Abstract:
System and method for LED packaging. The present invention is directed to optical devices. More specifically, embodiments of the presentation provide LED packaging having one or more reflector surfaces. In certain embodiments, the present invention provides LED packages that include thermal pad structures for dissipating heat generated by LED devices. In particular, thermal pad structures with large surface areas are used to allow heat to transfer. In certain embodiments, thick thermally conductive material is used to improve overall thermal conductivity of an LED package, thereby allowing heat generated by LED devices to dissipate quickly. Depending on the application, thermal pad structure, thick thermal conductive layer, and reflective surface may be individually adapted in LED packages or used in combinations. There are other embodiments as well.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 61/241,459, filed Sep. 11, 2009; U.S. Provisional Patent Application No. 61/241,457, filed Sep. 11, 2009; and U.S. Provisional Patent Application No. 61/241,455, filed Sep. 11, 2009, commonly assigned and incorporated by reference herein for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention is directed to optical devices. More specifically, embodiments of the invention provide LED packaging having reflector surfaces, and in some implementations provide LED packages that include thermal pad structures for dissipating heat generated by the LED devices. In particular, thermal pad structures with large surface areas are used to provide heat transfer. In certain embodiments, thick thermally conductive material is used to improve overall thermal conductivity of an LED package, thereby allowing heat generated by LED devices to dissipate quickly. Depending on the application, thermal pad structure, thick thermal conductive layer, and reflective surface may be individually adapted in LED packages or used in combinations. 
         [0003]    In the late 1800&#39;s, Thomas Edison invented the light bulb. The conventional light bulb, commonly called the “Edison bulb,” has been used for over one hundred years. The conventional light bulb uses a tungsten filament enclosed in a glass bulb sealed in a base, which is screwed into a socket. The socket is coupled to an AC power or DC power source. The conventional light bulb can be found commonly in houses, buildings, and outdoor lightings, and other areas requiring light. Unfortunately, the conventional Edison light bulb dissipates much thermal energy. More than 90% of the energy used for the conventional light bulb dissipates as thermal energy. Additionally, the conventional light bulb eventually fails due to evaporation of the tungsten filament. 
         [0004]    Fluorescent lighting overcomes some of the drawbacks of the conventional light bulb. Fluorescent lighting uses an optically clear tube structure filled with a noble gas, and typically also contains mercury. A pair of electrodes is coupled between the gas and to an alternating power source through a ballast. Once the mercury has been excited, it discharges to emit UV light. Typically, the optically clear tube is coated with phosphors, which are excited by the UV light to provide white light. Many buildings use fluorescent lighting and, more recently, fluorescent lighting has been fitted onto a base structure, which couples into a standard socket for household use. 
         [0005]    Solid state lighting techniques have also been developed. Solid state lighting relies upon semiconductor materials to produce light emitting diodes, commonly called LEDs. At first, red LEDs were demonstrated and introduced into commerce. Modern red LEDs use Aluminum Indium Gallium Phosphide or AlInGaP semiconductor materials. Most recently, Shuji Nakamura pioneered the use of InGaN materials to produce LEDs emitting light in the blue color range for blue LEDs. The blue LEDs led to innovations such as solid state white lighting, the blue laser diode, which in turn enabled the Blu-Ray™ DVD player (trademark of the Blu-Ray Disc Association), and other developments. Blue, violet, or ultraviolet-emitting devices based on InGaN are used in conjunction with phosphors to provide white LEDs. Other colored LEDs have also been proposed. 
         [0006]    To take advantage of LED devices, well designed LED packages that house LED devices and provide electrical connections are essential. Numerous types of conventional LED packages have been used, however, they suffer from various disadvantages. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    This invention is directed to optical devices. More specifically, embodiments of the invention provide LED packaging having reflector surfaces, and in some implementations provide LED packages that include thermal pad structures for dissipating heat generated by the LED devices. In particular, thermal pad structures with large surface areas are used to provide heat transfer. In certain embodiments, thick thermally conductive material is used to improve overall thermal conductivity of an LED package, thereby allowing heat generated by LED devices to dissipate quickly. Depending on the application, thermal pad structure, thick thermal conductive layer, and reflective surface may be individually adapted in LED packages or used in combinations. 
         [0008]    In one embodiment, this invention provides an optical device with a substrate and an interior surface. The device also includes a conductive layer having a thickness of at least 15 um overlying a portion of the surface. The optical device further includes an LED electrically coupled to the conductive layer, and a layer of insulating material overlaying the interior surface. A reflective layer overlies the insulating material, and preferably has a reflectivity of at least 95%. 
         [0009]    According to another embodiment, the invention provides an optical device which includes a substrate with a flat surface. A first conductive region overlies a first portion of the flat surface, and a separate second conductive region overlies a second portion of the flat surface. The optical device includes a first via structure on the first conductive region, and an electrically separated second via structure on the second conductive region. The device additionally includes a thermal pad structure overlaying a third portion of the flat surface. The thermal pad structure is electrically insulated from the first conductive region and the second conductive region. The third portion preferably covers at least 50% of flat surface. 
         [0010]    According to another embodiment, the invention provides an optical device. The device includes a substrate with an interior surface and a bottom side. A top conductive layer, preferably having a thickness of at least 15 um overlies a portion of the interface surface. A bottom conductive layer overlies a portion of the bottom side of the substrate. An LED is electrically coupled to a location of the top conductive layer, and a reflective layer overlies the top conductive layer. The reflective layer preferably has a reflectivity of at least 95%. 
         [0011]    The device and method of manufacture provide for improved lighting with improved efficiency. They are easier to manufacture using conventional technologies. In certain embodiments, thermal management structures are able to greatly improve thermal conductivity of the LED package. In a specific instance, thermally conductive material as a part of the conductive layer can improve thermal conductivity by almost 50%. For example, the thermal conductivity of a conventional LED package is about 13° C./W. By increasing the thickness of conductive layers according to an embodiment of the present invention (e.g., 50 microns of copper material illustrated in  FIG. 11 ), the thermal spreading resistance is lowered to 7° C./W. The use of insulating layers between the reflective layer and the conductive layer greatly improves reflectivity, which translates to more usable light coming out of the LED package. These and other benefits are further described throughout the present specification and more particularly below. 
         [0012]    A further understanding of the nature and advantages of the present invention may be realized by reference to following portions of the specification and attached drawings 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The patent application file contains at least one drawing executed in color. Copies of this patent application publication with color drawings will be provided upon request and payment of the necessary fee. 
           [0014]      FIG. 1  is a simplified diagram illustrating a conventional LED package with a silicon cavity. 
           [0015]      FIGS. 2(   a ) and  2 ( b ) are simplified diagrams illustrating an enhanced reflector according to embodiments of the present invention. 
           [0016]      FIG. 3  is a simplified diagram illustrating reflectivity of an enhanced surface according to embodiments of the present invention. 
           [0017]      FIG. 4  is a simplified diagram illustrating an LED package according to embodiments of the present invention. 
           [0018]      FIG. 5  is a simplified diagram illustrating a conventional LED package. As shown, LED device is positioned within a silicon package. 
           [0019]      FIG. 6  is a simplified diagram illustrating a thermal pad positioned on a conventional LED package. 
           [0020]      FIGS. 7(   a ) and  7 ( b ) are simplified diagrams illustrating thermal management structures according to embodiments of the present invention. 
           [0021]      FIG. 8  shows a thermal simulation of the HyLED 700/750 package on aluminum MCPCB. 
           [0022]      FIG. 9  shows a thermal simulation of an LED package having thermal pad configuration according to embodiments of the present invention. 
           [0023]      FIG. 10  is a simplified diagram illustrating a conventional LED package. 
           [0024]      FIG. 11  is a simplified diagram illustrating an LED package with thick conductive layer according to embodiments of the present invention. 
           [0025]      FIG. 12  is a simplified diagram illustrating an LED package with a thermal management structure according to embodiments of the present invention. 
           [0026]      FIG. 13  shows the thermal spreading resistance on a 150 um silicon thick silicon substrate on an aluminum board with a relatively low contact resistance of about 6×10 −6  (° C.-m 2 )/W. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    As explained above, LED chips are often used as a light source. For LED chips to function, they are secured into a package and electrically coupled to an energy source. The optical efficiency of an LED package is related the reflectivity of the cavity surfaces.  FIG. 1  is a simplified diagram illustrating a conventional LED package silicon cavity. In the silicon cavity package illustrated in  FIG. 1 , certain regions (such as the isolation areas between electrical contacts, portion of exposed gold) have low optical reflectivity. As a result, these areas lower the overall reflectivity of the cavity and thus, the optical efficiency of the package. In addition, conventional reflectors are often inadequate. 
         [0028]    According to an embodiment, the invention provides an improved reflector.  FIGS. 2(   a ) and  2 ( b ) are simplified diagrams illustrating an enhancement reflector according to embodiments of the present invention. These diagrams are merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives.  FIG. 2(   a ) illustrates a conventional reflector where reflective surface is provided by silver coating. Alternatively, the reflective surface can also be provided by aluminum or other types of coating. It is to be appreciated that embodiments of the present invention provides enhanced reflective surfaces, thereby improving the efficiency. More specifically, an enhanced reflective surface comprises of a layer of metal material (such as aluminum or silver over), and the metal material is coated with one or more layers of dielectric material. For example, a layer of ¼λ of SiO 2  and followed by another ¼λ or TiO 2  is provided on top of the metal material as shown in  FIG. 2(   a ). Depending on the application, other types of materials and thickness may be used. 
         [0029]      FIG. 3  is a simplified diagram illustrating reflectivity of an enhanced surface according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives. As shown in  FIG. 3 , an enhanced silver surface (e.g., with SiO 2  and TiO 2  coatings) is characterized by a much higher level of reflectivity at relatively shorter wavelength. More specifically, at 400 nm, the enhanced silver surface is characterized by a 93% reflectivity while the bare silver surface is characterized by an 86% reflectivity. 
         [0030]    It is to be appreciated that embodiments of the present invention provides other ways to enhance reflectivity, thereby improving overall LED package performance. In certain embodiments, reflectivity of an LED package is enhanced by increasing the coverage of reflective areas.  FIG. 4  is a simplified diagram illustrating an LED package according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives. An electrically insulating layer (such as SiO 2 ) is provided underneath the reflector layer as shown in  FIG. 4 . Over certain portions, the insulating layer is provided over the inside of the gaps between the electrical leads. On top of this insulating layer, a high reflectivity mirror layer is deposited. Among other things, the highly reflective layer covers the low reflectivity areas. Depending on the application, the reflective layer may comprise different types of metal layer such as aluminum, silver, and/or others. In a specific embodiment, the metal reflectivity is enhanced by coating the metal reflective surface with one or more layers of dielectric material. For example, a layer of ¼λ of SiO 2  and followed by another ¼λ or TiO 2  is provided on top of the reflective metal material as described above. 
         [0031]    In certain embodiments, the present invention provides thermal management means for the LED packages. More specifically, thermal management pads are provided for surface mount LED packages. For surface mount LED packages, the mounting surfaces typically have three contacts: two electrical contacts and an electrically isolated thermal contact. For example, an LED device is mounted onto a circuit board (such as a metal clad PCB) with matching contact pads. In operation, the LED device often generates a large of heat that needs to be dissipated, as the heat might cause problem even device break down. 
         [0032]    In an embodiment, the heat generated by the LED package is conducted across one or more thermal pad structures. More specifically, the thermal pad structures with relatively large surface area are positioned in proximity of the electrical contacts. In principle, the area of the thermal pad directly affects the contact resistance. For example, a larger the thermal pad area translates a lower the thermal resistance, as heat can dissipate through the large area. 
         [0033]    In an LED package that is mounted onto a metal clad PCB, the metal clad PCB has a dielectric layer that electrically isolates the electrical circuits from the base metal. For example, the base metal can be aluminum or copper. Unfortunately, the dielectric layer often has a very low thermal resistance of about 2.2° C./W compared to thermal resistance of copper at 400° C./W. As a result, the dielectric layer can be a thermal choke point. In particular for silicon packages where the VIA structures for the electrical contacts are formed by etching silicon substrate material, the VIA location can limit the size of the thermal pad. 
         [0034]      FIG. 5  is a simplified diagram illustrating a conventional LED package. As shown, LED device is positioned within a silicon package. For example, the package is a HyLED 700/750 package from Hymite™. In the design of  FIG. 5 , there are two electrical VIAs per electrical contact and the contacts are on opposing sides of the package. For example, VIAs  501  and  502  are both a part of the electrical contact  510 . In the configuration illustrated in  FIG. 5 , there is very little spacing for a thermal pad.  FIG. 6  is a simplified diagram illustrating a thermal pad positioned on a conventional LED package. As shown, the thermal pad  601  in  FIG. 6  only occupies a small portion of total area, as the electrical pads occupy a large amount of area. For example, the thermal pad may be able to occupy about 30% of the total base area, which is often insufficient for thermal dissipation. 
         [0035]    It is therefore to be appreciated that embodiments of the present invention provide improved thermal management structures, such as thermal pad structure with large surface area.  FIGS. 7(   a ) and  7 ( b ) are simplified diagrams illustrating thermal management structures according to embodiments of the present invention. These diagrams are merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 7(   a ), the relative size of thermal pads (relative to electrical pad) is larger that relative size of thermal pad shown in  FIG. 6 . More specifically, increased thermal pad size is made possible by moving VIAs to the same side of the package. In certain embodiments, only a single VIA is used per contact. As shown in  FIGS. 7(   a ) and  7 ( b ), the thermal pad coverage is up to 80% of the total surface area. 
         [0036]    It is to be appreciated that the increase thermal pad area greatly increases the rate of heat dissipation by the thermal pad structure.  FIG. 8  shows a thermal simulation of the HyLED 700/750 package on aluminum MCPCB. As shown in  FIG. 8 , the contact resistance is set at 3.6×10 −5  (K-m 2 )/W. For example, in this configuration the HyLED device is characterized by a very high thermal resistance of about 32° C./W. 
         [0037]    In comparison, thermal pad structures according to embodiments of the present invention reduces thermal resistance by a substantial amount.  FIG. 9  shows a thermal simulation of an LED package having thermal pad configuration according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 9 , when the area of the thermal pad is increased to about 75% of the total area, the thermal resistance is decreased to about 22° C./W. 
         [0038]    It is to be appreciated that the specific designs of the thermal pad can be varied depending on the application. In certain embodiments, heat dissipation is accomplished by increasing the thickness of thermal conductive material. 
         [0039]    Usually, silicon based LED packages are advantageous as they can processed in large wafer scaled formats. For example, up to 8″ silicon wafers are being processed today verses only 2″×4″ ceramic tiles used in the early days. However, the conductivity of silicon (80-140 W/m/k) is relatively low when compared to copper (3090 W/m/k) and other metal materials. Further, in some implementations, the thickness of silicon on which the LED is bonded can be rather thin. Sometimes, the thickness of the silicon material can be as low as 150 um. As a result, the thickness and the thermal conductivity of the silicon material limit the thermal spreading and increase thermal resistance, which is highly undesirable for high power densities applications. 
         [0040]      FIG. 10  is a simplified diagram illustrating a conventional LED package. As shown in  FIG. 10 , an LED device  1001  is positioned within a silicon package with “V” shaped vias etched on the silicon plane. It is to be understood that the “V” shaped vias are shown for illustrative purposes only; this invention applies equally to other via geometries such as those with relatively straight side walls. More specifically, in  FIG. 10  the LED device  1001  is positioned on a conductive layer  1002 . Silicon material  1004  is positioned between the conductive layers  1002  and  1003 . As explained above, the silicon material  1004  is characterized by a relatively poor thermal conductivity. Additionally, since the conductive layers  1002  and  1003  are relative thin (often less than 10 um), the conductive layers do not help dissipate heat. 
         [0041]    In various embodiments, the present invention provides LED packages with thick conductive layers that help dissipate heat from the LED package.  FIG. 11  is a simplified diagram illustrating an LED package with thick conductive layer according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 11 , an LED device  1101  is provided on the top side of the conductive layer  1102 . For example, the conductive layer  1102  comprises copper material and is characterized by a thickness of at least 25 um. Similarly, the bottom conductive layer  1103  is characterized by a thickness of at least 25 um that provides good thermal conductivity. Having thick layers of thermally conductive material at both top and bottom sides of the silicon substrate  1104 , the LED package in  FIG. 11  is able to dissipate heat generated by the LED device  1101  through the conductive layer. 
         [0042]    It is to be appreciated that other variations of thermal management structures are available as well.  FIG. 12  is a simplified diagram illustrating an LED package with a thermal management structure according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 12 , an LED device  1201  is coupled to a conductive layer  1202 . a portion (which is in direct contact with the LED device  1201 ) of the conductive layer  1202  is thicker than other portions of the conductive layer. For example, the conductive layer  1202  comprises copper material and its thick portion is characterized by a thickness of at least 25 um. It is to be appreciated that the thick portion of conductive material increases thermal conductivity and thermal spreading, and it helps heat generated by the LED device to dissipate quickly. As shown, the conductive layer  1202  is substantially thin and can be uniformly plated across all metal surfaces as shown in  FIG. 12 . Only certain selective regions (e.g., region directly under the LED device  1201 ) is thick as shown. 
         [0043]    LED packages with thermal management structures as shown in  FIGS. 11 and 12  greatly improves thermal conductivity compared to conventional LED packages.  FIG. 13  shows the thermal spreading resistance on a 150 um silicon thick silicon substrate on an aluminum board with a relatively low contact resistance of about 6×10 −6  (° C.-m 2 )/W. The thermal conductivity of the a conventional LED package is about 13° C./W. Increasing thickness of conductive layers (e.g., 50 microns of copper material illustrated in  FIG. 11 ), the thermal spreading resistance lowered to 7° C./W. 
         [0044]    In one or more preferred embodiments, the thermal management structures such as thermal pad and conductive layers can be arranged different or constructed using different types of materials. Similarly, various types of material may be used for the insulating later to improve the reflectivity of the reflective layer. Of course, there can be other variations, modifications, and alternatives. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims.