Abstract:
A light emitting diode (LED) packaging structure includes a base, a transparent layer, and an LED chip. The transparent layer is provided between the LED chip and the base to increase a relative distance between the LED chip and the base. With an increased relative distance between the LED chip and the base and the light transmitting function of the transparent layer, the overall LED packaging structure can have enhanced light extraction efficiency. Further, the transparent layer provides good thermal conductivity and accordingly, forms no harm to the heat dissipation efficiency of the LED.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a light emitting diode (LED) packaging structure, and more particularly to an LED packaging structure that enables enhanced light extraction efficiency of an LED chip. 
       BACKGROUND OF THE INVENTION 
       [0002]    Due to its advantages of small volume, low power consumption, and long service life, the conventional light emitting diode (LED) has gradually replaced the traditional bulbs and been widely applied in traffic signs, vehicle directional signals, flashlights, mobile phones, lamps, and large-scale outdoor signboards. Therefore, it is desirable to develop the techniques of enhancing the light extraction efficiency of LED to thereby further expand its application in different fields. In the conventional manner of packaging an LED, the substrate for the LED is thinned and the LED chip is bonded to a lead frame, a printed circuit board, a silicon substrate, or a metal substrate. Then, wire bonding to electrodes and encapsulating of LED chip are performed. Generally, the chip bonding layer has a relative small thickness. As a result, it is difficult to extract photons emitted toward the base. In the case the thickness of the chip bonding layer is increased, the problem with heat dissipation will occur due to the low thermal conductivity of the bonding layer. Therefore, most part of the incident light projected toward the substrate can not be efficiently extracted to adversely affect the light output efficiency of the LED. 
         [0003]      FIG. 1  schematically shows a conventional LED packaging structure, in which an LED chip  13  is bonded to a lead frame  11  via a chip bonding layer  12 , a wire is electrically bonded to each of a first electrode  131  and a second electrode  132  on the LED chip  13  through wire bonding, and the LED chip  13  is encapsulated to complete the LED packaging process. The lead frame  11  can be a printed circuit board, a silicon substrate, or a metal substrate. The LED chip  13  includes a light emitting layer  133 , which emits a light source  14 . Since the chip bonding layer  12  has a relative small thickness, the light source  14  emitted from the light emitting layer  133  and reflected by the lead frame  11  tends to be blocked by the LED chip  13  or be absorbed by the substrate of the LED  13 , resulting in reduced brightness of the LED chip  13 . 
       SUMMARY OF THE INVENTION 
       [0004]    A primary object of the present invention is to provide an LED packaging structure that enables enhanced light extraction efficiency of the LED packaging structure to solve the problem of poor light extraction efficiency as found in the conventional LED packaging structure. 
         [0005]    Another object of the present invention is to provide an LED packaging structure that includes a base, a transparent layer, an LED chip, and a bonding layer. The transparent layer is provided atop the base, the LED chip is provided atop the transparent layer, and the bonding layer is provided between the transparent layer and the LED chip to bond the LED chip to the transparent layer. 
         [0006]    A further object of the present invention is to provide an LED packaging structure that includes a metal substrate, a conductive transparent layer, an LED chip, and a bonding layer. The conductive transparent layer is provided atop the metal substrate, the LED chip is provided atop the conductive transparent layer, and the bonding layer is provided between the conductive transparent layer and the LED chip to bond the LED chip to the conductive transparent layer. 
         [0007]    A still further object of the present invention is to provide an LED packaging structure that includes a base, a transparent layer, and an LED chip. The transparent layer is provided between the LED chip and the base to increase a relative distance between the LED chip and the base. With an increased relative distance between the LED chip and the base and the light transmitting function of the transparent layer, the probability for photons emitted toward the base to extract from side walls of the transparent layer is increased, which in turn enhances the light extraction efficiency of the overall LED packaging structure. Further, the transparent layer provides good thermal conductivity and accordingly, forms no harm to the heat dissipation efficiency of the LED chip. 
         [0008]    The LED packaging structure according to the present invention provides one or more of the following advantages: 
         [0009]    (1) The provision of the transparent layer in the LED packaging structure increases the relative distance between the LED chip and the base to thereby enhance the light extraction efficiency of the overall LED packaging structure. 
         [0010]    (2) With the light transmitting property of the transparent layer provided in the LED packaging structure, photons emitted from the LED chip can pass through side walls of the transparent layer to thereby increase the brightness of the LED chip. 
         [0011]    (3) With the good thermal conductivity of the transparent layer provided in the LED packaging structure, the increased relative distance between the LED chip and the base does not adversely affect the heat dissipation efficiency of the LED chip, so that the problems of shortened service life and reduced brightness of the LED due to overheat can be avoided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings, wherein 
           [0013]      FIG. 1  is a schematic view showing a conventional LED packaging structure; 
           [0014]      FIG. 2  is a schematic view showing an LED packaging structure according to a first embodiment of the present invention; 
           [0015]      FIG. 3  is a schematic view showing an LED packaging structure according to a second embodiment of the present invention; 
           [0016]      FIG. 4  is a schematic view showing an LED packaging structure according to a third embodiment of the present invention; 
           [0017]      FIG. 5  is a schematic view showing an LED packaging structure according to a fourth embodiment of the present invention; 
           [0018]      FIG. 6  is a schematic view showing a first structure for the transparent layer of the LED packaging structure of the present invention; and 
           [0019]      FIG. 7  is a schematic view showing a second structure for the transparent layer of the LED packaging structure of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Please refer to  FIG. 2  that is a schematic view showing a light emitting diode (LED) packaging structure according to a first embodiment of the present invention. As shown, the LED packaging structure in the first embodiment includes a base  21 , a transparent layer  22 , an LED chip  23 , and a bonding layer  24 . The transparent layer  22  is provided atop the base  21 , the LED chip  23  is provided atop the transparent layer  22 , and the bonding layer  24  is applied between the transparent layer  22  and the LED chip  23  to bond the LED chip  23  to the transparent layer  22 . 
         [0021]    A first electrode  231  and a second electrode  232  are provided on the top of the LED chip  23 . Then, wire bonding is performed for the first electrode  231  and the second electrode  232 , so that a wire (not shown) is bonded to each of the first and the second electrode  231 ,  232  for inputting electric current thereto. 
         [0022]    Then, the LED chip  23  is encapsulated to complete the packaging process. An adhesive compound  25  is further applied between the transparent layer  22  and the base  21  to bond the transparent layer  22  to the base  21 . It is noted the transparent layer  22  can be otherwise directly deposited on the base  21  or directly coated on the base  21  instead of being bonded to the base  21  using the adhesive compound  25 . Therefore, in the present invention, the transparent layer  22  can be connected to the base  21  in a manner as suggested in the first embodiment without being limited thereto. 
         [0023]    The transparent layer  22  can be deposited in a predetermined area on the base  21  through standard exposure and development process, local deposition process, etc. Preferably, the base  21  is a lead frame, a printed circuit board (PCB), a silicon substrate, a transparent substrate or a metal substrate, and can be made of a metal material, a semiconductor material, a ceramic material, a plastic material or a glass material. 
         [0024]    The LED chip  23  has a light emitting layer  233 , which emits a light source  234 . By providing the transparent layer  22 , a relative distance between the LED chip  23  and the base  21  is increased. As a result, the light source  234  reflected by the base  21  can be transmitted to external environment more easily. Further, the transparent layer  22  is formed using a light transmitting material containing zinc oxide (ZnO), including but not limited to zinc oxide (ZnO), indium zinc oxide (InZnO), gallium zinc oxide (GaZnO), and aluminum zinc oxide (AlZnO). The transparent layer  22  can be a single-layer structure or a multi-layer structure, and has good transmittance of light to enable enhanced light extraction efficiency of the LED chip  23 . 
         [0025]    Further, the transparent layer  22  containing ZnO provides good thermal conductivity, which in turn allows degree of freedom in increasing the relative distance between the LED chip  23  and the base  21 . Therefore, the LED chip  23  is not subject to the drawback of reduced brightness or shortened service life due to any increase in the relative distance between it and the base  21 . It is also noted the thickness of the transparent layer  22  corresponds to the wavelength of the light source  234 . Thus, the thickness of the transparent layer  22  can be adjusted according to the wavelength of the light source  234 . 
         [0026]    Please refer to  FIG. 3 , which is a schematic view showing an LED packaging structure according to a second embodiment of the present invention. As shown, the LED packaging structure in the second embodiment includes a base  21 , a transparent layer  22 , an LED chip  23 , and a bonding layer  24 . The transparent layer  22  is provided atop the base  21 , the LED chip  23  is provided atop the transparent layer  22 , and the bonding layer  24  is applied between the transparent layer  22  and the LED chip  23  to bond the LED chip  23  to the top of the transparent layer  22 . A first electrode  231  and a second electrode  232  are provided on the top of the LED chip  23 . Then, wire bonding is performed for the first electrode  231  and the second electrode  232 , so that a wire is bonded to each of the first and the second electrode  231 ,  232  for inputting electric current thereto. Then, the LED chip  23  is encapsulated to complete the packaging process. The base  21  can be electroplated on one side of the transparent layer  22  directly, or by depositing a thin metal layer on the side of the transparent layer  22  first and then electroplating the base  21  on the transparent layer  22 . The LED chip  23  is then bonded to another side of the transparent layer  22  opposite to the base  21 . Thereafter, wire bonding is performed for the first and the second electrode  231 ,  232 , and the LED chip  23  is encapsulated. The transparent layer  22  is inlayed into the base  21 . 
         [0027]    A reflective layer  26  can be further formed between the transparent layer  22  and the base  21 . The reflective layer  26  can be of a distributed Bragg reflector (DBR), which reflects the light source emitted from the LED chip  23 . Further, the reflective layer  26  can be a single-metal material or a multi-metal material, or be a multi-layer dielectric material to enhance its reflectivity of the light source  234  and the light extraction efficiency of the overall LED packaging structure. 
         [0028]    The base  21  is a lead frame, a printed circuit board (PCB), a silicon substrate, a transparent substrate or a metal substrate, and can be made of a metal material, a semiconductor material, a ceramic material, a plastic material or a glass material. The transparent layer  22  is formed using a light transmitting material containing zinc oxide (ZnO), including but not limited to zinc oxide (ZnO), indium zinc oxide (InZnO), gallium zinc oxide (GaZnO), and aluminum zinc oxide (AlZnO). 
         [0029]      FIG. 4  is a schematic view showing an LED packaging structure according to a third embodiment of the present invention. As shown, the LED packaging structure in the third embodiment includes a metal substrate  31 , a conductive transparent layer  32 , an LED chip  33 , and a conductive bonding layer  34 . The conductive transparent layer  32  is provided atop the metal substrate  31 , the LED chip  33  is provided atop the conductive transparent layer  32 , and the conductive bonding layer  34  is applied between the conductive transparent layer  32  and the LED chip  33  to bond the LED chip  33  to the top of the conductive transparent layer  32 . A first electrode  331  is provided on the top of the LED chip  33 , and a wire (not shown) is bonded to the first electrode  331  for inputting electric current thereto. An adhesive compound  35  is further applied between the conductive transparent layer  32  and the metal substrate  31  to bond the conductive transparent layer  32  to the metal substrate  31 . It is noted the conductive transparent layer  32  can be otherwise directly deposited on the metal substrate  31  or directly coated on the metal substrate  31  instead of being bonded to the metal substrate  31  using the adhesive compound  35 . Therefore, in the present invention, the conductive transparent layer  32  can be connected to the metal substrate  31  in a manner as suggested in the third embodiment without being limited thereto. 
         [0030]    The conductive transparent layer  32  can be deposited in a predetermined area on the metal substrate  31  through standard exposure and development process, local deposition process, etc. The conductive transparent layer  32  is formed using a light transmitting material containing zinc oxide (ZnO), including but not limited to zinc oxide (ZnO), indium zinc oxide (InZnO), gallium zinc oxide (GaZnO), and aluminum zinc oxide (AlZnO). The conductive transparent layer  32  can be further doped with aluminum (Al) or gallium (Ga) to thereby have good electric conductivity. The metal substrate  31  can be made of nickel (Ni), copper (Cu), or alloys of nickel and/or copper. 
         [0031]    The LED chip  33  has a light emitting layer  332 , which emits a light source  333 . By providing the conductive transparent layer  32 , a relative distance between the LED chip  33  and the metal substrate  31  can be increased. As a result, the light source  333  reflected by the metal substrate  31  can be transmitted to external environment more easily. The conductive transparent layer  32  has good transmittance of light to enable enhanced light extraction efficiency of the LED chip  33 . 
         [0032]    Further, the conductive transparent layer  32  containing ZnO provides good thermal conductivity, which in turn allows degree of freedom in increasing the relative distance between the LED chip  33  and the metal substrate  31 . Therefore, the LED chip  33  is not subject to the drawback of reduced brightness or shortened service life caused by exceeded temperature due to any increase in the relative distance between it and the metal substrate  31 . It is also noted the thickness of the conductive transparent layer  32  corresponds to the wavelength of the light source  333 . Thus, the thickness of the conductive transparent layer  32  can be adjusted according to the wavelength of the light source  333 . 
         [0033]      FIG. 5  is a schematic view showing an LED packaging structure according to a fourth embodiment of the present invention. As shown, the LED packaging structure in the fourth embodiment includes a metal substrate  31 , a conductive transparent layer  32 , an LED chip  33 , and a conductive bonding layer  34 . The conductive transparent layer  32  is provided atop the metal substrate  31 , the LED chip  33  is provided atop the conductive transparent layer  32 , and the conductive bonding layer  34  is applied between the conductive transparent layer  32  and the LED chip  33  to bond the LED chip  33  to the top of the conductive transparent layer  32 . A first electrode  331  is provided on the top of the LED chip  33 , and a wire (not shown) is bonded to the first electrode  331  for inputting electric current thereto. The metal substrate  31  can be electroplated on one side of the conductive transparent layer  32  directly, or by depositing a thin metal layer on the side of the conductive transparent layer  32  first and then electroplating the metal substrate  31  on the conductive transparent layer  32 . The LED chip  33  is then bonded to another side of the conductive transparent layer  32  opposite to the metal substrate  31 . Thereafter, wire bonding is performed for the first electrode  331 , and the LED chip  33  is encapsulated. The conductive transparent layer  32  is inlayed into the metal substrate  31 . 
         [0034]    The conductive transparent layer  32  is formed using a light transmitting material containing zinc oxide (ZnO), including but not limited to zinc oxide (ZnO), indium zinc oxide (InZnO), gallium zinc oxide (GaZnO), and aluminum zinc oxide (AlZnO). The conductive transparent layer  32  can be further doped with aluminum (Al) or gallium (Ga) to thereby have good electric conductivity. The metal substrate  31  can be made of nickel (Ni), copper (Cu), or alloys of nickel and/or copper. The metal substrate  31  is a lead frame, a printed circuit board (PCB), a silicon substrate, a transparent substrate or a metal substrate, and can be made of a metal material, a semiconductor material, a ceramic material, a plastic material or a glass material. A reflective layer  36  can be further formed between the conductive transparent layer  32  and the metal substrate  31 . The reflective layer  36  can be of a distributed Bragg reflector (DBR), which reflects the light source emitted from the LED chip  33 . Further, the reflective layer  36  can be a single-metal material or a multi-metal material, or be a multi-layer dielectric material to enhance its reflectivity of the light source  333  and the light extraction efficiency of the overall LED packaging structure. 
         [0035]    Please refer to  FIG. 6  that shows a first structure for the transparent layer of the LED packaging structure of the present invention. As shown, a transparent layer  43  with the first structure is formed atop a base  41 , and an adhesive compound  42  can be used between the transparent layer  43  and the base  41  to firmly bond them to each other. According to the first structure, one or both of two opposite faces of the transparent layer  43  are roughed to enhance the light extraction efficiency of the LED packaging structure of the present invention. The base  41  can be a lead frame, a printed circuit board (PCB), a silicon substrate, a transparent substrate or a metal substrate. It is noted the transparent layer  43  can doped with aluminum (Al) or gallium (Ga) to thereby become electrically conductive. The adhesive compound  42  can also be electrically conductive. 
         [0036]      FIG. 7  shows a second structure for the transparent layer of the LED packaging structure of the present invention. As shown, a transparent layer  43  with the second structure is formed atop a base  41 , and an adhesive compound  42  can be used between the transparent layer  43  and the base  41  to firmly bond them to each other. According to the second structure, side walls of the transparent layer  43  are formed into beveled surfaces, so that more parts of the light source can be reflected out to the external environment to enhance the light extraction efficiency of the LED packaging structure of the present invention. The base  41  can be a lead frame, a printed circuit board (PCB), a silicon substrate, a transparent substrate or a metal substrate. It is noted the transparent layer  43  can doped with aluminum (Al) or gallium (Ga) to thereby become electrically conductive. The adhesive compound  42  can also be electrically conductive. 
         [0037]    The present invention has been described with some preferred embodiments thereof and it is understood that many changes and modifications in the described embodiments can be carried out without departing from the scope and the spirit of the invention that is intended to be limited only by the appended claims.