is then mounted on the first clip  40 . A fourth layer of solder paste  84  is then applied to the top surface  52  of second die  50  and the second clip  60  is positioned on top the second die  50 . Next, a fifth layer of solder paste  85  is applied to the die pad  18  forward of the first die  30 . The third die  70  is thereafter mounted on the centrally positioned die pad  18 , sandwiching the fifth layer of solder paste  85  between it and the die pad  18 . In addition to the layers of solder paste  81 - 85  which are sandwiched between dies and adjacent leadframe and clips, other portions of the various components may also be coated with solder paste to provide mechanical and/or electrical connection of components. For example the leg portions of the clips  40 ,  60  and the leads  20  which they engage may have a layer of solder paste (not shown) applied therebetween. 
         [0021]    In order for the layers of solder paste  81 - 85  to bind with adjacent surfaces, the solder paste must be heated to free flow temperature and subsequently cooled. As noted previously, applicants have discovered that when the stacked semiconductor package subassembly  10  is moved to a reflow oven, the adhesive properties of the solder paste layers  81  through  85  are not sufficient to prevent moderate shifting of the various components laterally, i.e., in forward and rearward directions  90  and/or side to side directions  91 . Such lateral shifting may cause defects in the subsequently produced stacked semiconductor package  110 . Such defects may include insufficient electrical contact between the surfaces of certain components or short circuits between areas of components which improperly come into contact. 
         [0022]    Applicants have developed techniques to prevent such lateral shifting of components. These techniques are implemented before the subassembly  10  is moved to a reflow oven. One technique for preventing lateral shifting is best illustrated in  FIG. 2 . According to this technique the first and second clips  40 ,  60  are held in laterally fixed relationship by abutting engagement between surface portions thereof and surface portions of a set of leads  20 . In  FIG. 2 , each of the several leads  20  which are engaged by the first clip leg portion  46  is provided with a half etch  21 . These half etches  21  may be positioned in longitudinal alignment such that an end projection  47  of the leg portion  46  is received within a more or less continuous half etch groove or trench. Thus, the recessed surface of the half etch  21  co-acts with the surface of the end projection  47  to form a mechanical lock that positively prevents lateral displacement of the first clip  40  relative to the leadframe  12 . Stabilizing the clip  40  in this manner also helps to laterally stabilize the first die  30  on which it is mounted. The second clip  60  may be stabilized in the same manner. An end projection  67  of the second clip leg portion  66  is received in the series of half etches  23  which are provided in another set of leads  20 . Thus second clip  60  is prevented from shifting laterally relative to the leadframe  12  which also laterally stabilizes the second die  50  upon which it is mounted. Again, the abutting surfaces of the recesses formed by the half etching  23  and the surface portion of the end projection  67  abut one another to prevent lateral shifting. 
         [0023]      FIGS. 3 and 4  illustrate another technique for preventing lateral shifting of the first clip  40 . In some embodiments, the first clip  40  has a laterally projecting tie bar  48  integrally formed therewith. The tie bar  48  is typically used during pick and place operations for mounting and otherwise handling of the clip  40 . In this embodiment, the tie bar  48  is also used to laterally stabilize the clip  40 . The tie bar  48  extends out over the leads  20  such that an end portion of the tie bar  48  is positioned directly above one of the leads  20 . UV-curing epoxy  49  is applied between the lead  20  and the end of tie bar  48 . Next, the UV-curing epoxy  49  is exposed to UV light which causes the epoxy to cure and rigidly attach the tie bar  48  to the lead  20 . This attachment forms another type of mechanical bond for holding the first clip  40  in laterally stable position on top the first die  32 . As illustrated in  FIG. 4 , a tie bar  68  on the second clip  60  extends out over the first clip  40 . Thus, UV-curable epoxy  69  positioned between tie bar  68  and the top surface  42  of first clip  40  can be exposed to UV light to cure the epoxy  69  and adhere tie bar  68  to first clip  40 . Because of the mechanical lock formed by epoxy  69 , second clip  60  is prevented from laterally shifting. 
         [0024]    The mechanical locks formed by abutting engagement of surfaces or by UV-curable epoxy attachment of surfaces described above may be used singlely or in combination to laterally stabilize the first and second clips  40 ,  60  and the first and second dies  30 ,  50 . It will of course be appreciated that these same clip stabilizing techniques may be used in die stacks containing only one clip or any number of clips. 
         [0025]    Next, the stabilized stacked semiconductor package assembly  10 , along with the other identical subassemblies on the leadframe sheet  11 , is moved to a reflow oven (not shown) where the entire leadframe sheet  11  and attached components are heated to reflow temperature, e.g., between about 100° C. and 250° C. for a period of about 8 minutes to 25 minutes. The heating causes the solder in the first, second, third, fourth, and fifth solder paste layers  81  through  85  to liquefy and bond with adjacent surfaces of the dies and clips. The structure coming out of the reflow oven is thus generally the same as the structure of the subassembly  10  that entered the reflow oven except that the solder paste layers  81 - 85  have now become solder layers  81 A,  82 A,  83 A,  84 A,  85 A. Next, as illustrated by  FIG. 5 , contact pads  36 ,  56  and  76  on the three dies are attached to one another and/or to leads  20  by bond wires  86 . The technique of wire bonding is well known in the art and will thus not be further described herein. Next, the leadframe sheet  11 , which includes leadframe  12 ,  FIG. 1 , is moved to a transfer mold. In the transfer mold the lead frame sheet  11 , including the leadframe  12  and associated clips and dies, is covered with encapsulation material  100 . The encapsulation material  100  is indicated by dashed lines in  FIG. 4 . Next, this encapsulated assembly is singulated along die streets, shown at AA, BB, CC and DD in  FIG. 1 , to provide a plurality of identical stacked semiconductor packages  110  having a plurality of exposed leads  20 ,  FIG. 6 . 
         [0026]    It will be appreciated by those skilled in the art that the leadframe  12 , dies  30 ,  50 ,  70  and clips  40 ,  50  may be constructed from various materials. In one exemplary and nonlimiting embodiment each die top has a surface coating of nickel/gold compound and each clip is made of Copper. The lead frame may be formed from a sheet of Copper. The solder paste may be indium lead. 
         [0027]    While illustrative embodiments of a stacked semiconductor package and methods of making a stacked semiconductor package have been described in detail herein, it is to be understood that the inventive concepts set forth in this disclosure may be otherwise variously embodied and employed. The appended claims are intended to be construed to include such variations except insofar as limited by the prior art.