Abstract:
A liquid cooled electronic device and a method for sealing a liquid cooled electronic device are disclosed. The liquid cooled electronic device has at least one heat generating electronic device suspended in an electrically insulative heat transfer fluid. The heat generating device or devices are electrically connected to at least two electrodes, which pass through and are sealed in electrically insulating portion of a sealed housing that encloses the electrically insulative heat transfer fluid. At least one thermally conductive surface is in direct contact with the electrically insulative heat transfer fluid, and at least one thermally conductive surface is sealed to the remainder of the housing, for example.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/340,821, filed Oct. 29, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to electronic devices and more specifically relates to a package for electronic devices having improved cooling. 
     BACKGROUND OF THE INVENTION 
     Semiconductor die such as diodes, transistors, thyristors and the like are usually mounted within a protective housing, frequently a plastic molded structured which encloses the die. The protective packages are made of electrical insulation materials which reduce the ability to remove heat generated by the die over its full surface area and from localized hot spots on the die. 
     It would be desirable to provide a semiconductor device package which provides excellent electrical insulation properties for the die while providing improved cooling of the die and reducing hot spot heating on the die. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention a heat-generating electronic device is suspended in an electrically insulative heat transfer fluid, which is sealed in a housing. The lead frame terminals are sealed in and passed through the housing wall for external connection. At least a portion of the housing is thermally conductive and in direct contact with the electrically insulative heat transfer fluid, which is sealed in the interior volume within the housing. An insulative heat transfer fluid such as any of the well known liquids, for example, GALDEN® PFPE 0001 , a perfluoropolyether, which has the chemical formula shown in FIG. 7, then fills at least a portion of the interior of the housing and is in contact with an exposed surface of one or more of the die of the heat generating electronic device. Thus heat produced by the device is carried by conduction and convection through the fluid (preferably a liquid) to the thermally conductive plate or plates, and then to the ambient exterior of the package. 
     Herein the term thermally conductive is defined as having a coefficient of thermal conductivity of at least 170 W/m K, the thermoconductivity of aluminum nitride. Some other examples of thermally conductive materials are aluminum and alloys of aluminum that are thermally conductive (about 200 W/m K), beryllium oxide (260 W/m K), and copper (393 W/m K), for example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-section of the package of one embodiment of the invention, taken across section line  1 — 1  in FIG.  2 . 
     FIG. 2 is a cross-section of FIG. 1 taken across section line  2 — 2  in FIG.  1 . 
     FIG. 3 shows a side view of another embodiment of the housing. 
     FIG. 4 shows a side view of another embodiment of the housing. 
     FIG. 5 shows an end view of yet another embodiment of the housing. 
     FIG. 6 shows a cross section along the plane into the page shown as line  3  on FIG. 5 of one embodiment. 
     FIG. 7 shows the chemical formula of perfluoropolyether. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a thin conductive lead frame  10  which has a paddle section  11  and leads  12  and  13  which extend integrally from paddle  11 . Other lead patterns could be used. 
     Semiconductor die  20 , which may be a power MOSFET has its bottom mount electrode  24  electrically connected to the top of paddle  11 , for example, by soldering. A second die  21  may be fixed on the same side of paddle  11 , or, as shown, on the bottom side of paddle  11 . 
     A closed insulation housing wall  30  of any desired shape and material is then prepared and has through conductors  35 ,  36 ,  37 ,  38 ,  39  and  40  sealed therein. The top and bottom surfaces of ring  30  may be metallized by metallizing rings  31  and  32  which are spaced planar parallel rings. Note that only the top of housing  30  need be open, with the ring having the shape of a cup with a closed bottom. 
     The lead frame  10  may then be mounted by soldering or otherwise fixing the outer ends of terminals  12  and  13  to the interiorly projecting contacts  36  and  39  respectively thus suspending the lead frame and die within the volume  41  within ring  30 . For example, a gate electrode and source electrode of die  10  are then wire bonded to the interior projections of terminals  35  and  37 , respectively, by wire bonds  45  and  46 , respectively. Similar connections will be made for die  21  to terminals  38  and  40 , for example. 
     Thermally conductive, but electrically insulative plates  50  and  51 , which may have metallized annular rings  52  and  53 , respectively, are fixed to, and are sealed to, for example, by welding or brazing or by epoxy adhesive, to rings  31  and  32  respectively. Alternatively, thermally conductive plates  50  and  51  may be electrically conductive, for example, aluminum, and may be bonded directly to rings  31  and  32 , respectively. 
     The interior of volume  41  and the full volume surrounding lead frame  10  is then sealed, and is filled with a suitable electrically insulative heat transfer fluid, preferably a liquid such as a perfluoropolyether, e.g., GALDEN® PFPE with a high temperature boiling point that is greater than the operational temperature of the heat-generating electronic device. The liquid can be loaded into the sealed volume  41 , as through a filling tube  60  (FIG. 2) which can be clamped or sealed closed after filling. 
     In operation, the heat generated by die  20  and  21  will be coupled directly to the liquid in volume  41  to the thermally conducive plates  50  and  51 , which may be, for example, beryllium oxide ceramics or the like. The liquid will circulate by natural convection to conduct heat away from hot spots and from the whole exposed area of die  20  and  21  and lead frame  10  and into heat exchange contact with the interior surfaces of plates  50  and  51 . The heat may be removed from the outer surfaces of plates  50  and  51  by convection to the ambient. Alternatively, desired, massive copper conductive plates  70  and  71  can be pressed into contact with plates  50  and  51 . In another alternative embodiment passive or active heat sinks may be mounted to plates  50  and  51 . 
     FIG. 4 shows another embodiment of the present invention having a heat generating device suspended in a heat transfer fluid that is an electrically insulative liquid which is sealed in the housing shown in the figure. The housing has a top plate  84 , a bottom plate  85  and a central housing wall  81 . Also, a plurality of through conductors  90 ,  91 ,  92 ,  93  extends through the central housing wall  81 . The through conductors  90 ,  91 ,  92 ,  93  are electrically conductive and are sealed within the central housing wall  81 . For example, the top and bottom plate are sealed to the central housing wall by a sealing means  83 , which can be a metallic seal, a compression seal or an adhesive, for example. In one embodiment, the sealing means  83  comprises the same structure as shown in FIG. 1, which uses, for example, metallized annular ring  52  and  53  and rings  31  and  32 . In this case, sealing means  83  represents the result of the sealed metallized ring  53  and ring  32 , for example by fusing the two rings. In FIG. 3, the material used for the top plate  84 , the bottom plate  85  and the central housing wall  81  is the same material, which is both electrically insulative and thermally conductive. FIG. 4 shows another embodiment of the present invention, which uses a different material for the top plate  82  and the central housing unit  81 . For example, the top plate  82  is an electrically and thermally conductive material, and the central housing wall  81  is an electrically insulating but thermally conductive material. In an alternative embodiment, the central housing wall  81  may be selected as an electrically insulative and thermally insulative material. In this alternative embodiment, heat extraction primarily occurs through the thermally conductive plates. For example, both the top plate and bottom plate are thermally conductive. In yet another embodiment, the thermally conductive top plate  82  can be electrically conductive. For example, the material of the top plate  82  may be an aluminum alloy, pure aluminum, beryllium oxide, or aluminum nitride. 
     FIG. 5 shows an end of yet another embodiment of the present invention. In this embodiment, the housing comprises a sealed enclosure having an electrically insulative portion  81  and a thermally conductive portion  82 , wherein the electrically insulative portion is sealed in the thermally conductive portion. A plurality of electrically conductive through conductors  90 ,  91 ,  92  and  94  extend through the electrically insulative portion and are sealed therein. 
     FIG. 6 shows a cross-section taken along the plane indicated by line  3 , which extends into the page. FIG. 6 shows a heat generating electronic device suspended in the heat transfer fluid which fills the sealed enclosure. The electrically insulative portion  81  is sealed to the thermally conductive portion  82  by the sealing means  83 . The heat generating device has a plurality of electrodes, for example, a source, a gate and a drain. In another example, the drain electrode may be a bottom mount electrode, which is located on the bottom of the heat generating device. FIG. 6 shows one of the electrodes wire bonded to a through conductor  92 . Another of the electrodes of the heat generating device of FIG. 6 is surface mounted to the pad of the suspension structure  100 , for example. In this example, the suspension structure  100  is physically and electrically attached to through conductor  91 . In another embodiment, a second heat generating device having a plurality of electrodes is attached on the opposite side of the suspension device  100 . The electrodes of the second device may be attached to one or more of the plurality of through conductors or to one or more of the electrodes of the first device. Also, an electrode can be electrically connected to the suspension structure  100 . 
     The distance between the housing and the suspended heat generating electronic devices may be selected such that the heat transfer fluid is capable of convectively displacing around the heat generating electronic devices. Thereby, the heat transfer fluid that is heated by the heat generating devices can readily flow around the heat generating devices, removing heat and carrying it to the thermally conductive portion of the housing. 
     Optionally, a heat sink  70  may be in contact with the thermally conductive portion of the housing, efficiently removing heat from the sealed liquid cooled electronic device. In one alternative embodiment of FIG. 6, the material for the thermally conductive portion  82  and the electrically insulative portion  81  may be the same material. In this case, the sealed enclosure is thermally conductive and electrically insulative, with possible exception for the sealing means  83 . This alternative embodiment is advantageous, because heated liquid is convectively displaced under the influence of gravity, and the convectively displaced heated liquid encounters a thermally conductive surface in the alternative embodiment, regardless of the orientation of the sealed liquid cooled electronic device. However, if the thermally conductive material is electrically conductive, such as aluminum, then the electrically insulative material must be a different material from the thermally conductive material. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein.