Abstract:
An LED assembly includes a substrate and a plurality of LEDs mounted on the substrate. Each LED comprises an LED die, a base supporting the LED die thereon and thermally contacting the substrate to take heat generated by the LED die to the substrate, a pair of leads electrically connecting the LED die to input a current to the LED die, and an encapsulant enveloping the LED die. The pair of leads hover above the substrate to separate an electrical route of the LED assembly from a heat conducting pathway thereof. Furthermore, each LED has a plurality of legs extending raidally from the base thereof to fit in the base of an adjacent LED, to thereby engagingly lock with the adjacent LED.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a light emitting diode (LED) assembly, and more particularly to an LED assembly having separated thermal and electrical structures for ensuring a constant lighting brightness and a long lifetime thereof. 
         [0003]    2. Description of Related Art 
         [0004]    LED have been available since the early 1960&#39;s. Because of the relatively high efficiency of LEDs, nowadays LED usage has been increased in popularity in a variety of applications, e.g., residential, traffic, commercial, industrial settings. However, when the LED is in operation, it generates a great deal of heat, which may result in the LED overheating or even malfunction if the heat cannot be dissipated timely. Therefore, many attempts are tried in order to effectively cool the LED. 
         [0005]    U.S. Pat. No. 6,561,680 B1 discloses a type of LED assembly, which comprises a metal substrate and an LED attached on the substrate (see FIG. 1B of U.S. Pat. No. 6,561,680 B1). The LED has an anode portion and a cathode portion electrically connecting an LED die to a power source to forward a current into the LED die. The anode portion and the cathode portion also thermally contact the substrate in order to dissipate heat from the LED die. When the LED is working, the current is carried to the LED die via the cathode and anode portions from the substrate; simultaneously, the heat generated by the LED die is also transferred to the substrate by the cathode and anode portions, thereby to ensure that a temperature of the LED die is controlled within its normal range. 
         [0006]    Nonetheless, there is a problem in this type of LED assembly: since both of the current flux and the heat flux are transferred via the anode and the cathode portions and the substrate, a current transferring pathway of the LED assembly substantially overlaps a heat conducting pathway thereof; thus, the current is easily influenced by the heat. As the temperature of the anode and the cathode portions reaches a high level, the influence of the heat acting on the current becomes significant and results in the current flowing through the anode and cathode portions unstable and fluctuating. The unstable current renders the light emitted from the LEDs flickering, whereby an illumination provided by the LED assembly deteriorates. Furthermore, this unstable current input into the LED may significantly lower a lifetime of the LED; thus, this type of conventional LED assembly cannot be utilized for a sufficiently long period of time. 
         [0007]    What is needed, therefore, is an LED assembly which can overcome the above-mentioned disadvantages. 
       SUMMARY OF THE INVENTION 
       [0008]    An LED assembly includes a substrate and a plurality of LEDs mounted on the substrate. Each LED comprises an LED die, a base supporting the LED die thereon and thermally contacting the substrate to transfer heat generated by the LED die to the substrate, a pair of leads electrically connecting the LED die to supply a current to the LED die, and an encapsulant enveloping the LED die therein. The pair of leads extends above the substrate so that an electrical route of the LED assembly is separated from a heat conducting pathway thereof. Thus, during operation of the LEDs, the heat generated by the LED die has only a little influence at the power supplied to the LED die. Accordingly, a steady current can be supplied to the LED die, a good illumination of the LED assembly can be achieved and a life span of the LEDs can be extended. Furthermore, each LED has a plurality of legs extending radially from the base thereof to fit in the base of an adjacent LED, to thereby engagingly combine the plurality of LEDs together. 
         [0009]    Other advantages and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Many aspects of the present apparatus 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 apparatus. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0011]      FIG. 1  is an assembled, isometric view of an LED unit of an LED assembly in accordance with a first embodiment of the present invention; 
           [0012]      FIG. 2  is a top view of  FIG. 1 ; 
           [0013]      FIG. 3  is a bottom view of  FIG. 1 ; 
           [0014]      FIG. 4  is an enlarged, perspective view of an LED of the LED unit of  FIG. 1 , wherein an encapsulant of the LED is removed for clarity; 
           [0015]      FIG. 5  is a top view of  FIG. 4 ; 
           [0016]      FIG. 6  is a bottom view of  FIG. 4 ; 
           [0017]      FIG. 7  is a side view of  FIG. 4 ; 
           [0018]      FIG. 8  is a side view of  FIG. 1  with a substrate of the LED assembly placed below the LED unit; 
           [0019]      FIG. 9  is a schematic block view of a power supply circuit connected between a power source and the LED assembly, wherein a snubber circuit is connected between a DC circuit and a driving circuit of the power supply circuit; 
           [0020]      FIG. 10  is a view similar to  FIG. 9 , but the snubber circuit is positioned between the driving circuit and the LED assembly; 
           [0021]      FIG. 11  is a view similar to  FIG. 5 , showing an LED in accordance with a second embodiment of the present invention, wherein a first lead, a second lead and legs of the LED are shorter than those of the LED of  FIG. 5 ; 
           [0022]      FIG. 12  is a top view of an LED unit in accordance with a third embodiment of the present invention; 
           [0023]      FIG. 13  is a bottom view of  FIG. 12 ; 
           [0024]      FIG. 14  is a top view of an LED unit in accordance with a forth embodiment of the present invention; 
           [0025]      FIG. 15  is a bottom view of  FIG. 14 ; 
           [0026]      FIG. 16  is a top view of an LED unit in accordance with a fifth embodiment of the present invention; 
           [0027]      FIG. 17  is a bottom view of  FIG. 16 ; 
           [0028]      FIG. 18  is a top view of an LED unit in accordance with a sixth embodiment of the present invention; 
           [0029]      FIG. 19  is a bottom view of  FIG. 18 ; 
           [0030]      FIG. 20  is a top view of an LED in accordance with a seventh embodiment of the present invention; 
           [0031]      FIG. 21  is a bottom view of  FIG. 20 ; 
           [0032]      FIG. 22  is a cross-sectional view of an LED assembly in accordance with an eighth embodiment of the present invention; 
           [0033]      FIG. 23  is a view similar to  FIG. 22 , wherein a substrate of the LED assembly contains a coolant therein; and 
           [0034]      FIG. 24  is a view similar to  FIG. 23  with a part of the substrate stamped to form a concave. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    Referring to  FIGS. 1 and 8 , an LED assembly in accordance with a first embodiment of the present invention comprises a substrate  10  and an LED unit  200  mounted on the substrate  10 . 
         [0036]    The substrate  10  is solid and entirely made of metal, such as copper, aluminum or an alloy thereof. The substrate  10  has a large and flat top surface for mounting the LED unit  200  thereon. 
         [0037]    Also referring to  FIGS. 4-7 , the LED unit  200  is constructed by soldering a plurality of LEDs  20  together. Each LED  20  comprises a hexagonal base  22  defining a hexagonal cavity  220  therein, an LED die  23  adhesively attached in the cavity  220 , a first electrode lead  24  and a second electrode lead  26  extending into the base  22  and electrically coupled to the LED die  23  via bonding wires (not shown), three legs  28  extending radially and outwardly from the base  22 , and a hemispherical encapsulant  29  (see  FIG. 1 ) encapsulating the LED die  23  therein. The base  22  is made of a material having a good heat conducting and electrically insulating capability, which is well known by those skilled in the related art, such as ceramic. The base  22  is in direct thermal contact with the top surface of the substrate  10  to direct heat generated by the LED die  23  to the substrate  10 . The cavity  220  is defined in a central area of a top face of the base  22 , being concentric with the base  22 . A hexagonal ring-shaped step  222  is formed between the cavity  220  and a circumferential periphery of the base  22 . The cavity  220  is used for receiving the LED die  23  therein, and retaining a bottom of the encapsulant  29  as well. 
         [0038]    The first and second leads  24 ,  26  are substantially planar and parallel to the substrate  10 ; they are extended into the base  22  from two opposite sides of the base  22 . Each of the first and second leads  24 ,  26  comprises a sheet  240 ,  260  penetrating through the step  222  to be located in the cavity  220 , and a block  242 ,  262  (illustrated in  FIG. 5 ) extending outwardly and horizontally from an extremity end of the sheet  240 ,  260  to be located outside of a corresponding side of the base  22 . The first and second leads  24 ,  26  are located above the substrate  10  when the LED  20  is mounted on the substrate  10  (see  FIG. 8 ). Each sheet  240 ,  260  consists of an arced inner part (not labeled) at the bottom of the cavity  220  for electrically connecting to the LED die  23  via the bonding wires, a middle part (not shown) fixedly received in the step  222  to secure the first and second leads  24 ,  26  in position, and a rectangular outer part (not labeled) extending beyond the step  222  to couple with the block  242 ,  262 . An area of each sheet  240 ,  260  is far larger than that of each block  242 ,  262 , to thereby enhance a reliability of the electrical connection of the first and second leads  24 ,  26  with the bonding wires. The block  242  of the first lead  24  forms an approximately circular groove  244  on a top face thereof, while the block  262  of the second lead  26  forms a nearly circular tab  264  projecting outwardly and horizontally from an extremity end thereof. As viewed from  FIG. 1 , the plurality of LEDs  20  are so electrically connected together that the grooves  244  of the first leads  24  of the LEDs  20  fittingly accommodate the tabs  264  of the second leads  26  of adjacent LEDs  20  therein. Since a current supplied to the LED die  23  travels through the first and second leads  24 ,  26  hovering above the substrate  10 , wherein the substrate  10  is responsible for dissipating heat generated by the LED die  23  from the base  22 , a current transferring pathway of the LED assembly is separated from a heat conducting pathway by a clearance between the substrate  10  and the first and second leads  24 ,  26 ; thus, an effect of the heat generated by the LED die  23  in influencing the current supplied to the LED die  23  can be reduced to an acceptable range. A depth of the groove  244  of the first lead  24  is slightly larger than a thickness of the tab  264  of the second lead  26 , whereby when the tab  264  is retained into the groove  244 , a space  248  is formed between a top of the tab  264  and a lateral of the groove  244  (shown in  FIG. 1 ). During soldering of the first and second leads  24 ,  26  together, the space  248  is capable of receiving excess solder (not shown) therein to prevent the excess solder from overflowing to the substrate  10 . 
         [0039]    The encapsulant  29  is partially retained into the cavity  220 , and projects upwardly in a manner that a size of a cross-section thereof gradually decreases from bottom toward top. The encapsulant  29  envelops the LED die  23  and the inner parts of the sheets  240 ,  260  of the first and second leads  24 ,  26  therein, to protect the inner parts of the sheets  240 ,  260  and the LED die  23  from external influence, e.g., contamination and humidity. The encapsulant  29  is made of a transparent material, such as epoxy, glass, silicone or the like, to guide light emitted by the LED die  23  to radiate out of the LED  20 . A fluorescent material (not shown) which is in particulate form, can be dotted in the encapsulant  29 , to help the LED  20  to exhibit a certain colorful characteristic, when it is desired. 
         [0040]    The three legs  28  are uniformly distributed around the circumferential periphery of the base  22  and located in vicinities of a bottom face of the base  22 . The legs  28  are spaced from the first and second leads  24 ,  26  to avoid direct thermal contact therebetween. Each leg  28  is formed integrally with the base  22  and comprises a rectangular strip  280  and a trapezoidal locking portion  282  extending horizontally and outwardly from the strip  280  (see  FIG. 6 ). Three cutouts  224  are equidistantly designed in the bottom face of the base  22  to alternate with the three legs  28 , wherein each cutout  224  is trapezoidal in shape so as to receive the locking portion  282  of a corresponding leg  28  therein, thus engagingly securing adjacent LEDs  20  with each other. 
         [0041]    In assembly, the LEDs  20  are fixed to each other to form the LED unit  200 ; the locking portion  282  of each leg  28  of each LED  20  is accommodated into a corresponding cutout  244  of an adjacent LED  20 , and the tab  264  of the second lead  26  of each LED  20  is received in the groove  244  of the first lead  24  of the adjacent LED  20 . Then the locking portions  282  of the legs  28  are soldered in the cutouts  224  of the bases  22 , whereby the LED unit  200  is securely formed. The LED unit  200  is attached on the substrate  10  to cooperatively construct the LED assembly. At last, the first and second leads  24 ,  26  are soldered together to electrically connect the plurality of LEDs  20 . 
         [0042]    With the help of the space  248 , during soldering of the first and second leads  24 ,  26 , even if a quantity of the solder offered to the first and second leads  24 ,  26  exceeds a predetermined amount, the excess portion of the solder would not overflow to contaminate the LED assembly, particularly, the substrate  10 . Thus, the top surface of the substrate  10  which supports the LEDs  20  thereon can be kept clean, and the problem caused by the overflowed solder such as a short circuit can be avoided. 
         [0043]    In use, when the LED die  23  is activated to lighten, the heat generated by the LED die  23  is conducted to the substrate  10  via the base  22 , while the current is conveyed to the LED die  23  through the first and second leads  24 ,  26 . Since the first and second leads  24 ,  26  are separated from the substrate  10 , the pathway through which the current is supplied is separated from the pathway through which the heat is dissipated, the heat would not significantly affect the set value of the current supplied to the LED die  23 ; thus, the current is capable of being maintained in a relatively steady level. The steadily input current insures the LED die  23  emitting light with a constant intensity; a good illumination of the LED assembly is thus obtained, and a lifetime of the LED assembly is accordingly prolonged. 
         [0044]    Alternatively, during assembling, the process of soldering the first and second leads  24 ,  26  together can be implemented before the LED unit  200  attached on the substrate  10 ; thus, the LED unit  200  is substantially completed prior to the LED assembly. Since the LED unit  200  has the first and second leads  24 ,  26  connected with each other to realize electrical connections, and the legs  28  locked with the bases  22  to realize mechanical connections; thus, if the bases  22  of the LED unit  200  can provide sufficient heat dissipation to the LED dies  23 , no substrate  10  is needed, and the LED unit  200  can be utilized individually. The LEDs  20  of the individual LED unit  200  can be freely arranged in various patterns according to different demands, without being restricted by a shape or a size of the substrate  10 ; therefore, a versatility of the LED unit  200  is obtained. 
         [0045]    Furthermore, the relationship and the number of the first and second leads  24 ,  26  and the legs  28  can be changed in accordance with different requirements. For example, the first lead  24  can be located adjacent to the second lead  26 , and one of the three legs  28  can be removed for reducing a manufacture cost. 
         [0046]    As illustrated in  FIG. 9 , the LED assembly can be connected to a snubber circuit to protect the LED die  23  from being damaged at the time when initially, electrically connecting the LED assembly to a power source. Conventionally, the LED assembly is sequentially connected to a driving circuit, a direct current (hereinafter is predigested to DC) circuit, an alternating current (hereinafter is predigested to AC) circuit, and a power source (such driving circuit, DC circuit and AC circuit are cooperatively called a power supply circuit). The AC circuit inputs an AC into DC circuit from the power source, the DC circuit converts the AC to a DC, which is then conveyed to the LED assembly via the driving circuit. However, such power supply circuit can only control a current type and a current value, and cannot control a varied range of the current. Following a suddenly, electrically connecting action, the current varies dramatically from zero to a large value in a relatively short time, which input into the LED die  23  from the circuit supply circuit would damage the LED die  23 . Therefore, in  FIG. 9  of the present invention, the sunnber circuit is added between the DC circuit and the driving circuit, to relieve the sharply varied current to a gradually increased current, which smoothly input into the LED assembly would not damage the LED die  23 . In operation, as soon as the LED assembly electrically connected, a sharply varied AC is produced and is conveyed to the DC circuit via the AC circuit. The DC circuit converts the sharply varied AC into a sharply varied DC, which is input into the snubber circuit. After passing through the snubber circuit, the sharp variation of the DC is relieved, and the DC becomes gradually increased. The gradually increased DC is then input into the LED assembly via the driving circuit, and enables the LED die  23  to lighten from dark to bright slowly. Therefore, the damage caused by the current to the LED die  23  is prevented. Also, it can be apprehended by those skilled in the related art that such snubber circuit can be connected between the driving circuit and the DC circuit as well, which is shown in  FIG. 10 . 
         [0047]    A distance between two adjacent LEDs  20  is associated with a length of the strip  280  of the leg  28  and a sum of lengths of respective portions of the first lead  24  and the second lead  26  extending outside the base  22 . In order to obtain much more light per area, the LEDs  20  should abut against each other more intimately without the distance existing therebetween, or only a little distance therebetween.  FIG. 11  schematically shows a second embodiment of the present invention, in which the configurations of the first and second leads  24   a ,  26   a  and the legs  28   a  are changed so as to hold adjacent LEDs  20   a  in intimate contact with each other. The first lead  24   a  of the LED  20   a  comprises a sheet  240   a  entirely received in a base  22   a . The second lead  26   a  comprises a sheet  260   a  accommodated within the base  22   a , and a tab  264   a  extending outwardly beyond the base  22   a . For realizing a connection between the first lead  24   a  and the second lead  26   a  of two adjacent LEDs  20   a , a portion of the base  22   a  is recessed to form a trough  226   a , in which an outmost part of the sheet  240   a  of the first lead  24   a  is exposed. Accordingly, each leg  28   a  only remains a locking portion  282   a  to be snugly fitted into a corresponding cutout (not shown) in an adjacent base  22   a . Since the tab  264   a  of the second lead  26   a  of each LED  20   a  is fully received in the trough  226   a  of the base  22   a  of an adjacent LED  20   a  to electrically connect with the first lead  24   a  of the adjacent LED  20   a , and the leg  28   a  completely received in the corresponding cutout of the base  22  of the adjacent LED  20   a , lateral sides of the two adjacent LEDs  20   a  are capable of abutting against each other intimately. Thus, LEDs  20   a  can be arranged in a higher density, and an overall output intensity of the light is accordingly increased. On the other hand, supposed that the light produced per area can meet a lighting intensity requirement, the first and second leads  24 ,  26  and the legs  28  of the LEDs  20  of the first embodiment only need to be shorten, to avoid redesigning and remanufacturing of the base  22 . 
         [0048]    Note that a shape and a size of the base  20  of the first embodiment can be changed to various types according to different occasions. For example,  FIGS. 12-13  illustrate a third embodiment of the present invention, in which the base  22   b ,  22   b ′ is in the form of a square, and has two legs  28   b ,  28   b ′ formed from two opposite sides thereof and two cutouts  224   b ,  224   b ′ defined at other two opposite sides thereof. In this embodiment there are two types of LEDs  20   b ,  20   b ′: the first type LEDs  20   b  each have two troughs  226   b  defined at the two opposite sides of the base  22   b  to expose outermost parts of a first and second leads  24   b ,  26   b ; the second type LEDs  20   b ′ each have two opposite tabs  244   b ′,  246   b ′ projecting outwardly from the first and second leads  24   b ′,  26   b ′ beyond the other two opposite sides of the base  22   b ′. The tabs  244   b ′,  264   b ′ of the second type LED  20   b ′ are received in the troughs  226   b  of an adjacent first type LED  20   b  and contact the first and second leads  24   b ,  26   b  of the first type LED  20   b , to thereby electrically connect the first type and second type LEDs  20   b ,  20   b ′ together. The first type LEDs  20   b  and the second type LEDs  20   b ′ are so locked together that the legs  28   b  of each first type LED  20   b  are retained into the cutouts  224   b ′ of two adjacent second type LEDs  20   b ′, and the cutouts  224   b  of each first type LED  20   b  receive the legs  28   b ′ of other two adjacent second type LEDs  20   b ′ therein. Such mutual connections of the plurality of LEDs  20   b ,  20   b ′ can also be characterized in that: the legs  28   b  of the first type LEDs  20   b  realize transverse connections, and the legs  28   b ′ of the second type LEDs  20   b ′ realize longitudinal connections. Furthermore, the LEDs  20   b ,  20   b ′ with square bases  22   b ,  22   b ′ can also be modified to have different configurations, which are illustrated in  FIGS. 14-15  of the forth embodiment of the present invention, for facilitating a uniform design and manufacture of the LEDs  20   b ,  20   b ′. As shown in  FIGS. 14-15 , each LED  20   c  of the forth embodiment has two legs  28   c  formed from two adjacent sides of the base  22   c , and two cutouts  224   c  defined at other two adjacent sides of the base  22   c.  Each LED  20   c  further forms a tab  264   c  of a second lead  26   c  extending outwardly beyond a side of the base  20   c , and a trough  226   c  defined at an opposite side of the base  22   c  to expose an outmost part of a first lead  24   c . The trough  226   c  receives the tab  264   c  of an adjacent LED  20   c . In assembly, the LEDs  20   c  mechanically engage with each other by fitting the legs  28   c  of each LED  20   c  into corresponding cutouts  224   c  of two adjacent LEDs  20   c , and electrically couple with each other by soldering the tab  264   c  of each LED  20   c  in the trough  226   c  of an adjacent LED  20   c . Due to the legs  28   c  of each LED  20   c , both transverse and longitudinal connections of the LEDs  20   c  can be realized, meanwhile the LEDs  20   c  have the same configuration and structure. 
         [0049]      FIGS. 16-17  show a fifth embodiment of the present invention, in which the base  22   d ,  22   d ′ is shaped to be triangle. Considering electrical and mechanical connections between the plurality of LEDs  20   d ,  20   d ′, the LEDs  20   d ,  20   d ′ should be designed to have two different configurations: a first type LEDs  20   d  each form three legs  28   d  respectively extending outwardly from three sides of the base  22   d , and define two troughs  226   d  at two sides of the base  22   d  to expose outmost parts of a first lead  24   d  and a second lead  26   d  thereof; a second type LEDs  20   d ′ each define three cutouts  224   d ′ around a periphery of the base  22   d ′ corresponding to the three legs  28   d , and have two tabs  264   d ′ extending outwardly from first and second leads  24   d ′,  26   d ′ beyond two sides of the base  22   d ′. In assembly, the three legs  28   d  of each first type LED  20   d  are respectively engaged in corresponding three cutouts  224   d ′ of three adjacent second type LEDs  20   d ′, and the two troughs  226   d  respectively accommodate two corresponding tabs  264   d ′ of two adjacent second type LEDs  20   d ′ therein. In this embodiment, the first type LED  20   d  acts as an active mechanical connector, and the second type LED  20   d ′ is employed as a passive mechanical connector. Alternatively, one leg  28   d  of the first type LED  20   d  can be moved to the second type LED  20   d ′, whereby each of the first type LED  20   d  and the second type LED  20   d ′ acts as the active mechanical connector. Such variations of the legs  28   d ,  28   d ′ between the first type LED  20   d  and the second type LED  20   d ′ are shown in  FIGS. 18-19  in accordance with a sixth embodiment of the present invention. A first type LED  20   e  forms a cutout  224   e  in a bottom of a base  22   e  at a location where the original leg  28   d  of the fifth embodiment is formed. Accordingly, a second type LED  20   e ′ forms a leg  28   e ′ at a location where the original cutout  224   d ′ of the fifth embodiment is defined. 
         [0050]    Furthermore, the base  22  of the first embodiment can be modified to another structure which is shown in  FIGS. 20-21  in accordance with a seventh embodiment of the present invention. The differences of the seventh embodiment relative to the first embodiment are given below. 
         [0051]    The base  22   f  of the LED  20   f  of the seventh embodiment is made of a thermally isolating and electrically insulating material such as epoxy, plastic and so on. A through hole (not labeled) is defined in a center of the base  22   f  of the LED  20   f . Three channels (not labeled) are radially formed in a bottom of the base  22   f , communicating with the through hole. A metal structure (not labeled), which is integrally formed by a post  27   f  and three legs  28   f , is filled in the through hole and the three channels, and separated from the first lead  24   f  and the second lead  26   f  by the base  22   f . Each leg  28   f  has an interior portion (not labeled) retained in a corresponding channel and coupling with a circumferential periphery of the post  27   f , and an exterior portion (not labeled) extending outside the base  22   f  for locking with an adjacent LED  20   f . The post  27   f , the legs  28   f  and the base  22   f  contact the substrate  10  by bottoms thereof engaging with the top surface of the substrate  10 . An LED die  23   f  is fixed on a top of the post  27   f  via a thermally conducting grease or glue. By using this construction, nearly all of heat generated by the LED die  23   f  is conducted to the substrate  10  via the post  27   f  and the legs  28   f , while a current given to the LED die  23   f  is conveyed through a first and second leads  24   f ,  26   f  which are spaced from the post  27   f  and the legs  28   f  by the base  22   f ; thus, a current conducting pathway of the LED assembly and a heat conducting pathway thereof are substantially separated from each other, and the current can be maintained in a steady state without being affected by the heat. Note that all of other components in this embodiment are the same as those described in the first embodiment. 
         [0052]    Referring to  FIG. 22 , which shows an LED assembly in accordance with an eighth embodiment of the present invention. In order to conduct heat generated by an LED die  23   g  to the substrate  10  more rapidly, a portion of a base  22   g  placed between the LED die  23   g  and the substrate  10  is omitted, while the LED die  23  is directly bonded on the top face of the substrate  10  via a kind of heat conducting adhesive  25   g . Preferably, a thickness of the heat conducting adhesive  25   g  is selected to be less than 0.01 inches, for obtaining a balance between a good heat conducting capability and a sufficient gluing force. The base  22   g  of each LED  20   g  is annular to spacedly surround the LED die  23   g . A first lead  24   g  and a second lead  26   g  are respectively inserted into the base  22   g  with inner parts (not labeled) thereof being exposed and outer parts (not labeled) thereof hovering above the substrate  10 . The LED die  23   g  is electrically connected to the first and second leads  24   g ,  26   g  via golden wires (not labeled). An encapsulant  29   g  envelops the LED die  23   g  and the inner parts of the first and second leads  24   g ,  26   g  therein to protect the LED die  23   g  from contamination and damage. Referring to  FIG. 23 , for more efficiently deriving heat from the LED die  23   g , such solid metal substrate  10  can be replaced by a hollow substrate  10   g  which transfers heat by using a phase change of working fluid. The substrate  10   g  can be a flat vapor chamber or a heat pipe. The substrate  10   g  can further connect with a heat sink (not shown) to disperse heat absorbed thereby to atmosphere, whereby the heat can be dissipated timely and sufficiently because of a large heat dissipating area of the heat sink. 
         [0053]    As viewed from  FIG. 24 , furthermore, the substrate  10   g  can be stamped to form a plurality of concaves  12   g  (only one shown) at spots where the LED dies  23   g  are mounted, whereby the LED dies  23   g  can be accurately positioned at predetermined locations of the substrate  10   g . On the other hand, such concave  12   g  is able to contain the adhesive  25   g  therein for preventing the adhesive  25   g  from overflowing to other parts of the substrate  10   g.    
         [0054]    It is believed that the present invention and its advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.