Patent Publication Number: US-2012043886-A1

Title: Integrated Heat Conductive Light Emitting Diode (LED) White Light Source Module

Description:
The current invention claims a priority to U.S. 61/374,938 filed on Aug. 18, 2010 and all references are incorporated herewith. 
    
    
     FIELD OF INVENTION 
     The present invention generally relates to the light emitting diodes (LEDs) white light source module, and more specifically to improve the overall performance/cost ratio of the LED white light source module by an advanced integrated approach to improve its light extraction through ultra thin film coating of phosphor as well as a structure to improve its heat conduction through eutectic bonding onto a small Silicon base. 
     BACKGROUND OF THE INVENTION 
     The energy-sensitive developments of LED technology has supported by the strong demand of low power consumption, high efficiency and high brightness lightening source. Compared to traditional white light sources, the LED has advantages such as small size, good illumination efficiency, long service life, high response speed, high reliability, and good wear resistance. 
     Typically, the LED dice is assembled into a sealed package containing electrical connections between the dice and terminal pads exposed on an outer surface of the package. For planar LED pads, which is commonly used today, electrical connections are either by (1) using wire-bonding of LED dice pads (on the top side of LED where EPI layers are), hence, in this case, the dice&#39;s bottom (substrate) has to be die-bonded to an electrically insulation base, or, by (2) using die-bonding of LED dice pads, hence, in this case, the dice&#39;s top is directly connected to a circuitry base. For vertical LED pads, both of the above (1) and (2) have to be applied to make electrical connections work. Such a package/module enables simple connection of the LED dice to external circuitry and, due the sealing properties of the package, protects the dice from external damage. It is potentially the ultimate electricity-saving solution for global lighting needs if further engineered correctly to maximize the overall performance/cost ratio of a LED white light source module. 
     Recently, LEDs are commonly used to generate light in a variety of applications ranging from simple low-power indication lights, higher-power LED traffic light clusters, vehicle dashboards, the backlight source of liquid crystal display devices, interior illumination and exterior lights ad LED matrix video displays. Among these applications, the white light source module (which typically adopts blue LEDs coated with, or immersed in, relatively thick film of phosphor) is perhaps the most wanted one due to its service in both BLU (Back Lighting Unit for LCD applications) and general lighting sect. 
     However, LED dice also is relatively a tiny point source of heat as well as light. Heat dissipation means may affect its quality and reliability, with respect to performance and life. LED dice deteriorates quickly when its temperature rises beyond certain level. Furthermore, trapped light not being emitted from a package (containing LED dice(s)) would convert to heat and increase the burden of heat dissipation. 
     To make a LED white light source module, the typical current approach is to (1) surface mount LED dice(s) onto a printed circuit board (PCB), (2) then wire-bond to connect electrodes of the LED to a pre-designed circuit, (3) then assemble it in a cup and encapsulate the package with a transparent material, (4) then apply phosphor coating to covert blue light to white light, and is tested and sorted and labeled according to its color temperature, (5) then attach other parts, such as a lens, to it for optical consideration if any. Common practices may combine (3) and (4) by mixing phosphor powder in silicone before curing, and may also include (a) making the cup inside surface reflective to increase photon extraction from the package, (b) applying minimum phosphor coating to reduce photon absorption (hence reducing heat somewhat). 
     Typically, heat dissipation can be achieved by fixing the LED white light source onto a heat conductive plate which is either connected to, or a part of, external means of heat dissipater such as a metal heat sink. Depending on application designs, the heat sink may dissipate heat via the natural, or forced, convection means. 
     A drawback with this type of LED light source module is its inefficiency in dissipating heat away from the LED dice. The circuit board substrate and the transparent encapsulate material are typically made from thermally insulating materials which trap heat in LED dice. For example, the commonly used PCB substrate which serves as the conduit for heat to flow to the heat sink is FR4 whose material has a thermal conductivity coefficient of 0.2-0.3 watts per meter Kelvin, while metals (copper or aluminum) would have a thermal conductivity coefficient 1000 times that of FR4. In addition, the rate of heat dissipation is also severely limited by the LED die-bonding material such as silicone glue and/or silver epoxy paste whose thermal conductivity only up to a few percent of metal aluminum. Thus, current LED light source module suffers, in general, 3 heat flow problems: (1) bonding LED dices to PCB by thermally insulating material, (2) PCB itself not good in heat conduction to convey heat to heat sink, (3) inefficient photon extraction rate by current packaging means, especially in the case of blue-to-white light conversion when an extra of thick non-conductive phosphor coating is added. 
     The poor heat dissipation properties of the current LED light source module cause LED dice temperature rising rapidly and eventually break the dice. 
     The current invention disclosed an advanced integrated approach as well as a novel structure to provide much superior heat conduction for LED and to remove all the limitations mentioned above and further provide low cost, highly reliable and brighter LED white light source modules. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to overcome the drawbacks in both photon extraction and heat dissipation of current LED white light source modules. The primary objective of the present invention is to provide such a superior LED white light source module to reach the highest performance/cost ratio by using a low-cost mature manufacturing process for easy mass production. 
     To achieve that objective, the present invention has made following improvements:
     (A) applying direct eutectic metal bonding by adopting semiconductor grade Silicone wafer (Si) as the base plate to hold LED dice(s) to make a LED/Si package and to connect such a package to heat-sink directly through eutectic/soldering means, in order to enjoy the high thermal conductivity of Si (about ⅔ of aluminum) for much better heat conduction, as well as the electro-insulation capability of Si;   (B) then, applying ultra thin film coating of phosphor directly on LED/Si package to convert it to a white light source module (at pre-defined color temperature);   (C) or alternatively, such a phosphor coating could be made directly on blue light LED dices before applying (A), making these a stand-alone white light LED dices (at pre-defined color temperature).   

     The major difficulties of current practices of making LED packages are the limited choices in selecting bonding material which needs to be heat-conductive and electro-insulative at the same time, in order to make the LED package efficient in heat conduction. Adopting the current invention would easily keep electro contacts from shorting as some of the connecting circuits and components can be designed, processed and embedded in the Si wafer, while eutectic bonding would keep heat flowing easily from the LED dice to the Si and from the Si to the heat-sink, since metallic joints pose no blockage to heat conduction and Si itself is a good heat conductive material. 
     The other major difficulties which current LED white light source makers are facing is the un-controllability of blue photon&#39;s optical path and intensity when interacting with phosphor powder particles in a relatively thick film to generate an uniform white light, thus the current practices have ended up modules with a wide range of white light in terms of color temperature, which has resulted in painful number of sorting bins. Adopting the current invention would significantly turn the blue LEDs into white light ones in according to designated color temperature lots within allowable tolerance, since the ultra thin film coating of phosphor is a process with specific and controllable parameters which makes blue-to-white light conversion under an optically much well-defined and tighter condition. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the top and side views of the LED/Si packaging of the present invention. 
         FIG. 2  illustrates the top and side views and the pattern of contact area of LED housing of the present invention. 
         FIG. 3  illustrates the top and side views of the light source module schematic and various shapes depending on application designs of the present invention. 
         FIG. 4  illustrates the option for ph-coating on LED/Si package of the present invention. 
         FIG. 5  illustrates the “T/T” packaging the white LED dice of the present invention. 
         FIG. 6  illustrates the “B/T” packaging the white LED dice of the present invention. 
         FIG. 7  illustrates the overall schematic of a LED white light source module of the present invention. 
     
    
    
     DETAIL DESCRIPTIONS OF THE INVENTION 
     The detailed embodiments of the present invention, a highly efficient heat-conductive LED white light source module, comprise many subtle features and improvement. However, the basic embodiments comprise the following two routes (A) and (B). Route (A) describes one preferred embodiment of the invention of related structures and procedures to make a highly efficient heat-conductive LED/Si package for white light source module starting from blue LED dice(s). Route (B) describes another preferred embodiment of the invention of related structures and procedures to make a highly efficient heat-conductive LED/Si package for white light source module starting from white LED dice(s). 
     (A) Starting from Blue LED Dice(s): 
     (A.I) Making a LED/Si Package 
       FIG. 1  illustrates the top and side view drawing of the LED light source module on Silicon wafer  101  according to a preferred embodiment of the present invention, where a predesigned circuit  103  is processed in a Silicon wafer  101  as an assembly base to house LED dice(s)  104 .
         Such a Si base  102  has circuit  103  (and electronic components if suitable cost-wise) processed (from semiconductor foundry services) on the top side. The bottom side of the Si base  102  is coated with eutectic bondable metal  109  layers to provide thermal passage linking the Si base  102  to a heat-sink.   Hereafter the present invention adopts a commonly used coating composition of bondable metal  111  layers, Ti—Ni—Ag  106 , as the eutectic means  109 , with adding a Sn layer on top of Ag as optional.   Depending on the application design, each of the top side of a Si base  102  has a set of corresponding areas (each area slightly bigger than a LED dice) to house LED dice(s)  104 , with each area designated to 1 LED dice.       

     (A.I.1) for Direct Electro-Connection between Planar LED Pads and Si Pads
           FIG. 2  illustrates the pattern of contact area of LED dice of the present invention. In this case, all of the top side areas of Si base  201  have electro-pads  207  (but bigger in size) patterned  210  the same way as the LED dice pads  205   206  so as LED pads and Si pads can be bonded together. These Si pads  207  are made with coating of eutectic bondable metal layers to provide electro passages directly linkable to LED pads  205   206 . Si pads  207  also serve as the thermal passage linking top side of LED dice to the top side area of Si base  201 .       

     (A.I.2) for Wire-Bonding of Planar LED Pads to Provide Electro-Connection
         In this case, all of the top side areas of Si base  201  are coated with eutectic bondable metal layers  203  to serve as the thermal passage linking bottom side of LED dice to the top side area of Si base  201 .       

     (A.I.3) for Wire-Bonding of Vertical LED Pads to Provide Electro-Connection
         In this case, all of the top side areas of Si base  201  are coated with eutectic bondable metal layers  203  to serve as the thermal passage linking bottom side of vertical LED dice (which the bottom side is itself a connecting pad) to the top side area of Si base  201 .       

     Then, a plurality of LED dice bonded onto the Si base  201  by applying usual Sn soldering technique (fine Sn—Sb flux preferred) through SMT with reflow or ultrasonic means; and 
     wherein the said plurality of LED dice bonded to the said predesigned Si base  201  to make it a LED/Si package, hereafter referred to as a “T/T” packaging for a (A.I.1) type and, a “B/T” packaging for (A.I.2) or (A.I.3) type. 
     (A.II) Making a LED White Light Source Module 
       FIG. 3  illustrates the top and side views of the light source module schematic and various shapes depending on application designs of the present invention, where a LED/Si package coated with a conformal ultra thin film of phosphor only on its LED  302  side.  FIG. 4  illustrates the option for ph-coating  403  on LED/Si package of the present invention. 
     (A.II.1) for T/T Packaging
         The LED/Si package is ready to be phosphor-coated directly. After phosphor-coating, then put on an electrically isolation cup  303  configured to contain LED dice(s) area, followed by the later processes including filling in encapsulation filler material  304  and/or lens for secondary optics. Note that the addition of cup and/or lens are optional depending on secondary optical designs.       

     (A.II.1) for B/T packaging
         The LED/Si package is (1) to be put on an electrically isolation cup  405  configured to closely confine dice(s) area, (2) wire-bond  406  the LED pads to predesigned circuitry  103 , (3) fill in encapsulation filler material  404  and cure the filler to fix/protect the wiring  406 . Then it is ready to be phosphor-coated. After phosphor-coating  403 , then put on lens  407  etc for optional secondary optics. Note that the above (1) and (3) are optional depending on designs if protection of connection wires can be secured.   Depending on application designs, the number of LED dices can vary a lot, so are the matching Si base  400  designs. The present invention calls for an integrated module of total LED power at 1 W to 100 W, with phosphor-coating thickness averaged at 10 to 100 micron (depending on the particle size distribution of phosphor powder supply), and cup  405  height at 1 to 3 mm.       

     Then, a phosphor-coated LED/Si package either stand-alone, or its bottom soldered to a predesigned larger piece of metal base sheet  401  (copper preferred) through SMT; and 
     wherein the said phosphor-coated LED/Si package either by itself, or its bottom soldered to the said predesigned metal base  401 , to make it a LED white light source module, ready to be further assembled with other predesigned parts including a simple sheet metal heat sink to make a general lighting device. 
     (B) Starting from White LED Dice(s): 
     (B.I) Making a White LED Dice 
       FIG. 5  illustrates the “T/T” packaging the white LED dice of the present invention and  FIG. 6  illustrates the “B/T” packaging the white LED dice of the present invention, where a blue LED dice coated with conformal ultra thin phosphor film  507  to make it a white LED dice; 
     (B.I.1) T/T Type White LED Dice
         In this case, blue LED dices  504  are lined up with at least  200  microns gap in between each other and are phosphor-coated  507   604  for pre-tuned film thickness on five sides except their EPI  508  top sides. The present invention calls for naked, un-coated LED substrate sides to be phosphor coated in order to enjoy the maximum white light extraction upon photons leaving the phosphor layer. These white LED dices so made up are then ready to be bonded to the Si base made up following steps as per (A.I.1).       

     (B.I.2) B/T Type White LED Dice
         In this case, blue LED dices are lined up with at least 200 microns gap in between each other and are phosphor-coated  604  for pre-tuned film thickness on five sides except their substrate  600  bottom sides. Then all of the LED pads  601   602  are to be opened up by only removing the coated phosphor on them. These white LED dices so made up are then ready to be bonded to the Si base made up following steps as per (A.I.2)/(A.I.3).       

     (B.II) Making a LED White Light Source Module, 
     A plurality of white LED dice bonded onto the Si base per steps as defined in (A.II) only without phosphor-coating procedures; and 
     wherein the said white LED dice bonded to the said Si base to form a LED/Si package and then, with or without its bottom soldered to a piece of predesigned metal sheet base to make it a LED white light source module, ready to be applied along with other predesigned parts to make a general lighting device. 
       FIG. 7  illustrates the overall schematic of a LED white light source module of the present invention, where a Printed Circuit Board fixture  701  is the assembly base with heathink metal  702  connected to the Si base  703  as described above and power supply  706 ,  708 . 
     A large number of light facilities applications can be enabled by the invention, two simple examples are described for basic reference. 
     (GL) Applying a LED White Light Source Module to Make General Lighting Devices 
     A white light of uniform color temperature (defined in according to order, say 2500 K to 10000 K) within tolerance can be accomplished by directly applying the said LED white light source module for producing general lighting devices, such as commonly used various kinds of light bulbs or tubes. 
     (BLU) Applying White LED Dices to Make BLU 
     A white light of uniform color temperature (defined in according to order, say 8000 K to 15000 K) within tolerance can be accomplished for a BLU by directly replacing the current BLU&#39;s blue LED dices with the said white LED dices, and save the blue-to-white light conversion process during the production of current BLU assemblies. 
     Although illustrative embodiments have been described herein with reference to the accompanying drawings is exemplary of a preferred present invention, it is to be understood that the present invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.