Patent Publication Number: US-2007122943-A1

Title: Method of making semiconductor package having exposed heat spreader

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to the packaging of integrated circuits (ICs) and more particularly to a method of making a semiconductor package having an exposed heat spreader.  
      Package reliability is compromised when heat generated within a semiconductor package is inadequately removed. To prevent package failure due to from overheating, a number of thermal management techniques have been devised. One common thermal management technique involves the use of a heat spreader to dissipate the heat generated by an integrated circuit (IC) die.  FIG. 1  shows a conventional semiconductor package  10  with an exposed heat spreader  12 . The semiconductor package  10  comprises an IC die  14  attached and electrically connected to a top surface  16  of a substrate  18 . More particularly, the IC die  14  is attached to the substrate  18  with a die attach material  20 , and electrically connected to the substrate  18  via a plurality of wire bonded wires  22 . The heat spreader  12  is placed over the IC die  14  and is attached to the substrate  18  with a heat spreader attach material  24 . The IC die  14 , the wire bonded wires  22 , a portion of the substrate  18  and a portion of the heat spreader  12 , including its sides  26 , are encapsulated with a molding compound  28 . A plurality of solder balls  30  is attached to a bottom surface  32  of the substrate  18 . During the encapsulation process, a substantial clamping pressure is applied to the heat spreader  12  to prevent flashing or bleeding of the molding compound  28 . To prevent the IC die  14  from cracking as a result of the high compressive stress exerted on the heat spreader  12 , the IC die  14  is separated from the heat spreader  12  by a layer of the molding compound  28  as shown in  FIG. 1 . However, as the molding compound  28  is typically a poor thermal conductor, the rate at which heat is conducted from the IC die  14  through the molding compound  28  to the heat spreader  12  is usually slower than that at which it is generated. Hence, the heat generated by the IC die  14  is often not adequately removed, and the semiconductor package  10  tends to fail due to overheating.  
      In view of the foregoing, it would be desirable to have a method of making a semiconductor package having an exposed heat spreader directly attached to an IC die that is capable of effectively dissipating heat generated by the IC die. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The following detailed description of preferred embodiments of the invention will be better understood when read in conjunction with the appended drawings. The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. It is to be understood that the drawings are not to scale and have been simplified for ease of understanding the invention.  
       FIG. 1  is an enlarged cross-sectional view of a conventional semiconductor package with an exposed heat spreader;  
       FIG. 2  is an enlarged cross-sectional view of a plurality of integrated circuit (IC) dice having respective bottom surfaces attached to a base carrier and respective top surfaces attached to a heat spreader in accordance with an embodiment of the present invention;  
       FIG. 3  is an enlarged top plan view of a patterned adhesive layer in accordance with an embodiment of the present invention;  
       FIG. 4  is an enlarged top plan view of a patterned adhesive layer in accordance with another embodiment of the present invention;  
       FIG. 5  is an enlarged cross-sectional view of the dice and the heat spreader of  FIG. 2  encapsulated with an encapsulant;  
       FIG. 6  is an enlarged cross-sectional view of the base carrier of  FIG. 5  having a plurality of solder balls attached thereto;  
       FIG. 7  is an enlarged cross-sectional view of the heat spreader of  FIG. 6  being detached from a laminate to expose a surface thereof;  
       FIG. 8  is an enlarged cross-sectional view of a semiconductor package formed in accordance with an embodiment of the present invention; and  
       FIG. 9  is an enlarged cross-sectional view of a semiconductor package formed in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention. In the drawings, like numerals are used to indicate like elements throughout.  
      The present invention provides a method of making a semiconductor package including the steps of attaching a bottom surface of an integrated circuit (IC) die to a base carrier and electrically connecting the die to the base carrier. A first surface of a heat spreader is attached to a top surface of the die. The heat spreader has a laminate attached to a second surface thereof. The die, the heat spreader, the laminate and at least a portion of the base carrier are encapsulated. The laminate is detached from the heat spreader, thereby exposing the second surface of the heat spreader.  
      The present invention also provides a method of making a plurality of semiconductor packages including the steps of attaching respective bottom surfaces of a plurality of integrated circuit (IC) dice to a base carrier and electrically connecting the dice to the base carrier. Respective bottom surfaces of a plurality of heat spreaders are attached to respective top surfaces of the dice. The heat spreaders have a laminate attached to respective top surfaces thereof. The dice, the heat spreaders, the laminate and at least a portion of the base carrier are encapsulated. The laminate is detached from the heat spreaders, thereby exposing the top surfaces and side surfaces of the heat spreaders.  
      The present invention further provides a method of making a plurality of semiconductor packages including the steps of attaching respective bottom surfaces of a plurality of integrated circuit (IC) dice to a base carrier and electrically connecting the dice to the base carrier. Respective first surfaces of a plurality of heat spreaders are attached to respective top surfaces of the dice. The heat spreaders have a laminate attached to respective second surfaces thereof. The dice, the heat spreaders, the laminate and at least a portion of the base carrier are encapsulated. A singulating operation is performed to separate adjacent ones of the dice such that side surfaces of the heat spreaders are exposed by the singulating operation. The laminate is detached from the heat spreaders, which exposes the second surfaces of the heat spreaders.  
       FIGS. 2 and 5 - 7  are enlarged cross-sectional views that illustrate a method of making a plurality of semiconductor packages  50  in accordance with an embodiment of the present invention. The semiconductor packages  50  preferably are made with a Molded Array Process (MAP), thereby achieving high throughput.  
      Referring now to  FIG. 2 , a plurality of integrated circuit (IC) dice  52  having respective bottom surfaces  54  attached to a base carrier  56  and respective top surfaces  58  attached to respective ones of a plurality of heat spreaders  60  is shown. The dice  52  are electrically connected to the base carrier  56 .  
      The dice  52  may be processors, such as digital signal processors (DSPs), special function circuits, such as memory address generators, or circuits that perform any other type of function. The dice  52  are not limited to a particular technology such as CMOS, or derived from any particular wafer technology. Further, the present invention can accommodate dice of various sizes, as will be understood by those of skill in the art. A typical example is a memory die having a size of about 15 mm by 15 mm. The dice  52  may be attached to the base carrier  56  with an adhesive material  62 . The adhesive material  62  may be any suitable adhesive material, such as an adhesive tape, a thermo-plastic adhesive, an epoxy material, or the like. Such adhesives for attaching an IC die  52  to a base carrier  56  are well known to those of skill in the art. The dice  52  are electrically connected to the base carriers  56  via a plurality of wire bonded wires  64 . The wires  64  may be made of gold (Au) or other electrically conductive materials as are known in the art and commercially available. As can be seen from  FIG. 2 , the wire bonded wires  64  in this particular embodiment are attached to the IC dice  52  with ball bonds. However, it should be understood that the present invention is not limited to a particular wire bonding technique or to wire bond type connections. In alternative embodiments, the dice  52  may be, for example, electrically connected to the base carrier  56  via flip chip bumps (see flip chip bumps  156  in  FIG. 9 , described below).  
      Respective first or bottom surfaces  66  of the heat spreaders  60  are attached to the respective top surfaces  58  of the dice  52 . The heat spreaders  60  have a laminate  68  attached to respective second or top surfaces  70  thereof. A conductive adhesive  72  such as, for example, silicone is used to attach the respective heat spreaders  60  to respective ones of the dice  52 . The conductive adhesive  72  is dispensed onto the respective top surfaces  58  of the dice  52  then the heat spreaders  60  are placed, as a gang, on the respective top surfaces  58  of the dice  52  and attached by curing the conductive adhesive  72 . Because the heat spreaders  60  are attached to the dice  52 , and not to the base carrier  56 , no restrictions are imposed on the design of the base carrier  56 . Therefore, existing base carriers can be used in the present invention. The heat spreaders  60  are made of a thermally conductive material such as, for example, copper, aluminium or alloys thereof, while the laminate  68  is preferably a high temperature tape and has a thickness of about 50 microns.  
      A patterned adhesive layer  74  is used to attach the laminate  68  to the top surfaces  70  of the heat spreaders  60 . The adhesive layer  74  may be made of silicone and is patterned to facilitate subsequent separation of the laminate  68  from the heat spreaders  60 , as described below. In this particular embodiment, the patterned adhesive layer  74  comprises an adhesive tape having at least one perforation  76 .  FIG. 3  is an enlarged top plan view of the patterned adhesive layer  74  of  FIG. 2 . As can be seen, the adhesive layer  74  includes one ( 1 ) perforation  76  proximate to a centre thereof. In another embodiment shown in  FIG. 4 , the patterned adhesive layer  78  includes a plurality of perforations  80  distributed throughout the adhesive layer  78 . Accordingly, it should be understood that the present invention is not limited by the number or location of the perforations in the adhesive layer.  
      Referring now to  FIG. 5 , the dice  52 , the heat spreaders  60 , the laminate  68  and at least a portion of the base carrier  56  of  FIG. 2  are encapsulated with an encapsulant  82 . A molding operation such as, for example, an injection molding process is performed to encapsulate the dice  52 , the heat spreaders  60 , the laminate  68  and the portion of the base carrier  56 . The encapsulant  82  may comprise well known commercially available molding materials such as plastic or epoxy. As can be seen, the heat spreaders  60  are completely encapsulated by the encapsulant  82  and are not in direct contact with the mold during the molding operation. Consequently, the heat spreaders  60  and the dice  52  to which they are attached are protected from the clamping pressure applied during the molding operation by the encapsulant  82 . This reduces the risk of die cracking during the molding operation.  
      Referring now to  FIG. 6 , a plurality of solder balls  84  is attached to the base carrier  56 . As shown in  FIG. 6 , the encapsulated dice  52 , heat spreaders  60  and base carrier  56  are positioned in a “dead bug” orientation (upside-down) for the attachment of the solder balls  84 . The solder balls  84  may be attached to the base carrier  56  using known solder ball attach processes. The encapsulated dice  52 , heat spreaders  60  and base carrier  56  are mounted on a tape  86 , such as a Mylar® film as part of a singulating operation, for example, saw singulation. More particularly, the tape  86  is attached to an exposed surface  88  of the encapsulant  82  parallel to the base carrier  56 . The singulating operation is performed along the vertical lines A-A, B-B and C-C to separate adjacent ones of the dice  52  and expose side surfaces  90  of the heat spreaders  60 . In this particular example, the singulating operation is performed after the attachment of the solder balls  84  to the base carrier  56 . However, those of skill in the art will understand that the singulating operation can also be performed before the attachment of the solder balls  84  to the base carrier  56 .  
      Referring now to  FIG. 7 , the heat spreaders  60  are detached from the laminate  68  to expose the top surfaces  70  of the heat spreaders  60 . More particularly, each of the semiconductor packages  50  is picked up, and de-taped in the pick-up process to expose the top surfaces  70  of the heat spreaders  60 . As shown in  FIG. 7 , a top portion or layer  92  of the encapsulant  82  is peeled off together with the laminate  68  to expose the top surfaces  70  of the heat spreaders  60 . As can be seen, the tape  86  is used to detach the laminate  68  from the heat spreaders  60 . The tape  86  facilitates the detachment process by adhering to the encapsulant  82 . Because a layer  92  of the encapsulant  82  is peeled off, ultra-thin semiconductor packages  50  can be formed with the present invention. Bleeding and flashing of the encapsulant  82  over the top surfaces  70  of the heat spreaders  60  are prevented because the laminate  68  protects the top surfaces  70  of the heat spreaders  60  during the encapsulation process.  
      Although  FIGS. 2 and 5 - 7  show only two (2) dice  52 , it will be understood that more or fewer dice  52  may be attached to the base carrier  56 , depending on the size of the base carrier  56 , the size of the dice  52 , and the required functionality of the resulting semiconductor packages  50 .  
      Referring now to  FIG. 8 , an enlarged cross-sectional view of a semiconductor package  100  formed in accordance with the procedure described above is shown. The semiconductor package  100  comprises an integrated circuit (IC) die  102  attached on a bottom surface  104  to a base carrier  106  and on a top surface  108  to a heat spreader  110 . In this embodiment, the base carrier  106  is a substrate. The IC die  102  is attached to the substrate  106  with an adhesive material  112 , while the heat spreader  110  is attached to the IC die  102  with a conductive adhesive  114 . The IC die  102  is electrically connected to the substrate  106  via a plurality of wire bonded wires  116 . The IC die  102 , a bottom surface or underside  118  of the heat spreader  110  and at least a portion of the substrate  106  (i.e., a top surface of the substrate  106 ) are encapsulated with an encapsulant  120 . A plurality of solder balls  122  is attached to an underside  124  of the substrate  106 . As shown in  FIG. 8 , a top surface  126  and side surfaces  128  of the heat spreader  110  are exposed.  
      Referring now to  FIG. 9 , an enlarged cross-sectional view of a semiconductor package  150  formed in accordance with another embodiment of the present invention is shown. The semiconductor package  150  comprises an integrated circuit (IC) die  152  placed on a base carrier  154 , in this embodiment, a lead frame. The IC die  152  is electrically connected to the lead frame  154  via flip chip bumps  156 . A heat spreader  158  is attached to a top surface  160  of the IC die  152  with a conductive adhesive  162 . The IC die  152 , a bottom surface or underside  164  of the heat spreader  158  and at least a portion of the lead frame  154  are encapsulated with an encapsulant  166 , leaving a top surface  168  and side surfaces  170  of the heat spreader  158  exposed. The semiconductor package  150  is strengthened by having top and bottom surfaces made of metal.  
      As can be seen from  FIGS. 8 and 9 , the heat spreader in the present invention is directly attached to the IC die. Consequently, a direct thermal path is provided from the IC die to the heat spreader. This facilitates dissipation of the heat generated by the IC die, thereby reducing the likelihood of package failure due to overheating.  
      Further, because the heat spreader of the present invention is exposed to the ambient environment on the top and side surfaces, the semiconductor package of the present invention provides a substantial surface area for the convection of heat away from the semiconductor package. This enhances the thermal performance of the semiconductor packages made in accordance with the present invention. With improved thermal performance, the power capability of the semiconductor packages can be increased, for example, from about 2 Watts (W) to about 3 W. Alternatively, the temperature of the semiconductor packages can be reduced, for example, by about half.  
      As is evident from the foregoing discussion, the present invention provides an inexpensive method for volume production of reliable and thermally enhanced semiconductor packages. The present invention can be implemented using current semiconductor assembly equipment. Hence, there is no need for additional capital investment. Package rigidity and reliability are enhanced with the provision of the heat spreader. The heat spreader of the present invention is simply shaped, and is therefore easy to manufacture and can be readily incorporated into the assembly process. Additionally, the heat spreader design is suitable for use in all package types and sizes.  
      The description of the preferred embodiments of the present invention have been presented for purposes of illustration and description, but are not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. For example, the present invention is applicable to molded packages, including but not limited to MapBGA, PBGA, QFN, QFP and FC devices. In addition, the die sizes and the dimensions of the steps may vary to accommodate the required package design. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.