Patent Publication Number: US-8115385-B2

Title: Multi-chip packaged LED light source

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/622,753, filed Jan. 12, 2007, the entire disclosure of which is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Light-emitting diodes (LEDs) are attractive candidates for the replacement of conventional light sources based on incandescent and fluorescent lights. LEDs have significantly higher power efficiencies than incandescent lights and have much greater lifetimes. In addition, LEDs do not require the high voltage systems associated with fluorescent lights and can provide light sources that more nearly approximate “point sources” than fluorescent fixtures. The latter feature is particularly important for light sources that utilize collimating or other imaging optics. 
     LEDs emit light in a relatively narrow spectral band. Hence, to provide a light source of an arbitrary perceived color, the light from a number of LEDs must be combined in a single light fixture or some form of phosphor conversion layer must be used to convert the narrow band of light to light having the desired color. While this complicates the construction of some LED light sources, it also provides the basis for light sources having a color that can be varied by altering the ratios of the light emitted by the various colored LEDs or an intensity by varying the power to all of the LEDs. In contrast, conventional light sources based on fluorescent tubes emit light of a fixed color and intensity. 
     To replace conventional light sources, several LEDs are typically needed. Typically, LEDs have power dissipations that are less than a few watts. Hence, to provide a high intensity light source to replace conventional light fixtures, a relatively large number of LEDs must be used in each light source. 
     In addition, LEDs age with use. Typically, the light output deceases with use and, in some cases, the spectrum emitted by the LED shifts with age giving rise to color shifts. In general, LEDs that emit different colors of light have different aging characteristics, since the aging profile of an LED depends on the fabrication process and materials, as well as other factors. In a light source based on three different color LEDs, the shift in intensity and/or spectrum causes the light emitted by the source to shift in color. To correct for these problems in a packaged LED source based on multiple LED dies, access must be provided to each die or group of dies that emit the same color of light so that the current through each die or group of dies can be adjusted separately over the life of the light source. 
     Heat dissipation is also a significant problem in the design of high-powered LED light sources. The efficiency with which an LED converts electrical power to light decreases with the temperature of the p-n junction in the LED. The shift in efficiency can lead to color shifts in a multi-LED light source based on LEDs of different colors. Also, the lifetime of the LED also decreases if the LED is operated at a high temperature. Hence, some mechanism for efficiently removing heat from the dies must be incorporated in the LED package. In general, the package includes some thermal path that thermally connects the LED dies to a larger heat-radiating surface such as the core of a printed circuit board on which the packaged light source is mounted. Providing such a thermal path in multi-chip LED packages presents problems, since each LED is normally mounted on a separate heat conducting pad that is connected to the printed circuit board core by a path that has a relatively high thermal resistance. To reduce the thermal resistance, the size of each of the thermal conductors must be increased, which, in turn, increases the size of the packaged light source. 
     SUMMARY OF THE INVENTION 
     The present invention includes a light source having a lead frame, a body, and a plurality of dies, each die having an LED thereon. The body includes a top surface, a bottom surface and a plurality of side surfaces. The lead frame includes first, second, and third sections, the first section includes a die mounting area having a first protrusion extending therefrom that passes through the body and terminates in a pad on the bottom surface. The second and third sections each include a lateral portion and a protrusion extending from the lateral portion, the protrusion being bent to form first and second leads that run along one of the side surfaces. Each of the dies is powered by applying a potential between the first and second contacts on the die. Each die is bonded to the die mounting area such that the first contact is electrically connected to the die mounting area, the second contact being connected to one of the second and third sections. The lead frame is embedded in the body such that light from the LEDs exits the body. The first protrusion of the first section has a lower thermal resistance than that of the first or second leads. The second and third leads can also terminate in pads on the bottom surface of the body to form a surface mounting packaged light source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are perspective views of prior art light source  20 . 
         FIG. 3  is a top view of prior art light source  20 . 
         FIG. 4  is a perspective view of the lead frame used in light source  60  to provide a clearer view of the lead frame  70  in the completed light source. 
         FIG. 5  is a top perspective view of light source  60 . 
         FIG. 6  is a bottom perspective view of light source  60 . 
         FIG. 7  is a cross-sectional view of lead frame  70  through line  7 - 7  shown in  FIG. 4 . 
         FIG. 8  is a cross-sectional view of a portion of a light source  90  according to another embodiment of the present invention. 
         FIG. 9  is a top view of another embodiment of a light source according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     The manner in which the present invention provides its advantages can be more easily understood with reference to  FIGS. 1-3 , which illustrate a prior art multi-LED packaged light source  20 .  FIGS. 1 and 2  are perspective views of light source  20 , and  FIG. 3  is a top view of light source  20 . To simplify the drawings, the details of the die placements and connections have been omitted from  FIG. 1 . 
     Light source  20  is constructed from a lead frame that is cut to provide leads  21 - 24  in the finished light source. LED dies  31 - 33  are mounted on leads  22 - 24 , respectively. The dies are powered by applying a potential to a first contact on the bottom of each LED and a second contact on the top of each LED. The second contacts are connected to lead  21  by wire bonds such as wire bond  35 . The contacts are accessed by the portion of the leads that extend outside the package. 
     The package includes two parts. The lower part  28  is molded around the leads after the dies have been attached to the leads and the various wire bonds formed. The upper portion  26  includes a cavity  25  having a reflective wall  27 . The upper portion is bonded to the lower portion after the lower portion is molded. The lead frame is then cut to leave the leads extending outside the package. Finally, the leads are bent to provide pads that extend under the package. These pads can be used to surface mount the packaged light source. 
     The heat generated by each die must be conducted away from that die by the lead on which that die is mounted. The heat is routed to the printed circuit board on which the package is mounted. The heat circuit can be viewed as consisting of a heat source, i.e., the LED, on one end of a linear heat conductor with the other end held at a constant temperature, i.e., the temperature of the core of the printed circuit board on which the package is mounted. The thermal resistance of this path depends on the length and cross-sectional dimension of the lead. For typical lead frame dimensions, this path has sufficient resistance to lead to a rise in the temperature of the LED die when a high power die is utilized. 
     In principle, the dimensions of the leads could be increased. Increasing the thickness of the leads results in other problems when the leads are bent around the package. Hence, any such dimensional increase must be accomplished by increasing the width of the leads outside of the package. This solution leads to an increase in the size of the package, which is also objectionable in many applications. In addition, as the number of LEDs in the package increase, the package size must increase even further. 
     Refer now to  FIGS. 4-7 , which illustrate an LED light source according to one embodiment of the present invention.  FIG. 4  is a perspective view of the lead frame used in light source  60  after the lead frame has been bent and with the various molded portions of light source  60  removed to provide a clearer view of the lead frame  70  in the completed light source.  FIG. 5  is a top perspective view of light source  60  in which the details of the lead frame have been omitted to simplify the drawing.  FIG. 6  is a bottom perspective view of light source  60 . 
     Light source  60  has three LED dies  71 - 73 . For the purposes of the present discussion, it will be assumed that these LEDs emit light in the red, blue, and green regions of the spectrum, respectively. However, other color combinations could be utilized depending on the particular application. The dies are mounted on a lead frame  70  that has 4 sections shown at  61 - 64 , respectively. All of the dies are attached to a die mounting area  67  on section  64  by bonding the bottom surfaces of the die to section  64  using a conductive adhesive. Each die has first and second contacts for powering the LED, or LEDs, on the die. The first contact is assumed to be on the bottom surface of the die. Hence, all of the first contacts of the dies are connected to a common electrode that can be accessed through the external pads  64 A,  65 A or  66 A on the bottom surface of the completed light source. The second contacts of dies  71 - 73  are on the top surface of the dies, and are connected to sections  61 - 63 , respectively, of lead frame  70  by wire bonds such as wire bond  75 . These contacts are accessible via pads  61 A- 63 A on the bottom surface of light source  60  as shown in  FIG. 6 . 
     Section  64  of lead frame  70  can be viewed as having 4 sub-sections. The first sub-section is die mounting area  67 . The second sub-section is a protrusion that is bent to form a lead that terminates in pad  64 A after wrapping around the outside of the package. The third and fourth sub-sections shown at  65  and  66  are protrusions that are bent directly downward from die mounting area  67  and terminate in pads  65 A and  66 A, respectively. Sub-sections  65  and  66  form two heat paths that directly connect the die mounting area  67  to the bottom surface of light source  60 . These heat paths are substantially shorter than the heat path that connects die mounting area  67  to the bottom surface of light source  60  through pad  64 A since that heat path must traverse the outside surface of light source  60 . Refer now to  FIG. 7 , which is a cross-sectional view of lead frame  70  through line  7 - 7  shown in  FIG. 4 . As can be seen from  FIG. 7 , the minimum length, h, of the heat conducting path from die mounting area  67  to the surface of pads  65 A and  66 A is determined by the parameters of the bending equipment that bend the lead frame leads and the minimum amount of encapsulant that must be provided under the die mounting area to provide adequate bonding of the lead frame in the final encapsulated product. In contrast, the length of the heat paths through a conventional package is determined by the lateral dimensions of the package as well as h. In addition, the cross-sectional area of the heat paths  65  and  66  can be significantly larger than that of sections  61 - 64 , since these heat paths can occupy more than half of the lateral dimension of the package without interfering with the signal leads in sections  61 - 64 . Accordingly, the present invention provides significantly better heat conduction than that obtained using a conventional multi-die package. 
     Refer again to  FIGS. 4-6 . Lead frame  70  is encapsulated in a package that can be viewed as having a bottom section  81  and a top section  82 . Top section  82  includes an opening extending from the top surface of section  82  to die mounting area  67 . In one embodiment, the wall  83  of this opening has slanted sides and is coated with a reflective material. Many LED dies emit a significant fraction of the light from the side walls of the dies. This light is light that was trapped within the LED due to the large difference in index of refraction of the layers used to construct the LED and the material outside the LED. To improve the efficiency of the light source, this light is re-directed into the forward region by reflecting wall  83 . 
     The opening in top section  82  can be filled with a layer of transparent material such that the LED dies are encapsulated between the lead frame and the layer of material. This material can include a diffusing material such as scattering particles that mix the light from various LED dies to provide a light source that appears to have the dimensions of the opening and is uniform in intensity. The encapsulating material can be an epoxy, other polymer, or silicone. Diffusing materials, consisting of particles of TiO 2  or SiO 2  having dimensions of the order of wavelength of the light generated by the LEDs, could be utilized. In addition, the encapsulating material could include phosphors or luminescent materials that convert the light generated by one or more of the LEDs to light having a different spectrum. If a diffusing material is incorporated in the encapsulating layer, the walls of the opening can have a white matte finish. This finish can be applied to the walls or the material from which top section  82  is constructed could be a white polymer or similar material. 
     Refer now to  FIG. 8 , which is a cross-sectional view of a portion of a light source  90  according to another embodiment of the present invention. Light source  90  includes two LEDs  93  and  94  that are mounted on the die mounting area of the lead frame within a top section  97  having a reflector that is similar to the reflector described above. Each LED is encapsulated in a first layer of material shown at  95  and  96 . This layer of material could include phosphors or other color modifying materials such as luminescent materials or dyes. The reflector is filled with a second layer of encapsulating material  91  that could also include a diffusing material as discussed above. Layer  91  could also have optical features such as lens  92  molded into the top surface of the layer. 
     Refer again to  FIGS. 4-6 . Bottom section  81  encapsulates lead frame  70 . To assure that lead frame  70  is securely bound within the encapsulating material, lead frame  70  includes a number of openings such as opening  76  through which the encapsulating material flows during the molding process. An additional cutout  77  can be provided in die mounting area  67  to provide an anchor point for the encapsulating layer. 
     The top and bottom sections of the housing can be molded either before or after the dies have been attached to the lead frame. In the latter case, the dies would be mounted through the opening in top section  82 . 
     The above-described embodiments of the present invention utilize three LED-containing dies. However, embodiments having different numbers of dies could also be constructed. In addition, one or more of the dies could be wired in parallel by connecting the contacts on the top surfaces of the dies to the same lead in the lead frame. Refer now to  FIG. 9 , which is a top view of another embodiment of a light source according to the present invention. Light source  100  includes 5 dies  101 - 105  that are mounted on a lead frame having a die mounting area  109  that is connected to lead  108 . Dies  102 - 104  have the top contacts thereon connected to leads  106 - 108 , respectively. Dies  101  and  105  have the top contacts thereon connected to lead  106 . This arrangement is useful in light sources in which one of the LED colors is not available in a power rating similar to the other dies, and hence, multiple dies must be used to obtain the desired light intensity. 
     The above-described embodiments of the present invention utilize an arrangement in which the die mounting area is connected to a signal lead and a heat conduction pad. However, embodiments in which the direct downward heat conduction pad is also used as the signal lead for the contacts on the bottom surfaces of the dies that are mounted on the die mounting area, could also be constructed without deviating from the teachings of the present invention. 
     Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.