Abstract:
A method of directly contacting a light emitting diode with an electrically nonconductive, substantially transparent liquid and an LED package including an LED in contact with an electrically nonconductive, substantially transparent liquid dissipates heat generated by the LED via thermal convection of the liquid. The method and the light emitting diode package may use a first heat-dissipating element in contact with the liquid and a second heat-dissipating element connected to the first heat-dissipating element to further conduct heat away from the LED. The method and the LED package may have excellent heat dissipation properties and may reduce or eliminate the decomposition and aging of a fluorescent powder due to heat and may further reduce light attenuation and color temperature differences.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This patent application relates and claims priority to Chinese patent application no. 201010593513.X filed Dec. 17, 2010, which is herein incorporated by reference for all purposes. 
       TECHNICAL FIELD  
       [0002]    This disclosure relates generally to the cooling and heat dissipation of a light emitting diode (“LED”) and, more specifically, to a method and package for cooling the LED by encapsulating an LED die with one or more liquids. 
       BACKGROUND  
       [0003]    With the developments in LED technologies, high-powered LEDs are more frequently being designed for lighting applications, for example, household lighting applications. In contrast with conventional light sources, such as incandescent light bulbs, LEDs possess various advantages, such as sufficient brightness, low energy consumption, high reliability, long lifetime, etc. Furthermore, LEDs have many additional advantages over a traditional incandescent lamp, such as smaller volume, lower heat output, greater reaction rate, and being more environmentally friendly. These advantages have lead to the widespread use of LEDs in various applications in the lighting field. 
         [0004]    LEDs are light emitting display devices made of semiconductor materials and are capable of directly converting electrical energy into optical energy. The previously recited advantages of lower power consumption and greater brightness have caused LEDs to be widely used as indicator lights and display panel lights in a wide variety of equipment, such as electronic circuits, household appliances, meters, and the like. LEDs, however, can be negatively affected by high temperatures and their energy conversion efficiencies can rapidly fall at higher temperatures, thereby consuming more electricity and producing more heat, which, in turn, further increases the temperature in the LEDs. Thus, a vicious circle begins and the longevity of the LEDs may be greatly reduced. 
         [0005]    If heat from the LEDs is not effectively dissipated, the longevity of the LEDs may be greatly shortened and energy consumption may increase, effectively eliminating two of the greatest advantages of LEDs over incandescent bulbs. Thus, an improved LED heat dissipation technology is desired and is an important task in the field of LED lighting. 
       BRIEF SUMMARY  
       [0006]    The present disclosure addresses heat dissipation issues, provides a method for cooling an LED with a liquid, and provides an LED package for cooling an LED with a liquid. 
         [0007]    A method for cooling an LED with a liquid includes contacting an LED with the liquid and transferring heat generated by the LED from the LED to the liquid thereby dissipating heat via thermal convection. The liquid may be a substantially transparent liquid, and the liquid may be electrically non-conductive. The liquid may comprise water, oil, a chemical polymer, or a combination thereof 
         [0008]    According to an aspect, the substantially transparent liquid and the LED may be encapsulated within a vessel. The vessel may be a substantially transparent vessel. Fluorescent powder may be applied to the outer wall of the substantially transparent vessel by spraying the powder on the outer wall or by applying a layer of fluorescent powder membrane on the outer wall of the vessel. The light emitted by turning on the LED may be transmitted through the vessel, to the fluorescent power, and through the fluorescent powder to the outside of the LED package. 
         [0009]    According to another aspect, the substantially transparent liquid and the substantially transparent vessel may be provided between a die of the LED and a fluorescent powder applied to the outside of the substantially transparent vessel such that the heat generated from the LED may not be directly transferred to the fluorescent powder, thereby preventing the fluorescent powder from decomposing and aging due to the negative effects of the heat. The light emitted after turning on the LED may transmit through the substantially transparent liquid and through the substantially transparent vessel to the fluorescent powder, and through the fluorescent powder to the outside of the LED package. The light from the LED may excite the fluorescent powder, which serves as a secondary light source, thereby intensifying the light from the LED and/or changing the wavelength (and thus color) of the light from the LED. 
         [0010]    According to another aspect, the heat generated by the LED may be conducted via a first heat-dissipating element connected to the substantially transparent vessel. The first heat-dissipating element may be at least partially in contact with the substantially transparent liquid. The heat generated by the LED may be further conducted away from the first heat-dissipating element via a second heat-dissipating element connected to the first heat-dissipating element. The first and second heat-dissipating elements may be heat sinks and, more specifically, may be aluminum or ceramic heat sinks 
         [0011]    According to another aspect, the first heat-dissipating element may be provided with one or more protrusions on the surface of the first heat-dissipating element that is in contact with the substantially transparent liquid. The increased surface area of the first heat-dissipating element in contact with the substantially transparent liquid increases the efficiency of the first heat-dissipating element. 
         [0012]    According to another aspect, the LED may be soaked together with a plurality of leads in the substantially transparent liquid. The leads soaked in the liquid may be manufactured from metal and may comprise a printed circuit board. According to another aspect, the substantially transparent liquid used in the present disclosure may be water, oil, chemical polymer, or any combination thereof. 
         [0013]    The present disclosure also provides an LED package that may comprise an LED die and its leads; a substantially transparent liquid in contact with the LED die; a substantially transparent encapsulating vessel containing the LED die, the leads, and the substantially transparent liquid; a heat sink connected to the substantially transparent encapsulating vessel; and a fluorescent powder located on the outer wall of the substantially transparent encapsulating vessel. The LED package may achieve excellent heat dissipation of the LED die, and thereby may prevent the heat generated by the LED die from directly conducting with the fluorescent powder. By preventing the heat generated by the LED die from directly conducting with the fluorescent power, the powder may be substantially prevented from prematurely decomposing and aging, and the LED may substantially avoid light attenuation and color temperature drifting. 
         [0014]    According to another aspect, the LED package may further comprise a secondary heat-dissipating element connected to the primary heat sink, which in turn could be connected to the substantially transparent vessel, to further conduct heat away from the heat sink. The secondary heat-dissipating element is thus located outside the substantially transparent vessel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  illustrates a cross-sectional drawing of a conventional LED and heat sink; 
           [0016]      FIG. 2  illustrates a schematic drawing of conventional heat conduction relationship of an LED; 
           [0017]      FIG. 3A  illustrates a cross-sectional drawing of an LED package cooled by a liquid, in accordance with the present disclosure; 
           [0018]      FIG. 3B  illustrates a cross-sectional drawing of an LED package with one or more protrusions on a top surface of a first heat sink, in accordance with the present disclosure; 
           [0019]      FIG. 4  illustrates a schematic drawing of a heat conduction relationship in LED packages, in accordance with the present disclosure; and 
           [0020]      FIG. 5  illustrates a process for cooling LED packages, in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    LEDs are well known in the art and are currently used in a wide variety of lighting applications. However, LEDs may be negatively affected by high temperatures, resulting in increased power consumption and lower energy conversion efficiency, which results in even higher temperatures. 
         [0022]      FIG. 1  illustrates a cross-sectional drawing of a conventional LED  100  using the existing heat dissipation technology. The LED  100  includes an aluminum substrate  101 , a heat sink  102 , adhesive materials  103 ,  104 , an LED die  105 , a plurality of leads  106 , a mixture of silicone gel and fluorescent powder  107 , an anode  108 , a cathode  109 , and an encapsulate  110 . The existing heat dissipation technology used by the LED  100  cannot dissipate the heat from the LED die  105  sufficiently, which may cause heat to accumulate, electrical current to rise, and the fluorescent powder  107  to decompose due to the heat, which may result in light attenuation and color temperature drifting. 
         [0023]    The heat sink  102  is mounted on the aluminum substrate  101  by means of the adhesive material  103 . The encapsulate  110  surrounds the anode  108  and the cathode  109 , which are connected to the LED die  105  by the plurality of leads  106 . The LED die  105  is connected to the heat sink base  102  by means of the adhesive material  104 . The adhesive materials  103 ,  104  may include a silver paste, a silicone gel, or a chemical polymer, and are in contact with the LED die  105  to conduct heat energy and transfer the heat away from the LED die  105 . In the process of heat transfer, different materials have different heat transfer coefficients; thus, a heat conduction bottleneck may result when using materials with a low heat transfer coefficient (such as die attachment adhesives), resulting in a decreased efficiency of heat conduction. 
         [0024]    Furthermore, the source of the heat of the LED die  105  is at a top surface  115  of the LED die  105 , rather than on a bottom surface  116  of the LED die  105 , and thus having the heat dissipation mechanism at the bottom surface  116  of the LED die  105  has inherent defects. By having the source of the heat at a top surface  115  of the LED die  105 , this indirect method of heat dissipation is significantly limited in design, both by heat dissipation indirect interfaces and by the area and volume of the heat dissipation material. As a result, the heat from the LED die  105  accumulates, the efficiency of converting electric energy into optical energy decreases, and light attenuation continuously occurs. Also, because the mixture of fluorescent powder and silicone gel  107  is immediately attached to and in contact with the luminescent layer of the LED die  105 , the fluorescent powder and silicone gel  107  absorbs a high level of heat, which may cause the fluorescent powder  107  to decompose due to long term heating. This can cause light attenuation and color temperature drifting. 
         [0025]      FIG. 2  illustrates a schematic view of the heat conduction relationship in conventional LED encapsulation technology, for example as used by the LED  100  shown in  FIG. 1 .  FIG. 2  shows a transparent silicone gel layer  211 , a fluorescent powder silicone gel mixture layer  207 , an LED heat source (die)  205 , an LED base  216 , a die attachment silver paste  204 , a heat sink  202 , a heat sink attachment silver paste  203 , and an aluminum substrate  201 , which in combination constitutes an LED package  200 . 
         [0026]    In the LED package  200 , an upper layer  213  of the LED die  205  generates light (blue, for example) and heat. A mixture of fluorescent power and silicone gel  207  is put on the upper layer  213  of the LED die  205 . The emitted blue light from the LED die  205  will go through the fluorescent powder  207  which may change the blue light to white light. A transparent silicone gel layer  211  may be put on top  214  of the fluorescent powder layer  207  to protect the fluorescent powder  207 . 
         [0027]    The LED die  205  is small in size and further heat dissipation is typically desired. Therefore, the LED die  205  is usually put on a larger heat sink  202  to further direct the heat away from the LED die  205 . The LED die  205  is usually put on the heat sink  202  and attached to the heat sink  202  with a thermal conductive paste  204 , such as a silver paste. 
         [0028]    As shown in  FIG. 2 , the direction of heat conduction travels away from the LED heat source  205  in at least two directions. The heat may travel in other directions as well, but for illustrative purposes,  FIG. 2  shows two directions. The heat conduction may travel from the LED heat source  205  “upwards” (i.e., through the upper layer  213  of the LED die  205 ) through the fluorescent powder silicone gel layer  207  to the transparent silicone gel layer  211 , and may also travel “downwards” (i.e., through the bottom  215  of the LED die  205 ) through the LED base  216 , the die attachment silver paste  204 , the heat sink  202 , the heat sink attachment thermal paste  203 , and finally through the aluminum substrate  201 . 
         [0029]    Different layers/elements shown in  FIG. 2  may have different thermal conductivity. The transparent silicone gel layer  211  may be made of material such as Dow Corning OE6650, which is available from Dow Corning Corporation, and may have a thermal conductivity of 4.0 W/(mK) (watts per meter kelvin). The fluorescent powder silica gel  207  may have a thermal conductivity of 22.0 W/(mK). The LED heat source  205  is the heat source of the LED package  200 , and therefore its thermal conductivity is not applicable. The LED base  216  may be made of sapphire and may have a thermal conductivity of 42.0 W/(mK). The die attachment silver paste  204  may be made of DX-20-4, which is available from Emerson &amp; Cuming, and may have a thermal conductivity of 17.8 W/(mK). The heat sink  202  may be made of iron and may have a thermal conductivity of 80.0 W/(mK). The heat sink attachment silver paste  203  may have a thermal conductivity of 40.0 W/(mK). The substrate  201  may be made of aluminum and may have a thermal conductivity of 237.0 W/(mK). 
         [0030]    As discussed in the foregoing, the prior-art structure of LED package  100 ,  200 , as shown in  FIGS. 1 and 2 , has the disadvantage that heat dissipation is insufficient, which causes heat to accumulate, electrical current to rise, and the fluorescent powder to decompose due to heat, which may result in light attenuation and color temperature drifting. 
         [0031]      FIG. 3A  illustrates a cross-sectional diagram of an LED package  300 . The LED package  300  may include an LED die  301 , a fluorescent powder silicone gel layer  302 , a first heat-dissipating element  303 , a substantially transparent liquid  304 , a substantially transparent vessel  305 , positive and negative leads  306 , and a second heat-dissipating element  307 . 
         [0032]    According to an embodiment, the LED package  300  may be configured with the LED die  301  at least in partial contact with the substantially transparent liquid  304 . In an alternative embodiment, the LED die  301  may be completely surrounded by the substantially transparent liquid  304 . The substantially transparent liquid  304  may be water oil, a chemical polymer, or any combination thereof. The heat generated by the LED die  301  may be transferred from the LED die  301  to the substantially transparent liquid  304 . The heat generated by the LED die  301  may be conducted by the liquid  304 . Because a portion of the liquid  304  may be heated by the heat generated from the LED die  301 , thermal convection may occur, where heat may be transferred throughout the liquid  304  enclosed in the substantially transparent vessel  305  to the first and second heat-dissipating elements  303 ,  307 . 
         [0033]    The LED die  301  and the substantially transparent liquid  304  may be enclosed within the substantially transparent vessel  305 . The substantially transparent vessel  305  may be made from a plastic, glass, or a combination thereof, and may allow light emitted from the LED die  301  to be transmitted outside the LED package  300 . The substantially transparent vessel  305  may be dome-shaped, cylindrical, rectangular, or any other shape that may enclose the substantially transparent liquid  304  and the LED die  301  within the substantially transparent vessel  305 . 
         [0034]    The first and second heat-dissipating elements  303 ,  307  may be heat sinks in an embodiment. The first heat-dissipating element  303  may be connected to an open end of the substantially transparent vessel  305 , thereby encapsulating the substantially transparent liquid  304  within the substantially transparent vessel  305 . The first heat-dissipating element  303  may further be connected to the second heat-dissipating element  307 , which may be located opposite the substantially transparent vessel  305  and not in contact with the substantially transparent liquid  304 . 
         [0035]    The first heat-dissipating element  303  may be at least partially in contact with the substantially transparent liquid  304  and may be configured to dissipate heat away from the LED die  301  through the substantially transparent liquid  304  to the first heat-dissipating element  303 . The second heat-dissipating element  307  may be configured to further dissipate heat away from the first heat-dissipating element  303  to the second heat-dissipating element  303  and outside the LED package  300 . 
         [0036]    The LED package  300  may further comprise the plurality of leads  306 . The plurality of leads  306  may be connected to the LED die  301  within the substantially transparent vessel  304  and may run outside the LED package  300  through both the first heat-dissipating element  303  and the second heat-dissipating element  307 . The plurality of leads  306  may be made from metal or traces on a printed circuit board. A portion of the plurality of leads  306  may be soaked in the substantially transparent liquid  304 . 
         [0037]    The fluorescent powder silicone gel layer  302  may be located on the outer wall of the substantially transparent vessel  305 . The LED die  301  may emit light and produce heat after being activated by connecting the plurality of leads  306  to a power source. The LED die  301  may transmit light through the substantially transparent liquid  304  and the substantially transparent vessel  305  to and through the fluorescent powder silicone gel layer  302 , exciting the fluorescent powder  302 . The excited fluorescent powder  302  may function to intensify and/or change the wavelength/color of the light emitted from the LED die  301 . The substantially transparent liquid  304  and the substantially transparent vessel  305  may form a barrier between the LED die  301  and the fluorescent powder  302 , which may advantageously prevent the fluorescent powder  302  from decomposing and aging due to excessive heat. 
         [0038]      FIG. 3B  illustrates a cross-sectional drawing of the LED package  300  of  FIG. 3A  with one or more protrusions  308  on a top surface  309  of the first heat sink  303 . In an embodiment, the one or more protrusions  308  on a top surface  309  of the first heat-dissipating element  303  may be in contact with the substantially transparent liquid  304 . These protrusions  308  may enlarge the surface area of the top surface  309  that is in contact with the substantially transparent liquid  304 , thus further enhancing the efficiency of heat dissipation of the LED package  300 . In an embodiment, the one or more protrusions  308  may be dome-shaped, pyramid-shaped, cylindrical, rectangular, or any other shape that may increase the surface area of the first heat-dissipating element  303 . 
         [0039]      FIG. 4  illustrates a schematic drawing of a heat conduction relationship of the LED package  300  shown in  FIGS. 3A and 3B .  FIG. 4  shows an LED die  401 , a fluorescent powder silicone gel layer  402 , a first heat-dissipating element  403 , a substantially transparent liquid  404 , a substantially transparent vessel  405 , a second heat-dissipating element  407 , and an LED base  410 . 
         [0040]    In the LED package  400 , the LED die  401  may be attached to the LED base  410 . The LED die  401  may generate light (blue, for example) and heat. The LED die  401  may be encapsulated in the substantially transparent vessel  405  and surrounded by the substantially transparent liquid  404 . The fluorescent powder silicone gel layer  402  may be applied to the outside of the substantially transparent vessel  405 . The light from the LED die  401  may be emitted through the substantially transparent liquid  404  and the substantially transparent vessel  405  to and through the fluorescent powder silicone gel  402 , which may change the color of the light (blue light to white light, for example). 
         [0041]    The first heat-dissipating element  403  may be at least partially in contact with the substantially transparent liquid  404 . The first heat-dissipating element  403  may also be connected to the substantially transparent vessel  405 . The second heat-dissipating element  407  may be connected to the first heat-dissipating element  403  to further dissipate heat away from the LED die  404  and the fluorescent powder silicone gel  401 . In an embodiment, both the first heat-dissipating element  403  and the second heat-dissipating element  407  may be heat sinks. 
         [0042]    As shown in  FIG. 4 , the direction of heat conduction may travel away from the LED die  401  (i.e., the heat source) in at least two directions. The heat conduction may travel from the LED heat source  401  through the LED base  410 , both of which may be surrounded by the substantially transparent liquid  404 . The heat may also travel through the substantially transparent liquid  404 , traveling “downwards” (i.e., through the bottom of the LED die  401  and the LED base  410 ) through the first heat-dissipating element  403  and the second heat-dissipating element  407 , and traveling “upwards” (i.e., through the top of the LED die  401 ) to the wall of the substantially transparent vessel  405  to the fluorescent powder silicone gel  402 . The heat may travel in other directions as well, but for illustrative purposes downwards and upwards are shown in  FIG. 4 . 
         [0043]    The thermal conductivity (watts per meter kelvin, W/(mK)) of each layer in  FIG. 4  may differ. By way of example only, the fluorescent powder silicone gel  402  may have a heat conductivity of 22.0 W/(mK). In an embodiment, the wall of the substantially transparent vessel  402  may be made of plastic and may have a heat conductivity of 0.87 W/(mK). The substantially transparent liquid  404  may be water and may have a heat conductivity of 0.62 W/(mK). The LED base  410  may be made of sapphire and may have a heat conductivity of 42.0 W/(mK). The first heat-dissipating element  403  may be connected to the substantially transparent vessel  405 , and in contact with the substantially transparent liquid  404 . The second heat-dissipating element  407  may be connected to the first heat-dissipating element  407 . Both the first and the second heat-dissipating elements  403 ,  407  may be made of aluminum and may have a heat conductivity of 237 W/(mK). 
         [0044]    Referring now to  FIGS. 3A ,  3 B, and  4 , the substantially transparent vessel  305 / 405  wall and the substantially transparent liquid  304 / 404  may be present between the LED die  301 / 401  (i.e., the heat source) and the fluorescent powder silicone gel  302 / 402 . In an embodiment, the wall of the substantially transparent vessel  305 / 405  and the substantially transparent liquid  304 / 404  may provide a non-conductive barrier between the LED die  301 / 401  and the fluorescent powder silicone gel layer  302 / 402 . 
         [0045]    Thus, the heat generated by the LED die  301 / 401  may not be directly conducted to the fluorescent powder silicone gel layer  302 / 402 . This may substantially minimized the decomposing and aging effects of the LED die  301 / 401  on the fluorescent powder silicone gel layer  302 / 402 . The presence of the first heat-dissipating element  303 / 403 , which may be at least partially in contact with the substantially transparent vessel  305 / 405 , may further enhance this effect. The contact between the first heat-dissipating element  303 / 403  and the substantially transparent liquid  304 / 404  may accelerate the heat dissipation from the LED die  301 / 401  through the substantially transparent liquid  304 / 404 , to the first heat-dissipating element  303 / 403 . The contact between the first heat-dissipating element  303 / 403  and the second heat-dissipating element  307 / 407  may further accelerate the heat dissipation from the LED die  301 / 401  to outside the LED package  300 / 400 , as the second heat-dissipating element  307 / 407  may provide additional channels of heat dissipation. 
         [0046]      FIG. 5  illustrates a process  500  for cooling the LED packages of  FIGS. 3A and 3B  by encapsulating an LED die with one or more liquids. At step  510 , the LED is put in contact with a liquid. The liquid may be a substantially transparent and electrically non-conductive liquid, and may comprise water, oil, a chemical polymer, or a combination thereof. At step  520 , the LED is put in contact with the substantially transparent liquid. At step  530 , heat generated by the LED is transferred from the LED to the liquid. The cooling process  500  is a continuous process, wherein steps  510 ,  520 , and  530  may occur concurrently and not sequentially. 
         [0047]    In some embodiments, at step  515 , the LED and the liquid may be enclosed in a vessel. The vessel may be a substantially transparent vessel. In some embodiments, at step  525 , a first heat-dissipating element may be connected to the vessel. The first heat-dissipating element may be a heat sink. At step  535 , heat from the LED and the liquid may be transferred to the first heat-dissipating element, wherein the first heat-dissipating element is at least partially in contact with the liquid. The first heat-dissipating element may be provided with one or more protrusions on a surface of the first heat-dissipating element in contact with the liquid, increasing the surface area of the first heat-dissipating element that is in contact with the liquid. Increasing the surface area of the first heat-dissipating element that is in contact with the liquid increases the efficiency of the first heat-dissipating element. 
         [0048]    In some embodiments, at step  526 , a second heat-dissipating element may be connected to the first heat-dissipating element. The second heat-dissipating element may be a heat sink. At step  536 , heat from the first heat-dissipating element may be transferred to the second heat-dissipating element, 
         [0049]    In some embodiments, a plurality of leads may be connected to the LED and the LED and the leads may be soaked in the liquid within the vessel. The plurality of leads may be made of metal or traces on a printed circuit board. 
         [0050]    While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the claims and their equivalents for any patent that issues claiming priority from the present provisional patent application. 
         [0051]    Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” such claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Brief Summary” to be considered as a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.