Abstract:
An assembly for high-powered LEDs provides a direct attachment of the LED to a ceramic thermal conductor/electrical insulator sealed in a housing with a compression element between a portion of the housing and ceramic heat sink to provide a predetermined range of biasing force locating the ceramic heat sink against the portion of the housing with dimensional changes in the ceramic heat sink caused by thermal expansion of the ceramic heat sink.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a National Phase of International Application Number PCT/US2011/064952, filed Dec. 14, 2011, and claims the benefit of U.S. Provisional Application 61/423,153 filed Dec. 15, 2010, and the benefit of U.S. Provisional Application 61/521,178 filed Aug. 8, 2011. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to light emitting diodes and in particular to an improved heat sink assembly for high-powered light emitting diodes. 
     BACKGROUND OF THE INVENTION 
     High-powered light emitting diodes are increasingly replacing other light technologies, including incandescent and fluorescent lamps, for general illumination purposes. Such high-powered light emitting diodes accept currents in excess of 100 milliamps and typically at least one ampere to provide a light output for illumination of a space or area, for example, in an appliance such as a refrigerator or the like. 
     High-powered light emitting diodes normally require a heat sink to prevent destructive overheating. For this purpose, the light emitting diode will be placed on a carrier such as a printed circuit board holding conductive traces to connect the light emitting diode to other circuitry or power leads. The substrate may be attached to a heat sink, for example, of molded or extruded aluminum to conduct heat generated by the light emitting diode away from the diode into ambient air or other medium. 
     SUMMARY OF THE INVENTION 
     The present invention provides a high-powered LED assembly in which the LED is attached directly to the heat sink without the need for an intervening printed circuit board. By eliminating the thermal resistance of the substrate, improved cooling of the LED may be provided permitting higher power or longer life illumination systems. Electrical connection between the LED and a wiring harness is provided by a spring clamp system that both retains and attaches to a wire conductor and provides a positive electrical connection to a trace on the surface of the heat sink that may in turn attach to the LED. 
     Specifically, the present invention provides an LED assembly having a LED dissipating during operation at least 500 mW attached in thermal communication with the first surface of the ceramic heat sink. The LED is attached to a first and second conductive lead. A sealed housing receives the ceramic heat sink and LED therein and has a first portion fitting over the LED to permit the passage of light therethrough and a second portion providing for sealed ingress of the first and second electrical conductors through a housing wall for providing power to the LED. A compression element extends between a portion of the housing and ceramic heat sink to provide a predetermined range of biasing force, locating the ceramic heat sink against the portion of the housing with dimensional changes in the ceramic heat sink caused by thermal expansion of the ceramic heat sink. 
     It is thus a feature of at least one embodiment of the invention to provide for a fluid tight housing protecting and LED attached to a ceramic substrate that may accommodate thermal expansion of the substrate. 
     The first portion of the sealed housing may be a light-transmissive thermoplastic and the second portion of the sealed housing is a thermoplastic fused to the first portion. Similarly, the electrical conductors may have integral coaxial thermoplastic insulation and where the second portion of the sealed housing may be fused to the thermoplastic insulation. 
     It is thus a feature of at least one embodiment of the present invention to provide a simple method for producing a sealed housing amenable to mass production through injection molding. 
     The spring clamp may be a flexible metal ring. 
     It is thus a feature of at least one embodiment of the present invention to provide a fatigue resistant spring element that may handle multiple cycles of thermal expansion. 
     The first portion of the sealed housing may provide a ledge abutting a portion of the first surface of the ceramic heat sink and a collar extending around a portion of the ceramic heat sink behind the first surface and the spring clamp may include cantilevered teeth portions providing a wedging engagement with an inner surface of the collar to hold the flexible metal ring in abutment with a second surface of ceramic heat sink behind the first surface. 
     It is thus a feature of at least one embodiment of the present invention to provide a method of staking the components together prior to sealing of the housing that may make use of the spring element. 
     The metal ring may further include protrusions contacting the second surface and limiting the force applied thereto. 
     It is thus a feature of at least one embodiment of the present invention to control the maximum force is applied to the ceramic to prevent damage thereto. The protrusions limit maximum force and by providing a space between the ring and the ceramic allow greater compliancy of the spring force. 
     The first surface may include metal traces attached directly to the first surface and electrically communicating between the LED and first and second conductors. 
     It is thus a feature of at least one embodiment of the present invention to allow close integration of the LED to the ceramic substrate. 
     The metal traces may be printed conductive ink. 
     It is thus a feature of at least one embodiment of the present invention to provide a simplified electrical connection to the LED die possible in the sealed environment of the housing. 
     The ceramic may be Steatite. 
     It is thus a feature of at least one embodiment of the present invention to provide a readily manufactured ceramic material that is thermally conductive and yet electrically insulated. 
     The LED assembly may further include a first and second spring clamp insertable through apertures in the ceramic heat sink each having first portions, contacting different conductive traces when so inserted, and clamping elements receiving and retaining different of the first and second conductors. 
     It is thus a feature of at least one embodiment of the present invention to provide a method allowing electrical communication between wires and conductive traces with simple mechanical assembly. 
     The first portion of the spring clamps may be spring biased by the spring clamp against the conductive trace. 
     It is thus a feature of at least one embodiment of the present invention to provide a positive connection between a conductive material on the ceramic and a conductor, with thermal expansion and contraction of the ceramic. 
     The clamping elements of the spring clamps may provide flexible opposed spring elements slidably receiving ends of the conductors in electrical engagement. 
     It is thus a feature of at least one embodiment of the present invention to provide a simple connector-like attachment of conductive wires to the LED 
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary perspective view of a ceramic heat sink used in the present invention supporting an LED die for direct mounting thereon and showing conductive metal traces attached to the ceramic surface; 
         FIG. 2  is a cross-sectional view along the line  2 - 2  of  FIG. 1  showing incorporation of the ceramic heat sink into a protective molded assembly; 
         FIG. 3  is an exploded, perspective fragmentary view of the top of the heat sink showing a spring clamp insertable into a slot in the heat sink; 
         FIG. 4  is a cross-section along line  4 - 4  of  FIG. 1  showing the spring clamp before insertion into the heat sink and the wire conductor before insertion into the spring clamp; 
         FIG. 5  is a detailed fragmentary view of  FIG. 4  in unexploded form showing a spring biasing of the spring clamp to engage a trace on the surface of the heat sink; 
         FIG. 6  is a perspective partial cross-section view of an alternative embodiment of the invention employing a chip on board (COB) construction; 
         FIG. 7  is a perspective view and partial cross-sectional view of a compression element retaining the heat sink in the protective molded assembly while accommodating thermal expansion; 
         FIG. 8  is a figure similar to that of  FIG. 1  showing alternative direct soldering connection of the conductors to traces on the heat sink; 
         FIG. 9  is a figure similar to that of  FIG. 2  showing the direct soldering connection of  FIG. 8 . 
     
    
    
     Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , an LED assembly  10  per one embodiment of the present invention may provide for a ceramic substrate/heat sink  12  having an upper planar surface  14  supporting a high-powered LED  16 . The LED  16  may include conductive pads  18  attached to conductive traces  20  attached directly to the upper planar surface  14  of the ceramic substrate/heat sink  12  for example by printing. The conductive traces  20  may communicate from the conductive pads  18  to spring clamps  22  connected to power lead conductors  24  as will be described. 
     The ceramic substrate/heat sink  12  in one embodiment may be Steatite or soapstone that has been formed by a compression molding and fired to a high temperature to impart good thermal conductivity and high electrical resistance. Steatite is predominately a hydrated magnesium silicate. In one embodiment, the coefficient of thermal expansion of the ceramic substrate/heat sink  12  is approximately matched to that of the traces  20  and, for example, when the ceramic is Steatite, an L3 grade may be selected. The traces  20  are preferably formed of a silkscreened silver/platinum ink that is baked onto the surface of the ceramic substrate/heat sink  12  for improved adhesion. 
     The ceramic substrate/heat sink  12  extends away from the upper planar surface  14  to a fluted heat sink body portion  26  to provide thermal coupling to ambient air or the like through the increased surface area of fins according to well-known techniques. 
     Referring now to  FIG. 2 , the LED assembly  10  may further provide a supporting package including a lens assembly  36  providing a hemispherical dome  37  over the LED  16 . The hemispherical dome  27  may be of transparent thermoplastic having dimensions that follow a portion of the sphere substantially centered on the LED  16  with a diameter slightly smaller than the upper planar surface  14 . A radially extending flange  38  may pass outward from the periphery of the hemispherical dome  37  having a lower surface substantially equal to the height of the upper planar surface  14 . This flange  38  may extend over a sheet-metal console  40  or the like having an opening size to accept the LED assembly  10  therein. Clips  42  may extend rearward from the bottoms of the flanges  38  to fit through the hole in the sheet-metal console  40 . The clips  42  provide a spring-loaded outward cantilevered arm to retain the LED assembly  10  against the sheet-metal console  40 , the latter sandwiched between a lower portion of the flange  38  and upper portion of each cantilevered arm of the clips  42 . 
     A cylindrical collar or socket  44  may also extend rearward from the flange  38  inside of the clip  42  to receive the upper planar surface  14  of the ceramic substrate/heat sink  12  therethrough. The periphery of the upper planar surface  14  of the ceramic substrate/heat sink  12  within the cylindrical socket  44  may abut against a lip  46  of the dome  37 . The ceramic substrate/heat sink  12  may present a flange surface  50  at the periphery of its rearward face that may receive a compression washer  52  fitting between the inner walls of the cylindrical socket  44  and the flange surface  50  to retain the ceramic substrate/heat sink  12  abutting the lip  46 . 
     Referring also to  FIG. 7 , the compression washer  52  may be formed from a flexible metal sheet and may provide a ring  53  having outwardly splayed pawls  55  that operate to stake the compression washer  52  within the cylindrical socket  44  with a press-fitting operation so that the ring  53  closely abuts the flange surface  50  and the splayed pawls  55  are braced against the inner surface of the cylindrical socket  44  in the manner of a ratchet pawl to prevent shifting of the ring  53  away from the surface  50 . The ring  53  may provide for embossed dimples  57  that serve to limit the force of the compression washer  52  against the surface  50  and by providing a space between the ring  53  and the surface  50 , allow flexure of the compression washer  52  accommodate thermal expansion of the ceramic substrate/heat sink  12  within the sealed housing. 
     Referring again to  FIG. 2 , a remainder of the cylindrical socket  44  beyond the flange surface  50  and compression washer  52  may be filled with a low temperature thermoplastic  59  such as nylon to seal out moisture from the dome  37  by fusing with the inner wall of the cylindrical socket  44 . The low temperature thermoplastic  59  may also be molded into strain relief arms  54  extending rearward along the conductors  24  to seal the conductors  24  by fusing with the insulation covering of the conductors  24 . The strain relief arms  54  have circumferential ribs providing controlled flexibility and limiting a radius of curvature of the bending of the conductors  24 . 
     A bridge of thermoplastic  56  between the strain relief arms  54  abuts a rear surface of the ceramic substrate/heat sink  12  to further prevent its disengagement. 
     Referring now to  FIGS. 3 and 4 , the conductive traces  20  leading from the LED  16  may extend toward the periphery of the upper surface  14  toward two separated rectangular openings  60  extending downward into the surface  14  which may receive the power lead conductors  24  upward therethrough. A spring clamp  22  may be inserted into the rectangular openings  60  to provide electrical connection between the power lead conductor  24  and the conductive traces  20  for each lead of the LED  16 . 
     The spring clamp  22  may be formed from a single strip of metal such as a brass or bronze and provides an upper bridge  61  that will ultimately lie generally parallel to the upper surface  14  of the ceramic substrate/heatsink  12  and which has left and right legs  62   a  and  62   b  extending downward there from to fit into rectangular openings  60 . The width of the legs  62  (generally perpendicular to the long extent of the bridge  61 ) conforms to the width (shortest cross-sectional dimension) of the rectangular opening  60  and the separation of the legs  62   a  and  62   b  conform generally to the length (longest cross-sectional dimension) of the rectangular opening  60  so that the spring clamp  22  may fit snugly within the rectangular opening  60 . 
     The center of the bridge  61  provides a ring portion  64  which will be coaxial around the upward extending conductor  24  when the spring clamp  22  is in place in the rectangular opening  60  with the bridge  61  substantially flush against the top of the surface  14  and the conductor  24  engaged with the spring clamp  22  after passing upward through the ceramic substrate/heatsink  12 . The ring portion  64  includes left and right downwardly extending dimples  66  bisecting the ring portion  64  and the bridge  61 . The dimples  66  are separated from each other by a distance greater than the width of the rectangular openings  60  so as to straddle the rectangular openings  60  when the spring clamp  22  is in place in the rectangular opening  60 . One dimple  66  will contact the upper surface of the conductive trace  20  closest to the rectangular opening  60  and the other dimple  66  will contact the upper surface  14  directly on the opposite side of the rectangular opening  60  to stabilize the ring portion  64  against torsion (the thickness of the conductive traces  20  is greatly exaggerated in  FIG. 3 ). 
     Referring now to  FIGS. 4 and 6 , each of the downwardly extending legs  62  near its lower extent may have an outwardly extending spring biased tooth  70  which in a relaxed state projects beyond the length of the rectangular opening  60  but which may flex inward together with inward flexing of the legs  62  to allow insertion of the spring clamp  22  into the rectangular openings  60 . A lower portion of the rectangular opening  60  beneath the surface  14  expands to a greater length providing outwardly extending and downwardly facing ledges  71 . When the spring clamp  22  is fully inserted into the rectangular opening  60 , the inwardly compressed teeth  70  may relax outward to engage these ledges preventing removal of the spring clamp  22  by upward directed forces. 
     Referring momentarily to  FIG. 5 , when the spring clamp  22  is inserted into the rectangular openings  60  and held downward by engagement of the ledges  71  and teeth  70 , the bridge  61  and ring portion  64  are flexed upward indicated by arrows  78  to provide a downward spring force  80  providing a positive engagement between the dimple  66  and the conductive trace  20  that is better resistant to vibration. 
     Referring again to  FIGS. 4 and 6 , lower ends of the legs  62  curve upward and toward each other to provide cantilevered conductive fingers  76  that approach each other at an angle and contact along a vertical axis passing through a midpoint of the bridge  61  below the ring portion  64 . The cantilevered conductive fingers  76  may thus provide a sliding electrical engagement with the conductor  24  inserted upward through the ceramic substrate/heat sink  12  and between the endpoints of these cantilevered conductive fingers  76 . The angled approach of the cantilevered conductive fingers  76  provide for resistance against extraction of the conductor  24  whose frictional engagement with the ends of the cantilevered conductive fingers  76  tends to tighten to tighten their grip on the conductor  24  when it is withdrawn. The same flexure permits some accommodation of thermal expansion that might otherwise unduly increase the tension in the conductor  24 . 
     Referring to  FIG. 6 , in an alternate embodiment, the LED  16  may be placed on top of one of the traces  20  and a bonding wire  82  attached between the upper surface of the LED  16  and the second trace  20  in a chip on board (COB) configuration. 
     Referring now to  FIGS. 8 and 9 , in an alternative embodiment the conductor  24  may pass upward through the ceramic substrate/heatsink  12  with the slot  60  of  FIG. 3  replaced with a vertical bore  80  sized to receive the conductor  24  and its surrounding insulation. A bared end of the conductor  24  may then pass upward through a conductive doughnut  82  joining with traces  20  to be attached thereto with solder  84  with a simple solder joint. 
     Various features of the invention are set forth in the following claims. It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.