Abstract:
A light-emitting diode (LED) assembly includes a heat dissipation device ( 30 ) and at least one LED ( 10 ). The heat dissipation device has a connecting surface ( 33 ) with circuitry ( 20 ) being directly formed thereon. The at least one LED is electrically connected with the circuitry, and is maintained in thermal and mechanical contact with the connecting surface to dissipate heat generated thereby through the heat dissipation device. A method for making the LED assembly includes steps of: (A) providing a heat dissipation device having a surface for the LED to be mounted thereon; (B) insulating the surface; (C) forming circuitry on the insulated surface directly to obtain a connecting surface; (D) attaching the LED to the connecting surface of the heat dissipation device thermally and mechanically, and connecting the LED with the circuitry electrically to form the LED assembly.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to light-emitting diode (LED) assemblies, and more particularly to an LED assembly with improved heat dissipation ability so that heat generated by the LEDs of the assembly can be effectively removed. The present invention relates also to a method for making the LED assembly. 
   2. Description of Related Art 
   Light-emitting diode (LED) is a highly efficient light source currently used widely in such field as automobiles, screen displays, and traffic light indicators. When the LED gives off light, heat is also produced. 
   Generally an LED assembly includes a plurality of LEDs mounted on and electronically connected with a printed circuit board (PCB). A heat sink made of metal, such as aluminum or copper, is arranged under the PCB to remove the heat generated by the LEDs. To reduce thermal resistance between the heat sink and the PCB, thermal interface material, such as thermal grease, is often applied between the heat sink and the PCB. However, the thermal grease has a heat transfer coefficient generally not larger than 5 W/(m·K), which is much smaller than that of the metal. Furthermore, as the PCB is made of FR-4, which is produced by glass fiber impregnation into ethoxyline, thermal resistance of the PCB is very large. The heat generated by the LEDs is thus is only very slowly transferred to the heat sink through the PCB and the thermal grease. The heat thus cannot be rapidly and efficiently removed, which results in significant reductions in the lifespan of the LEDs. 
   Therefore, it is desirable to provide an LED assembly wherein one or more of the foregoing disadvantages may be overcome or at least alleviated. 
   SUMMARY OF THE INVENTION 
   The present invention relates, in one aspect, to a light-emitting diode (LED) assembly. The LED assembly includes a heat dissipation device and at least one LED. The heat dissipation device has a connecting surface with circuitry directly formed thereon. The at least one LED is electrically connected to the circuitry, and is maintained in thermal and mechanical contact with the connecting surface to dissipate heat through the heat dissipation device. 
   The present invention relates, in another aspect, to a method of making the LED assembly. The method includes steps of: (A) providing a heat dissipation device having a surface for the LED to be mounted thereon; (B) insulating the surface by applying an insulating layer thereon; (C) forming circuitry on the insulating layer thereby obtaining a connecting surface; (D) attaching the LED to the connecting surface of the heat dissipation device thermally and mechanically, and connecting the LED with the circuitry electrically to form the LED assembly. 
   Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the present light-emitting diode (LED) assembly can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present LED assembly. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views: 
       FIG. 1  is an exploded, isometric view of an LED assembly in accordance with a first embodiment of the present invention; 
       FIG. 2  is a cross-sectional view of the LED assembly of  FIG. 1  taken along a traverse direction thereof; 
       FIG. 3  is a is a sketch indicating circuitry of the LED assembly of  FIG. 1 ; 
       FIG. 4  is an exploded, isometric view of the LED assembly in accordance with a second embodiment of the present invention; 
       FIG. 5  is a cross-sectional view of  FIG. 4  taken along a transverse direction thereof; 
       FIG. 6  is an assembled, isometric view of the LED assembly in accordance with a third embodiment of the present invention; 
       FIG. 7  is an assembled, isometric view of the LED assembly in accordance with a fourth embodiment of the present invention; 
       FIG. 8  is a cross-sectional view of the LED assembly in accordance with a fifth embodiment of the present invention; 
       FIG. 9  is an exploded, isometric view of the LED assembly in accordance with a sixth embodiment of the present invention; and 
       FIG. 10  is a flow chart showing a preferred method of making the LED assembly. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-2  illustrate a light-emitting diode (LED) assembly in accordance with a first embodiment of the present invention. The LED assembly includes a plurality of LEDs  10 , and a heat dissipation device for removing heat generated by the LEDs  10 . In this embodiment, the heat dissipation device is a fin-type heat sink  30 . The heat sink  30  is made of highly thermally conductive material, such as copper, aluminum, or their alloys. The heat sink  30  as shown in this embodiment is an extruded aluminum heat sink, including a chassis  31  and a plurality of pin fins  32  extending upwardly from the chassis  31 . Apparently, the fins  32  are used for increasing the heat dissipation area of the heat sink  30 . Alternatively, the fins  32  can be flat shaped. The fins  32  and the chassis  31  can be formed separately, and then connected together by soldering. 
   Referring also to  FIG. 3 , a bottom surface of the chassis  31  of the heat sink  30  forms a connecting surface  33  for the LEDs  10  to be mounted thereon. Circuitry  20  is directly formed on the connecting surface  33  of the chassis  31 . It is to be understood that the circuitry  20  is formed according to the number and mode of the LEDs  10 , and is not limited to this embodiment. Each LED  10  includes an LED die  14  being electrically connected with the circuitry  20 , and a packaging layer  12  being provided to encapsulate the LED die  14 . The packaging layer  12  can transmit light and is generally made of polymeric material such as resin. The packaging layer  12  also functions to firmly secure the LED die  14  in place. When the LEDs  10  are mounted on the heat sink  30 , the LED dies  14  are directly attached to the connecting surface  33  of the heat sink  30 , and are electrically connected with the circuitry  20  of the connecting surface  33  of the heat sink  30  through wire bonding or flip chip. In this embodiment, the LED dies  14  are electrically connected with the circuitry  20  through wire bonding, in which a pair of gold threads  16  of each LED  10  are electrically connected with contacts  22  of the circuitry  20  of the heat sink  30 . 
   Since the circuitry  20  is directly formed on the connecting surface  33  of the chassis  31  of the heat sink  30 , the LED dies  14  are maintained in thermal and mechanical contact with the chassis  31  of the heat sink  30 . The heat resistance formed either between the LEDs  10  and the printed circuit board (PCB), or between the PCB and the heat sink  30  of a conventional LED assembly is thus avoided. During operation, the heat generated by the LEDs  10  can be directly and timely transferred to the chassis  31 , and then dissipated to ambient air through the fins  32  rapidly and efficiently. In this way the heat of the LEDs  10  can be quickly removed, thus significantly improving lifespan of the LEDs  10 . 
     FIGS. 4-5  illustrate a second embodiment of the LED assembly. The difference between the second embodiment and the first embodiment is that the heat dissipation device further includes a plate-type heat pipe  40 . The heat pipe  40  generally includes a hollow and vacuumed pipe body (not labeled) containing a working fluid, such as water or alcohol therein. A wick structure  42  is connected to an inner surface of the heat pipe  40 . Top and bottom outer surfaces of the pipe body are both planar-shaped. The bottom surface of the heat pipe  40  forms a connecting surface  43  as the connecting surface  33  of the chassis  31  of the heat sink  30  of the first embodiment. The circuitry  20  of  FIG. 3  is also formed on the connecting surface  43  of the heat pipe  40 . A heat sink  30   a  is thermally attached to the top surface of the heat pipe  40 . For increasing a contacting area of the heat pipe  40  and the heat sink  30   a , a chassis  31   a  of the heat sink  30   a  defines an elongated groove  311  in a bottom surface thereof. The groove  311  has a shape conforming to that of the heat pipe  40  and thus can receive the heat pipe  40  therein. 
   During operation, as the heat generated by the LEDs  10 , which are attached to the connecting surface  43 , is transferred to the heat pipe  40 , the working fluid contained therein absorbs the heat and evaporates into vapor. Since the vapor spreads quickly, it quickly fills an interior of the heat pipe  40 , and whenever the vapor comes into contact with cooler walls of the heat pipe  40  (i.e., the top surface of the heat pipe  40 ) which thermally contact with the heat sink  30   a , it releases the heat to the heat sink  30   a . After the heat is released, the vapor condenses into liquid, which is then brought back by the wick structure  42  to the evaporating region, i.e., the connecting surface  43  of the heat pipe  40 . Since the heat pipe  40  transfers the heat by using phase change mechanism involving the working fluid, the heat transferred to the heat pipe  40  from the LED dies  14  is thus rapidly and evenly distributed over the entire heat pipe  40  and is then conveyed to the heat sink  30   a  through which the heat is dissipated into ambient air. Obviously, multiple heat pipes  40  can be used to increase the heat dissipation efficiency for the LED assembly. For example, two or more heat pipes  40  can be thermally attached to the chassis  31   a  of the heat sink  30   a . Also the chassis  31   a  of the heat sink  30   a  can define more grooves  311  for receiving the heat pipes  40  therein. 
   The heat pipe  40  is a heat transfer device having a relatively high heat transfer capability due to the phase change mechanism used. The heat pipe  40  has the advantage of low thermal resistance and is capable of transferring a large amount of heat while maintaining a low temperature gradient between different sections thereof.  FIGS. 6-7  show third and fourth embodiments of the LED assemblies which also use heat pipes  40   c ,  40   d  to transfer the heat of the LEDs  10 . 
   As shown in  FIG. 6 , the heat pipe  40   c  has an elongated, substantially rectangular shape. Two opposite ends of the heat pipe  40   c  respectively form an evaporating section  41  and a condensing section  42 . Top and bottom surfaces of the evaporating section  41  of the heat pipe  40   c  are planar-shaped, and each forms a connecting surface  43   c  on which the circuitry  20  of  FIG. 3  is directly formed. Thus more LEDs  10  can be mounted on a signal heat pipe  40   c  for the two connecting surfaces  43   c  thereof. A heat sink  30   c  includes a plurality of flat fins  32   c  stacked together. Each of the fins  32   c  defines a cutout in a lateral side thereof. Cooperatively the cutouts define a groove  37  receiving the condensing section  42  of the heat pipe  40   c  therein. 
     FIG. 7  shows the fourth embodiment of the LED assembly, which includes a heat dissipation device having a heat spreader  50 , a heat pipe  40   d  and a heat sink  30   d . Top and a bottom surfaces of the heat spreader  50  form connecting surfaces  53   d  for the LEDs  10  to be mounted thereon. The circuitry  20  of  FIG. 3  is directly formed on each of the connecting surfaces  53   d  of the heat spreader  50  with the LED dies  14  being electrically connected thereto. An evaporating section  41  of the heat pipe  40   d  is received in the heat spreader  50  to absorb the heat generated by the LEDs  10 . The heat sink  30   d  includes a plurality of flat fins  32   d . Each fin  32   d  defines a hole (not labeled) in a central portion thereof. Cooperatively the holes of the fins  32   d  define a groove  38  for extension of a condensing section (not labeled) of the heat pipe  40   d  therethrough. Thus the fins  32   d  surround the condensing section of the heat pipe  40   d , and the heat of the heat pipe  40   d  absorbed from the LEDs  10  can be more evenly distributed over the fins  32   d.    
     FIG. 8  illustrates a fifth embodiment of the present LED assembly, in which a vapor chamber  30   b  is provided. The vapor chamber  30   b  has a much larger size than the heat pipe  40 ,  40   c ,  40   d  shown in the previous embodiments. The vapor chamber  30   b  has a chassis  31   b  with a top surface from which a plurality of fins  32   b  extend upwardly and a flat bottom surface to form a connecting surface  313  on which the circuitry  20  of  FIG. 3  is formed directly. The LEDs  10  are electrically connected with the circuitry  20  and are maintained in thermal and physical contact with the connecting surface  313  of the vapor chamber  30   b . The vapor chamber  30   b  also contains a working fluid therein and also employs a phase change mechanism to transfer heat. The heat from the LED dies  14  is directly transferred to the vapor chamber  30   b  and then is transferred from the vapor chamber  30   b  to the fins  32   b  for dissipation. As the vapor chamber  30   b  has a much larger size, more LEDs  10  can be provided to the assembly so as to increase the overall lighting brightness. In this embodiment, there are three LED arrays mounted on the connecting surface  313  of the vapor chamber  30   b  for heat dissipation. 
     FIG. 9  shows a sixth embodiment of the present LED assembly. The LED assembly in this embodiment employs a liquid cooling system to dissipate the heat generated by the LEDs  10 . The liquid cooling system includes a cold plate  70  through which a working fluid such as water (hereafter called “coolant”) is circulated, a pump  210 , a heat exchanger  230  and a plurality of connecting pipes  220 . The cold plate  70  includes a base  71  and a cover  73  mounted on the base  71 . Cooperatively the base  71  and the cover  73  define a hollow body receiving the coolant therein. A flow channel  201  is defined in the hollow body of the cold plate  70  for passage of the coolant. The flow channel  201  is wave-shaped in order to increase heat exchange area between the coolant and the cold plate  70 . A top surface of the cover  73  of the cold plate  70  forms a connecting surface  72 . The circuitry  20  of  FIG. 3  is directly formed on the connecting surface  72 . The LEDs  10  are electrically connected with the circuitry  20  and are maintained in thermal and mechanical contact with the connecting surface  72  of the cover  73  of the cold plate  70 . As the coolant passes through the flow channel  201 , it receives the heat generated by the LEDs  10 . The pump  210  drives the coolant to flow into the connecting pipes  220  through which the coolant is guided to the heat exchanger  230  where the heat in the coolant is released. Thereafter, the cooled coolant is sent back by the pump  210  to the cold plate  70  where it is again available for absorbing heat from the LEDs  10  again. 
   According to the foregoing embodiments of the present LED assembly, the circuitry  20  is directly formed on the heat dissipation device (i.e., the heat sink  30 , the heat spreader  50 , the heat pipe  40 ,  40   c , the vapor chamber  30   b , or the cold plate  70 ). The LED dies  14  of the LEDs  10  are electrically connected with the circuitry  20  and are maintained in thermal and mechanical contact with the connecting surface  33 ,  43 ,  43   c ,  53   d ,  313 ,  72  of the heat dissipation device. The PCB is thus no longer needed. The heat resistance formed either between the LEDs and the printed circuit board (PCB), or between the PCB and the heat dissipation device of the conventional LED assembly is thus avoided. The heat of the LEDs  10  may therefore be quickly removed to improve lifespan of the LEDs  10 . In particular, the heat pipe  40 , the vapor chamber  30   b , or the cold plate  70  has a hollow body which is filled with working fluid to directly absorb the heat from the LEDs  10 . The heat is rapidly transferred from the LEDs  10  to the heat dissipation device for dissipation via a phase change mechanism or a rapid circulation of the working fluid in the heat-absorbing member, whereby the heat generated by the LED dies  14  of the LEDs  10  is efficiently and effectively removed. Also, the PCB can be omitted, which reduced cost in production and assembly of the LED assembly. 
     FIG. 10  shows a preferred method in accordance with the present invention for producing the LED assembly. Firstly, a heat dissipation device, such as a fin-type heat sink, a heat pipe, a vapor chamber or a cold plate is provided. In this embodiment, the heat dissipation device is the fin-type heat sink  30  of  FIGS. 1-3 , which includes the chassis  31  and the plurality of fins  32  extending therefrom. The heat sink  30  forms a planar-shaped bottom surface which is processed with cleaning, caustic scrubbing or burring so that the bottom surface can be firmly attached with an insulating layer. Then a very thin insulating layer is formed on the bottom surface of the heat sink  30  through vacuum sputtering, vaporization or anodizing. 
   The circuitry  20  of  FIG. 3  is then formed on the insulating layer of the bottom surface of the heat sink  30  by the following steps. Firstly, a thin layer of copper foil is applied onto the insulating layer of the bottom surface so as to evenly cover the insulating layer. The copper foil layer can be formed on the insulating layer through sputtering, hot-pressing, electroless copper deposition, or electrodeposition. As material does not easily adhere to the insulating layer, surface activation is usually needed before forming the copper foil layer on the insulating layer. The surface activation usually includes silver spraying, sandblasting, and coursing. Thus the copper foil layer is easily applied to the insulating layer after surface activation. Then the circuitry  20  is formed on the bottom surface of the heat sink  30  by the copper foil layer through photoresist coating, exposing and etching. Alternatively, the circuitry  20  can be formed by stamping when the circuitry  20  has a relatively large thickness. The bottom surface of the heat sink  30  with the insulating layer and the circuitry  20  directly formed thereon forms the connecting surface  33 . 
   The LEDs  10  now can be mounted onto the heat sink  30  to form the LED assembly. The LED dies  14  can be firstly packaged with the packaging layers  12  to form the LEDs  10 . Then the LEDs  10  are maintained in thermal and mechanical contact with the connecting surface  33  of the heat sink  30  with the gold threads  16  of the LED dies  14  being electrically connected with the contacts  22  of the circuitry  20  through wire bonding. Alternatively, the LED dies  14  can be mounted on the connecting surface  33  of the heat sink  30  firstly, and the gold threads  16  of the LED dies  14  are electrically connected with the contacts  22  of the circuitry  20 ; then packing the LED dies  14  is processed to form the packaging layers  12  to encapsulate the LED dies  14 . 
   It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.