Abstract:
A molded leadless package (MLP) semiconductor device includes a heat spreader with a single connecting projection extending from an edge of a cap of the heat spreader to a leadframe. The heat spreader can include additional projections on its edges that act as heat collectors and help to secure the spreader in the MLP. The connecting projection is attached to a lead of the leadframe so that heat gathered by the cap can be transferred through the connecting projection to the lead and to a printed circuit board to which the lead is connected. In embodiments, the heat spreader includes a central heat collector projection from the cap toward the die, preferably in the form of a solid cylinder, that enhances heat collection and transfer to the cap. The cap can include fins projecting from its top surface to facilitate radiant and convection cooling.

Description:
BACKGROUND AND SUMMARY 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a packaged semiconductor, and more particularly, to packaged semiconductor devices and integrated circuits for removing excess heat from the devices and integrated circuits. 
         [0003]    2. Description of the Related Art 
         [0004]    Generally, semiconductor devices include and are not limited to integrated circuit (IC) devices, diodes, thyristors, or MOS gate devices, for example, metal-oxide-semiconductor field effect transistors (MOSFET) and insulated gate bipolar transistors (IGBT). Each are formed in a silicon semiconductor die. In vertical MOSFET devices, the die includes a metal drain electrode at its lower portion, and a metal source electrode, and a gate electrode on its top surface. In that special case, the die pad of the MOSFET may also be a lead. The device die or IC die is attached to a surface of a leadframe pad, and electrodes on the die are electrically connected to leads of the leadframe by a wire bonding. The leadframe temporarily holds the leads in place. A typical leadframe has two parallel rails with a number of cross rails. A die pad in the center is supported by tie bars that extend from the rails toward the center of the frame. Other leads extend from The leads extend from the rails toward the center of the frame but do not support the die. Consequently, the electrodes on the die are electrically connected to proximate ends of leads of the leadframe. The distal ends of the leads protrude out of a molded housing. The silicon semiconductor die and the wire are completely molded in the housing. 
         [0005]    A conflict arises since there is a demand for smaller and smaller semiconductor devices, while speed, power, and capacity are expected to increase. Various solutions have arisen to provide compromise solutions. For example, one problem that can arise is heat dissipation. Since electronic components, such as diodes and ICs, produce heat, it is important to have a way to remove heat from the components to prevent overheating, which can adversely affect performance of the components or even cause them to fail. Prior art arrangements offer heat dissipation arrangements, but it is always desirable to have more efficient heat dissipation allowing electronic components to operate at lower temperatures when feasible and not overly expensive. 
         [0006]    An example of such a heat dissipation solution is shown in  FIG. 1 . In  FIG. 1 , a semiconductor device  10  includes a die  11  attached to a premolded leadframe  12  via a die attach die bonding material  13 , such as an epoxy based bonding material. The premolded leadframe  12  includes a plurality of leads  14  and an attach area  15  on the top surface of the premolded leadframe  12  and is underfilled with a molding compound, such as an epoxy resin, electronic molding compound (EMC), or the like, so that the leads on the leadframe are supported by the material. The die  11  is attached to the attach area  15  via wires  22  bonded to the die  11  by known wire bonding techniques. The die  11 , premolded leadframe  12 , and bonded wires are encased in EMC and the leads are cut to form terminals  23  of the device  10 . While the leads  14  and terminals  23  conduct heat away from the die  11  and transfer it to conductors to which they are attached, heat can still undesirably build up in the device  10  since the only heat path from the die to the leads is the premold material on the leadframe and the EMC applied to the entire assembly, reducing performance and possibly resulting in failure of the device  10 . 
         [0007]    Another approach is disclosed in U.S. Pat. No. 7,190,066 to Huang et al. in which a round top heat spreader is disposed over a die, but the spreader only extends part way across the die and the device as a whole. Further, the substrate does not allow for transfer of heat from the spreader to the bottom of the device since it employs ball grid array (BGA) balls and does not have leads extending through the entire depth of the substrate. 
         [0008]    A further approach is disclosed in U.S. PreGrant Publication No. 20070132091 to Wu et al. in which an upper heat sink includes a depression over the die and is connected to the substrate by four leads. However, while there is dissipation via the top surface of the upper heat sink, no provision is made to conduct heat from the upper heat sink to the bottom of the device for enhanced heat dissipation. 
         [0009]    Embodiments disclosed herein provide a thermally enhanced MLP that uses heat conductive material shaped into a heat spreader and placed in the MLP so as to conduct more heat away from the die embedded therein, enhancing performance and lengthening the life of the device. The heat spreader is preferably in thermal communication with at least one of the leads of the device and occupies at least a portion of the top of the device, allowing radiant cooling or connection of the spreader to heat pipes or other cooling arrangements. In one form, the spreader is simply a layer of thermally conductive material with projections extending toward the leadframe, at least one of the projections serving as a support during assembly and as a connector and thermal conduit from one or more of the leads. In another form, the heat spreader projections are eliminated save for the support/conduit/connector and a central heat collector positioned in the center of the spreader over the center of the die. The support/conduit/connector is sized so that the heat collector is spaced apart from the die, but can collect heat produced by the die and direct it to the top surface of the device. The top surface can then radiate heat or be connected to heat pipes or other cooling arrangements. Fins can be employed in embodiments to enhance cooling, such as by convection and radiation. The resulting device allows the die to operate at a lower temperature than prior arrangements. 
         [0010]    Advantageously, an improved leadframe can be employed in embodiments to further enhance thermal communication between the die, the heat spreader, and the leadframe. The improved leadframe includes lead portions that are in direct thermal communication with the die and preferably also includes at least one enlarged lead in direct thermal communication with the heat spreader. The enlarged lead(s) provide additional heat carrying capacity to better dissipate heat transferred to the lead. The improved leadframe even further lowers operating temperature of the die. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic perspective view that illustrates a prior art molded leadless package (MLP). 
           [0012]      FIG. 2  is a schematic perspective view of a completed MLP according to embodiments disclosed herein. 
           [0013]      FIG. 3  is a schematic perspective view of the bottom of a frame bearing two heat spreaders according to embodiments disclosed herein. 
           [0014]      FIG. 4  is a schematic perspective view of the top of a frame bearing two heat spreaders according to embodiments disclosed herein, the heat spreaders including fins. 
           [0015]      FIG. 5  is a schematic perspective view of the top of a leadframe of a MLP according to embodiments disclosed herein. 
           [0016]      FIG. 6  is a schematic perspective view of the bottom of a leadframe of a MLP according to embodiments disclosed herein. 
           [0017]      FIG. 7  is a schematic illustration of a portion of a method of assembling a semiconductor device according to embodiments disclosed herein. 
           [0018]      FIGS. 8 and 9  schematically illustrate the joined components including a heat spreader according to embodiments disclosed herein and prior to molding. 
           [0019]      FIG. 10  is a schematic top view of the joined components of  FIGS. 11 and 12  after molding and indicating saw lines. 
           [0020]      FIG. 11  is a schematic perspective view of a completed MLP including a heat spreader according to embodiments disclosed herein. 
           [0021]      FIG. 12  is a schematic perspective view of the completed MLP of  FIG. 14  showing the bottom of the MLP. 
           [0022]      FIG. 13  is a schematic perspective view of the bottom of a frame bearing two heat spreaders that each include a collector according to embodiments. 
           [0023]      FIG. 14  is a schematic perspective view of the bottom of a frame bearing two heat spreaders that each include a collector with an alternate shape according to embodiments. 
           [0024]      FIGS. 15 and 16  schematically illustrate a complete MLP including a heat spreader with a collector as seen is  FIG. 16  and showing an interior of the MLP according to embodiments. 
           [0025]      FIGS. 17 and 18  are schematic perspective views of the completed MLP with a heat spreader including a collector showing the top and bottom of the MLP according to embodiments. 
       
    
    
     DESCRIPTION 
       [0026]    With reference to the accompanying FIGS., and  FIG. 2  in particular, a semiconductor device in the form of a molded leadless package (MLP)  100  according to embodiments includes a die  102  attached to a thermally enhanced substrate  104 , such as a premolded leadframe, via a bonding material  106 , such as an epoxy encapsulating material, also called die bonding material. The substrate  104  includes a plurality of leads  108  and is underfilled with a molding compound, such as electrically insulating electronic mold compound  107  or the like. 
         [0027]    The die  102  is preferably electrically connected to the leads  108  via wires  134  bonded to the die  102  by known wire bonding techniques. While normal wire bonding is preferred, other techniques can be applied. For example, ball stitch on bump bonding can be used instead to reduce bond wire loop height by as much as 50% should such reduction be required or desired. While copper and aluminum wires can be used, embodiments preferably employ gold wires since gold wires produce stronger bonds and lower electrical resistance than other metals. While wires and wire bonding are shown in the exemplary embodiment, other electrical connections could be used instead. It is within the purview of the invention to employ flip chip techniques, for example. 
         [0028]    As seen particularly in  FIGS. 2-4 , a heat spreader  200  according to embodiments includes a cap  202  comprising a layer of thermally conductive material, preferably rectangular in shape, placed over the die  102 . A connecting projection  204  extends from the bottom of the cap  202  to connect to one of the leads  108 . Preferably, the connecting projection  204  is located along an edge  206  of the cap, such as about half way along such an edge  206 . Additional projections  208 , which act has peripheral heat collectors, can be included along one or more edges of the layer  202  to collect additional heat from the body of the MLP and to better secure the spreader  200 . The particular number, size, and shape of the projections can vary depending on the needs of a particular MLP, but preferably the projections  204 ,  208  are rectangular in shape. 
         [0029]    The projections  204 ,  208  can be formed by various methods, such as by cutting, stamping, precise casting and molding, or even by MEMS-related technologies, such as chemical etching, but preferably are formed by stamping. For example, the projections  204 ,  208  can be stamped from a sheet at a uniform length after which all but the connecting projection  204  are trimmed to their final lengths. The bottom of the connecting projection  204  is preferably connected to a respective lead  108  with thermally conductive bonding material, such as solder, epoxy, thermal paste, or other suitable material. As indicated above, to enhance the connection from the spreader  200 , the lead to which the connecting projection  204  is attached can be extended to match or exceed the dimension of the connection projection  204 . 
         [0030]    The die  102 , substrate  104 , bonded wires  134 , and heat spreader  200  are encased in molding compound  109 , such as electronic molding compound, to form the body of the MLP  100 , as seen in  FIG. 2 , and the leads  108  are cut to form terminals  146  of the device  100 . If a premolded leadframe is used as the substrate  104 , the compound used to form the body of the MLP  100  need not be the same as the compound employed in the premolded leadframe if the requirements of a particular application would be better met by employing different materials. The completed MLP has one heat path from the die  102  through the heat collectors  208  to the cap  202 . From the cap  202 , heat can be transferred to the surroundings, such as air, but heat also can move through the at least one connecting projection  204  to the substrate  104 . Another heat path of the MLP  100  includes the die attach areas  110  of the leads  108 : heat passes from the die to the die attach areas  110 , then through the leads  108  to the surroundings, such as a printed circuit board. 
         [0031]    To ease assembly of the device  100  with the spreader  200 , with particular reference to  FIGS. 3 and 4 , two or more spreaders  200 ,  200 ′ are formed in a frame  214  for assembly with a premolded leadframe  160  seen in later FIGS. with leads and attachment areas for respective numbers of dies  102 ,  102 ′. The spreaders  200 ,  200 ′ are preferably supported by tie bars as is known in the art. As seen in  FIG. 4 , fins  209 ,  209 ′ can be formed projecting from the top surface of each cap  202 ,  202 ′ to enhance heat dissipation. The fins  209 ,  209 ′ can be formed from any of the methods by which the projections  204 ,  208  can be formed and additionally can be formed separately and then attached, such as with adhesive, but are preferably formed by stamping. 
         [0032]    As seen particularly in  FIGS. 5-7 , a preferred substrate can be a premolded leadframe  160  that includes two sets of leads  108 ,  108 ′ each arranged in an asterisk-like pattern and supported by tie bars. As mentioned, while two sets are shown and two spreaders are shown, more or fewer can be used as appropriate. The asterisk-like pattern of the leads  108 ,  108 ′ results in part from extension of the leads  108 ,  108 ′ toward the center of the set of leads to form die attach areas  110 ,  110 ′. The die attach areas  110 ,  110 ′ are in thermal communication with the die  102 ,  102 ′ once the die  102 ,  102 ′ is attached. The inclusion of die attach areas  110 ,  110 ′ on the leads  108 ,  108 ′ allows the premolded leadframe  160  to have better thermal performance than prior art arrangements with the added benefit of better accommodation of the attachment of the die  102 . To even further enhance thermal performance of the leadframe  160 , one or more of the leads can be enlarged, such as enlarged lead  111 ,  111 ′, to provide greater heat carrying capacity as will be explained below. Embodiments can also include terminals of the leads  108 ,  108 ′ that extend to the bottom surface of the leadframe, providing an additional path for heat dissipation and/or for electrical connection to the die. As such, this embodiment has no die pad, per se, but rather uses the ends of the leads proximate the die to support the die. The leads are longer than conventional leads and extend below the die pad and provide a surface for receiving and supporting a portion of the die. 
         [0033]    The electrically conductive portions of the leadframe  160  are preferably coated with a material such as nickel, zinc, gold, palladium, and/or another suitable material or an alloy or other combination thereof. The EMC  107  of the premolded leadframe  104  is preferably applied substantially prior to attachment of the die  102  so that the premolded leadframe is supplied ready for attachment of the die  102 . Preferably, the MLP  100  of at least one embodiment has six attach areas  110  at the ends of six respective leads  108  of the leadframe  160 , though more or fewer attach areas and/or leads can be employed as befits a particular arrangement. 
         [0034]    Preferably, as seen particularly in  FIG. 7 , assembly comprises providing a substrate  104 , such as a premolded leadframe  160  that includes two sets of leads  108 ,  108 ′ with die attach areas  110 ,  110 ′, placing die attach material  106 ,  106 ′, such as an encapsulating die attach material, on the premolded leadframe  160  and die attach areas  110 ,  110 ′. Alternatively, the premolded leadframe  160  can be supplied with die attach material  106 ,  106 ′ already in place. However the die attach material  106 ,  106 ′ is provided, assembly proceeds by bonding the dies  102 ,  102 ′ to the premolded leadframe  160  via the die attach material  106 ,  106 ′, and attaching wires  134 ,  134 ′ to the dies  102 ,  102 ′ and to the leads  108 ,  108 ′. As mentioned above, the wires  134 ,  134 ′ are preferably attached via known wire bonding techniques that can include ball stitch on bump techniques to reduce wire loop height. Bonding material  162  is then placed on the leads to which the spreaders  200 ,  200 ′ will be attached, such as enlarged leads  111 ,  111  ′. The spreaders  200 ,  200 ′ are then placed on the bonding material  162 , which connects the spreaders  200 ,  200 ′ to the premolded leadframe  160  and forms an intermediate assembly  180  seen in  FIGS. 8 and 9 . The intermediate assembly  180  includes two complete sets of components for two packages  100 ,  100 ′. Electrically insulating molding compound  109 , which need not be the same material as the premolded leadframe EMC  107 , is injected into the intermediate assembly  180  to fill the spaces between the components and the leadframe and to provide a body assembly  164  for the MLPs, as seen in  FIG. 10 . The body assembly  164  is then singulated, such as by sawing to remove the frames, separate the MLPs  100 ,  100 ′, and expose the terminals of the MLPs, as seen in  FIGS. 11-12 . 
         [0035]    In embodiments, as seen particularly in  FIGS. 13-18 , the cap  202  can include a collector  210  extending from the cap  202  toward the die  102 . Preferably, the collector  210  is centrally located on the bottom of the cap. While the collector  210  is shown as being circular, it can have any suitable shape. One alternative shape for the collector  210  is a rectangular shape as seen in  FIG. 14 . In embodiments including a collector  210  and a connecting projection  204  to connect the heat spreader cap  202  to the bottom lead. Other projections can be omitted, though the cap  202  will preferably include tabs  208  to secure the spreader  200  in the device  100  as in  FIG. 3 . 
         [0036]    With the heat spreader  200  of embodiments in place, heat produced by the die  102  is gathered by the cap  202  and transferred to the surroundings via a first heat path. Additionally, heat is transferred through a second heat path including the connecting projection  204 , the lead to which the projection  204  is connected, and a printed circuit board to which the lead is attached. Simulations of embodiments in operation indicate significant improvement in thermal dissipation. For example, the prior art design shown in  FIG. 1  yields a thermal resistance of approximately 456° C./W. However, embodiments, those that use the collector  210  and those that do not, yield a thermal resistance of approximately 268° C./W. Thus, employing the heat spreader  200  produces an improvement in thermal resistance on the order of 40%. Additionally, embodiments employing a heat collector  210  should provide even more reduction of thermal resistance for larger chips. 
         [0037]    It should further be noted that the heat spreader  200  need not be used with the asterisk-like leadframe  160 , but can advantageously be used with many other leadframes. Likewise, the asterisk-like leadframe  160  of embodiments can be used without the heat spreader  200 . However, the preferred implementation is to use both the heat spreader  200  and the asterisk-like leadframe  160  as disclosed herein. 
         [0038]    It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. It will also be understood that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.