Abstract:
In accordance with the invention, an illumination device comprises a highly thermally conductive substrate having a surface, a plurality of light emitting diodes (LEDs) supported by the surface and arranged in an array to provide illumination. At least one reflective barrier at least partially surrounds each LED. The reflective barrier is shaped to reflect away from the LED light emitted by other LEDs in the array. Advantageously the LEDs and reflective barrier are thermally coupled to a heat spreader to dissipate heat generated by the LEDs. The substrate preferably comprises an LTTC-M heat spreader, and the reflective thermal barriers preferably comprise metal ridges or cups.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/822,191 filed on Apr. 9, 2004 (“Light Emitting Diode Arrays With Improved Light Extraction”). U.S. patent application Ser. No. 10/822,191 is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to light emitting diode (LED) arrays and, in particular, to LED arrays with integral reflective barriers and methods for making same.  
       BACKGROUND OF THE INVENTION  
       [0003]     Light emitting diodes (LEDs) are being used as light sources in an increasing variety of applications extending from communications and instrumentation to household, automotive and other visual displays. LED arrays comprise a plurality of LEDs arranged on a common substrate. One problem with LED arrays is the significant heat generated by dense concentrations of LEDs. Solutions to the thermal problems associated with LED arrays are the subject of a related application entitled, “ Light Emitting Diodes Packaged For High Temperature Operation ” U.S. patent application Ser. No. 10/638,579, filed Aug. 11, 2003. The 10/630,579 application is incorporated herein by reference.  
         [0004]     Another problem in LED arrays concerns illumination efficiency. Illumination efficiency is a measure of the percentage of generated light that actually leaves an LED package and that can serve as useable light in the intended application.  FIGS. 1 and 2  show a typical LED array  10 . LED dies (semiconductor chips)  12  generate light. LED dies  12  are typically box-like in structure with 6 sides. Since they are almost always mounted on one of the light surfaces, the other 5 surfaces are capable of emitting light generated by the device. Some of the light is absorbed by nearby walls  22  of array package  11 , some is reflected back to the emitting die, and some is absorbed directly by nearby LED die  12  in the array. The remainder of the light exits the package.  
         [0005]     There is a relationship between illumination efficiency and the thermal problems of LED arrays. Self-heating by absorption contributes to thermal problems. Thus, there is a need for an LED packaging arrangement that can increase the illumination efficiency of LED array devices and reduce the thermal problems produced by absorption.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with the invention, an illumination device comprises a highly thermally conductive substrate having a surface, a plurality of light emitting diodes (LEDs) supported by the surface and arranged in an array to provide illumination. At least one reflective barrier at least partially surrounds each LED. The reflective barrier is shaped to reflect away from the LED light emitted by other LEDs in the array. Advantageously the LEDs and reflective barrier are thermally coupled to a heat spreader to dissipate heat. The substrate preferably comprises an LTTC-M heat spreader, and the reflective thermal barriers preferably comprise metal ridges or cups. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings:  
         [0008]      FIG. 1  shows a typical LED array according to the prior art;  
         [0009]      FIG. 2  shows a side view of the LED array of  FIG. 1 ;  
         [0010]      FIG. 3  shows an exemplary LED package of the prior art;  
         [0011]      FIG. 4  shows accordion-tapered reflective thermal barriers;  
         [0012]      FIG. 5  shows a side view of an LED array with integral tapered thermal barrier reflectors;  
         [0013]      FIG. 6  shows a metal cup tapered reflective barrier;  
         [0014]      FIG. 7  shows a multi-cup tapered reflective barrier;  
         [0015]      FIG. 8  shows wedge shaped cup tapered barriers arranged to form a circular array;  
         [0016]      FIGS. 9-11  show exemplary reflecting surfaces on an LED cavity wall, according to embodiments of the present invention;  
         [0017]      FIG. 12  shows an assembly including a mushroom-like heat sink;  
         [0018]      FIG. 13  illustrates an exemplary LED package comprising a stamped reflector adapted for a directional lighting array, according to an embodiment of the present invention; and  
         [0019]      FIG. 14  illustrates an exemplary LED package, according to an embodiment of the present invention.  
         [0020]     It is to be understood that these drawings are for illustrating the concepts of the invention and are not to scale. 
     
    
     DETAILED DESCRIPTION  
       [0021]     This description is divided into two parts. Part I describes the structure and features of light emitting diodes (LEDs) packaged in an array for high illumination efficiency in accordance with the invention and illustrate exemplary embodiments. Part II provides further details of the LTCC-M packaging technology as applicable to LED arrays.  
         [0000]     I. LEDs PACKAGED FOR HIGH ILLUMINATION EFFICIENCY  
         [0022]      FIG. 4  illustrates, a tapered barrier reflector  40  fabricated as a periodic array of troughs  43  and tapered reflective ridges  42 . This accordion-like structure is a particularly cost effective to manufacture. Metal reflective material can be folded in an accordion-like manner to form the tapered reflective barriers  42 . LED dies  12  can be affixed in the troughs  43  between reflective barriers  42 .  
         [0023]     The barrier reflector  40  can provide a connection to the anode or cathode of LED dies  12 . The barrier  40  also serves a thermal cooling function. Heat from the LED die  12  can be channeled by the barrier reflector  40  to associated thermal spreaders and to a supporting LTCC-M substrate  
         [0024]      FIG. 5  shows an alternative high efficiency LED array  50  comprising discrete reflective barrier structures  52 . The reflective thermal barriers  52  are advantageously shaped as fins causing the heat to move from the bottom of the LED  12  through the thermally conductive material (such as solder or silver epoxy  53  ) to the top of the fin. The length and angle of the fin can be modified by those skilled in the art. The thermal resistance of a LED array package is inversely proportional to the heat dissipating area. Thus the more and longer reflective thermal barriers  52  are, the larger the area for heat dissipation. The LED devices are subsequently encapsulated as by an optically matched clear epoxy  55  formed in a domed shaped in order to increase light extraction and to minimize total internal reflection (TIR).  
         [0025]     LED dies  12  are disposed in an array pattern overlying substrate  54 . Tapered discrete reflective barriers  52  cause light that would have been absorbed by other die or walls of the package to reflect out of the array package, thus increasing the illumination efficiency. LED dies  12  can be affixed to substrate  54  by solder or epoxy  53 .  
         [0026]     Substrate  54  may be non-conducting or conducting. In the case where substrate  54  is conducting, or has overlaying conductive patterns and traces (not shown), an electrical connection can be made on the mounting surface to either the anode or cathode of LED dies  12 . In the case of conductive traces, both the anode and cathode connections can be made on the mounting surfaces of dies  12 .  
         [0027]      FIG. 3  shows further detail of possible electrical connections to die  12 . The electrical connections can be made via wire bonds  32  to the LED anode and cathode. Alternatively either the anode or cathode can make electrical contact with a conductor on an insulating substrate, or a conductive substrate. In this case, the remaining terminal can then be connected to dies  12  by a single wire bond. LED die can be soldered (to a metal substrate  36  or overlying conductor  35 ) or they can be epoxied  33  to substrate  36 . A typically translucent or transparent package wall  34  can support a translucent or transparent LED encapsulate  31 .  
         [0028]     Turning to  FIG. 6 , it can be seen that liquid epoxy  53  can be deposited on a metal conductor  56  and conductor  56  can be attached to substrate  54 . Conductor  56  can be thick film, thin film, electro-deposited, a metal laminate, or other suitable electrical and thermal conductor. If no electrical contact is required conductor  56  can be omitted; however additional heat spreading from the die can be accomplished if conductor  56  is used. Substrate  54  can be a ceramic, multilayer printed wire board, low temperature cofired ceramic (LTCC), LTCC on metal (LTCC-M), high temperature cofired ceramic (HTCC), or other suitable electrical insulator and thermal conductor. Substrate  54  can be an electrically conducting material if electrical contact to die  12  is desirable, or it can be an electrically insulating layer formed between the substrate and die  12 .  
         [0029]      FIG. 6  is an exploded view that shows a device  60  where the tapered barrier is a reflective cup  61 . LED dies  12  can then be affixed within each cup  61 . Each cup  61  is affixed to substrate  54  by solder or epoxy. A small hole  62 , with a diameter slightly smaller than the width of the die, can be formed in cup  61 . The hole  62  allows some of the liquid epoxy or solder to seep into the cup. Die  12  is then placed into the cup on top of the epoxy or solder. Similarly, LED die  12  can be placed in cup  61  and bonded directly through hole  62  to substrate  54  by epoxy or solder. While hole  62  is not required in assembly  60 , the hole facilitates fabrication because the additional step of adding epoxy or solder in the cup for the die can be avoided.  
         [0030]     Cup  61  can be fabricated from aluminum, stainless steel, tin, nickel, or other reflective material. The cups can be formed by stamping, etching, coining, machining, or other manufacturing method. Assembly of an array of LED die in reflective cup tapered barriers can be accomplished using inexpensive pick-and-place assembly.  
         [0031]      FIG. 7  illustrates an advantageous embodiment  70  using an array of cups  71 . This multi-cup assembly can be pressed, stamped, or otherwise formed from a sheet of metal  72 . Cups  71  need not be round and can be elliptical, square, or rectangular in shape. The metal can gauge from 0.002″ to 0.030″, and it can comprise brass, copper, phosphorous bronze, beryllium copper, stainless steel, titanium, Inconel, carbon or alloy steel or precious metal.  
         [0032]      FIG. 8  shows a circular array  80  created from wedge-shaped sector cups  81 . Each wedge  81  can include an LED die  12 , and the sector cups can be arranged in the form of a circle.  
         [0033]     Referring to  FIGS. 9, 10  and  11 , yet another method to create a reflecting surface on an LED cavity wall is to metallize the wall of the cavity with a thick or thin reflective metal film  101 , such as silver. Then a molten reflecting metal  92  such as solder is applied to use the cavity edge as a capillary holder ( FIG. 9 ). The molten metal will conform to the edge capillary and will shape the reflector cup wall into a parabola shape as seen in assemblies  100  and  110 . The middle of the cup can have a metal insert such as a high temperature solder ball  91  ( FIG. 10 ) or a metal/solder column  111  ( FIG. 11 ).  
         [0034]     In all of the above mentioned embodiments, the materials used to form the reflective barriers should have low absorption characteristics in the 300 to 800 nm wavelength range. The barrier surfaces can be dispersive or non-dispersive depending on the application.  
         [0035]     It is sometimes desirable to direct light that comes out of the LED on to a certain target.  FIG. 12  shows a conventional assembly  120  composed of multiple subassemblies  121  illuminating from the periphery of a mushroom-like heat sink  122 . The target illumination profile of this subassembly is 360 degrees at an inclination angle θ. In order to direct light at this angle, a truncated cone shape heat sink would have to be machined. Placing the subassemblies on the angled surface of the heat sink is difficult. Standard surface mount (SMT) technology can not be used. Thus fabrication becomes a complex, laborious and expensive process.  
         [0036]     In accordance with this aspect of the invention, reflector walls are used as guides for reflecting light directionally from an array. The barrier surfaces can be shaped as light guides to reflect the light at a particular target angle.  FIG. 13  depicts the outline of a stamped reflector cup  131  adapted for a directional lighting array. Direction at an angle θ is accomplished by shaping the cup. The die will sit at an angle due to the slope of the cup. The LED die will be connected with the substrate using conductive epoxy and wired using a wirebond though hole  132 . The die/cup assemblies can be mounted on a planar substrate by SMT assembly.  
         [0037]     Another advantageous embodiment is depicted in  FIG. 14 . The LED  12  can be placed using standard SMT equipment parallel with the thermally conductive substrate. A glob of conductive epoxy  142  is dispensed on the bottom of the cup  141 , attaching to the board both the cup and the LED die. The cup  141  is shaped so that the majority of the rays come out at angle θ. The electrical connection can be made with a gold wirebond  32  through a hole  132  in the cup. This embodiment is advantageous due to its simplicity and ease of volume manufacturing. Cup  141  can be fabricated from aluminum, stainless steel, tin, nickel, or other reflective material. It can be formed by stamping , etching, coining, machining, or other manufacturing method.  
         [0000]     II. LTCC-M PACKAGING  
         [0038]     LTCC-M packaging is particularly suitable for dispensing heat generated by densely packed arrays of LED die. This section highlights some of the important aspects of LTCC-M packaging applicable to fabricating LED arrays with reflective barriers.  
         [0039]     Multilayer ceramic circuit boards are made from layers of green ceramic tapes. A green tape is made from particular glass compositions and optional ceramic powders, which are mixed with organic binders and a solvent, cast and cut to form the tape. Wiring patterns can be screen printed onto the tape layers to carry out various functions. Vias are then punched in the tape and are filled with a conductor ink to connect the wiring on one green tape to wiring on another green tape. The tapes are then aligned, laminated, and fired to remove the organic materials, to sinter the metal patterns and to crystallize the glasses. This is generally carried out at temperatures below about 1000° C., and preferably from about 750-950° C. The composition of the glasses determines the coefficient of thermal expansion, the dielectric constant and the compatibility of the multilayer ceramic circuit boards to various electronic components. Exemplary crystallizing glasses with inorganic fillers that sinter in the temperature range 700 to 1000° C. are Magnesium Alumino-Silicate, Calcium Boro-Silicate, Lead Boro-Silicate, and Calcium Alumino-Boricate.  
         [0040]     More recently, metal support substrates (metal boards) have been used to support the green tapes. The metal boards lend strength to the glass layers. Moreover since the green tape layers can be mounted on both sides of a metal board and can be adhered to a metal board with suitable bonding glasses, the metal boards permit increased complexity and density of circuits and devices. In addition, passive and active components, such as resistors, inductors, and capacitors can be incorporated into the circuit boards for additional functionality. Where optical components, such as LEDs are installed, the walls of the ceramic layers can be shaped and/or coated to enhance the reflective optical properties of the package, or reflective barriers as described herein in Part I, can be used to further improve both the illumination and thermal efficiency of the LED array package.  
         [0041]     This system, known as low temperature cofired ceramic-metal support boards, or LTCC-M, has proven to be a means for high integration of various devices and circuitry in a single package. The system can be tailored to be compatible with devices including silicon-based devices, indium phosphide-based devices and gallium arsenide-based devices, for example, by proper choice of the metal for the support board and of the glasses in the green tapes.  
         [0042]     The ceramic layers of the LTCC-M structure must be matched to the thermal coefficient of expansion of the metal support board. Glass ceramic compositions are known that match the thermal expansion properties of various metal or metal matrix composites. The LTCC-M structure and materials are described in U.S. Pat. No. 6,455,930, “Integrated heat sinking packages using low temperature co-fired ceramic metal circuit board technology”, issued Sep. 24, 2002 to Ponnuswamy, et al and assigned to Lamina Ceramics. U.S. Pat. No. 6,455,930 is incorporated by reference herein. The LTCC-M structure is further described in U.S. Pat. No. 5,581,876, 5,725,808, 5,953,203, and 6,518502, all of which are assigned to Lamina Ceramics and also incorporated by reference herein.  
         [0043]     The metal support boards used for LTCC-M technology do have a high thermal conductivity, but some metal boards have a high thermal coefficient of expansion, and thus a bare die cannot always be directly mounted to such metal support boards. However, some metal support boards are known that can be used for such purposes, such as metal composites of copper and molybdenum (including from 10-25 % by weight of copper) or copper and tungsten (including 10-25 % by weight of copper), made using powder metallurgical techniques. Copper clad Kovar®, a metal alloy of iron, nickel, cobalt and manganese, a trademark of Carpenter Technology, is a very useful support board. AlSiC is another material that can be used for direct attachment, as can aluminum or copper graphite composites.  
         [0044]     Another instance wherein good cooling is required is for thermal management of flip chip packaging. Densely packed microcircuitry, and devices such as decoder/drivers, amplifiers, oscillators and the like which generate large amounts of heat, can also use LTCC-M techniques advantageously. Metallization on the top layers of an integrated circuit bring input/output lines to the edge of the chip so as to be able to wire bond to the package or module that contains the chip. Thus the length of the wirebond wire becomes an issue; too long a wire leads to parasitics. The cost of very high integration chips may be determined by the arrangement of the bond pads, rather than by the area of silicon needed to create the circuitry. Flip chip packaging overcomes at least some of these problems by using solder bumps rather than wirebond pads to make connections. These solder bumps are smaller than wire bond pads and, when the chip is turned upside down, or flipped, solder reflow can be used to attach the chip to the package. Since the solder bumps are small, the chip can contain input/output connections within its interior if multilayer packaging is used. Thus the number of die in it, rather than the number and size of bond pads will determine the chip size.  
         [0045]     However, increased density and integration of functions on a single chip leads to higher temperatures on the chip, which may prevent full utilization of optimal circuit density. The only heat sinks are the small solder bumps that connect the chip to the package. If this is insufficient, small active or passive heat sinks must be added on top of the flip chip. Such additional heat sinks increase assembly costs, increase the number of parts required, and increase the package costs. Particularly if the heat sinks have a small thermal mass, they have limited effectiveness as well.  
         [0046]     In the simplest form of the present invention, LTCC-M technology is used to provide an integrated package for a semiconductor component and accompanying circuitry, wherein the conductive metal support board provides a heat sink for the component. A bare semiconductor die, for example, can be mounted directly onto a metal base of the LTCC-M system having high thermal conductivity to cool the semiconductor component. In such case, the electrical signals to operate the component must be connected to the component from the ceramic.  
         [0047]     Indirect attachment to the metal support board can also be used. In this package, all of the required components are thermally coupled to a metal support board, that can also incorporate embedded passive components such as conductors and resistors into the multilayer ceramic portion, to connect the various components, i.e., semiconductor components, circuits, heat sink and the like, in an integrated package. In the case of LED arrays, where electrical circuit considerations would dictate an insulating material be used, thermal conduction can be problematic. Here the inventive reflective barriers further serve as thermal spreading devices to help transfer heat received by conduction and radiation through the insulating layer to the metal base.  
         [0048]     For a more complex structure having improved heat sinking, the integrated package of the invention combines a first and a second LTCC-M substrate. The first substrate can have mounted thereon a semiconductor device, and a multilayer ceramic circuit board with embedded circuitry for operating the component; the second substrate has a heat sink or conductive heat spreader mounted thereon. Thermoelectric (TEC) plates (Peltier devices) and temperature control circuitry are mounted between the first and second substrates to provide improved temperature control of semiconductor devices. A hermetic enclosure can be adhered to the metal support board.  
         [0049]     The use of LTCC-M technology can also utilize the advantages of flip chip packaging together with integrated heat sinking. The packages of the invention can be made smaller, cheaper and more efficient than existing present-day packaging. The metal substrate serves as a heat spreader or heat sink. The flip chip can be mounted directly on the metal substrate, which is an integral part of the package, eliminating the need for additional heat sinking. A flexible circuit can be mounted over the bumps on the flip chip. The use of multilayer ceramic layers can also accomplish a fan-out and routing of traces to the periphery of the package, further improving heat sinking. High power integrated circuits and devices that have high thermal management needs can be used with this new LTCC-M technology.  
         [0050]     It is understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.