Abstract:
In one embodiment, an LED bulb includes a plurality of metal lead frame strips, including at least a first strip, a second strip, and a third strip. First LED dies have their bottom electrodes electrically and thermally connected to a top surface of the first strip. Second LED dies have their bottom electrodes electrically and thermally connected to a top surface of the second strip. The top electrodes of the first LED dies are wire bonded to the second strip, and the top electrodes of the second LED dies are wire bonded to the third strip to connect the first LED dies and second LED dies in series and parallel. The strips are then bent to cause the LED dies to face different directions to obtain a wide emission pattern in a small space. The strips are then enclosed in a thermally conductive bulb having electrical leads.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a §371 application of International Application No. PCT/IB2014/062347 filed on Jun. 18, 2014 and titled “Bonding LED Die to Lead Frame Strips,” which claims the benefit of PCT/CN2013/000784, filed Jun. 28, 2013. Both PCT/IB2014/062347 and PCT/CN2013/000784 are incorporated herein. 
     FIELD OF THE INVENTION 
     This invention relates to the packaging of light emitting diodes (LEDs) and, in particular, to a technique for forming an LED module containing a plurality of interconnected LED dies and having good heat sinking capability. 
     BACKGROUND 
     High power, high brightness LEDs generate high heat in a very small area (e.g., 1 mm 2 ) and this heat must be efficiently dissipated while maintaining a small package (or module) size. Further, to generate a very high flux, there may be multiple LEDs in the same package. Such LEDs are usually connected in series and/or parallel within the package. 
     In a common package for a high power LED, the bottom metal pad of the LED die is soldered to a top metal pad of a thermally conductive submount (e.g., a ceramic), and the LED die is encapsulated. The external submount metal pads (electrically connected to the anode and cathode of the LED die) are then soldered to pads of a printed circuit board (PCB), where the PCB helps sink heat from the LED and provides power to the LED die. The LED&#39;s thermal coupling to the submount and PCB is not as good as a thermal coupling directly to a metal heat sink. When there are multiple LEDs in a single package connected in series, the thermal design is more complex since the LEDs cannot share a large metal pad for spreading heat. 
     This problem of heat dissipation is especially severe in the field of automobile headlights, where multiple high power LEDs must deliver high flux in a small space near a focal point in an enclosed headlight parabolic reflector. 
     Various prior art LED modules have been proposed for packaging multiple LEDs but there are still problems with heat sinking and size that make the packages unsuitable for an automobile headlight. Examples of this prior art include U.S. Pat. Nos. 7,806,560; 8,371,723; 2013/0005055; and 2008/0074871. 
     Therefore, what is needed is a technique for forming an LED module that has a relatively small size, has excellent heat sinking capability, and can contain an array of high power LEDs that may be used in automobile applications, such as headlights, indicator lights, or taillights. 
     SUMMARY 
     In one embodiment, high power vertical LEDs are used in a module that is inserted into a socket of a headlight, indicator light, or tail light assembly, or for another application. Each of the LED dies has a bottom metal pad which serves as a cathode and a thermal conductor. A top electrode is a wire bond pad and serves as an anode. 
     To provide the desired flux and voltage drop (such as for a 12V supply in an automobile), multiple LEDs are connected in parallel, and the parallel groups are connected in series. 
     In a simple example, it is assumed that 12 LED dies are required to generate the desired flux, where there are four groups of LED dies connected in parallel (three in each group), and the four groups are connected in series across the power terminals. The LEDs may emit white light by being blue GaN based LEDs with a yellow phosphor over them, such as a YAG phosphor. 
     The three LED dies in a first group have their bottom pads directly soldered or silver-epoxied to a common first copper strip, which is part of a lead frame. The three LED dies in a second group have their bottom pads directly soldered to a common second copper strip, which is part of the lead frame. The three LED dies in a third group have their bottom pads directly soldered to a common third copper strip, which is part of the lead frame. The anodes of the LED dies in the first group are wire-bonded to the second copper strip. The anodes of the LED dies in the second group are wire-bonded to a third copper strip. The anodes of the LED dies in the third group are wire-bonded to a fourth copper strip. All the copper strips have a reflective silver layer for reflectance. 
     In another embodiment, the LED dies have a bottom metal pad which serves as an anode and a thermal conductor. A top electrode is a wire bond pad and serves as a cathode. The cathodes of the LED dies in the first group are wire-bonded to the second copper strip. The cathodes of the LED dies in the second group are wire-bonded to a third copper strip. The cathodes of the LED dies in the third group are wire-bonded to a fourth copper strip. A transparent encapsulant is molded around the LED dies and the copper strips, such as at three locations, to allow the strips to be bent between the encapsulation castings yet providing a mechanical coupling between the strips. In one embodiment, the transparent encapsulant is used as an optically transmissive enclosure. 
     The copper strips are then bent, such as in a triangular or U-shape, so that the ends of the copper strips are in the same plane to form anode and cathode end leads of the array of LEDs. A current controller is coupled between the ends of the strips and power leads of the module. The ends of the strips may snap into a flange for connection to the current controller. 
     The strips are much wider than needed to carry the required current since they are also used for heat sinking and reflection. In one embodiment, the strips are more than five times the width of the LED dies. 
     Therefore, all the LED dies have their light emitting surfaces facing outward and around a wide angle. Since there is side emission from the LED dies, the light emission may be generally spherical. 
     The electrical structure is then enclosed in a transparent protective bulb for protection of the electrical structure and to give the module a standard form, such as a T20 bulb form, for connection to a standard socket. The bulb may be a clear, thermally conductive plastic to encapsulate the LEDs and lead frame. The wide emission pattern of the bent electrical structure is particularly suitable for use in a reflective assembly. 
     In another embodiment, the electrical structure is covered with the optically transmissive enclosure to form a primary optics, and then the electrical structure with the optically transmissive enclosure is enclosed in a thermal conductive plastic for protection of the electrical structure, dissipation of the heat and to give the module a standard form. The thermal conductive plastic encapsulates the lead frame, and contacts the optically transmissive enclosure with out-light surfaces of the electrical structure exposed. 
     In another embodiment, the bulb may allow air to circulate within the bulb to cool the copper strips and LED dies. Since there is an air gap above, below, and on the sides of each oversized copper strip, the LED dies are cooled even in a headlight or tail light reflector assembly. 
     This concept may be applied to any number of LED dies connected in parallel and series. Different color LEDs or different types of LEDs may be mounted on the same copper strip or mounted on different strips in the same lamp. 
     The resulting module uses relatively few parts, improving its reliability. 
     In another embodiment, the copper strips are not bent and are supported in a reflective cup. The strips spread heat and connect the LED dies in series and parallel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a section of a module, showing four LED dies mounted on copper strips (part of a single lead frame), where the LED dies are connected in series and parallel. 
         FIG. 2  is a cross-sectional view of one of the strips of  FIG. 1  with a layer of silver epoxy in an LED die area. 
         FIG. 3  illustrates an LED die electrically and thermally attached to the strip via the silver epoxy after a curing step. 
         FIG. 4  illustrates a gold ball placed on the top electrode of the LED die for a wire bond. 
         FIG. 5  illustrates a gold wire connecting the top anode of the LED die to the adjacent copper strip. 
         FIG. 6  is a perspective view of raised sections of the copper strips, with one LED dies mounted on the raised section. 
         FIG. 7  is a perspective view of a lead frame with raised sections. 
         FIG. 8  is a top down view of three copper strips, with six LED dies (connected in parallel) on each of two of the strips, and the two sets of LED dies connected in series with wire bonds. 
         FIG. 9  is a perspective view of a portion of the module after the strips of  FIG. 8  have been bent to form a triangle shape. In an actual embodiment, the strips and LED dies would have been encapsulated at the three flat sections of the triangle prior to the bending. 
         FIG. 10  is a side view of a portion of the module of  FIG. 9  showing a molded epoxy casting around a portion of the strips (for mechanical stability) and how a transparent silicone or epoxy lens encapsulates the LED dies. 
         FIG. 11  is a perspective view of the module portion of  FIG. 10  showing two strips and the encapsulant over two LED dies connected in parallel. 
         FIG. 12  is a cross-sectional end view of the three strips of  FIG. 9  and the encapsulant over the LED dies. 
         FIG. 13  is a perspective view of the module portion of  FIG. 12  showing two encapsulated LED dies connected in series. 
         FIG. 14  is a side view of the module portion of  FIG. 9  with the castings have been formed, which further shows the current controller portion of the module and a transparent bulb for protection of the electrical structure. 
         FIG. 15  is a front view of the module of  FIG. 14 . 
         FIGS. 16-18  are perspective views of the lead frames attached with the electric components. 
         FIG. 19  is a front view of a mechanical reference on the module. 
         FIG. 20  is a side view illustrating the module is installed in a luminaire. 
         FIG. 21  is a perspective view illustrating the copper strips bent in a U-shape for a different emission pattern. The strips may then be mounted in the way shown in  FIGS. 12 and 13 . 
         FIG. 22  is a side view of the strips of  FIG. 14 . 
         FIG. 23  illustrates how the strips may narrow under the LED dies and then widen to allow the LED dies in a row to be closer together without sacrificing heat sinking by the strips. 
         FIG. 24  is a cross-sectional view of a conical reflective cup containing the copper strips and LED dies, such as from  FIG. 1 . 
     
    
    
     Elements that are the same or similar are labeled with the same numeral. 
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a section of a module  10 , showing four LED dies  12  mounted on copper strips  14 A and  14 B (part of a lead frame), where the LED dies  12  are connected in series and parallel. The strip  14 C is used as an interconnect and is part of the lead frame. The strip  14 C could be narrower than the strip  14 A and the strip  14 B. The strips  14 , during fabrication of the module  10 , are provided in a large, stamped sheet along with strips for other modules. The strips  14  are connected together by thin cross-pieces of copper that will later be cut off during singulation. 
     The strips  14  will typically be on the order of 0.25-0.5 mm thick and at least 2 mm wide for good heat sinking. Since the LED dies  12  are typically less than 1 mm wide, the strips  14  are much wider than required for carrying the LED current. In one embodiment, the strips  14  are over four times the width of the LED dies  12 . The strips  14  are plated with silver to be highly reflective, so providing wide strips  14  also provides the synergy of good heat conduction, good reflectance of side light, and low resistance. 
     For an automobile headlight application, the LED dies  12  may be white light LEDs comprising a blue emitting LED with a YAG phosphor. Other types of LEDs may also be used depending on the application. Red or amber LEDs may be used for tail lights. 
     The LED dies  12  in the examples are vertical types, with a metal layer on the bottom acting as a cathode and a smaller electrode on top acting as an anode. Other types of LED dies may be used, including vertical types with an anode on top and a cathode on the bottom, lateral LEDs with both electrodes on top and a metal thermal pad on the bottom. Flip chips may also be used, but there is additional complexity in electrically connecting to the bottom electrodes. 
     The LED dies  12  on a single strip  14 A or  14 B are connected in parallel since their bottom electrodes are bonded to their respective strips  14 , and their top electrodes are wire bonded, via gold wires  16 , to an adjacent strip  14 . This also results in the groups of parallel LED dies  12  to be connected in series. 
     Although only two groups of parallel-connected LED dies  12  are shown in  FIG. 1 , there may be any number of LED dies  12  mounted on a single strip  14 , and there may be any number of strips  14  for connecting the groups of LED dies  12  in series. The number of series connections affects the voltage drop across the module. The optimum voltage drop across the module and the number of LED dies  12  used depends on the particular application. 
     In another embodiment, there are four groups of LED dies, each group being mounted on a single copper strip, and three LED dies in each group connected in parallel on the strip. The groups are connected in series. Five copper strips are used in that embodiment. 
     The LED dies on a single strip or in a single row may be different colors. For example, some LED dies may emit red light, some may emit amber light, and some may be blue LEDs with a phosphor to emit white light. The LED dies on a single strip may be different wavelength, different color temperature or different flux also. 
     If one LED die on a strip is different from another LED die on the strip and has a different voltage drop, their anode wires  16  may be connected to different voltages. This may be done by connecting the anode wires to different strips, since each strip is at a different voltage. 
     In one embodiment, ends of the strips are electrically connected to terminals of the module for connection to a power supply, and power may be selectively applied to different combinations of the strips to turn some LED dies on but not others. For example, in  FIG. 1 , applying power across strips  14 A and  14 C will turn all the LED dies  12  on, but applying power across only strips  14 A and  14 B will only turn on the LED dies  12  mounted to the strip  14 A. Similarly, applying power across strips  14 B and  14 C will only turn on the LED dies  12  on the strip  14 B. This technique can be used to create different color emissions from a single module, and colors on different strips may be selectively combined to create a wide range of colors. There may be any number of groups of LED dies  12  (each group on a different strip) connected in series. If all LED dies emit the same color, power may be selectively applied across different combinations of strips to achieve a desired brightness. 
     In an example of a tail light that is to emit bright red light for indicating a stop, a bright amber light for turning, and a less bright amber light that is continuously on, such red and amber LED dies may be mounted on respective strips, and power may be selectively applied across different combinations of the strips, so that the strip(s) with the red LED dies are turned on when there is a stop, all of the amber LED dies are turned on when there is a turn, and only some of the amber LED dies are turned on continuously. 
     A driver  20  is shown supplying the required current through different combinations of the strips  14 . 
     Ultimately, the LED dies  12  and strips  14  are encased in a transparent, molded epoxy casting  22  at various sections with optional openings for lenses over the LED dies  12 . The casting  22  provides mechanical support, and protection of the LED dies  12  as well as wire bonds  16 . 
     In another embodiment, the casting  22  could also be formed directly in a particular shape as the transparent encapsulant (primary optics) to realize different emission patterns, as well as to provide mechanical support, and protection of the LED dies  12  and wire bonds  16 . The casting  22  material should have good optical transmissivity of at least 50%. The transparent encapsulant could cover part of LED dies, or all LED dies. Preferably, the transparent encapsulant covers at least two LED dies. Multiple transparent encapsulants could be formed on the lead frame to cover the LED dies, and to further form an optically transmissive enclosure. 
       FIG. 2  is a cross-sectional view of one of the strips  14 A of  FIG. 1  with a layer of thermally conductive silver epoxy  24  in an LED die area. The epoxy  24  may be printed or deposited in other ways. 
       FIG. 3  illustrates an LED die  12  electrically and thermally attached to the strip  14 A via the silver epoxy  24  after a heat curing step. 
       FIG. 4  illustrates a gold ball  26  ultrasonically bonded to the top electrode of the LED die  12  for a wire bond. Similar gold balls may be bonded to the adjacent strip  14 B. 
       FIG. 5  illustrates a gold wire  16  ultrasonically bonded between the gold ball  26  and the adjacent strip  14 B, creating a parallel and series connection as discussed above. 
       FIGS. 6-7  illustrate lead frame strips  14  having a raised section where the LED dies  12  are to be mounted to simplify the epoxy casting step. The strips  14  may also have indented areas to define where bends are to occur in a later step. 
       FIG. 8  is a top down view of a larger portion of the three copper strips  14 A,  14 B, and  14 C, with six LED dies  12  (connected in parallel) on strip  14 A, and six LED dies  12  on strip  14 B. The two sets of LED dies  12  are connected in series with the wires  16 . At this stage, the full lead frame sheet is still in-tact, supporting perhaps hundreds of LED dies. In another embodiment, there are four strips  14  and three groups of LED dies  12 , with four LED dies  12  mounted on three of the strips. 
     A current controller (not shown), for regulating current through the LED dies  12  in a single module, may be connected to the strips  14  of each module at this time. The current controller may consist of one or more resistors connected to different power terminals of the module, and a brightness of the emission depends on which resistor conducts the current. A reverse voltage protection circuit and ESD protection may also be included in the module. 
     The transparent epoxy castings  22  ( FIG. 1 ) may then be molded around the LED dies  12  and strips  14  for mechanically keeping the group of strips  14  together. 
     The lead frame sheet is then cut to separate out the individual modules. 
     Dashed lines  32  and  34  illustrate where the strips  14  will be bent to form the triangular shape shown in  FIG. 9 .  FIG. 9  does not show the castings  22 . 
       FIG. 10  illustrates a portion of the strip  14 B, showing the casting  22  and lenses  38  (primary optics) formed over the LED dies  12  (connected in parallel). The lenses  38  may be molded over the LED dies  12 , or a preformed lens is positioned over each LED die  12  and affixed with a transparent silicone for encapsulating the LED dies  12 . If lenses are used, the castings  22  may have openings for the LED dies  12 . The optical transmissivity of the lenses  38  or encapsulant should be over 50%. 
       FIG. 11  is a perspective view of the module portion of  FIG. 10 , showing two strips  14 B and  14 C. 
       FIG. 12  is a cross-sectional end view of the three strips  14 A,  14 B, and  14 C of  FIG. 9 , the casting  22  and the lenses  38  over two LED dies connected in series. 
       FIG. 13  is a perspective view of the module portion of  FIG. 12 . 
       FIG. 14  is a side view of the entire module  10 , showing the epoxy castings  22  around potions of the strips  14 . The side view shows strip  14 A. One end of the strip  14 A is inserted into a connector  40  that shorts the end to another portion of the strip  14 A and maintains the triangular shape. The ends of the strips  14 B and  14 C are similarly inserted into the connector  40  for shorting the ends of the respective strips to other portions of the strips and maintaining the triangular shape. By shorting the end of a strip to another portion of the strip after bending, the current is more evenly conducted along the strips. The connector  40  may be a flange that compresses the parts of the strips  14  together when portions of the flange are snapped together by the manufacturer. In some embodiments, it is not necessary to short the ends of a strip, and keep one of the ends of the strip open, which may be fixed by the bulb  44  finally. 
     The strips  14 A and  14 C are connected to a current controller  42  for regulating the current through the strips  14 A,  14 B, and  14 C. In one embodiment, the current controller  42  may comprise ESD protection elements and resistors for limiting current. In an embodiment where the module  10  is used for both a stop light and a tail light, the current controller uses a first resistor for the stop light function to provide an increased current (and flux), and second resistor for the tail light function to provide a reduced current (and flux). 
     The resulting electrical structure is then placed in a transparent plastic or glass bulb  44  for protection. Or the resulting electrical structure could also be over molded with a thermal conductive plastic. While a non-transparent plastic is used, the module  10  should be encapsulated with out-light surfaces of the transparent encapsulant exposed. The shape of the bulb  44  may be a standard T20 bulb size for use in an automobile headlight or tail light assembly, or other application. The bottom of the bulb  44  may include electrical connectors that contact the strips  14 A and  14 C to provide standardized leads for a T20 socket. 
       FIG. 15  is simplified front view of the module  10  of  FIG. 14 . The bulb  44  material may be a thermally conductive plastic that is molded around the casting  22  and strips  14  with out-light surfaces of the casting  22  exposed. The casting  22  in  FIG. 15  are formed directly in a particular shape as the transparent encapsulant to realize different emission patterns, as well as to provide mechanical support, and protection of the LED dies and the wire bonds. The wide copper strips  14  spread the heat to cool the LED dies. The bulb  44  provides uniform temperature distribution. 
     In another embodiment, the bulb  44  is hollow and allows air to flow inside. Air openings  45  may be at the bottom of the bulb  44  for air circulation to remove heat from the strips  14 . The leads  46  also thermally couple the strips  14  to the socket. 
     After the bulb  44  encases the electrical portion of the module  10 , any lead frame strips  14  extending from the bottom of the bulb  44  may be cut if the bulb  44  itself provides the leads for the socket. The bulb  44  may also be a bulb that meets standards of H series, P series, T series, W series or R series in a bulb form. The resulting lamp (or module or bulb) may have more than two terminals if different brightnesses are desired or different color combinations are desired 
       FIGS. 16-18  demonstrate that the strips  14  are attached with different electric components, including resistors  60 , diodes  61 , and capacitors  62  to realize a current driving function, a reverse polarity protection function, or an ESD protection. 
     The location of the LED dies  12  above the bottom of the bulb  44  may be set to be near the focal point of a reflective cavity, such as a headlight or tail light, after the lamp is plugged in. 
       FIGS. 19-20  show an example of a mechanical reference  63  formed on a thermal conductive plastic as a reference for fixing an optical center of the bulb  44 . The bulb  44  itself provides the leads for the socket. While the bulb  44  is installed or plugged in the luminaire  64 , the mechanical reference  63  on the bottom part of the bulb  44  may be taken as a reference to match the optical center of the module  10  and the optical center of the luminaire  64 . The mechanical reference  63  also can be formed on a transparent bulb or a glass bulb. The mechanical reference  63  could be a wedge-shape or a snap joint. 
       FIG. 21  is a perspective view illustrating the copper strips  14 A,  14 B, and  14 C bent in a U-shape for a different emission pattern. The strips may then be mounted in the same way shown in  FIGS. 14 and 15 . 
       FIG. 22  is a side view of the strips  14  of  FIG. 21 . 
     The modules of  FIGS. 14 and 21  are particularly suitable for use in a parabolic reflector assembly, such as for a headlight or tail light, where the wide emission pattern is directed in a forward direction by the reflector. 
     The modules have relatively few parts, resulting in increased reliability. 
     Any number of LED dies  12  in any number of groups in series and parallel may be provided in a small module size. There is no thermally insulating material between the LED dies  12  and the copper strips  14 , causing the heat to be spread by the relatively long and wide copper strips  14 . The circulating air, or the thermally conductive bulb  44  material, removes heat from the front, back, and sides of the strips. 
       FIG. 23  illustrates a portion of a set of copper strips  47 A- 47 E on which a single row of LED dies  12  is mounted and connected in series (the wiring is not shown). More LED dies  12  may be mounted on the strips  47  for connecting in parallel. In order to place the LED dies  12  close together while not sacrificing the heat sinking of the copper strips  47 , the strips  47  don&#39;t need to be rectangular and can be narrowed in the vicinity of the LED dies  12  and widen out to provide a high thermal mass with a large surface area for cooling. An area of the LED die means a minimum area for mounting the LED die on the strip. The minimal pitch between two adjacent LED dies  12  is no more than 2 mm, as is shown in  FIG. 23 . The pitch is a distance between centers of two adjacent LED dies. The relationship of areas of the strips and the minimum area of the LED die could satisfy a below formula, in which A1, A2, A3 and A4 represent the minimum area for each LED dies  12 , and B1, B2, B3 and B4 represent the area of each strip for thermal dissipation: 
     
       
         
           
             
               
                 
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     Due to a compact arrangement of the LED dies in each row, their photometrical and electrical characteristics merge together, so that different types of LED dies could be used in one row. For example, LED dies with different dominant wavelength, different forward voltage, or different flux could be formed in one row. By combining different bins (wavelength, voltage, flux) of LED dies in one row, the cost for the module could be largely reduced. 
       FIG. 24  illustrates another embodiment of a lamp where a conical reflective cup  50  contains the copper strips  14 A,  14 B, and  14 C and LED dies  12 , such as from  FIG. 1 . The cup  50  may be a thermally conductive ceramic, or metal with an insulating layer, having reflective walls  52 . A light ray  53  is shown being directed upward by the reflective walls  52 . The LED dies  12  are encapsulated by a transparent encapsulant  54  or an encapsulant that contains a phosphor. Metal leads  56  extend from the strips  14 A and  14 C to terminate in large pads or leads at the bottom of the cup  50  for connection to a thermally conductive PCB. The strips  14  spread the heat over the cup  50  to remove heat from the LED dies  12 , and provide a reflector. 
     In the various embodiments, the strip  14 C is only used for interconnecting the LED dies  12  and does not support any of the LED dies  12  for heat sinking. However, it is convenient to interconnect the LED dies  12  using a portion of the lead frame that also forms the strips  14 A and  14 B. In some applications, an interconnector other than the strip  14 C may be used, and strip  14 C is not used. 
     Other lamp designs are envisioned using the present technique. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.