Abstract:
A method for forming a light emitting device includes providing a light emitting diode (LED) configured to emit light of a first color and providing a plurality of semi-spherical lenses made of a silicone material that contains no phosphor material. Each of the lenses has a layer of phosphor material attached thereto. The method also includes testing the plurality of lenses to select a subset of lenses that converts light of the first color to light of a second color. The method further includes forming the light emitting device using the LED, one of the selected subset of lenses, and a heat conductive substrate. In an embodiment, after the testing of the plurality of lenses, one of the selected subset of lenses is disposed overlying the LED. In another embodiment, the testing of the plurality of lenses is conducted with a light source other than the LED.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to light emitting diodes, and, more particularly, to methods for packaging light emitting devices and related microelectronic devices. 
     2. Description of Related Art 
     Introduced as an electronic element in 1962, a light emitting diode (LED) may be used as indicator lamps in many devices, and used for lighting. Modern LEDs emit visible, ultraviolet and infrared wavelengths, with very high brightness. 
     LEDs are getting to replace automotive lighting (particularly brake lamps and turn signals) as well as traffic signals. With the advantages of compact size, narrow bandwidth, high switching speed, and improved reliability, LEDs are becoming more and more popular in the art. 
     SUMMARY OF THE INVENTION 
     Embodiments of this invention relate to semiconductor light emitting devices (LED) as well as associated passive and active devices that are integrated part of a complete LED system, and fabricating methods therefore. More particularly, some embodiments of the present invention provide packaging and packaging methodologies for semiconductor light emitting devices. Certain embodiments of the present invention describe methods of packaging LED, wherein LED chips are placed on metal heat sink for thermal dissipation, alternate material and method for its attachment, where phosphor encapsulation structure is mounted in the assembly for light conversion. Further, in some embodiments, an optical component which is typically used as secondary optics for LED lighting module may be integrated into the package according to some of embodiments of the present invention. This LED package platform has high-efficiency thermal dissipation, and substantially reduces cost in packaging as well by eliminating several packaging steps as conducted in traditional methods. 
     In an embodiment, a method for forming a light emitting device includes providing a light emitting diode (LED) configured to emit light of a first color and providing a plurality of semi-spherical lenses made of a silicone material that contains no phosphor material. Each of the lenses has a layer of phosphor material attached thereto. The method also includes testing the plurality of lenses to select a subset of lenses that converts light of the first color to light of a second color. The method further includes forming the light emitting device using said LED, one of the selected subset of lenses, and a heat conductive substrate. In an embodiment, the lenses may be selected according to a specific wavelength range of the second color. In an embodiment, after the testing of the plurality of lenses, one of the selected subset of lenses is disposed overlying said LED. In another embodiment, the testing of the plurality of lenses is conducted with a light source other than said LED. 
     In another embodiment, a light emitting device includes a heat conductive substrate, at least one semiconductor light emitting diode (LED), and an encapsulant overlying the at least one LED. The encapsulant includes a substantially uniform layer of wavelength-converting material. Moreover, the encapsulant is preformed and pre-characterized for optical properties prior to being disposed overlying the LED. In an embodiment, the encapsulant may be tested according to a specification for converting light of the first color to light of a second color. In an embodiment, the encapsulant may be selected according to a specific wavelength range of the second color. In another embodiment, the substantially uniform layer of wavelength-converting material is preformed into the encapsulant, e.g. a lens. 
     These and other features and advantages of embodiments of the present invention will be more fully understood and appreciated upon consideration of the detailed description of the preferred implementations of the embodiments, in conjunction with the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are simplified cross-sectional views of LED packages wherein LEDs are placed on a metal heat sink, and phosphor encapsulation structure is mounted for light conversion; 
         FIGS. 2A and 2B  are simplified cross-sectional views of LED packages wherein LEDs are placed on a metal heat sink, and an array of phosphor encapsulation structures are mounted for light conversion; 
         FIG. 3  illustrates simplified cross-sectional views of different types of LED phosphor encapsulation structures that may be attached on an LED package according to embodiments of the present invention; 
         FIG. 4  is simplified cross-sectional views of LED package wherein LED is placed on a metal heat sink, and an integrated reflector with heat sink. The reflector may have highly reflective coating to increase light extraction through phosphor encapsulation structures; and 
         FIG. 5  is simplified cross-sectional views of LED package wherein LED is vertical type of chip, mounted on an insulating layer placed on metal heat sink, and a phosphor encapsulation structure is attached for light conversion. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention describe a new LED package platform to address issues of thermal dissipation, and provide significant cost reduction for semiconductor light emitting devices. As used herein, the term semiconductor light emitting device may include a light emitting diode, laser diode, or/and other semiconductor device which may include one or more semiconductor layers, and which may include silicone, sapphire, silicon carbide, or/and other semiconductor material. In the description below, an LED is used as an example to illustrate details of packaging methods. 
     In current LED packaging methods, LED chip is assembled into a carrier (level-1) such as ceramic, silicone or plastic. The level-1 package is usually necessary for phosphor process for color conversion, and color binning of the white color points. Next, a set of level-1 LED products are integrated on PCB (level-2), and then mounted on a heat sink such as copper, aluminum or other thermal conductive materials for thermal dissipation of the light engine. 
     In such packaging methods as the above described, there exists several thermal barriers (interfaces like chip-carrier, carrier-PCB, and PCB-heat sink) to dissipate the heat from LED chip to heat sink in the design of LED light engine. Although several different package concepts were proposed to resolve the thermal issues, these methods usually add cost to package. One of solutions is so-called Chip-on-board (COB), wherein LED chip is directly mounted on circuit board to reduce thermal barriers and packaging cost. Although COB technique simplifies assembly process, skipping level-1 process, however, it is known that poor thermal conductivity of PCB material limits LED application in high power regime. In addition, dispensing phosphor-silicone mixture used for white light conversion in COB package has difficulties in achieving good color quality such as color consistency, color uniformity over angle, color rendering index, and so on—where product optimization through color binning of the white color points is not possible. 
     Features of new package platforms provided in embodiments of the present invention described below, which can be applied to LED lamp package or/and light engine structure, include the following:
         LED chip is mounted on metal heat sink to maximize capacity of heat dissipation   Different mounting technique, where conventional conductive attachment is accompanied by heat pipe—therefore increasing the thermal dissipation and reducing total thermal resistance by an order of magnitude.   Eliminating traditional level-1 and level-2 packaging gives cost advantage.   Integrate secondary optics with heat sink structure. The surface of optics may have highly reflective coating material.   Pre-characterized phosphor encapsulations such as phosphor lens or phosphor sheet can be attached on the packaging to significantly improve color quality such as color consistency, color uniformity, color rendering, and so on, for the packaging structure.   The phosphor encapsulation such as lens may be attached individually to the package or attached in an array to the light engine.   Attachment procedure to overlay phosphor layer over individually or arrayed wire bonded LEDs to make the light engine.       

     In some embodiments, an LED chip is directly mounted on a heat sink, which can be made of a material having suitable heat conductivity, for example, a metal or other conductors. In other embodiments, an insulating layer having suitable heat conductivity can be placed between the LED die and the heat sink. Such an insulating layer can allow more flexible wiring connections. In some embodiments, electrical circuit connections can be formed on a circuit board attached on the heat sink. In an example, the circuit board can be placed adjacent to the LED chip. In another example, the LED chip can be placed in an opening of a circuit board. 
     In some embodiments, the LED chip is directly mounted on a heat sink and integrated with a phosphor encapsulation structure. In an embodiment, the phosphor encapsulation structure can be a lens with a phosphor-containing light conversion layer built in. In some embodiments, the phosphor encapsulation structures can be pre-characterized and pre-sorted. Some examples of the phosphor encapsulation structures are described in our previous patent applications. For example, U.S. Patent Application No. 61/216,374 on May 15, 2009 and U.S. Patent Application No. 61/273,129 on Jul. 30, 2009. 
     Some alternative embodiments provide a lighting apparatus that includes an LED die configured to emit light within a first wavelength range and a phosphor encapsulation structure overlying the LED die. In an embodiment, the phosphor encapsulation structure is configured to transmit light of target color when receiving input light within the first wavelength range of LED die. The phosphor encapsulation structure contains phosphor material which is pre-measured for color point to match specific wavelength of LED for desired white color point. 
     In some embodiments, the LED chips can be pre-tested and pre-binned, and the phosphor encapsulation structure can also be pre-binned. Then matching LED chips and phosphor encapsulation structure can be selected to form a lighting device for emitting certain target light color. 
     In some embodiments, the LED die may have two metal contacts on the same side of the die or opposite sides of the LED die. For LED with metal contacts on opposite sides, an insulating layer is necessary to connect the LED&#39;s in series applications. 
     In some embodiments, a flat bottom phosphor encapsulation is used. The loop of wire bond is lower than the height of circuit board for not damaging wire during attachment. In an embodiment, the flat bottom phosphor encapsulation can be mounted on the edge of a circuit board. 
     Embodiments of the present invention will be now described below with reference to various examples illustrated in the figures. 
       FIG. 1A  illustrates a light lamp or light engine structure, in where in LED  101  is placed on the top of a metal heat sink  301  with an epoxy or solder material. Because LEDs are directly placed on a metal heat sink, this structure gives best-achievable thermal conductivity for LED lamp or LED light engine. The heat sink may be one of any materials commonly adapted in LED packaging such as copper, aluminum, ceramic, and so on. A layer of printed circuit board (PCB)  201  is laminated on the top of heat sink as an insulating layer for electrical connection. The thickness of PCB material is higher than the loop of wire bond, so that phosphor encapsulation can be placed on the top without damaging the bonded wire  202 . The space between LED chip and encapsulation is filled up with a predetermined amount of silicone gel. Alternatively, the space may be filled in with silicone material prior to phosphor encapsulation or lens attachment. 
     The phosphor encapsulation structure  102  containing phosphor particles  103  may be a lens or a phosphor sheet as shown in  FIG. 3 . The color properties such as correlated color temperature (CCT) or color points of phosphor encapsulation is pre-measured and selected to match LED for desired color properties of LED lamps and light engines. 
       FIG. 1B  illustrated a light lamp or light engine structure with an inclusion of recess in heat sink  301 . The recess in the heat sink includes a surface  302  which may serve as a light reflector to increase amount of blue photons for white light conversion in phosphor encapsulation structure  102 . The reflector surface  302  may contain a highly reflective coating material for example but not limited to such as TiOx coating or whitish PCB material. 
       FIGS. 2A and 2B  illustrates a light lamp or light engine structures as described in  FIGS. 1A and 1B . In this embodiment, phosphor encapsulation structure  104  is an array structure which is aligned on LED chips placed on the heat sink. Each encapsulation is still connected with encapsulation material which may have or may not have phosphor particles in the connection  105 . Keeping phosphor encapsulation structure  104  in an array form can simplify process steps during formation of phosphor encapsulation, as well as increase throughput of encapsulation placement in manufacturing. 
     As shown in  FIG. 3 , methods of forming phosphor encapsulation structure are described in our previous patent applications. The phosphor encapsulation structures mounted on heat sink may be in various shapes or in a sheet form depending on desired light quality such as color uniformity or light radiation pattern. Materials of phosphor encapsulation structure may be epoxy or silicone material, or thermoplastic or thermosetting material, or ceramic plate, or glass or any materials that can be used for LED encapsulation. Phosphor particles  103  may contain single or multi-layers of phosphors which may have different optical properties depending on desired optical properties. 
       FIG. 4  illustrates a method of packaging LED in one embodiment of the present invention. LED  101  is placed on a heat sink  301  with an epoxy or solders material  203 . A reflector  303  is mounted on heat sink  301  with solder material  305 . Alternatively the reflector  303  may be formed together with heat sink  301  to eliminate thermal barrier of solder material  305 . The heat sink material may be copper, aluminum, ceramic, silicon, or other thermal conductive materials commonly used for LED package. The height of reflector may or may not be greater than the loop of bonded wire  202 , ensuring that the wires  202  are not damaged during encapsulation placement. The surface of reflector  303  may be coated with a layer of highly reflective material  304 , such as TiO x  or whitish PCB material to increase light extraction. PCB material  201  is attached and may or may not be laminated on the surface of heat sink as an insulator for electrical connection. The phosphor encapsulation  102  as the above described may be in various shapes or a sheet as shown in  FIG. 3 . A predetermined amount of silicone gel is injected to fill up the space  106  after encapsulation placement. Alternatively, the space  106  may be filled with silicone material prior to phosphor encapsulation or lens attachment. Alternatively, the space  106  may also be the bottom part of over layered  103  which fills up  106  with silicone material prior to lens attachment. The packaging structure may contain one of more LEDs covered with single phosphor encapsulation. 
     For LED chips with two metal contacts (p and n contacts) located on the same side, LEDs may be connected in series or in parallel in the package as the-above illustrated. The electrical connection can be proper layout on laminated PCB  201  for connection in series or parallel depending on application. 
     For LED chips with two metal contacts (p and n contacts) located on the opposite side so called vertical chip, LED mounted on metal heat sink which serves as common ground for LED chips, results in parallel connection. Therefore, an insulating layer  204  is required in between LED bottom contact, and the metal heat sink  301  for series connection of vertical chips, as illustrated in  FIG. 5 . The electrical connection can be proper layout on laminated PCB  201  for electrical connection. Other than insulating layer  204 , all embodiments described in the above for LEDs with the metal contact on the same side in  FIGS. 1 to 4  can be applied to the vertical chip as well. 
     The insulating layer  204  may include metal bond pads  204   a  on AlO x    204   b /aluminum  204   c , or SiO 2    204   b /silicone  204   c , or ceramic, or PCB material or other insulating materials with good thermal conductivity. In some embodiments of the present invention, material  204   b  may be the same material as heat sink  301  such as Aluminum, copper, ceramic, silicon or other thermal conductive materials commonly used for LED package. 
     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the invention.