Abstract:
An LED package creates a narrow beam in a very compact package without use of a lens. A plastic is molded around a metal lead frame ( 12, 14 ) to form a molded cup ( 26 ), where the cup has parabolic walls extending from a bottom area of the cup to a top thereof. The lead frame forms a first set of electrodes exposed at the bottom area of the cup for electrically contacting a set of LED die electrodes ( 18, 20 ). The lead frame also forms a second set of electrodes outside of the cup for connection to a power supply. A reflective metal ( 28 ) is then deposited on the curved walls of the cup. An LED die ( 16 ) is mounted at the bottom area of the cup and electrically connected to the first set of electrodes. The cup is then partially filled with an encapsulant ( 64 ) containing a phosphor ( 66 ).

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to packaged light emitting diodes (LEDs) and, in particular, to a reflective cup used as a complete and compact package for an LED die. 
       BACKGROUND 
       [0002]    It is common to mount an LED die on a printed circuit board (PCB), or other substrate, for electrically connecting electrodes of the LED to conductive traces on the PCB. Then, a molded reflector cup with a center hole is affixed to the PCB and surrounds the LED die. The cup is then filled with a phosphor mixture, or a clear material, and cured to encapsulate the LED die. The cup limits the side light emission of the LED die and directs it in a generally forward direction. Therefore, the LED die package is a combination of the PCB, the cup, and the encapsulant. 
         [0003]    In some cases, a hemispherical lens containing an encapsulant is affixed over the LED die to improve light extraction. This requires a large center hole in the cup to accommodate the lens. 
         [0004]    The resulting package is fairly large since the PCB must extend beyond the reflective cup. Further, there are multiple parts to the package, which require handling and which reduce the reliability of the package. 
         [0005]    Another drawback of using the molded reflector cup with the center hole is that the inner edges of the cup facing the LED die form short vertical walls, rather than angled knife edges. Knife edges are not achievable with a standard molding process. Therefore, the inner edges block some of the light rather than reflect it in a forward direction. 
         [0006]    Additionally, conventional reflective cups are relatively shallow, which produces a wide beam, since the purpose of the cup is just to reflect the side light and contain the encapsulant. The cup is not used to shape the beam. To shape the beam, such as to collimate the beam, a lens is provided over the LED die prior to the cup being affixed to the PCB. Therefore, the lens and the added handling add further cost to the package. The lens also attenuates the light. 
         [0007]    Some examples of shallow reflective cups are shown in US publication 2013/0228810, where the cups are completely filled and, in some cases, a lens is molded over the cup and LED die. 
         [0008]    What is needed is a more compact, less expensive, and more reliable package for an LED die in an application that requires a collimated beam. 
       SUMMARY 
       [0009]    In one example of the invention, a plastic cup is molded over a lead frame, where the center of the cup has metal pads for connection to electrodes of an LED die. The curved wall of the cup is coated with a highly reflective film, such as silver or aluminum. The cup is deep (e.g., at least 5 mm) to create a narrow collimated beam. The lead frame may extend out from the sides of the plastic cup, or the lead frame may lead to bottom electrodes on the cup. The lead frame may be copper for good electrical and heat conduction, and the metal pads/electrodes may be plated with gold, have gold bumps, wetted with solder, or otherwise prepared for being suitable for bonding to LED electrodes and to pads on a substrate (including a PCB). 
         [0010]    The LED die is then mounted on the cup&#39;s base and electrically connected to the lead frame. The lead frame and cup material conduct heat from the LED die. Wire bonding may also be used for non-flip-chip dies. 
         [0011]    After the LED die is mounted in the cup, the cup is partially filled with an encapsulant, which may be clear or a phosphor mixture. The encapsulant is then cured. 
         [0012]    Since the cup does not have a center hole, it can be easily molded so that there is no vertical portion of the wall next to the LED die that blocks the light or reflects light back into the die. All light incident on the reflective wall is reflected in a forward direction. 
         [0013]    Accordingly, the entire package is a single unit that is the size of the reflective cup. The deep cup is used to shape the beam so no lens is needed. The package has a high reliability since it is a single piece. 
         [0014]    Other embodiments are described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a cross-sectional view of a flip-chip LED die mounted on a lead frame molded into a deep reflective cup in accordance with one embodiment of the invention. 
           [0016]      FIG. 2  is a cross-sectional view of a wire-bonded LED die mounted on a lead frame molded into a deep reflective cup in accordance with another embodiment of the invention. 
           [0017]      FIG. 3  is a cross-sectional view of a flip-chip LED die mounted on a lead frame molded into a deep reflective cup, where the lead frame forms bottom pads of the cup, in accordance with another embodiment of the invention. 
           [0018]      FIG. 4  is a cross-sectional view of a wire-bonded LED die mounted on a lead frame molded into a deep reflective cup, where the lead frame forms bottom pads of the cup, in accordance with another embodiment of the invention. 
           [0019]      FIG. 5  is a top down view of the cup portion of any of the embodiments of  FIGS. 1-4 , showing the location of bonding pads in the cup and the location of the LED die within the cup. 
           [0020]      FIG. 6  illustrates the cup and LED die of  FIG. 1  after a phosphor mixture or clear mixture is deposited in the cup to encapsulate the LED die. 
           [0021]      FIG. 7  illustrates the cup and LED die of  FIG. 3  after a phosphor mixture or clear mixture is deposited in the cup to encapsulate the LED die. 
           [0022]      FIG. 8  is a light intensity vs. angle profile for any LED die wavelength between 400-750 nm, illustrating the relatively narrow beam that results with the deep cup of  FIGS. 1-7 . 
       
    
    
       [0023]    Elements that are the same or similar are labeled with the same numeral. 
       DETAILED DESCRIPTION 
       [0024]      FIG. 1  illustrates a reflective cup package  10  in accordance with one embodiment of the invention. A copper lead frame is stamped from a sheet or sheets to form the leads  12  and  14  of the package  10 . There may be an array of lead frames connected together to simplify processing of the packages, and the lead frames are cut after forming the packages to separate out the individual packages. 
         [0025]    The area where the leads  12  and  14  are to be bonded to the LED die  16  electrodes  18  and  20  may be plated with a suitable metal, such as gold or alloys, to form bonding pads  22  and  24 . Gold balls, solder wetting, or other techniques, if required, may also be used to allow bonding to the die electrodes  18 / 20 . Any portion of the lead frame that is used for an electrical connection is referred to herein as a bonding pad or an electrode, whether the connection is by solder, ultrasonic weld, wire bond, conductive epoxy, etc. 
         [0026]    Over the lead frame is molded a plastic cup  26 . An identical plastic cup is simultaneously molded over each lead frame in the array. Compression molding or injection molding may be used. Preferably, the plastic is thermally conductive. If the plastic is also electrically conductive, for example, due to containing metal particles (for increasing its thermal conductivity), the portion of the lead frame in contact with the plastic has a dielectric coating (not separately shown) formed over it prior to the molding step. 
         [0027]    The cup  26  generally forms a parabola which is orthogonal to the plane of the top light emitting surface of LED  16 , with a circular cross-section which is parallel to the plane of the top light emitting surface of LED  16 , such as shown in  FIG. 5 . The shape can also be a compound parabolic concentrator (CPC). In one embodiment, the parabola portion of the cup  26  is about 5 mm deep, its top opening is about 6-7 mm in diameter, and its bottom surface flat area for the LED die  16  is about 1-2 mm in diameter. The cup  26  slopes up from its bottom surface to its top edge to generally reflect all LED die light upward. The deeper the cup, the narrower the beam, so the beam shape is determined by the cup shape rather than any lens. In the preferred embodiment, no lens is used. 
         [0028]    The inside surface of the cup  26  is then coated with a reflective material  28 , such as a silver or aluminum film, by sputtering, evaporation, spraying, or other process. The reflection may be specular for the narrowest beam or may be diffusive (such as by using white paint) for a wider beam. A masking process may be used to ensure that that bonding pads  22 / 24  are not shorted or coated by reflective material  28 . In the alternative, the reflective material may be removed from bonding pad  22 / 24  and then plated with gold or any other suitable material. 
         [0029]    In another embodiment, the reflective film is a dichroic coating tuned to the LED die emission. A masking process may be used to ensure that that the electrodes are not coated with reflective material  28  or the alternative dichroic coating. 
         [0030]    The bottom electrodes  18 / 20  of the flip-chip LED die  16  are then bonded to the bonding pads  22 / 24  formed at the ends of the leads  12  and  14 . The bonding may be by ultrasonic welding, solder, solder paste, conductive epoxy, or by other means. LED dies are typically square and on the order of 0.5-1 mm per side. The leads  12  and  14  form anode and cathode leads for connection to a power supply. 
         [0031]    Depending on the application, the outer ends of the leads  12  and  14  may be soldered to metal pads on a printed circuit board (PCB) or other substrate to supply power to the LED die  16 . A light ray  30  emitted from the LED die  16  is shown reflecting off the wall of the cup  26  in a forward direction. Any light rays from the side walls of the LED die  16  will similarly be reflected upwards by the cup  26 . 
         [0032]    Heat from the LED die  16  is removed by a combination of the air over the LED die  16 , the leads  12  and  14 , and the package  10 . The bottom surface  32  of the package  10  may be thermally coupled to a substrate using a thermally conductive paste. The substrate and/or the cup  26  may have an aluminum core (not shown) that acts as a heat sink. 
         [0033]      FIG. 5  illustrates the location of the bonding pads  22  and  24  relative to the LED die  16  (shown by dashed lines as transparent). The bonding pads  22  and  24  may be as wide or wider than the LED die  16 . The leads  12 / 14  in  FIG. 1  may be much wider than the LED die  16  to better sink the heat from the LED die. 
         [0034]      FIG. 2  illustrates a package  36  similar to that of  FIG. 1  but the LED die  38  has a top electrode that is wire bonded to a bonding pad of the lead  40  via a wire  42 . The LED die  38  has a bottom electrode bonded to the bonding pad of the lead  44  for good thermal and electrically conductivity. The cup  26  is otherwise the same. 
         [0035]    The LED die may be the type that has two top electrodes, and both electrodes are wire bonded to bonding pads of the leads. The bottom thermal pad of the LED die would be thermally bonded to the plastic base of the cup  26  using a thermally conductive epoxy. 
         [0036]    In  FIGS. 1 and 2 , the width of the leads  12 / 14  or  40 / 44  may be at least as wide as the LED die, such as  2  mm wide or more, to provide a good thermal path to the substrate (e.g., a PCB). 
         [0037]      FIG. 3  illustrates an electrode pattern for a package  48  where the lead frame forms bottom bonding pads  50  and  52  after the plastic cup  54  is molded around the lead frame. The top and bottom surfaces of the leads may be plated with gold or other metal to enhance bonding to the LED die  16  electrodes and the substrate electrodes. Gold balls, solder, or other bonding techniques may be used instead of plating.  FIG. 3  shows top bonding pads  56  and  57  formed on the top surface of the lead frame. The bottom electrodes  50  and  52  may extend the entire width of the package  48  to maximize thermal contact with the substrate. The package  48  provides better thermal conduction between the LED die  16  and the substrate than the package  10  of  FIG. 1 . 
         [0038]    The bonding pad configuration shown in  FIG. 5  may also apply to  FIG. 3 . 
         [0039]      FIG. 4  illustrates a package  58  similar to that of  FIG. 3  but the top electrode of the LED die  38  is connected to the top bonding pad  60  by the wire  42 . 
         [0040]    The LED die may also have two top electrodes wire bonded to the top bonding pads of the lead frame, and the bottom thermal pad of the LED die is thermally coupled to the package  48  or  58  by a thermally conductive epoxy. 
         [0041]      FIGS. 6 and 7  illustrate the LED die  16  in the packages  10  and  48  being encapsulated after being mounted in the cup. The same encapsulation may also be used in the packages  36  and  58 . The encapsulation protects the LED die  16  and improves light extraction by typically having an index of refraction between the LED die material (e.g., GaN) and air. For wavelength conversion, the encapsulant  64  may be a silicone binder infused with phosphor powder, such as YAG phosphor or red and green phosphor. If the LED die  16  emits blue light, some of the blue light will leak through and combine with the phosphor light to produce white light. Any color may be generated by the selection of the phosphor. A phosphor particle  66  is shown emitting a yellow light ray  68  that mixes with the LED die&#39;s blue light ray  30  to create white light. 
         [0042]    The encapsulant  64  may instead be clear or diffusing. Silicone may be used. A diffusing material may be TiO 2  (white) particles in the silicone. 
         [0043]    The phosphor may even be a separate layer covering the LED die  16  prior to depositing the encapsulant  64 . 
         [0044]    In contrast to  FIGS. 6 and 7 , prior art shallow cups, which are used to restrict side light, are typically completely filled with an encapsulant due to the very small volume of the cup. A lens in then typically mounted over the shallow cup. 
         [0045]      FIGS. 6 and 7  also show the leads  12 / 14  and bottom pads  50 / 52  bonded (e.g., soldered) to respective pads or traces  70 - 73  on a substrate  76 / 78 , such as a PCB. The substrate  76 / 78  may have a metal core (not shown) for conducting the heat away from the LED die  16 . 
         [0046]    In another embodiment, the leads extend from a single side of the package and form male connectors (electrodes) for a socket or for other types of female connectors. 
         [0047]      FIG. 8  is a light intensity vs. angle profile of a 5 mm deep cup having parabolic reflective walls. The units on the y axis convey the relative flux rather than an absolute value. The beam is extremely well-defined, narrow, and symmetrical about the center axis of the LED die (at 90 degrees). The beam can be shaped by the cup rather than a lens. Cups having depths greater than 5 mm are also envisioned for a narrower beam. By using the deep cup package, even a low power LED may be used to generate a very bright but narrow beam. 
         [0048]    The resulting packages are essentially a minimum possible size, given that the cup must have certain dimensions for the desired light emission. 
         [0049]    If the plastic cup is formed of a white plastic, then no reflective film is required to be deposited on the cup walls if a diffused reflection is desired. 
         [0050]    Although plastic has been used in the example of the moldable material, any other suitable moldable material may be used for the cup. 
         [0051]    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.