Abstract:
Semiconductor packages and other electronic assemblies having an active heat sink are disclosed, along with methods of making the same. The active heat sink includes a cavity partially filled with a heat activated liquid. Heat generated during operation of a chip boils the heat activated liquid. The vapor condenses on an inner surface of the active heat sink and transfers heat to an outer, possibly finned, surface exposed to ambient to dissipate heat. In some embodiments, the active heat sink may be a closed vessel mounted on the chip. In some embodiments, the vessel of the active heat sink is formed from a die pad of a leadframe substrate. The die pad includes a recess that forms the active heat sink cavity when bonded to the back surface of the chip. The heat activated liquid directly contacts the back surface of the chip in these embodiments.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to packages for semiconductor chips or other electronic devices. 
     2. Description of the Related Art 
     A typical package for a semiconductor chip includes an internal leadframe, which functions as a substrate for the package. The leadframe includes a central metal die pad and a plurality of leads that radiate outward from the die pad. A hardened, insulative encapsulant material covers the semiconductor chip (or die), die pad, and an inner portion of each of the leads. The semiconductor chip is mounted on the die pad and is electrically connected to the leads. In particular, the chip includes a plurality of bond pads, each of which is electrically connected by a bond wire or the like to a bond finger that is at an inner end of one of the leads. An outer portion of each lead extends outward from the encapsulant material, and serves as an input/output (I/O) terminal for the package. The outer portion of the leads may be bent into various configurations, such as a J lead configuration or a gull wing configuration. 
     Semiconductor chips that have a high degree of functionality, such as microprocessors, or that are used in high power applications, generate large amounts of heat. Accordingly, packages for such semiconductor chips must have the capacity to dissipate such heat to avoid a malfunction of the packaged chip. 
     A conventional heat dissipation solution in semiconductor packages includes the provision of a solid, machined copper or aluminum slug, which may or may not have fins, that is embedded in the encapsulant material of the package. Such a heat sink, however, has the drawback of a relatively low efficiency of heat dissipation even when fins are provided. Accordingly, an improved semiconductor package with a more efficient integrated heat sink is needed. 
     SUMMARY 
     Embodiments of the present invention include semiconductor packages that have an active heat sink embedded in the package. The active heat sink is in a thermal connection with a semiconductor chip of the package. The encapsulant material of the package encapsulates the chip and a portion of the active heat sink. The chip is electrically connected to a plurality of external terminals of the package. The active heat sink includes a surface exposed to ambient and a cavity partially filled with a heat activated liquid. In some embodiments, an indirect thermal connection is provided between the heat activated liquid and a surface of the chip. Alternatively, a direct thermal connection can be provided between the heat activated liquid and a surface of the chip. 
     The heat activated liquid boils in response to heat generated during operation of the chip, thereby forming a vapor. The vapor condenses on a juxtaposed inner surface of the active heat sink and transfers heat to an opposite outer surface exposed to ambient to remove heat from the package. The inner and/or outer surfaces of the active heat sink can include fins to increase the surface areas of these surfaces. Optionally, an external heat sink including a plurality of fins can be thermally coupled to the outer surface that is exposed to ambient to further increase the surface area exposed to ambient. 
     By comparison to conventional heat dissipation solutions in semiconductor packages, the packages of the present invention provide much more efficient heat dissipation. This efficiency can be increased further through the use of fins, which may be included in or on the active heat sink and/or attached externally to the outer surface of the active heat sink that is exposed to ambient. 
     These and other aspects and features of the present invention will be better understood in new of the following detailed description of the exemplary embodiments and the drawings thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional side view of a leadframe semiconductor package having an active heat sink molded into the package according to one embodiment of the present invention. 
     FIG. 2 is a cross-sectional side view of the active heat sink of the package of FIG.  1 . 
     FIG. 3 is a cross-sectional side view of a ball grid array semiconductor package having an active heat sink molded into the package according to another embodiment of the present invention. 
     FIG. 4 is a cross-sectional side view of a leadframe semiconductor package having an active heat sink according to another embodiment of the present invention. 
     FIG. 5 is a cross-sectional side view of a semiconductor assembly having an active heat sink according to another embodiment of the present invention. 
     FIG. 6 is a cross-sectional side view of an assembly including a semiconductor package with an active heat sink and an external heat sink according to another embodiment of the present invention. 
     FIG. 7 is a cross-sectional side view of an assembly including a semiconductor package with an active heat sink and an external heat sink according to another embodiment of the present invention. 
    
    
     In the drawings, like or similar features are typically labeled with the same reference numbers. 
     DETAILED DESCRIPTION 
     In accordance with one embodiment of the present invention, FIG. 1 shows a semiconductor package  10  having an active heat sink  12  molded into the package. Package  10  includes a metal leadframe as a substrate. The leadframe includes a planar rectangular central metal die pad  18  and a plurality of leads  24  that extend outwardly adjacent to two or all four sides of die pad  18 . In view of the discussion below, however, practitioners will appreciate that packages made in accordance with the present invention may have any number of different substrate configurations. For example, instead of having a leadframe, package  10  may have a printed circuit board substrate, as in a BGA package. The present invention may be employed in virtually any encapsulated semiconductor chip application. 
     Returning to FIG. 1, a semiconductor chip  16  is mounted on die pad  18 . Chip  16  includes a plurality of bond pads  102  arranged in a row adjacent to two or all four peripheral sides of chip  16 . Bond pads  102  are each electrically connected by one of a plurality of wire bonds  22  to an inner portion of one the leads  24 . An external portion of leads  24  form I/O terminals of package  10 . 
     Active heat sink  12  is in thermal contact with an active surface  68  of chip  16 . Active heat sink  12  is a closed vessel that includes a heat activated liquid  46  that cools chip  16  as chip  16  generates heat during operation. A thermally conductive layer  14  thermally couples active surface  68  of chip  16  and a lower surface  74  of active heat sink  12 . Chip  16  is thus in an indirect thermal connection with heat activated liquid  46 . Thermally conductive layer  14  may comprise, for example, a heat dissipative epoxy or other adhesive. Conductive layer  14  is thermally conductive, but not necessarily electrically conductive. The heat dissipative epoxy embodiment may include, for example, silicon or quartz or other thermally conductive materials in an epoxy base. 
     Die pad  18 , inner portions of leads  24 , wire bonds  22 , chip  16 , thermally conductive layer  14 , and active heat sink  12  are encapsulated in an encapsulant  20 , which may be formed, for example, by molding an epoxy-based resin compound. The encapsulating process is performed so as to leave an upper portion of active heat sink  12  exposed to ambient to aid in heat dissipation. 
     FIG. 2 is a detailed cross-sectional side view of just active heat sink  12  of package  10  of FIG.  1 . Active heat sink  12  is a vessel formed of an upper portion  32  joined to a lower portion  34 . 
     Upper portion  32  includes a horizontal rectangular plate  76 , an oblique sidewall  82 , and a horizontal outward extending flange  36  at a lower end of sidewall  82  fully around upper portion  32 . Together, rectangular plate  76  and sidewall  82  define a recess  106  in upper portion  32 . A central orifice  104  extends through an upper first surface  72  of upper portion  32 . Orifice  104  is shown filled with plug  42 . Plug  42  may be a plug of epoxy or solder, or a screw inserted in orifice  104 . A plurality of inner fins  40  surrounding orifice  104  and extending into recess  106  increase the surface area of an inner surface  88 . Similarly, a plurality of outer fins  38  increase the surface area of first surface  72 . First surface  72  includes the outer surface of upper portion  32  extending from the edge of flange  36 , along sidewall  82 , along rectangular plate  76 , along the outer surface of outer fins  38 , up to and surrounding orifice  104 . As shown in FIG. 1, a portion of first surface  72  including outer fins  38  (as well as orifice  104  and plug  42 ) remains exposed to ambient in package  10 . 
     Lower portion  34  includes a horizontal rectangular plate  78 , an oblique sidewall  84 , and a horizontal outward extending flange  37  at an upper end of sidewall  84  fully around lower portion  34 . Together, rectangular plate  78  and sidewall  84  define a recess  108  in lower portion  34 . 
     Upper and lower portions  32  and  34  may be formed, for example, by stamping a sheet of stainless steel, nickel, copper, or other easily stamped heat dissipative materials. Alternatively, upper and lower portions  32  and  34  may be formed, for example, by machining aluminum, copper, or other easily machined heat dissipative metals. Outer fins  38 , inner fins  40 , and orifice  104  can be stamped or machined simultaneously with flange  36 , sidewall  82 , and rectangular plate  76  of upper portion  32 . 
     After forming, upper portion  32  and lower portion  34  are joined by spot welding or otherwise affixing juxtaposed flanges  36  and  37 . Recess  106  of upper portion  32  is juxtaposed with recess  108  of lower portion  34 , thereby forming a vessel with internal cavity  86 . Cavity  86  is partially filled with heat activated liquid  46 . Heat activated liquid  46  is a low boiling point liquid which may be, for example, ethylene glycol. 
     Active heat sink  12  may optionally include a baffle plate  44  in cavity  86  to reduce any sloshing of heat activated liquid  46  as package  10  is moved during handling or use. Baffle plate  44  can be made of, for example, a screen or a plate with drilled holes. 
     To make package  10  of FIG. 1, a metal leadframe having a die pad  18  and a plurality of external leads  24  (I/O terminals) is provided. Chip  16  is mounted on die pad  18  using an adhesive. Subsequently, bond pads  102  of chip  16  are each electrically connected to the inner portion of a respective one of leads  24  by a bond wire  22  using a conventional wire bonding machine. The vessel of active heat sink  12 , which has cavity  86  and open orifice  104  in first surface  72 , but no heat activated liquid  46  therein, is provided. Upper portion  32  of active heat sink  12  may or may not include outer fins  38 . The vessel of active heat sink  12  is then thermally coupled to active surface  68  of chip  16 , within bond pads  102 , using thermally conductive layer  14 , which may be a thermally conductive, electrically insulative adhesive, as mentioned above. 
     Next, the assembly of the leadframe, chip  16 , and vessel of active heat sink  12  is placed in a mold cavity. The inner surface of the top mold die contacts outer fins  38  of upper portion  32  of the vessel of active heat sink  12 . In particular, the top surface of outer fins  38  contacts the inner surface of the top mold die. Orifice  104  through first surface  72  is left open. 
     An encapsulant  20 , such as a mold compound, is then injected into the mold at high temperature (e.g., typically near 160° C.) and allowed to cool and harden. The molten encapsulant is prevented from reaching orifice  104 , which remains open. Having open orifice  104  through first surface  72  allows pressure in cavity  86  to equalize during the encapsulating process. As a result of the molding process, the central portion of first surface  72  is not covered by encapsulant  20 , but rather is left exposed to ambient. 
     Subsequently, the molded assembly is removed from the mold and cleaned, if necessary, to remove excess encapsulant. Next, cavity  86  is partially filled with heat activated liquid  46  through open orifice  104  through first surface  72 . Orifice  104  is subsequently sealed with plug  42 , thereby completing the assembly of active heat sink  12 . Conventional debar, dejunk, trim, and form steps may then be done to finish package  10 . 
     Cavity  86  of active heat sink  12  is filled only partially with heat activated liquid  46  in this embodiment. The partial filling allows room for heat activated liquid  46  to boil, at about 80-90° C., in response to heat generated in underlying chip  16  during operation. As a result of this boiling, a plurality of vapor molecules  48  of heat activated liquid  46  rise and condense on inner surface  88  of upper portion  32 , thereby transferring heat to upper portion  32 . A central portion of opposite first surface  72  of upper portion  32  is exposed to ambient allowing radiation and convection by airflow, which increases the temperature gradient that causes the condensation, to complete the heat dissipation process. The heat dissipation process can be made more efficient by increasing the surface areas of inner surface  88  (using inner fins  40 ) and first surface  72  (using outer fins  38 ) of upper portion  32 . 
     FIG. 3 is a cross-sectional side view of a ball grid array (BGA) package  30  with an active heat sink  12  molded into package  30  in accordance with another embodiment of the present invention. Package  30  is similar to package  10  of FIG.  1  and is labeled with many similar reference numbers. Accordingly, to avoid redundancy, our discussion will focus on differences between package  30  and package  10 . 
     The chief difference between package  30  of FIG.  3  and package  10  of FIG. 1 is that the lead frame substrate, including die pad  18 , is replaced by a circuit board substrate  110 , as is conventional in BGA style packages. In BGA package  30 , bond pads  102  of chip  16  are each electrically connected by a wire bond  22  to one of a plurality of conductive traces  116  on a first surface  112  of circuit board substrate  110 . Each conductive trace  116  on upper first surface  112  is electrically connected to a conductive trace  116  on a second surface  114  of substrate  110  using a via  94  that extends through substrate  110  from first surface  112  to second surface  114 . Metal contacts  26  (e.g., solder balls) are each electrically connected to respective conductive traces  116  on lower second surface  114  of circuit board substrate  110  and serve as I/O terminals of package  30 . Conductive traces  116  and vias  94  form conductive paths  92  on and through substrate  110  routing signals between chip  16  and the I/O terminals (e.g., metal contacts  26 ). 
     Similarly to package  10  of FIG. 1, package  30  includes an active heat sink  12  that is embedded in encapsulant  20  and is in thermal contact with chip  16 . As in package  10 , package  30  provides an indirect thermal connection between heat activated liquid  46  and chip  16 . The method of making package  30  is substantially similar to the above-described method of making package  10 , except for the change in the substrate. Minor changes to the assembly method to accommodate the circuit board substrate  110 , versus the leadframe substrate including die pad  18  of package  10 , will be apparent to practitioners. 
     In view of the discussion above, practitioners will appreciate that the present invention is not limited by the type of substrate upon which the chip to be cooled is mounted. For instance, FIG. 5 shows a cross-sectional side view of a semiconductor assembly  60  having an active heat sink  12  in accordance with another embodiment of the present invention. In assembly  60 , chip  16  is mounted in a flip chip configuration on a motherboard  118 . Each bond pad  102  of chip  16  is electrically connected to a respective conductive terminal  120  of motherboard  118  using a solder bump  124 . Active heat sink  12  is thermally connected to an inactive surface  122  of chip  16  opposite motherboard  118  using a thermally conductive layer  14 . A glob top encapsulant  126  covers the periphery of active heat sink  12  and chip  16 . Active heat sink  12  cools chip  16  during operation in the manner described above. As in package  10  of FIG.  1  and package  30  of FIG. 3, assembly  60  of FIG. 5 provides an indirect thermal connection between heat activated liquid  46  of active heat sink  12  and chip  16 . 
     In view of the discussion above, practitioners will appreciate that heat dissipation may be increased in packages in accordance with the present invention by using an optional external heat sink. For instance, FIG. 6 shows a cross-sectional side view of an assembly  70  including a semiconductor package  10 , as shown in FIG. 1, with an active heat sink  12  and an external heat sink  152 . Heat sink  152  includes a plurality of orthogonal fins  156  extending from an upper surface  162  of a horizontal rectangular base plate  158 . 
     A plurality of barbed projections  154  extending from a lower surface  164  of rectangular base plate  158  are spaced so as to extend into and engage outer fins  38  of active heat sink  12  of package  10  by a friction force. The barbed projections  154  compress upon engagement with outer fins  38  of package  10 , and the restoring spring force of compressed barbed projections  154  provides the resistance to removal of external heat sink  152 . The result is a “snap-on” attachment of external heat sink  152  to package  10 . Heat sink  152  may be formed, for example, by stamping aluminum, copper, or other metals. 
     When package  70  of FIG. 6 is in operation, heat generated by chip  16  is transferred, as described above, to outer fins  38 . Attachment of external heat sink  152  to active heat sink  12  of package  10  allows heat conduction (due to physical and thermal contact) from outer fins  38  of active heat sink  12 , to barbed projections  154 , rectangular base plate  158 , and fins  156  of external heat sink  152 . The surface area exposed to ambient is thereby increased, so as to now include the external surfaces of all of fins  156 , upper surface  162 , and a portion of lower surface  164  of external heat sink  152 . The larger surface area exposed to ambient increases the heat dissipation capability of package  10 . Practitioners will appreciate that external heat sink  152  may be attached to any package including a heat sink with outer fins  38 , including, for example, the embodiments of FIG.  3  and FIG.  5 . 
     In some embodiments of the present invention, the outer surface (that is exposed to ambient) of the active heat sink does not include fins but rather is planar. In such embodiments, external heat sink  152  can be modified for use by removing barbed projections  154 . The lower surface  164  of rectangular base plate  158  can be attached to the exposed surface of the active heat sink using a thermally conductive material, such as solder paste or thermally conductive epoxy. (See the discussion of FIG. 7 below.) 
     In some embodiments, for example as shown in FIG. 1, FIG. 3, and FIG. 5, active heat sink  12  is a closed vessel with lower surface  74  of lower portion  34  in thermal contact with both chip  16  (via thermally conductive layer  14 ) and heat activated liquid  46 . This provides an indirect thermal connection between heat activated liquid  46  of active heat sink  12  and chip  16 . It is possible to provide, however, a direct thermal connection between chip  16  and heat activated liquid  46 . 
     In an alternative embodiment of the present invention, an active heat sink  132  is provided wherein there is direct physical and thermal contact between chip  16  and heat activated liquid  46  of active heat sink  132 . For example, FIG. 4 is a cross-sectional side view of a leadframe package  50  with active heat sink  132  molded into package  50 . Package  50  is similar to package  10  of FIG.  1  and is labeled with many similar reference numbers. Accordingly, to avoid redundancy, our discussion will focus on differences between package  50  and package  10 . 
     An upper portion  134  of active heat sink  132  is formed directly from the metal die pad, which is analogous to die pad  18  of package  10  of FIG. 1, of the leadframe. Unlike in package  10 , however, the die pad of package  50  is stamped into upper portion  134  to include a central horizontal planar rectangular plate  142 , an oblique sidewall  144 , and an outward extending flange  136  at a lower end of sidewall  144  fully around upper portion  134 . Upper portion  134  includes a central recess  96  defined by plate  142  and sidewall  144 . An orifice  104 , shown filled with plug  42 , is provided through a central portion of rectangular plate  142  of upper portion  134 . 
     Upper portion  134  is bonded to inactive surface  122  of chip  16  with a seal  54 . In particular, seal  54  seals a peripheral portion of inactive surface  122  fully around chip  16  to flange  136  of upper portion  134 . Seal  54  may be, for example, a solder paste, an epoxy, or a solder, such as gold tin solder. Seal  54  may be a thermally conductive material so that inactive surface  122  of chip  16  is thermally connected to upper portion  134  of active heat sink  132  through seal  54 . Together, upper portion  134  and inactive surface  122  of chip  16  define a cavity  146  hat is accessed through orifice  104 . 
     A plurality of bond pads  102  on active surface  68  of chip  16  are each electrically connected, using respective wire bonds  22  and a conventional wire bonder, to inner portions of respective leads  24  of package  50 . Leads  24  form the I/O terminals of package  50 . 
     Chip  16  and upper portion  134  are encapsulated in encapsulant  20 , which may be formed by molding an epoxy or other resinous molding compound. As discussed above, the molding process is performed so as to leave a first surface  138  of rectangular plate  142  of upper portion  134  exposed to ambient to aid in heat dissipation. As with package  10  of FIG. 1, orifice  104  remains open during the encapsulation process, which allows the pressure in cavity  146  to equalize during the molding process, thus preventing explosion of the structure. 
     After molding, cavity  146  is partially filled through orifice  104  with heat activated liquid  46 . Orifice  104  is subsequently sealed with plug  42 , which may be a plug of epoxy or solder, or a screw inserted in orifice  104 . Cavity  146  of active heat sink  132  is closed by seal  54  and plug  42 , so that heat activated liquid  46  remains in cavity  146  between upper portion  134  and inactive surface  122  of chip  16 . Heat activated liquid  46  is directly exposed to inactive surface  122  of chip  16  thus providing a direct thermal connection between chip  16  and heat activated liquid  46  of active heat sink  132 . 
     As discussed above, cavity  146  of active heat sink  132  is only partially filled with heat activated liquid  46 , which allows room for heat activated liquid  46  to boil in cavity  146  in response to heat generated in underlying chip  16 . This allows vapor molecules  48  created by the boiling of heat activated liquid  46  to rise and condense on an inner surface  98  of rectangular plate  142 , thereby transferring heat from chip  16  to first surface  138  of rectangular plate  142  of upper portion  134 , which is exposed to ambient. Additional heat fins (not shown) can be bonded to first surface  138  of rectangular plate  142  of upper portion  134  to yield an increased surface area exposed to ambient, thereby increasing the efficiency of the heat dissipation. 
     FIG. 7 is a cross-sectional side view of an assembly  80  including a semiconductor package  50  with an active heat sink  132  and an external heat sink  152  according to another embodiment of the present invention. FIG. 7 shows the package embodiment of FIG. 4 combined with an embodiment of the external heat sink of FIG.  6 . The major difference is that barbed projections  154  have been removed from external heat sink  152 . Lower surface  164  of external heat sink  152  is attached to first surface  138  of active heat sink  132  of package  50  using a thermally conductive material  166 , which may be a solder paste or thermally conductive epoxy. Further, the top of plug  42  is made to be flush with, or substantially flush with, first surface  138 . Other aspects of package  50  and external heat sink  152  are as described above. 
     The exemplary packages and mountings described above include an embedded active heat sink to cool the semiconductor chip during operation. This improves the efficiency of heat dissipation compared to prior art solutions. Better heat dissipation makes for a more reliable package and reduces the likelihood of chip failure due to overheating. 
     Of course, the embodiments of the present invention provided above are exemplary only. Practitioners may well see variations possible in view of our teachings. Accordingly, the present invention includes all that fits within the literal and equitable scope of the appended claims.