HEAT SINK FOR INTEGRATED CIRCUIT PACKAGES

Abstract: A heat sink (52) for use in a molded integrated circuit package (38) includes a main body (54) formed of a thermally conductive material. The main body (54) has (i) an upwardly facing horizontally extending top surface (56) adapted to support either directly or indirectly an integrated circuit die (40), (ii) a downwardly facing horizontally extending bottom surface (58), (iii) an outer circumferential edge (60), and (iv) a slot (62) or recess (84) formed into the main body (54) of said heat sink (52). The slot (62) or recess (84) is adapted to receive some of an encapsulating material (50) during the molding of the integrated circuit package (38) so as to form a mechanical connection between the heat sink (52) and the package (38). And, the bottom surface (58) of the heat sink (52) is exposed on an external surface of the integrated circuit package (38) after the molding of the integrated circuit package (38) is complete.

Description: HEAT SINK FOR INTEGRATED CIRCUIT PACKAGES B AC GRQQNP QF THE INVENTION The present invention relates generally to integrated circuit packaging and more specifically to an integrated circuit package including a particularly designed heat sink which, in a preferred embodiment, has a surface thereof exposed to the exterior of the integrated circuit package. In the field of integrated circuit packaging, the cost and reliability of the components making up the package are major concerns. Any reduction in the cost or improvement in the reliability of these components making up the package can provide a significant commercial advantage. To this end, the packaging of integrated circuits is continuously being improved to provide more cost effective designs and methods of producing integrated circuit packages. For example, the prior art approaches to providing a heat sink have typically involved fairly complex shapes. These shapes are designed to allow one of the surfaces of the heat sink to be exposed to the exterior of the encapsulating material making up the integrated circuit package while providing an arrangement for locking the heat sink in place within the molded package. Also, when using this type of exposed heat sink, it is necessary to provide a durable seal between the encapsulating material and the heat sink in order to prevent contamination of the integrated circuit die or other components within the package. One example of such a complex heat sink is disclosed in U. S. Patent Number 5,381 ,042. In this case, the heat sink has several different features including multiple surfaces having different thicknesses, fins protruding outwardly from the main body of the heat sink, holes formed in the heat sink, and various other features. All of these relatively complex shaped features make this heat sink more expensive and relatively difficult to mass produce. Another example of a prior art heat sink is shown in Figure 1 which illustrates an lntegruted circuit package generally designated by reference numeral 10. Integrated circuit package 10 includes a heat sink 12 having a top surface 14 (which may be adapted to support an integrated circuit die or other device), a bottom surface 16, and an outermost peripheral edge surface 18. Peripheral edge surface 18 includes a fish-tail shaped protrusion 20 extending outwardly. Integrated circuit package 10 also includes an encapsulating material 22 surrounding all of heat sink 12 except bottom surface 16 which remains exposed at the exterior surface of integrated circuit package 10. Fish-tail protrusion 20 cooperates with encapsulating material 22 to mechanically hold heat sink 12 in position within integrated circuit package 10. Still another example of a prior art heat sink is shown in Figure 2 which illustrates an integrated circuit package generally designated by reference numeral 24. Integrated circuit package 24 includes a heat sink 26 having a top surface 28 (which may be adapted to support an integrated circuit die or other device), a bottom surface 30, a peripheral edge surface 32, and a curved pocket 34 formed around the periphery of bottom surface 30. Integrated circuit package 24 also includes an encapsulating material 36 surrounding all of heat sink 12 except bottom surface 16 which remains exposed at the exterior surface of integrated circuit package 10. Curved pocket 34 cooperates with encapsulating material 36 to mechanically hold heat sink 26 in position within integrated circuit package 24. Some of the above described prior art heat sinks perform better than others, however, there are still reliability issues and cost issues that may be improved upon. The present invention provides a reliable, easy to mass produce heat sink for use in an integrated circuit package in which the heat sink, in a preferred embodiment, has a surface of the heat sink exposed to the exterior of the integrated circuit package. SUMMARY OF THE INVENTION As will be described in more detail hereinafter, a heat sink for use in an integrated circuit package having at least one integrated circuit die and an encapsulating material for molding the tmαgruied υlrυuli die ami ui leant a part of the heat alnk Into the lntagruiad ølruuil package during the manufacture of the package is disclosed. The heat sink includes a main body formed of a thermally conductive material. The main body has (i) an upwardly facing horizontally extending top surface adapted to support the integrated circuit die either directly or indirectly, (ii) a downwardly facing horizontally extending bottom surface, (iii) an outer circumferential edge, and (iv) a slot extending a predetermined distance into the main body along the entire circumferential edge between the top surface and the bottom surface of the main body. The slot is adapted to receive the encapsulating material during the molding of the integrated circuit package thereby forming a mechanical connection between the heat sink and the package. Also, in a preferred embodiment, the bottom surface of the heat sink is exposed on an external surface of the integrated circuit package after the molding of the integrated circuit package is complete. In another embodiment, the main body of the heat sink has (i) a central portion , (ii) an outer peripheral portion, (iii) an upwardly facing horizontally extending top surface adapted to support the integrated circuit die either directly or indirectly, (iv) a downwardly facing horizontally extending bottom surface, (v) an outer circumferential edge, and (vi) a continuous downwardly facing V-shaped recess having an innermost and an outermost opposing sidewall. The recess is formed into the bottom surface of the outer peripheral portion of the main body adjacent to the outer circumferential edge. The outermost sidewall is shorter than the innermost sidewall such that the thickness of the heat sink at its outer circumferential edge is less than the thickness of the central portion of the main body. The recess is adapted to receive the encapsulating material during the molding of the integrated circuit package thereby forming a mechanical connection between the heat sink and the package. In a preferred embodiment, the bottom surface of the heat sink is exposed on an external surface of the integrated circuit package after the molding of the integrated circuit package is complete. BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention may best be understood by reference to the following denurlptlon of the prenenlly preferred embodiment* together with the auυomμanylng drawings in which: Figure 1 is a diagrammatic cross sectional view of a first prior art heat sink shown assembled into an integrated circuit package; Figure 2 is a diagrammatic cross sectional view of a second prior art heat sink shown assembled into an integrated circuit package; Figure 3 is a diagrammatic cross sectional view of an integrated circuit package including a first embodiment of a heat sink in accordance with the present invention; Figure 4 is a diagrammatic cross sectional view of an integrated circuit package including a second embodiment of a heat sink in accordance with the present invention; Figure 5 is a diagrammatic cross sectional view of an integrated circuit package including a third embodiment of a heat sink in accordance with the present invention; and Figures 6A-C are diagrammatic plan views of the undersides of several variations of an integrated circuit package including a heat sink in accordance with the present invention with the leads of the package not being shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Inasmuch as Figures 1 and 2 were discussed previously, attention is directed to Figures 3-6 which illustrate integrated circuit packages in accordance with the present invention wherein like components are represented by like reference numerals. Referring initially to Figure 3, a first embodiment of an integrated circuit package in accordance with the present invention and generally designated by reference numeral 38 will be described. Typically, as shown In Figure 3, Integrated circuit package 38 Includes (1) an Integrated circuit die 40 huvlng a plurality of input/output terminals 42, (ii) a lead frame 44 having a plurality of electrically conductive leads 46 for electrically connecting package 38 to other electrical components but absent its die attach pad, (iii) an array of bonding wires 48 electrically connecting certain integrated circuit die input/output terminals 42 to associated leads 46 of leadframe 44, and (iv) an encapsulating material 50 surrounding integrated circuit die 40, bonding wires 48, and portions of lead frame 44. Integrated circuit package 38 also includes a heat sink 52, which in this embodiment, directly supports die 40 and lead frame 44. In other embodiments, lead frame 44 may include a die attach pad for supporting die 40 and the die attach pad may be supported by the heat sink 52 which therefore would support die 40 indirectly. In accordance the present invention, heat sink 52 includes a main body 54 formed of a thermally conductive material. This material may be copper, aluminum, or a variety of other thermally conductive materials such as thermally conductive ceramics or other metal alloys. Main body 54 of heat sink 52 has an upwardly facing horizontally extending top surface 56 adapted to support integrated circuit die 40 (or as mentioned above a die attach pad of lead frame 44), a downwardly facing horizontally extending bottom surface 58, and an outer circumferential edge 60. In this embodiment of the present invention, heat sink 52 further includes a slot 62 extending a predetermined distance into main body 54 along the entire circumferential edge 60 between top surface 56 and bottom surface 58. This predetermined distance may vary widely depending on the specific application in which the heat sink is to be used. Slot 62 is filled with encapsulating material 50 during the molding of the integrated circuit package thereby forming a strong and reliable mechanical connection between heat sink „ „ T -, Λ PCT US96/03282 52 and overall package 38. As shown in Figure 3 and in accordance with the present invention, bottom surface 58 of heat sink 52 cooperates with an integrated circuit package mold (not shown) during the manufacture of package 38 such that bottom surface 58 of heat sink 52 is exposed on an external surface 64 of integrated circuit package 38 after the molding of the integrated circuit package is complete. Still referring to Figure 3, in this embodiment, slot 62 formed into main body 54 of heat sink 52 Includes beveled edges 66 along the entire circumferential edge 60 of main body 54. Beveled edges 66 provide an enlarged entrance opening into slot 62 for allowing encapsulating material 50 to more easily fill slot 62 during the molding process. It is to be understood that beveled edges 66 are not a requirement of the present invention and the invention equally applies to a heat sink including a slot without beveled edges although beveled edges are preferred. Also, it should be understood that the beveled edges may be a wide variety of sizes depending on the requirements of the application in which the heat sink is to be used. As illustrated, the bevels do not extend to the top and bottom surfaces 56 and 58 of main body 54 but rather define upper and lower square edges 67. These square edges ad structural rigidity to the overall configuration. Since slot 62 is filled with encapsulant material 50, a relatively long interface path between heat sink 52 and encapsulating material 50 is formed. This long interface path provides a reliable seal that prevents moisture or other contaminants from penetrating package 38 and damaging the internal components of the package. Also, since heat sink 52 is a relatively simple shape, it may be easily mass produced at a low cost by methods of casting or machining from a sheet stock of the appropriate material. Referring now to Figures 4 and 5, a second embodiment of an integrated circuit package generally designated by reference numeral 68 and including a heat sink in accordance with the present invention will be described. As described above for integrated circuit 38, integrated circuit 68 would also typically include die 40, lead frame 44 (with or without a die attach pad, bonding wires 48, and encapsulating material 50. However, this embodiment includes a heat sink 70 having a main body 72 formed of a thermally conductive material as described above for heat sink 52 but a different shape. As shown in Figures 4 and 5, main body 72 of heat sink 70 has a central portion 74, an outer portion 76, an upwardly facing horizontally extending top surface 78 adapted to support integrated circuit die 40 (or a die attach pad), a downwardly facing horizontally extending bottom surface 80, and an outer circumferential edge 82. In accordance with the present invention, heat sink 72 also includes a continuous downwardly facing V-shaped recess 84 having an innermost sidewall 86 and an outermost opposing sidewall 88. Recess 84 is formed into the bottom surface within outer peripheral portion 76 adjacent to outer circumferential edge 82. Outermost sidewall 88 is shorter than innermost sidewall 86 such that the thickness of heat sink 70 at its outer circumferential edge 82 is less than the thickness of central portion 74 of the main body 72. Although sidewalls 86 and 88 are shown in Figure 5 as having a slight curvature, it should be understood that the sidewall may also be flat. Recess 84 receives encapsulating material 50 during the molding of integrated circuit package 68 thereby forming a mechanical connection between heat sink 70 and package 68. Also, as shown in Figures 4 and 5, bottom surface 80 of heat sink 70 cooperates with an integrated circuit package mold (not shown) during the manufacture of the package such that bottom surface 80 of the heat sink 70 is exposed on an external surface 90 of integrated circuit package 68 after the molding of the integrated circuit package is complete. Since recess 84 is filled with encapsulating material 50 during the molding of the package, a relatively long interface path between heat sink 70 and encapsulating material 50 is formed. This long interface path helps prevent moisture and other contaminants from penetrating the overall package and damaging the internal components thereby improving the reliability of the package. With the shape for heat sink 70 described above, heat sink 70 may be formed by a variety of methods. For instance, in the case of the embodiment illustrated in Figure 4, heat sink 70 is formed by ether machining or casting an appropriate thermally conductive material into the described shape. As mentioned above, this material is typically copper or aluminum, however, other materials such as thermally conductive ceramics or other metal alloys may be used. Alternatively, as illustrated in Figure 5, this same basic heat sink shape may be formed using a stamping or punching technique where the material making up the heat sink lends itself to this. With this stamping or punching approach, the entire heat sink may be formed in one rapid step which stamps or punches heat sink 70 into its final shape including recess 84 from a sheei of the appropriate material stock. This approach, in accordance with the present Invention, ullo n heat sink 70 to bo mans produce in a very low cost manner while still providing a reliable heat sink that has a strong mechanical connection to the overall package and provides a reliable seal at the interface between the heat sink and the encapsulating material as described above. Referring now to Figures 6A-C, it should be understood that the heat sinks of the present invention described above may take on a wide variety of shapes when viewed in the plan view. These shapes include, but are not limited to, circular, square, or rectangular shapes as shown in Figures 6A, 6B, and 6C, respectively. Other shapes such as oval or other polygon shapes would also fall within the spirit and scope of the present invention. Although the embodiments described above include a single integrated circuit die 40, it is to be understood that the present invention equally applies to integrated circuit packages having multiple die or other electronic devices in place of the integrated circuit die. Also, although the embodiments described above used a lead frame attached to the heat sink, the present invention would also apply to an integrated circuit package using other arrangements for the lead frame. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.

Claims: WHAT IS CLAIMED IS: 1 . A heat sink for use in an integrated circuit package including at least one integrated circuit die and an encapsulating material for molding said integrated circuit die and at least part of said heat sink ultimately formed into the integrated circuit package, said heat sink comprising; a main body formed of a thermally conductive material, said main body having (1) an upwurdly fuclng horizontally extending top surface adapted to support said Integrated circuit die, (ii) a downwardly facing horizontally extending bottom surface, (iii) an outer circumferential edge, and (iv) a slot extending a predetermined distance into said main body from and along its entire outer circumferential edge between said top surface and said bottom surface, said slot being adapted to receive some of said encapsulating material so as to form a mechanical connection between said heat sink and the rest of said package. 2. A heat sink according to Claim 1 wherein said heat sink is formed from copper. 3. A heat sink according to Claim 1 wherein said heat sink is formed from aluminum. 4. A heat sink according to Claim 1 wherein said circumferential edge is generally circular in shape. . A heat sink according to Claim 1 wherein said circumferential edge is generally square in shape. 6. A heat sink according to Claim 1 wherein said circumferential edge is generally rectangular in shape. 7. A heat sink according to Claim 1 wherein said slot formed into said main body includes beveled edges along the entire circumferential edge of said main body so as to provide an enlarged entrance opening into said slot for said encapsulating material. T/US96/03282 8. A heat sink according to Claim 7 wherein the entire circumferential edge of said main body includes an uppermost and a lowermost edge portion which is substantially perpendicular with the lop and bottom surfaces of the main body above and below said beveled edges, respectively. 9. An integrated circuit package comprising: (a) at least one integrated circuit die; (b) means for electrically connecting said die to external components; (c) a heat sink including a main body formed of a thermally conductive material, said main body having (i) an upwardly facing horizontally extending top surface supporting said integrated circuit die, (ii) a downwardly facing horizontally extending bottom surface, (iii) an outer circumferential edge, and (iv) a slot extending a predetermined distance into said main body from and along its entire circumferential edge between said top surface and said bottom surface; and (d) an encapsulating material surrounding said integrated circuit die, part of said connecting means, and portions of said heat sink such that its bottom surface is exposed to the ambient surroundings, said encapsulating material filling said slot in said main body of said heat sink so as to form a mechanical connection between said heat sink and said package. 10. An integrated circuit package according to Claim 9 wherein said connecting means includes a lead frame having a die attach pad, said die attach pad supporting said integrated circuit die, and said heat sink supporting said die attach pad. 1 1. An integrated circuit package according to Claim 9 wherein said heat sink supports said integrated circuit die directly. 12. An integrated circuit package according to Claim 9 wherein said heat sink is formed from copper. 13. An integrated circuit package according to Claim 9 wherein said heat sink is formed from aluminum. 14. An integrated circuit package according to Claim 9 wherein said circumferential edge is generally circular in shape. 15. An integrated circuit package according to Claim 9 wherein said circumferential edge Is generully square In shape. 16. An integrated circuit package according to Claim 9 wherein said circumferential edge is generally rectangular in shape. 17. An integrated circuit package according to Claim 9 wherein said slot formed into said main body of said heat sink includes beveled edges along the entire circumferential edge of said main body so as to provide an enlarged entrance opening into said slot for said encapsulating material. 18. An integrated circuit package according to Claim 17 wherein the entire circumferential edge of said main body of said heat sink includes an uppermost and a lowermost edge portion which is substantially perpendicular with the top and bottom surfaces of the main body above and below said beveled edges, respectively. 19. A heat sink for use in an integrated circuit package including at least one integrated circuit die and an encapsulating material for molding said integrated circuit die and said heat sink into said integrated circuit package during the manufacture of said package, said heat sink comprising; a main body formed of a thermally conductive material, said main body including (i) a central portion , (ii) a peripheral portion, (iii) an upwardly facing horizontally extending top surface adapted to support said integrated circuit die, (iv) a downwardly facing horizontally extending bottom surface, (v) an outer circumferential edge, and (vi) a continuous downwardly facing, generally V-shaped recess having an innermost and an outermost opposing sidewall, said recess being formed into said bottom surface of said main body adjacent said outer circumferential edge, the outermost sidewall of said recess being shorter than said innermost sidewall such that the thickness of said main body at its outer circumferential edge is less than the thickness of said main body inwardly of said recess, said recess being adapted to receive said encapsulating material during the molding of said integrated circuit package so as to form a mechanical connection between said heat sink and ■aid pnokuøe. 20. A heat sink according of Claim 19 wherein said V-shaped recess is formed by means of stamping. 21. A heat sink according to Claim 19 wherein said heat sink is formed from copper. 22. A heat sink according to Claim 19 wherein said heat sink is formed from aluminum. 23. A heat sink according to Claim 19 wherein said circumferential edge is generally circular in shape. 24. A heat sink according to Claim 19 wherein said circumferential edge is generally square in shape. 25. A heat sink according to Claim 19 wherein said circumferential edge is generally rectangular in shape. 26. An integrated circuit package comprising: (a) at least one integrated circuit die; (b) means for electrically connecting said die to external components; (c) a heat sink including a main body formed of a thermally conductive material, said main body including (i) a central portion, (ii) a peripheral portion, (iii) an /US96/03282 upwardly facing horizontally extending top surface supporting said integrated circuit die, (iv) a downwardly facing horizontally extending bottom surface, (v) an outer circumferential edge, and (vi) a continuous downwardly facing, generally V-shaped recess having an innermost and an outermost opposing sidewall, said recess being formed into said bottom surface of said main body adjacent said outer circumferential edge, the outermost sidewall of said recess being shorter than said innermost sidewall such that the thickness of said main body at its outer oirouinfβrβntiul edge is less than the thickness of said main body inwardly of said recess; and (d) an encapsulating material surrounding said integrated circuit die, part of said connecting means, and portions of said heat sink such that its bottom surface is exposed to the ambient surroundings, said encapsulating material filling said recess in said bottom surface of said main body of said heat sink so as to form a mechanical connection between said heat sink and said package. 27. An integrated circuit package according to Claim 26 wherein said connecting means includes a lead frame having a die attach pad, said die attach pad supporting said integrated circuit die, and said heat sink supporting said die attach pad. 28. An integrated circuit package according to Claim 26 wherein said heat sink supports said integrated circuit die directly. 29. An integrated circuit package according to Claim 26 wherein said heat sink is formed from copper. 30. An integrated circuit package according to Claim 26 wherein said heat sink is formed from aluminum. 31. An integrated circuit package according to Claim 26 wherein said circumferential edge is generally circular in shape. 32. An integrated circuit package according to Claim 26 wherein said circumferential edge is generally square in shape. 33. An integrated circuit package according to Claim 26 wherein said circumferential edge is generally rectangular in shape.

Inventors: KIM HEE JHIN, MOSTAFAZADEH SHAHRAM, KIM, HEE, JHIN, MOSTAFAZADEH, SHAHRAM
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1996
Main classification: H01L23
IPC codes: H01L23