Abstract:
A mounting substrate ( 40 ) has a patterned metal layer defining a plurality of top metal bond pads for bonding to bottom metal bond pads of LED dies. A solder mask layer ( 52 ) is formed over the mounting substrate, where the mask has openings that expose the top metal bond pads and protects metal traces on the substrate. The mask layer is a highly reflective white paint. The exposed top metal bond pads are then wetted with solder. The LED dies&#39; bottom metal bond pads are then soldered to the exposed top metal bond pads, such that the mask layer surrounds each LED die to reflect light. A reflective ring ( 60 ) is affixed to the substrate to surround the LED dies. A viscous phosphor material ( 62 ) then partially fills the ring and is cured. All downward light by the LED dies and phosphor is reflected upward by the ring and solder mask layer.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to packages for phosphor-converted light emitting diodes (pcLEDs) and, in particular, to a package that increases light extraction. 
       BACKGROUND 
       [0002]    For high brightness applications, it is common to mount an array of LED dies on a substrate, where the substrate has metal traces that interconnect the LED dies and lead to anode and cathode electrodes for connection to a power supply. It is common for the LED dies to be GaN-based and emit blue light, where a phosphor (e.g., a YAG phosphor) is deposited over all the LED dies. The combination of the blue light leaking though the phosphor and the yellow-green phosphor light creates white light. 
         [0003]    Some light from the LED dies and some light from the phosphor is emitted in the downward direction and is partially absorbed by the substrate. Further, if the individual LED dies are also mounted on submounts (typically much larger than the LED die), and the submount electrodes are bonded to the substrate, the submount surface also absorbs some of the LED and phosphor light. Such absorption by the substrate and submount reduces the overall efficiency of the module. 
         [0004]    What is needed is a packaging technique for phosphor-converted LEDs that results in more light being emitted by the package. 
       SUMMARY 
       [0005]    In one example of the invention, a starting substrate comprises aluminum for sinking heat. A thin dielectric layer is formed over the top surface of the substrate, and metal traces are patterned over the dielectric. The metal traces define small-area solder pads for a plurality of LED dies, larger anode and cathode electrodes (also solder pads) for the module, and interconnections between the LED dies and electrodes. 
         [0006]    In one embodiment, screen printing is used to deposit a solder mask over the substrate. The solder mask deposits a dielectric that has openings exposing the various electrodes and LED die solder pads. The dielectric is a highly reflective material, such as a binder including TiO 2 , ZiO 2 , VO 2 , or other suitable reflective particles, that scatter and reflect any impinging light. The reflective material may be the same white paint that is used in integrating spheres, with a reflectivity greater than about 94%. 
         [0007]    Next, the LED die electrodes are soldered to the exposed solder pads. The solder mask openings may be small enough such that the peripheries of the LED dies align with or overhang the reflective material, so that any downward light from the LED dies will be reflected by the reflective material. 
         [0008]    A reflective ring, forming vertical walls, is then affixed to the substrate to surround the array of LED dies. 
         [0009]    A phosphor is then deposited within the ring (the ring is also used as a mold) to encapsulate the LED dies and wavelength convert the light emitted from the LED dies. The phosphor also serves to conduct heat away from the LED dies. In one embodiment, the LED dies emit blue light, and the blue light plus the phosphor light creates white light. 
         [0010]    The LED dies may or may not include a submount. A submount is typically used to simplify handling, add mechanical strength, and/or to provide robust and simple electrodes for soldering to a circuit board. If such a submount is used, the submount is as small as practical so as to not intercept the downward light from the LED semiconductor layers. 
         [0011]    Dome lenses may be molded over the LED dies prior to the deposition of the phosphor for improving light extraction. 
         [0012]    The reflective solder mask adds no additional steps, yet greatly improves the efficiency of the LED module. 
         [0013]    Other embodiments are described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a cross-sectional view of a prior art LED die. 
           [0015]      FIG. 2  is a cross-sectional view of an LED die mounted on a minimal-area submount. 
           [0016]      FIG. 3  is a bottom view of an LED die or a submount showing the electrode pattern. 
           [0017]      FIG. 4  is a top down view of an aluminum substrate having a dielectric layer and a patterned metal layer. 
           [0018]      FIG. 5  illustrates the substrate  FIG. 4  after deposition of a reflective solder mask, exposing the solder pads. 
           [0019]      FIG. 6  illustrates the substrate of  FIG. 5  after LED dies have been soldered to the solder pads. 
           [0020]      FIG. 7  is a magnified cross-sectional view, along line  7 - 7  in  FIG. 5 , of a portion of the substrate of  FIG. 5  showing two LED dies having optional lenses. 
           [0021]      FIG. 8  is a top down view of the substrate of  FIG. 6  after a reflective ring is affixed around the array of LED dies. 
           [0022]      FIG. 9  is a magnified cross-sectional view, along line  9 - 9  in  FIG. 8 , of a portion of the substrate of  FIG. 8  illustrating the ring being at least partially filled with an encapsulating phosphor mixture. 
       
    
    
       [0023]    Elements that are the same or similar are labeled with the same numeral. 
       DETAILED DESCRIPTION 
       [0024]      FIG. 1  illustrates a conventional LED die  12 . Although a flip-chip die is shown in the examples, the present invention is applicable to any type of LED die, including vertical LED dies, lateral LED dies, etc. 
         [0025]    The LED die  12  includes a bottom anode electrode  14  coupled to a p-layer  16  and a bottom cathode electrode  18  coupled to an n-layer  20  by a conductor  22  that fills in an etched opening, covered with a dielectric, in the p-layer  16  and active layer  24 . The electrode configuration may be more complex and include distributed electrodes for better spreading of current across the LED die  12 . The active layer  24  generates light having a peak wavelength. In one example, the peak wavelength is a blue wavelength, and the layers  16 ,  20 , and  24  are GaN-based. 
         [0026]    The layers  16 ,  20 , and  24  are epitaxially grown over a substrate  26 , such as sapphire. Alternatively, the growth substrate may be removed and replaced by a transparent support substrate affixed to the semiconductor layers by an adhesive or by other techniques. Alternatively, there is no support substrate, although handling of the thin LED semiconductor layers becomes more difficult. 
         [0027]    As shown in  FIG. 2 , the LED die  12  may be optionally mounted on a submount  28  for ease of handling, mechanical support, heat sinking, and simplifying the electrode structure for mounting on a printed circuit board. The submount  28  includes a thermally conductive body  30 , bottom electrodes  32  and  34 , and vias  36  and  37  connecting the bottom electrodes  32 / 34  to the LED die electrodes  14 / 18 . Additional pads (not shown) on the submount surface are used to connect the LED die electrodes  14 / 18  to the vias  36 / 37 . The typical submount is much larger than the LED die  12  to accomplish the above-mentioned functions. However, in the preferred embodiment of the invention, when using a submount, it is desirable to make the submount much smaller than the typical size to minimize light absorption by the submount surface. In one embodiment, the growth substrate  26  is removed after the LED die  12  is mounted on the submount  28 . 
         [0028]    The term “LED die” hereinafter refers to either the bare chip (e.g.,  FIG. 1 ) or the chip mounted on a submount (e.g.,  FIG. 2 ). 
         [0029]      FIG. 3  illustrates a possible bottom electrode configuration for either the LED die  12  or the submount  28 . 
         [0030]    In the remaining figures, it is assumed a submount is not used. However, the LED die/submount may be substituted for the bare LED die  12  in the remaining figures. 
         [0031]      FIG. 4  is a top down view of a substrate  40  for an array of LED dies  12 .  FIG. 7  shows a cross sectional view of the substrate  40 . The substrate  40  may include a body  41  formed of aluminum or an alloy for heat sinking. The substrate  40  may have any shape, such as rectangular, circular, etc. The length or diameter of the substrate  40  depends on the number of LED dies it supports and will typically range from  1  cm to  4  cm. The substrate  40  is typically connected in an array of substrates by narrow connectors that are later cut for singulation to simplify handling and speed processing. 
         [0032]    A non-conductive dielectric layer  42  ( FIG. 7 ) is formed over the body  41  for electrical insulation. 
         [0033]    A patterned metal layer, such as copper, is formed over the dielectric layer  42  to define small solder pad groups  44 A,  44 B,  44 C, and  44 D for each of the LED dies  12  in the array, and to define larger solder pads  46  and  48  for the array&#39;s anode and cathode electrodes. The solder pad groups  44 A-D may include one or more heat sink connections. The metal layer also forms interconnections  50  between the LED dies  12  and the electrodes  46  and  48 . In the example, only four LED dies  12  are connected in series and parallel. In other embodiments, more or fewer LED dies  12  may be interconnected in series and/or parallel to achieve the desired electrical characteristics and flux. 
         [0034]    A problem with the related prior art LED modules is that a significant portion of the LED light emitted downward is absorbed by the mounting substrate. The most absorption by the substrate occurs in the vicinity of each LED die  12 , since that is where the light is the brightest. The present invention minimizes such absorption. 
         [0035]    In  FIG. 5 , a white (diffusing) paint  52  is screen printed over the substrate  40  in all places except where solder is to be applied.  FIG. 7  is a discontinuous cross-section along line  7 - 7  in  FIG. 5  and shows the paint  52  in cross-section. The screen is a mesh that is patterned to block the deposition of the white paint  52  over the solder pad groups  44 A,  44 B,  44 C, and  44 D. The white paint  52  is a viscous dielectric that is then cured. Accordingly, the white paint  52  replaces a conventional solder mask, and no additional steps are required. A solder mask restricts any deposited solder, such as molten solder that flows over the substrate  40 , to areas that are exposed by the mask. 
         [0036]    Examples of a diffusing white paint include a binder (such as silicone) infused with particles of TiO 2 , ZiO 2 , VO 2 , or other suitable reflective scattering particles. 
         [0037]    In another embodiment, the white paint is applied, in conjunction with a mask, by jetting, spraying, a photolithography process with evaporation, or other technique. 
         [0038]    Commercially available white paints are greater than 94% reflective for visible wavelengths and are sometimes used in integrating spheres for light measurement. The white paint  52  material should be thermally conductive. Preferably, the reflectivity of the white paint  52  is at least 90% for visible light. 
         [0039]    By applying the white paint  52  as the solder mask, it is ensured that the reflective material extends to the edge of the LED dies  12  and even under the LED dies  12  between the electrodes. Accordingly, the white paint  52  maximizes the reflective surface area of the substrate, limits the applied solder to only the areas exposed by the solder mask, and acts as a corrosion barrier for the metal traces, without any added process steps, so there is synergy in the inventive process. 
         [0040]    In another embodiment, a conventional solder mask material (e.g., used for non-LED circuit boards) is applied directly over the substrate  40  followed by the deposition of the white paint  52  (using the same mask pattern) if the conventional solder mask material is desired for adequate protection of the substrate  40  and to reduce costs. In such a case, the white paint  52  protects the solder mask material from degrading due to the high intensity blue or UV light. 
         [0041]    A solder  54  ( FIG. 7 ) is then applied to the various solder pads for wetting. The solder  54  may be screen-printed, or applied with a solder mask, or applied in any other conventional way. The solder  54  may be a solder paste. 
         [0042]    In  FIG. 6 , the LED die  12  electrodes (or submount electrodes) are soldered to the substrate&#39;s solder pads using a heating process. As seen, there is no gap between the white paint  52  and the edge of the LED dies  12 . If the LED dies were vertical LED dies, only the bottom electrode would be directly soldered to one of the substrate&#39;s  42  solder pads, and the top electrode would be wire-bonded to another solder pad. If the LED dies were lateral LED dies, the bottom thermal pad would be soldered to one of the substrate&#39;s  42  solder pads and both top electrodes would be wire-bonded to associated solder pads. In virtually all LED dies, there is at least one bottom metal bond pad that serves as an electrical and/or thermal pad. 
         [0043]    In another embodiment, solder is not used. Instead the bonding may be by an ultrasonic weld, a conductive adhesive (electrically and thermally conductive), or using other techniques. In such cases, the “solder mask” would be referred to by a suitable name but would still define the areas on the substrate  40  where the bottom metal bond pad of the LED die is to be bonded, and the exposed areas would be approximately the size of the LED die. 
         [0044]      FIG. 7  is a compressed and magnified cross-sectional view of a portion of the substrate  40  along line  7 - 7  in  FIG. 5  cutting across two of the LED dies  12  in the horizontal direction. A solder  54  is shown connecting the LED die electrodes to the solder pad groups  44 A and  44 D. 
         [0045]    Domed lenses  58  may optionally be molded over the LED dies  12  for improved light extraction and protection of the LED dies  12 . In an alternative embodiment, LED dies  12  may be enclosed in domed lens prior to soldering, with or without a submount. 
         [0046]    In  FIG. 8 , a reflective ring  60  is affixed to the surface of the substrate  40 , surrounding the array of LED dies  12 . The ring  60  may be a reflective metal or a material coated with a reflective layer. The ring  60  may be affixed with silicone or epoxy. 
         [0047]      FIG. 9  is a compressed and magnified cross-sectional view of a portion of the substrate  40  along line  9 - 9  in  FIG. 8  cutting across two of the LED dies  12  in the horizontal direction and the ring  60 , where walls of the ring  60  extend above the tops of the LED dies  12  and reflect and mix the light. 
         [0048]    As also shown in  FIG. 9 , a viscous phosphor mixture  62  is deposited within the ring  60 , acting as a mold, and cured. The phosphor mixture  62  may be a mixture of silicone and phosphor particles. Many ways to deposit the phosphor mixture  62  are envisioned, such as by using a syringe, screen printing, positioning a pre-formed tablet in the ring that is then melted, etc. The phosphor mixture  62  encapsulates the LED dies  12  for added protection. 
         [0049]    The thickness of the phosphor mixture  62  and phosphor particle density are controlled to achieve the desired combination of blue light leakage and phosphor conversion. The phosphor may be a single phosphor (e.g., YAG) or a combination of phosphors (e.g., YAG and red, or green and red, etc.) to achieve the desired color emission. 
         [0050]    Preferably, the indices of refraction of the various layers are selected to provide a transition from the high index GaN to the low index of air to minimize total internal reflection (TIR). 
         [0051]    The reflective surface of the substrate  40  under the phosphor mixture  62  reflects back over 94% of all the phosphor light emitted in a downward direction. 
         [0052]    Any number of LED dies  12  may be mounted on the substrate  40 , and the diameter of the ring  60  is that needed to surround the LED dies  12  and may be adjusted accordingly. Any number of the resulting substrates  40  can be connected in a system to achieve any desired flux. In one embodiment, the resulting structure emits white light. Other emission colors are possible by selecting different LED dies and phosphors. A quantum dot material may be substituted for the phosphor mixture  62 . 
         [0053]    An improvement in efficiency of greater than 10% is typically achieved by using the invention. 
         [0054]    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.