Patent Publication Number: US-11036114-B2

Title: Thin LED flash for camera

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/104,220 filed Jun. 13, 2016, which is a 35 U.S.C. § 371 application of and claims the benefit of International Application No. PCT/IB2014/067204 filed Dec. 22, 2014, which claims the benefit of U.S. Provisional Application No. 61/923,925 filed Jan. 6, 2014, which are incorporated by reference as if fully set forth. 
    
    
     FIELD OF INVENTION 
     This invention relates to packaged phosphor-converted light emitting diodes (pcLEDs) and in particular, to a packaged pcLED that is useful as a flash for a camera. 
     BACKGROUND 
     In modern digital cameras, including smartphone cameras, it is common to provide a flash that uses a pcLED. A common flash is a GaN-based blue LED die mounted in a round reflective cup on a rigid printed circuit board. A layer of YAG phosphor (emits yellow-green) fills the cup. Since the LED die is very thin, almost all light is emitted from the top surface of the die. A circular Fresnel lens is then positioned over the cup to create a generally conical light emission pattern (having a circular cross-section) to illuminate a subject for the photograph. A cover plate, forming part of the camera body, typically has a circular opening for the lens. 
     Since the field of view of the camera is rectangular, much of the light emitted from the flash, having a circular cross-section, illuminates areas surrounding the subject and is wasted. Such unnecessary illumination may also be bothersome to those not in the picture. 
     Further, due to the shape of the cup and the phosphor in the cup, the phosphor is not uniform over the LED die, resulting in color non-uniformity vs. angle. 
     Further, due to the use of the rigid printed circuit board, the thinness of the flash module is limited. 
     Further, the lens must be spaced away from the top surface of the LED die by a certain minimum distance (e.g., the focal length) in order to properly redirect the light. This minimum distance significantly adds to the thickness of the flash module. 
     Further, there is substantial back-reflection from the lens back toward the cup and LED die. 
     Further, the bottom inner edge of the reflector cup facing the sides of the LED die has a thickness that is typically greater than the height of the LED semiconductor layers, so the inner edge of the cup blocks the side light or reflects it back into the LED die. 
     Further, since almost all light is emitted from the top surface of the LED die, the reflective cup has limited usefulness in shaping the beam, and the resulting beam is not very uniform across the field of view of the camera. 
     Further, since almost all light is emitted from the top surface of the LED die in a Lambertian pattern, the reflector cup has to have relative high walls to redirect and collimate the “angled” light emitted from the LED die. Any light rays that are not reflected (collimated) spread out at wide angles. The high walls of the reflector limit the minimum thickness of the flash module. 
     It is known to affix a lens over the LED die for a flash, where the lens has a cavity for the LED die, such as described in patent publication KR2012079665A. The lens has a rectangular top surface and curved side surfaces. However, a significant portion of the light escapes from the sides and is not reflected toward the subject. Also, the prior art lens is relatively thick, resulting in a thick flash module. 
     What is needed is a thin LED flash module for a camera that more uniformly anti efficiently illuminates a subject. 
     SUMMARY 
     In one example of the inventive flash module for a camera, a blue flip-chip LED die has a relatively thick transparent substrate on its top surface. This causes a significant portion of the light emission to be from the sides of the LED die, such as 50% of the total light emission. 
     A conformal coating of phosphor is deposited over the top and side surfaces of the LED die to create a uniform white light. 
     The pcLED is mounted on a supporting substrate having a metal pattern for connection to the bottom anode and cathode electrodes of the LED die. The substrate has bottom metal pads for bonding to a cameras printed circuit board. The substrate can be a very thin flex circuit or a rigid substrate such as ceramic. 
     A rectangular reflector, with rounded corners, is then mounted on the substrate surrounding the rectangular LED die. The rectangular reflector has curved walls for redirecting the side LED light into a generally pyramidal beam, having a rectangular cross-section, where the cross-section aspect ratio is similar to the standard aspect ratio of a cameras field of view. In one embodiment, the reflector is stamped aluminum, where the opening for the LED die has knife edges facing the sides of the LED die so virtually all side light is reflected upward rather than being blocked by the inner edges of the opening. Such a knife edge could not be achieved by a molded reflector cup. 
     A thin lens is affixed over the lop of the reflector, where the lens has a convex side that faces toward the LED die, so the convex portion does not add thickness to the module. The lens not only protects the LED die but increases light extraction due to the convex portion receiving most of the light from the LED die and reflective walls at a substantially normal angle. In contrast to a prior art conventional Fresnel lens, having a flat surface facing the LED die, there is much less back-reflection. 
     Due to the high percentage of side light being reflected upward by the reflector, the effective optical distance between the lens and the LED die is the sum of the horizontal distance between a side of the LED die and a curved wall of the reflector plus the vertical distance between the curved wall and the lens. Therefore, the lens can be spaced a focal length from the LED die&#39;s sides while being much closer to the top surface of the LED die. This allows the flash module to be even thinner. The reflector walls can be more widely spaced from the LED die to further reduce the thinness of the module. 
     Since a large portion of the light emitted from the LED die is from its sides, the reflector walls can be made relatively shallow, further reducing the thinness of the module. 
     The convex shape of the lens toward the LED die and the closeness of the lens to the LED top surface cause the lens to intercept a wide angle of the Lambertian light emitted from the top surface of the LED die and slightly redirect it toward the center of the beam, if necessary, to further improve the uniformity of the beam. The convex shape is designed to optimize the uniformity of light across a desired portion of the generally rectangular beam. The lens is not used to significantly shape the beam (but primarily improves uniformity), since the shape of the beam is primarily controlled by the shape of the reflector, in contrast to the prior art. 
     Accordingly, due to the large percentage of the LED die light being side light and redirected by the rectangular reflector, and the reflected light being blended with the light emitted from the top surface of the LED die, a more uniform rectangular beam of light is emitted by the flash, which generally matches the aspect ratio (e.g., 4:3) of the camera&#39;s field of view. Further, the flash module can be made very thin. 
     Other embodiments are described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of a Hash module in accordance with one embodiment of the invention. 
         FIG. 2  is a cross-sectional compressed view of an LED die, with a conformal phosphor coating, mounted on a flex circuit (or other supporting substrate) that may be used in the module of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the flash module of  FIG. 1 . 
         FIG. 4  is a top down view of the flash module of  FIG. 1 . 
         FIG. 5  is a bottom view of the flash module of  FIG. 1  showing the electrode pattern and thermal pad. 
         FIG. 6  is a perspective bisected view of the flash module of  FIG. 1 . 
         FIG. 7  is a perspective view of the flash module of  FIG. 1 . 
         FIG. 8  is a cross-sectional view of an embodiment of the flash module using a flex substrate and identifying various dimensions in millimeters. 
         FIG. 9  is a back view of a smartphone illustrating the rectangular flash module and camera lens. 
     
    
    
     Elements that are the same or similar are labeled with the same numeral. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
       FIG. 1  is an exploded view of a flash module  10  in accordance with one embodiment of the invention. A support substrate  12  may be a rigid substrate or a very thin flexible circuit. Using a flexible circuit as the support substrate  12  allows the module  10  to be thinner. 
     A metal trace  14  pattern is formed on the substrate  12  to define metal pads  15  and  17  for the bottom anode and cathode electrodes of the flip chip LED die  16  and to define metal pads  19  and  21  for the electrodes of an optional transient voltage suppressor (TVS) chip  18 . 
     The bare LED die  16 , such as a GaN-based blue LED die, is then electrically and thermally connected to the substrate  12 . The TVS chip  18  may also be electrically connected to the substrate  12 . 
     Typically, the LED die  16  is a flip chip die, although other die types, including those with bonding wires, may be used. To minimize the thickness of the flash module  10 , the LED dies bottom electrodes are directly bonded to the metal pads  15  and  17  of the metal trace  14 . In another embodiment, the bare LED die  16  may be first mounted on a submount with more robust bottom metal pads to simplify handling and enable the LED die  16  to be conformally coated with a phosphor layer  20  after bonding to the submount.  FIG. 1  shows an alternative embodiment where the phosphor layer  20  covers only the top surface of the LED die  16 . 
     If the bare LED die  16  is directly mounted on the substrate  12  (as shown in  FIG. 2 ), the phosphor layer  20  may be deposited over the entire substrate  12  and LED die  16  to coat the top and side surfaces of the LED die  16 . The phosphor layer  20  may be phosphor particles, such as YAG phosphor particles or red and green phosphor particles, infused in a silicone binder. The phosphor layer  20  may also serve as an adhesive layer for affixing a reflector  22  to the surface of the substrate  12 . In the alternative, a separate adhesive may be used to affix the reflector  22 . 
     Various details of the components in  FIG. 1  are described with respect to  FIGS. 2-8 . 
       FIG. 2  is a cross-sectional compressed view of the LED die  16  with the conformal phosphor layer  20 . 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. 
     The LED die  16  includes a bottom anode electrode  23  bonded to the metal pad  15  (defined as a portion of the metal trace  14  of  FIG. 1 ) and includes a bottom cathode electrode  25  bonded to the metal pad  17 . The pads  15  and  17  are electrically connected by vias  30  and  31  to associated bottom pads  32  and  34 , which may be used to solder the flash module  10  to a camera&#39;s printed circuit board. A thermal pad  36  is formed on the bottom surface of the substrate  12 , which may be soldered to a heat sink in the printed circuit board. 
     The LED die  10  semiconductor layers are grown on a relatively thick sapphire substrate  40 , which may be as thick as 1 mm. This is thicker than a typical growth substrate, since a manufacturer typically uses the thinnest growth substrate practical for reducing costs and maximizing the top emission. Frequently, in the prior art, the growth substrate is completely removed. The sapphire substrate  40  is much thicker than required for mechanically supporting the LED semiconductor layers. Other material for a growth substrate may instead be used. The top surface and growth surface of the growth substrate  40  may be roughened for increasing light extraction. 
     A typical width of the LED die  16  is on the order of 1 mm. 
     N-type layers  42  are epitaxially grown over the sapphire substrate  40 , followed by an active layer  44 , and p-type layers  46 . A portions of the active layer  44  and p-type layers  16  are etched away to gain electrical contact to the n-type layers  42  by means of a via  48  leading to the cathode elect rode. 
     The active layer  44  generates light having a peak wavelength. In the example, the peak wavelength is a blue wavelength, and the layers  42 ,  44 , and  46  are GaN-based. 
     Alternatively, the growth substrate  40  may be removed and replaced by a transparent support substrate, such as glass, affixed to the semiconductor layers by an adhesive (e.g., silicone) or by other techniques. 
     By using a thick growth substrate  40  (or other transparent substrate), the light exiting the sides of the LED die  16  is made to be preferably about 50% of the total light emission, with 50% of the total light being emitted from the top surface of the LED die  16 . In another embodiment, over 30% of all light emitted by the LED die  16  is from the sides, where the percentage of side light is based on the thickness of the substrate  40 . The more side light, the more the reflected light from the reflector  22  is adding to the overall beam and the thinner the Clash module can be. 
     In one embodiment, the thickness of the LED semiconductor layers is less than 100 microns (0.1 mm), and typically less than 20 microns, and the substrate  40  thickness is greater than 0.2 mm and up to 1 mm. 
     A portion of the blue light leaks through the phosphor layer  20 , and the combination of the blue light and phosphor light creates white light for the flash. Since the phosphor layer  20  has a uniform thickness, the color emission will be substantially uniform vs. angle. 
     The reflector  22  ( FIG. 1 ) is preferable formed by stamping an aluminum sheet. The stamp forms a rectangular opening  52  in the sheet and compresses the surrounding aluminum to form curved sidewalls  54 . The term “rectangular,” as used herein, includes a square, and includes rectangles with rounded corners. The edges of the opening  52  are knife edges (less than 50 microns thick) to limit any back reflection of the light emitted from the sides of the LED die  16 /phosphor. Typically, the opening  52  and curved sidewalls  54  have the same aspect ratio as the camera&#39;s field of view, such as 4:3, so the resulting beam will resemble the 4:3 aspect ratio. 
     The reflector  22  is then coated with a silver layer for high reflectivity, such as by plating, evaporation, sputtering, etc. 
     The footprint of the reflector  22  may be approximately that of the substrate  12  to minimize the size of the flash module  10 . The reflector  22  is then affixed to the substrate  12  using the phosphor layer  20  (containing silicone) as an adhesive. The reflector  22  adds rigidity to the module  10 . Tire phosphor layer  20  is then cured. 
     A preformed polycarbonate lens  56  is then affixed to the top surface of the reflector  22 , such as by silicone. The silicone is then cured to complete the flash module  10 . Typically, the lens  56  is rectangular with rounded edges to receive the generally rectangular emission from the reflector  22  and LED die  16 . 
     As shown by the cross-sectional view of the module  10  in  FIG. 3 , the lens  56  has a flat top surface and a bottom surface. A portion of the bottom surface is a convex surface  58  that faces the LED die  16 . Thus, the convex surface does not add to the thickness of the module  10 . Typically, the convex surface  58  is rectangular or rectangular with rounded corners, as shown in  FIG. 1 . 
     In prior art flash modules using LED dies that generate little side light, the lens had to be spaced relatively far front the top surface of the LED die to properly redirect the light. In one embodiment of present invent ion, around 40-50% of the light is emitted from the sides of the LED die  16 , and the effective optical distance from the LED die  16  to the lens  56  is the sum of the generally horizontal distance from an LED die side to a reflector wall  54  plus the generally vertical distance from the reflector wall  54  to the lens  58 . Accordingly, to make the module  10  even thinner, the reflector walls  54  can be further spaced from the LED die  16  while retaining the same effective optical distance between the sides and the lens  58 . The lens  56  is designed to improve the uniformity of light across a central portion of the rectangular beam. 
     In one embodiment, the effective optical distance between the sides of the LED die  16  and the lens  58  is approximately the focal length of the lens  58 . 
     Dry air (or other gas) fills the gap between the lens  56  and the LED die  16  to obtain a large difference in the indices of refraction at the interface of the lens  56  and the gap to achieve the desired refraction by the lens  56 . 
       FIG. 3  shows two sample light rays  60 A and  60 B. Rays, such as  60 A, from the top center surface of the LED die  16  are not substantially redirected by the lens  56 . Reflected rays, such as ray  60 B, that impinge the convex surface  58  at an angle are slightly redirected toward the center axis to improve the uniformity of the beam across at least a central portion of the 4:3 aspect ratio. The shape of the beam is primarily defined by the shape of the reflector  22 , since the reflector  22  reflects virtually all side light and some angled light from the top surface of the LED die  16 . 
       FIG. 3  also shows that the aluminum sheet for forming the reflector  22  is stamped to have a bottom cavity for the TVS chip  18 . 
     By using the phosphor layer  20  (a dielectric) as an adhesive for the aluminum reflector  22 , the bottom of the metal reflector  22  does not short out the metal traces  14 , and there is no separate step for depositing an adhesive. In another embodiment, the reflector  22  is formed to have a thin dielectric layer on its bottom surface before being mounted on the substrate  12 . 
       FIG. 4  is a top down view of the module  10  of  FIG. 3 . 
       FIG. 5  is a bottom view of the module  10  showing the cathode and anode bottom pads  32  and  34 , and the thermal pad  36 , also shown in  FIG. 2 . 
       FIG. 6  is a perspective bisected view of the flash module  10 . The phosphor layer  20  over the sides of the LED die  16  is not shown. 
       FIG. 7  is a perspective view of the flash module  10  of  FIG. 1 . 
       FIG. 8  is a cross-sectional view of an embodiment of the flash module  10  using a flex substrate  12  and identifying various dimensions in millimeters. Although the LED die  16  is about 1.0 mm in width, the height of the lens  56  above the top surface of the LED die  16  is only about 0.3 mm, since the optimal separation is based on the travel path of the side light to the lens  56  when being reflected off the reflector  22 . 
     The flex substrate  12  only adds 0.05 mm to the thickness of the module  10 . The phosphor layer  20  is shown as being 0.05 mm thick. The reflector  22  is shown as being 0.750 mm thick, and the lens  56  is shown as adding only 0.1 mm to the module  10 . The growth substrate  40  ( FIG. 2 ) may be about 0.25-0.5 mm thick. The total height of the flash module  10  of  FIG. 8  is less than 1 mm. It is envisioned that all practical flash modules of the invention, using a flex circuit, can be formed to have thicknesses less than 2 mm. 
     Note that the top surface area of the LED die  16  is about 1 mm2 and the combined area of the four sides of the LED die, using a 0.5 mm thick substrate  40 , is about 2 mm2. For a substrate  40  thickness of 0.25 mm, the side area equals the top surface area. So there is substantial side emission. 
       FIG. 9  is a back view of a smartphone  66 , illustrating the rectangular flash module  10  and camera lens  68 . 
     Accordingly, the present invention reduces the thickness of a flash module, improves the color uniformity across the beam, and increases the efficiency of the flash by creating a generally rectangular beam with a substantially uniform intensity across the relevant portion of the beam and by incurring less reflection of the LED light back toward the LED die. 
     The present invention may be used for other applications besides camera flashes, such as a flashlight module. 
     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.