Patent Publication Number: US-2005139846-A1

Title: High power light emitting diode package and fabrication method thereof

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a light emitting diode package, and more particularly, a high power light emitting diode package which can enhance heat radiation effect as well as omit a wire bonding procedure to simplify a package structure and reduce the package size.  
      2. Description of the Related Art  
      Light Emitting Diodes (LEDs) are widely used owing to several advantages such as low power consumption and high brightness, and in particular, recently utilized in illumination devices and as backlights for large-sized Liquid Crystal Displays (LCDs). The LEDs are provided in the form of packages to be easily mounted on the illumination devices and so on. LED protection ability, connection structures to main devices and heat radiation performance for radiating heat generated from LEDs are main bench-marks of the LED packages. High heat radiation performance is a more important package requirement in an industrial field such as common illumination devices and LCD backlights which adopt high power LEDs.  FIG. 1   a  is a perspective sectional view illustrating a conventional high power LED package.  
      Referring to  FIG. 1   a,  an LED package  10  includes a housing  1  having lead frames  2 , an LED  3  in the form of a chip, a heat sink  4  seating the LED  3  thereon, a silicon sealant  5  for sealing the LED  3  and a plastic lens  7  for covering the silicon sealant  5 . The LED  3  is connected to the lead frames  2  via wires  6  to be powered, and seated on the heat sink  4  via solders.  
      The LED package  10  in  FIG. 1   a  is mounted on a PCB  9 , as shown in  FIG. 1   b,  of an illumination device (not shown). The heat sink  4  of the LED package  10  can transfer heat generated from the LED  3  to the PCB  9  via a heat conductive pad  8  such as solders to suitably radiate the heat to the outside.  
      Fabrication of the high power LED package is difficult owing to a complicated process such as a die bonding and a wire bonding of the LED. In particular, its assembly/connection process such as wire bonding may have a high percent defective, and the wires may act as a factor for increasing the size of the overall package.  
       FIGS. 2   a  and  2   b  illustrate another conventional high power LED package.  
      Referring to  FIGS. 2   a  and  2   b,  a high power LED package  20  includes a lower ceramic board  11  having lead frames  13  and  14  and an upper ceramic board  12  having a circular cavity therein. On the lower ceramic board  11 , there is mounted an LED  15  to be connected to the lead frames  13  and  14 . A cylindrical reflector  12   a  is placed on the side wall of the cavity in the upper ceramic board  12 , and transparent resin is filled into the cavity to encapsulate the LED  15 .  
      Unlike  FIG. 1   a,  one electrode of the LED  15  in the LED package  20  shown in  FIG. 2   a  is connected to one of the lead frames  13  and  14  via a wire  16 . Alternatively, the LED  15  may be mounted via flip chip bonding.  
      Since the overall structure is simplified, there are advantages that a fabrication process is facilitated and percent defective is reduced, but heat radiation effect is degraded as a drawback.  
      More particularly, although the package shown in  FIG. 2   a  may also have a plurality of conductive via holes (not shown) formed in the lower ceramic board  11  to promote heat radiation from the LED  15 , the size and number of the conductive via holes is essentially restricted to stably support an LED chip while preventing unwanted contact with the lead frames As a consequence, the LED package has a relatively lower heat radiation effect than that of the package in  FIG. 1   a,  and thus cannot sufficiently endure the heat generated from the high power LED.  
      As described above, the conventional LED package tends to be defective owing to its complicated structure and fabrication process. To the contrary, the package of a simple structure has a problem that heat radiation effect, which is one of its major functions, is degraded.  
     SUMMARY OF THE INVENTION  
      Therefore the present invention has been made to solve the foregoing problems of the prior art.  
      It is an object of the present invention to provide a novel LED package having a simplified overall structure to facilitate its fabrication as well as more effectively radiate heat generated from an LED therein.  
      It is another object of the present invention to provide a fabrication method of the LED package of the invention.  
      According to an aspect of the invention for realizing the object, there is provided an LED package comprising: a lower board having a heat radiation member formed in an LED mounting area and filled with conductive material and at least one via hole formed around the heat radiation member; first and second bottom electrodes formed in the underside of the lower board and connected to the heat radiation member and the at least one conductive via hole, respectively; an insulation layer formed on the top of the lower board to cover at least the heat radiation member; first and second electrode patterns formed on the insulation layer and connected to the first and second bottom electrodes through the at least one conductive via hole, respectively; and an LED connected to the first and second electrode patterns.  
      Preferably, the LED may be connected to the first and second electrode patterns via flip chip bonding.  
      The present invention can realize various forms of vertical connection structures between the first and second electrode patterns and the first and second bottom electrodes.  
      According to another aspect of the present invention, the at least one conductive via hole may comprise first and second conductive via holes arranged in opposite positions around the heat radiation member, and wherein the first and second electrode patterns may be connected to the first and second bottom electrodes through the first and second conductive via holes, respectively. Further, the first and second conductive via holes may be provided in plurality, respectively.  
      According to an further another aspect of the present invention, the first electrode pattern may be connected to the first bottom electrode via the at least one conductive via hole, and the second electrode pattern may be connected to the second bottom electrode via the heat radiation member.  
      Preferably, one of the first and second electrodes may be leaded to the heat radiation member to more effectively induce heat radiation.  
      According to an further another aspect of the present invention, the heat radiation member has a sectional area matching at least 50% of that of the LED, and the heat radiation member has a sectional area larger than that of the LED.  
      Preferably, the insulation layer may have a thickness of about 100 μm or less so that heat can be effectively radiated through the heat radiation member.  
      Preferably, The LED package may further comprise an upper board formed on the lower board to surround the LED. In this embodiment, the upper board may have a reflector provided in an inside wall portion surrounding the LED, and the LED package of the invention may further comprise a transparent lens structure provided on the upper board.  
      According to still another aspect of the invention for realizing the object, there is provided a fabrication method of LED packages comprising the following methods of: preparing a lower board having a heat radiation member formed in an LED mounting area filled with conductive material and an at least one conductive via hole formed around the heat radiation member; forming an insulation layer on the top of the lower board to cover at least the heat radiation member; forming first and second bottom electrodes in the underside of the lower board to be connected to the heat radiation member or the at least one conductive via hole; forming first and second electrode patterns on the insulation layer to be connected to the first and second bottom electrodes through the heat radiation member or the at least one conductive via hole, respectively; and mounting an LED to be connected to the first and second electrode patterns.  
      As set forth above, the present invention proposes an approach of mounting the LED via flip chip bonding instead of wire bonding that is a main factor causing a complicated structure and assembly process as well as defects. Further, the present invention provides a novel structure capable of enhancing heat radiation effect while utilizing flip chip bonding LED.  
      In order to form an electrode connection structure together with a heat radiation structure filled with high heat conductivity metal in a flip chip mounting area, the present invention also proposes to provide a large area heat radiation member, cover the heat radiation member with an insulation layer, and then form electrode patterns necessary for flip chip bonding on the insulation layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1   a  and  1   b  are perspective sectional and side sectional views illustrating a conventional high power LED package;  
       FIGS. 2   a  and  2   b  are side sectional and perspective views illustrating another conventional high power LED package;  
       FIG. 3  is a sectional view illustrating a high power LED package according to a preferred embodiment of the invention;  
       FIG. 4  is a sectional view illustrating a high power LED package according to an alternative embodiment of the invention;  
       FIGS. 5   a  to  5   i  are perspective views illustrating a fabrication process of high power LED packages according to the invention;  
       FIG. 6  is a perspective view illustrating a lower board having a plurality of conductive via holes according to the invention; and  
       FIGS. 7   a  and  7   b  are perspective views illustrating a heat radiation member structure according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
       FIG. 3  is a sectional view illustrating a high power LED package according to a preferred embodiment of the invention.  
      Referring to  FIG. 3 , a high power LED package  30  includes a lower board  31  mounted with an LED  35  and an upper board  32  surrounding an area where the LED  35  is arranged.  
      The lower board  31  includes a heat radiation member  36  formed in a substantially central area and first and second conductive via holes  33   b  and  34   b  defining two vertical connection structures. Unlike the conductive via holes  33   b  and  34   b  of tens μm sizes, the heat radiation member  36  has a size corresponding to the that of the LED  35 . The heat radiation member  36  can be made by filling conductive material into a cavity of a sufficient size formed in the lower board  31 . The heat radiation member  36  has a sectional area preferably matching about 50% of that of the LED  35  to be mounted thereon, and more preferably larger than that of the LED  35 .  
      The lower board  31  is covered with an insulation layer  37 , which is sized to cover at least the heat radiation member  36 . On the insulation layer  37 , first and second electrode patterns  33   a  and  34   a  are formed to be connected to the first and second conductive via holes  33   b  and  34   b,  respectively. The insulation layer  37  functions to separate the electrode patterns for flip chip bonding from the filling material of the heat radiation member (e.g., mainly a conductive material such as metal). The insulation layer  37  is preferably formed at a thickness of about 100 μm not to excessively block the heat transfer from the LED to the heat radiation member by large quantities.  
      The LED  35  is so mounted that the electrodes thereof are connected to the first and second electrode patterns  33   a  and  34   a  via flip chip bonding. The first and second conductive via holes  33   b  and  34   b  are connected to first and second bottom electrodes  33   c  and  34   c,  respectively, and the first and second bottom electrodes  33   c  and  34   c  function as power supplying terminals of the LED package  30 .  
      In addition, transparent resin may be filled into the inner mounting area of the upper board to encapsulate the LED, and a transparent lens structure  39  may be mounted on the upper board  32  to more efficiently emit light generated from the LED  35 .  
       FIG. 4  is a sectional view illustrating a high power LED package according to an alternative embodiment of the invention. The LED package of this embodiment shown in  FIG. 4  has a configuration similar to that shown in  FIG. 3  except for vertical connection structures between LED mounting electrodes and power supplying electrodes.  
      Referring to  FIG. 4 , the LED package  40  includes a lower board  41  mounted with an LED  45  and an upper board  32  for surrounding an area where the LED  45  is placed. In addition, a transparent lens structure  49  may be mounted on the upper board  42  to efficiently emit light generated from the LED  45 .  
      The lower board  41  includes a heat radiation member  46  formed in a substantially central area and a conductive via hole  43   b.  The heat radiation member  46  can be made by filling conductive material into a cavity of a sufficient size formed in the lower board  41 . The heat radiation member  46  has a sectional area preferably matching about 50% of that of the LED  45  to be mounted thereon, and more preferably larger than that of the LED  45 .  
      On the lower board  41 , there is arranged an insulation layer  47 , which is sized to cover the heat radiation member  46 . On the insulation layer  47 , there are formed first and second electrode patterns  43   a  and  44   a.    
      The first electrode pattern  43   a  is connected to the conductive via hole  43   b  as in the embodiment shown in  FIG. 3 , whereas the second electrode pattern  44   a  is connected to the heat radiation member  46 . Therefore, this embodiment provides the conductive via hole  43   b  as means for connecting the first electrode pattern  43   a  to the first bottom electrode  43   c.  Then, the heat radiation member  46  of this embodiment also functions as vertical connecting means together with heat radiating means. Further, in this embodiment, the second bottom electrode  44   c  is leaded to the heat radiation member  46  to enhance heat radiation effect, and this structure can be similarly applied to the embodiment in  FIG. 3 .  
       FIGS. 5   a  to  5   i  are perspective views illustrating a fabrication process of high power LED packages according to the invention.  
      As shown in  FIG. 5   a,  a lower board  51  having a cavity C in a substantially central area and two via holes h 1  and h 2  formed around the cavity C is prepared. The lower board can be produced by laminating a plurality of green sheets for example 5 green sheets  51   a  to  51   e  as in this embodiment according to Low Temperature Cofired Ceramic (LTCC) technique or High Temperature Cofired Ceramic (HTCC) technique. While the lower board  51  is made of ceramic like this, it may be made of a PCB or other insulating material. The cavity C has a sectional area preferably matching about 50% of that of the mounted LED.  
      Then, as shown in  FIG. 5   b,  suitable conductive material is filled into the cavity C to form a heat radiation member  56 , and into the via holes h 1  and h 2  formed in the lower board  51  to form conductive via holes  53   b  and  54   b.  Since the high heat conductivity material filling the heat radiation member  56  generally has a predetermined value of electric conductivity, the heat radiation member  56  can be formed through the same procedure as the filling procedure of the conductive via holes  53   b  and  54   b.  This is a printing procedure using metal paste, and more particularly, may be realized as a printing procedure for the respective green sheets  51   a  to  51   e  in the lamination procedure shown in  FIG. 5   a.    
      Next an insulation layer  57  is formed on the lower board  51  as shown in  FIG. 5   c.  The insulation layer  57  is a constitutional element for forming electrode patterns for flip chip bonding as well as insulating the large-sized heat radiation member  56  arranged in a mounting area, and thus so formed to cover the area of the heat radiation member  56 . The insulation layer is preferably made at a thickness of about 100 μm or less. The insulation layer can be made through a conventional process such as lamination, spraying or printing, and for the purpose of stabilization, may be sintered after being laid on the lower board.  
      Then, electrodes are formed on the top and underside of the lower board  51  as shown in  FIG. 5   d.  On the insulation layer  57 , first and second electrode patterns  53   a  and  54   a  are first formed to be connected to the two conductive via holes  53   b  and  54   b,  respectively. Then, first and second bottom electrodes  53   c  and  54   c  are formed on the underside of the lower board  51  to be connected to the two conductive via holes  53   b  and  54   b,  respectively. The second bottom electrode  54   c  is leaded to the heat radiation member  56 . This electrode forming procedure can be implemented through a procedure such as printing, plating, vacuum deposition, sputtering or post-deposition photolithography, and sintering may be selectively added to stabilize the electrodes formed like this.  
      Next, as shown in  FIG. 5   e,  an upper board  52  having a cavity for surrounding the LED-mounting area is mounted on the lower board  51 . The upper board is not limited in its material, but may be made of metal, ceramic and/or plastic. Preferably, a reflector may be additionally formed on the inside wall of the cavity to improve reflectivity. Further, the upper board-mounting procedure may be alternatively performed following an LED-mounting procedure.  
      Then, LED mounting is performed on the first and second electrode patterns  53   a  and  54   a  via flip chip bonding. First, as shown in  FIG. 5   f,  solder bumps B 1  and B 2  are placed on the first and second electrode patterns  53   a  and  54   a  to which high power LED bonding electrodes are to be connected. Then, as shown in  FIG. 5   e,  a high power LED  55  is mounted on the electrode patterns  53   a  and  54   a  so that bonding electrodes (not shown) of the high power LED  55  are connected to the solder bumps B 1  and B 2 , respectively. Preferably, fluorescent material capable of converting light generated from the LED into different wavelength light may be applied to the surface of the LED  55 .  
      In addition, the cavity of the upper board  52  may be filled with transparent resin or fluid  58  as shown in  FIG. 5   h  to protect the LED  55 . Then, as shown in  FIG. 5   i,  a transparent lens structure  59  is mounted on the upper board  52 , and the transparent resin or fluid  58  can be mixed with the fluorescent material which can convert the wavelength of light generated from the LED.  
      This process is an illustrative example of providing the two conductive via holes of vertical connection structures, in which more conductive via holes can be formed if necessary. For example, at least two conductive via holes can be used as vertical connection structures for connecting the first electrode pattern to the first bottom electrode.  
       FIG. 6  is a perspective view illustrating a lower board  61  having at least two conductive via holes according to an embodiment of the invention.  
      Referring to  FIG. 6 , the lower board  61  applicable to the invention is depicted. The lower board  61  has a heat radiation member  66 . The lower board  61  also has five first via holes  63 ′ and five second via holes  64 ′, which are exposed from the top surface of the lower board  61  and arranged opposite positions around the heat radiation member  66 . This embodiment has an advantage that a sufficient conductive area can be realized between the electrode patterns to be formed in the top and the bottom electrodes to be formed in the underside. In particular, this embodiment comprises a structure suitable to a high power LED having a plurality of electrodes, and permits the flow of electric current by massive amount.  
      It is also possible to provide only one conductive via hole and utilize the heat radiation member as a vertical connection structure for the other electrode as in the above embodiment shown in  FIG. 4 . This purpose can be easily realized through a modification in which only one conductive via hole is formed in the procedures shown in  FIGS. 5   a  and  5   b  and both the second electrode pattern and the second bottom electrode are connected to the heat radiation member.  
       FIGS. 7   a  and  7   b  are perspective views illustrating a heat radiation member structure according to the invention. The embodiment shown in  FIGS. 7   a  and  7   b  is an example of heat radiation member which can be stably fixed to the lower board.  
      Referring to  FIG. 7   a,  a lower board  71  applicable to the invention is depicted. The lower board  71  can be adopted to the embodiment having a conductive via hole and a heat radiation member as shown in  FIG. 4 . As a consequence, one bottom electrode  73  is connected to a conductive via hole  73 ′ and the other bottom electrode  74  is connected to a heat radiation member  76  as shown in  FIG. 7   b    
      The heat radiation member  76  formed in the lower board  71  has a roughened face. Since the heat radiation member  76  of the invention has a large sectional area, there is a risk that it may escape out of the lower board  71 . In order to prevent undesired escape, at least one face of the heat radiation member may be roughened horizontally. Alternatively, if the lower board is of a plurality of sheets or layers, the heat radiation member may be roughened vertically by imparting different sizes of cavity regions to the respective sheets and filling metal paste into the cavity regions.  
      While the present invention has been described with reference to the particular illustrative embodiments and the accompanying drawings, it is not to be limited thereto but will be defined by the appended claims. It is to be appreciated that those skilled in the art can substitute, change or modify the embodiments into various forms without departing from the scope and spirit of the present invention.  
      As set forth above, the present invention replaces wire bonding with flip chip bonding to simplify the overall structure as well as facilitate its fabrication process, and utilizes the insulation layer provided with the electrodes for flip chip bonding to realize the large-sized heat radiation member thereby remarkably enhancing heat radiation effect.