Patent Publication Number: US-2013233710-A1

Title: Method of manufacturing light emitting diode packaging lens and light emmiting diode package

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a light emitting diode packing lens. More specifically, the present invention relates to a light emitting diode packing lens having a phosphor layer made by electrophoretic deposition. 
     BACKGROUND OF THE INVENTION 
     Light emitting diode (LED) has commonly been used in many fields for different purposes, such as lightening, signaling and displaying. Although LED industry made a great leap during last two decades, uniformity of light and lightening efficiency are still two aspects that are not easily controlled by current manufacturing process. This is because attachment of phosphors is done by gluing. It causes non-uniform phosphor distribution in the glue or an irregular gluing body. A common defect of such a LED shows annular yellow rings. 
     Please refer to  FIG. 1  (Prior Art). A conventional LED package  10  is shown. The LED package  10  has first and second lead frames  12  and  14 , by which electrical power is supplied to the LED package  10 . The lead frame  12  has a recessed reflector area  16  in which is disposed an LED  18 . The LED  18  is made from an indium-doped gallium nitride epitaxial layer on a transparent sapphire substrate. When activated by a direct current at an appropriate forward voltage, the top surface of the LED  18  of indium gallium nitride produces a blue light at approximately 470 nm wavelength. 
     The LED  18  is connected by a wire bond  20  to the lead frame  12  and by a wire bond  22  to the lead frame  14 . The LED  18  has a layer of fluorescent material  24  disposed over it. The fluorescent material  24  is generally a transparent epoxy resin containing particles of YAG/Gd:Ce phosphors. The entire assembly is embedded in a transparent encapsulation epoxy resin  26 . 
     Also shown in  FIG. 1  (Prior Art) are arrows  28  and  30 , which represent the light rays of an annular blue ring. The arrows  32  and  34  represent the light rays of an outer annular ring, and the arrows  36  and  38  represent an inner annular yellow ring. 
     Referring to  FIG. 2  (Prior Art), the lead frame  12  is shown with its reflector portion  16  which forms a cup holding the LED  18 . Shown closer up is the layer of fluorescent material  24  having thin areas  40  and  42  and a thicker area  44 . The final encapsulation epoxy resin  26  is not shown for purposes of simplicity. 
     Where the layer of fluorescent material was relatively thin at areas  40  and  42 , shown in  FIG. 2 , the blue light would generally provide a blue annular ring along the light ray lines  28  and  30  since there would be insufficient contribution of light from the phosphors. Inside and out of the annular blue ring would be yellow annular rings due to light rays  32  and  34  and light rays  36  and  38  where the phosphors would contribute some light but not enough to create a uniform white light. It has been determined that the surface tension of the material  24  over the LED  18  causes areas of various thickness which range from the thicknesses at areas  40  and  42  by the corners of the LED  18  and the thickness at area  44  above the center of the LED. This causes non-uniform reradiation of the blue light and the annular rings previously described. Here, we can see the problem of non-uniformity of phosphor layer. 
     In order to solve the aforementioned problem, U.S. Pat. No. 5,959,316 disclosed an improving structure for LED encapsulation. In the invention as shown in  FIG. 3 , a surface-mounted LED light  52  disposed on a device substrate  54  of a surface mount device. The LED  52  is encapsulated in a transparent spacer  56  which is further covered by a layer of fluorescent material  58  and a final transparent encapsulation layer  50 . It is possible to utilize surface tension (which at the size of an LED  52  is large relative to gravitational forces) in combination with viscosity to allow the drop of a hemispherical shape of a viscous, transparent ultraviolet (UV) light-cured resin over the LED  52  which forms the transparent spacer  56 . The resin would cover all the corners and then be cured by using UV light. This would then be followed with the layer of the fluorescent material  58 , a viscous UV cured resin. The deposition of the transparent spacer  56  would provide a hemisphere. Then the layer of fluorescent material  58  would flow to conform to the hemispherical shape of the transparent spacer  56  and be cured prior to the final encapsulation  58  are formed. Although the invention would not be easily subject to the annular ring problem, manufacturing process for forming the layer of fluorescent material  58  in practice often encounters non-uniform distribution of phosphors in the mixture of fluorescent material  58 . Shape of the transparent spacer  56  is not perfectly hemispherical. It is still difficult to provide a good yield rate of products. 
     Another invention described in U.S. Pat. No. 7,278,756 provides an innovative way to improve the problem of uniformity of light. Please refer to  FIG. 4 . It is a schematic cross-sectional view of the LED in accordance with the &#39;756. The LED  60  comprises a chip body  68  for emitting light, an encapsulation can  66  surrounding the chip body  68  and having a light emitting surface  62 , and a base  69  supporting the encapsulation can  66  and the chip body  68 . The encapsulation can  66  has numerous fluorescent particles  64  arranged there. 
     The fluorescent particles  64  are distributed in a region adjacent to the light emitting surface  62 , distal from the chip body  68 . The fluorescent particles  64  progressively increase in size with increasing distance away from a center axis of the region. The fluorescent particles  64  scatter light emitted from the chip body  68  to improve luminance and uniformity of illumination. However, in practice, it is also a challenge to achieve such particle arrangement. 
     Recently, U.S. Pat. No. 7,479,662 provides a method to overcome the defects of LED mentioned above. As illustrated in  FIG. 5 , an LED package  70  includes an LED chip  72  mounted on a substrate  74 , which in turn is mounted on a reflector  76 . A lens  78  surrounds the chip  72 , the substrate  74  and reflector  76 . Optionally filling space  82  between the lens  78  and the chip  72  is typically an epoxy or other transparent material. A phosphor coating  84  comprising phosphor particles is applied on the inside surface of the lens  78  and on the top surface of the reflector  76 . The top surface of the reflector  76 , which may be thought of as the bottom of the package, is preferably first coated with a reflective layer  80 , such as a high dielectric powder, such as alumina, titania, . . . etc. A preferred reflective material is Al 2 O 3 . The phosphor layer  84  is then placed over the reflective layer  80  on top of the reflector  76 . The use of the reflective layer  80  serves to reflect any radiation  86  that penetrates the phosphor layer  84  on this surface. 
     Alternately, instead of coating the transparent lens  78  with a separate phosphor layer  84 , the phosphor may instead be intimately dispersed within the material comprising the transparent hemisphere. The phosphor layer  84  over the reflective layer  80  on the reflector  76  is preferably relatively thick, i.e. &gt;5 layers of powder, while the phosphor layer on the curved top of the hemisphere may be adjusted to achieve a desired color and to absorb all radiation incident on it However, a proper approach to make the separate phosphor layer  84  is not disclosed. 
     Hence, a method for forming a uniform phosphor layer is desired. More specifically, a uniform phosphor layer formed in a curved shape is the key point to solve those problems mentioned above. The present invention uses electrophoretic deposition to form a uniform layer of phosphors. Meanwhile, the coated object is a pre-formed and transparent curved cup and free from the problem of lens shape. Therefore, the invention is a preferred solution for LED packaging lens to improve the uniformity of phosphor layer and lightening efficiency. 
     SUMMARY OF THE INVENTION 
     This paragraph extracts and compiles some features of the present invention; other features will be disclosed in the follow-up paragraphs. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. 
     In accordance with an aspect of the present invention, a method of manufacturing light emitting diode packaging lens, comprises the steps of: a) providing a cup having a curved portion; b) forming a transparent conductive coating on one surface of the cup; c) providing a first solution having a first group of phosphors for electrophoretic deposition and a first curved electrode immersed in the first solution, wherein surface shapes of the cup and the first curved electrode are the same; d) immersing the cup into the first solution; e) locating the cup and the first curved electrode when the first curved electrode and the transparent conductive coating have electric potential difference, equal electric potential forms the same shape as the surface shape between the cup and the first curved electrode; f) providing a first direct current to the transparent conductive coating and the first curve electrode for electrophoretically depositing the first group of phosphors onto the coated surface of the cup and forming a first phosphor layer; g) removing the cup from the first solution; and h) drying the cup. 
     Preferably, the surface shape is hemispherical. 
     Preferably, the cup is made of epoxy resin, silicone, polyetherimide, fluorocarbon polymer, polymethyl methacrylate (PMMA), polycarbonate (PC), cyclo olefin copolymer (COC) or glass. 
     Preferably, the transparent conductive coating comprises indium tin oxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO), zinc oxide, tin dioxide, or a mixture thereof. 
     Preferably, the first solution further comprises a first solvent and a first binder. 
     Preferably, the first solvent is isopropyl alcohol (IPA), ethanol, acetone, or water. 
     Preferably, the first binder is silver nitrate or magnesium nitrate. 
     Preferably, the cup further has a fixing portion formed on rim of the curved portion. 
     Preferably, the fixing portion is a cylinder having a thickness as that of the cup. 
     Preferably, a radius of the curved portion of the cup differs from a radius of the first curved electrode by at least one order. 
     Preferably, the method further comprising the steps of: f1) providing a second solution having a second group of phosphors for electrophoretic deposition and a second curved electrode immersed in the second solution, wherein the surface curve of the second curved electrode is the same as that of the cup; f2) immersing the cup into the second solution; f3) locating the cup and the second curved electrode wherein when a direct current applied to the second curved electrode and the transparent conductive coating, equal electric potential contour lines form the same shape as the surface shape between the cup and the first curved electrode; f4) providing a second direct current to the transparent conductive coating and the second curved electrode for electrophoretically depositing the second group of phosphors onto the first phosphor layer; and f5) removing the cup from the second solution; 
     Preferably, the second solution further comprises a second solvent and a second binder. 
     Preferably, the second solvent is isopropyl alcohol (IPA), ethanol, acetone, or water. 
     Preferably, the second binder is silver nitrate or magnesium nitrate. 
     In accordance with an aspect of the present invention, a light emitting diode package, comprises: a substrate; a reflecting layer formed on a top surface of the substrate; a one light emitting chip mounted on the reflecting layer; and a lens including: a cup having a curved portion enclosing the light emitting diode, fixed on the substrate; a transparent conductive coating formed on one surface of the cup; and at least one phosphor layer formed on the transparent conductive coating. 
     Preferably, the curved portion is hemispherical. 
     Preferably, the substrate is a silicon substrate, a ceramic substrate, an aluminum plate, a copper plate or a printed circuit board. 
     Preferably, the reflecting layer is made of a metal. 
     Preferably, the cup is made of epoxy resin, silicone, polyetherimide, fluorocarbon polymer, polymethyl methacrylate (PMMA), polycarbonate (PC), cyclo olefin copolymer (COC) or glass. 
     Preferably, the transparent conductive coating comprises indium tin oxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO), zinc oxide, tin dioxide, or a mixture thereof. 
     Preferably, the method further comprises a layer of polyvinyl alcohol (PVA) and sensitizers on the surface of the phosphor layer. 
     Preferably, the cup is fixed on the substrate by gluing. 
     Preferably, the cup further has a fixing portion formed on rim of the curved portion. 
     Preferably, the fixing portion is a cylinder having a thickness as that of the cup. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a first prior art of a conventional light emitting diode package. 
         FIG. 2  illustrates the detailed structure disclosed in the first prior art. 
         FIG. 3  illustrates a second prior art of light emitting diode structure. 
         FIG. 4  illustrates a third prior art of light emitting diode structure. 
         FIG. 5  illustrates a fourth prior art of light emitting diode structure. 
         FIG. 6  is a schematic cross-sectional view of a cup coated with a transparent conductive coating in a first embodiment of the present invention. 
         FIG. 7  illustrates an electrophoretic deposition tank used in the first embodiment. 
         FIG. 8  shows an electric potential distribution in a hemispherical portion of the cup after conducted with a direct current in the first embodiment. 
         FIG. 9  illustrates ion movement in the cup after conducted with a direct current in the first embodiment. 
         FIG. 10  shows a first phosphor layer formed on a concave surface of the cup in the first embodiment. 
         FIG. 11  shows a relation between deposition weight and time under different voltages. 
         FIG. 12  shows a relation between deposition weight and time for different YAG concentration under the same voltage (300V). 
         FIG. 13  is a plot of output light intensity versus wavelength for a uniform phosphor. 
         FIG. 14  is a plot of output light intensity versus wavelength for different combinations of uniform phosphors and LED lights. 
         FIG. 15  is an ideal correlated color temperature distribution in a CIE 1931 color space for a combination of a uniform phosphor and a LED light. 
         FIG. 16  shows the tested results of correlated color temperature distribution for the combination of a uniform phosphor and a LED light. 
         FIG. 17  shows a relation between deposition weight and thickness for a specified phosphor. 
         FIG. 18  shows a relation between correlated color temperature and thickness. 
         FIG. 19  shows a correlation of correlated color temperature, deposition weight and thickness. 
         FIG. 20  is a light emitting diode package of the first embodiment. 
         FIG. 21  is a curved cup of the first embodiment. 
         FIG. 22  shows a schematic cross-sectional view of a cup of a second embodiment. 
         FIG. 23  is a light emitting diode package of the second embodiment. 
         FIG. 24  shows a schematic cross-sectional view of a cup of a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For a better understanding of the present invention, three embodiments are described below. 
     First Embodiment 
     Please refer to  FIG. 6  to  FIG. 21 . A first embodiment is illustrated.  FIG. 6  shows a schematic cross-sectional view of a cup  100 . The cup  100  is transparent and made of polycarbonate (PC). It has a diameter of 4.5 mm and uniform thickness of 0.5 mm. The cup  100  has a shape of hemisphere. It means that a point A on  FIG. 6  keeps equally distant from each part of the concave surface of the cup  100 . A transparent conductive coating layer  102  is formed on the concave surface of the cup  100  and made of indium tin oxide (ITO). ITO particles are sputtered onto the cup  100 . Since ITO is transparent and conductive, its thickness can be 200-300 nm in practice. 
     Shown in  FIG. 7  is an electrophoretic deposition tank  200  used in the first embodiment. The tank  200  contains a first solution  202  which comprises a first group of phosphors (not shown). In this embodiment, the phosphor particles used are Y 3 A 15 O 12  (YAG):Ce. YAG particles stay in a suspension status in the first solution  202 . A first hemispherical electrode  304  immersed in the first solution is connected to a DC power source  300 . The DC power source  300  can provide a voltage of 100V-1000V. 
     Then, the cup  100  shown in  FIG. 6  is placed into the first solution  202 . With a holder  204  to fix its position, a hemispherical portion  3044  of the first hemispherical electrode  304  and the concave surface of the cup  100  are concentrically placed. For the reason of liquidity of the first solution  202 , the radius of the cup  100  is 15 times larger that of the first hemispherical electrode  304 . The DC power source  300  is further connected with the transparent conductive coating layer  102 . When the voltage is applied, electrophoretic deposition starts. In this embodiment, the first point electrode  304  acts as an anode while the transparent conductive coating layer  102  is a cathode. 
     Please refer to  FIG. 8  and  FIG. 9 .  FIG. 8  illustrates the electric potential distribution during electrophoretic deposition. As the dash lines show, equal electric potential locations form a hemi-circle in the cross-section. In a 3-dimensional view, the equal electric potential locations form a hemisphere. In  FIG. 8 , movement of cations  2022  and anions  2024  in the electrophoretic deposition tank  200  during electrophoretic deposition is illustrated. Cations  2022  move to the transparent conductive coating layer  102  while anions  2024  gather in the first point electrode  304 . In addition to the first group of phosphors, the first solution  202  further comprises isopropyl alcohol (IPA) as a solvent and magnesium nitrate as a binder. The magnesium nitrate is ionized in the first solution  202 . The suspended YAG particles combine with magnesium ions and become the cations  2022 . Therefore, when the DC voltage is applied (electrophoretic deposition starts), a phosphor layer  104  is gradually formed on the surface of the transparent conductive coating layer  102  shown in  FIG. 10 . 
       FIG. 11  to  FIG. 19  illustrate different factors affecting formation of a phosphor layer by using some experimental data.  FIG. 11  shows a relation between deposition weight and time under different voltages. It is obvious that the higher voltage is applied, the more deposition weight is formed. When time increases, deposition weight increases as well. Similar to  FIG. 11 ,  FIG. 12  shows a relation between deposition weight and time for different YAG concentration under the same voltage (300V). If YAG concentration is higher, during the same electrophoretic deposition time, the deposition weight is higher. 
     For a typical uniform light source, the light intensity distribution over its full wavelength is shown in  FIG. 13 . When the light source covered with a uniform layer of phosphor, the distribution will have two peaks. Please refer to  FIG. 14 . It shows distribution for different deposition weights. If the deposition weight is higher, the peak value will become higher. 
       FIG. 15  is an ideal correlated color temperature distribution in a CIE 1931 color space for a combination of a uniform phosphor and a LED light. A straight line links two ends fall on the curve. It means any correlated color temperature in the line can be found by using the combination of a specified light and a phosphor. However,  FIG. 16  indicates that experimental results don&#39;t fall on the expectation. A correction is desired. Please refer to  FIG. 17 . There is a linear relation between deposition weight and thickness for a specified phosphor when electrophoretic depositing. However, correlated color temperature has an inverse proportional relation with thickness shown in  FIG. 18 . By combining  FIGS. 17 and 18 ,  FIG. 19  reveal two best fit lines to indicate how to get a desired correlated color temperature. 
     Therefore, speed of formation of the phosphor layer  104  depends on applied voltage, concentration of phosphor and electrophoretic deposition duration. The higher voltage is applied, denser concentration of phosphor is used or the longer electrophoretic deposition duration lasts, the faster the phosphor layer  104  is formed. Besides, due to an isotropic distribution of electric potential, the phosphor layer  104  can be formed uniformly. 
     After the electrophoretic deposition stops, the cup  100  is removed from the first solution  202  and dried. The cup  100  is placed in a well ventilated place for drying. When the drying process finishes, the phosphor layer  104  becomes more compact and most of IPA have been removed. 
     Please refer to  FIG. 20 . In order to get a light emitting diode packaged for an illuminating device, the cup  100  is further assembled with a silicon substrate  400 . The silicon substrate  400  has a reflecting layer  402  which is made by coating a layer of silver onto a surface of the silicon substrate  400 . A light emitting diode  404  is mounted on the reflecting layer  402 . Glue  408  is used to bind the cup  100  and the silicon substrate  400  for packaging. The glue  408  is silicone. 
     It should be noticed that air or any gas can be filled in the package between the cup  100  and the silicon substrate  400 . Refraction index of the air or gas is 1 which is much smaller than 1.5 of silicone (a conventional material of LED package) and 2.4 of light emitting chips, so emission of light out of the LED package can be more efficient. This is because the light reflected from phosphors in the phosphor layer  104  can be reflected back to the phosphor layer  104  by the reflecting layer  402 . Besides, due to electrophoretic deposition, the phosphor layer  104  can be uniform. A white light of good quality can be generated with the phosphor layer  104  to be very thin. Thin phosphor layer  104  further brings less reflected light and causes an improved lightening effect. 
     In the first embodiment, the cup  100  is a hemisphere. It can be other curved shape as shown in  FIG. 21 . There is no more structure for holding the cup  100 . In practice, the reflecting layer  402  can be formed on the substrate  406  and the number of light emitting chip is not limited to one. Most of all, more than two kinds of phosphors can be used for forming the phosphor layer  104 . 
     The first embodiment shows that the radius of the first hemispherical electrode  304  is small than that of the cup  100 . In some extreme cases, if the first hemispherical electrode  304  is small enough, it can be replaced by a point electrode. According to the present invention, the cup  10  smaller than that of the first hemispherical electrode  304  by at least one order can be applied, too. Under this situation, the transparent conductive coating layer  102  is formed on the other surface of the cup  10 . Meanwhile, the first hemispherical electrode  304  is still concentrically placed with the cup  10  and covers the cup  10 . Therefore, the phosphor layer  104  will be formed on the same surface with the transparent conductive coating layer  102  (on the protruding surface). 
     Second Embodiment 
     Please refer  FIG. 22  and  FIG. 23 . A second embodiment is illustrated. A cup  500  which is transparent and has a hemispheric portion  5002  and a short cylinder portion  5004 . The cup  500  is made of polymethyl methacrylate (PMMA). Its size is 5.0 mm and has uniform thickness of 0.5 mm. A transparent conductive coating layer  502  is formed on the concave surface of the cup  500  and made of indium zinc oxide (IZO). IZO particles are sputtered onto the cup  500 . Since IZO is transparent and conductive, its thickness can be 200-300 nm in practice. By using the same process, solvent and binder of electrophoretic deposition in the first embodiment, a phosphor layer  504  is formed. Here, a Terbium-doped YAG and a Cerium-doped YAG are used as phosphors. During the electrophoretic deposition process, the phosphors combine with magnesium ions and become cations. Therefore, they can be coated on the cathode side. 
     Then, a gel-like fixing liquid is sprayed over the concave surface of the cup  500 . The fixing liquid comprises ethanol and sensitizers. After curing by ultraviolet (UV) beams, an attaching layer  506  is formed. The attaching layer  506  is used to help attachment of phosphors in the phosphor layer  504 . 
     In the assembly process, the cup  500  is connected with a substrate  508 . The substrate comprises a reflecting layer  510 . A light emitting chip group  512  is mounted on the substrate  508  with bonding wires. Four light emitting chips are arranged in the light emitting chip group  512 . A circular slot  516  used to accommodate the cylinder portion  5004  of the cup  500  is on the substrate  508 . By using glue  518 , the cup  500  is closely assembled with the substrate  508 . 
     Third Embodiment 
     According to the present invention, number of the phosphor layer is not limited to one. For a specific color light, two or more layers of phosphors can be applied. It can be achieved by electrophoretic deposition, too. 
     Please see  FIG. 24 . A third embodiment is illustrated. A cup  600  which is transparent and hemispherical and made of glass. A transparent conductive coating layer  602  is formed on the concave surface of the cup  600  and made of aluminum zinc oxide (AZO). AZO particles are sputtered onto the cup  600 . By using the same process, solvent and binder of electrophoretic deposition in the first embodiment, a first phosphor layer  604  is formed. Here, the first phosphor layer  604  has MgSiO 3 :Eu phosphors for generating a red light by excitation of LED light. Then, the cup  600  is removed from the first solution described in the first embodiment and immersed into a second solution. The second solution has Cerium-doped YAG phosphors, solvent of IPA and binder of silver nitrate. By using a second point electrode (not shown), the electrophoretic deposition process goes again. Then, a second phosphor layer  606  is formed. The cup  600  can be used as a packaging lens for any LED package. 
     In this embodiment, the first phosphor layer  604  is thinner than the second phosphor layer  606  for two reasons: 1) for the second electrophoretic deposition process, the first phosphor layer  604  can not be thick to affect current conduction, and 2) the MgSiO 3 :Eu phosphors are just used to make light color a little reddish. It is not necessary to apply MgSiO 3 :Eu phosphors more than Cerium-doped YAG phosphors. The factors to control thickness of phosphor layer are time for electrophoretic deposition, concentration of phosphor and voltage applied. 
     It should be noticed that materials of the cup  600  can be epoxy resin, silicone, polyetherimide, fluorocarbon polymer, or cyclo olefin copolymer (COC). Conductive coating layer  602  can be made of zinc oxide or tin dioxide. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is understood that the invention needs not be limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.