Patent Publication Number: US-9893243-B2

Title: LED-based light source utilizing asymmetric conductors

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of, and claims priority under 35 U.S.C. § 120 from, nonprovisional U.S. patent application Ser. No. 13/086,310 entitled “LED-Based Light Source Utilizing Asymmetric Conductors,” filed on Apr. 13, 2011, now U.S. Pat. No. 9,478,719, which in turn is a continuation-in-part of, and claims priority under 35 U.S.C. § 120 from, nonprovisional U.S. patent application Ser. No. 12/941,799, entitled “LED-Based Light Source Utilizing Asymmetric Conductors,” by Yan Chai and Calvin B. Ward, filed on Nov. 8, 2010, now U.S. Pat. No. 8,455,895. The subject matter of each of the aforementioned patent documents is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to packaging for light-emitting diodes, and more particularly, to a low-cost method of making an LED light source with improved light extraction characteristics. 
     BACKGROUND INFORMATION 
     Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy into light. Improvements in these devices have resulted in their use in lighting fixtures as replacements for conventional incandescent and fluorescent light sources. LEDs have significantly longer lifetimes than both incandescent bulbs and fluorescent tubes. In addition, the energy conversion efficiency of LEDs has now reached the same level as that obtained in fluorescent light fixtures and promises to exceed even these efficiencies. 
     A single LED produces too little light to be used as a replacement for a conventional lighting source in most applications. Hence, a replacement light source must utilize a large number of individual LEDs. The packaging costs and reliability problems that result from having to use large numbers of individual LEDs present challenges that must be overcome if LED-based light sources are to reach their full potential as replacements for conventional light sources. 
       FIG. 1  (prior art) is a cross-sectional view of a portion of a prior art, phosphor-converted, LED light source. Light source  20  includes a plurality of LEDs of which LED  21  is typical. The LEDs are mounted on a printed circuit board  32  that includes a heat-spreading layer  33 , an insulating layer  34 , and a conducting layer that is patterned to provide electrical conductors such as conductor  35 . The LEDs are mounted in depressions having reflecting walls  36  that re-direct light leaving the side surfaces of the LEDs such that the light leaves the light source in the vertical direction as shown by the arrows. The surface of heat-spreading layer  33  is typically covered with a reflecting material that redirects any light that is emitted in a downward direction into the upward direction. The LEDs are covered by a layer of phosphor  37  that converts a portion of the blue light generated by the LEDs to light in the yellow region of the optical spectrum. The combination of the blue and yellow light is perceived as “white” light by a human observer. 
     The LEDs include a light-emitting structure  22  that is deposited on a sapphire substrate  23 . The light-emitting structure can be viewed as an active layer  24  that is sandwiched between an n-type GaN layer  28  that is deposited on substrate  23  and a p-type GaN layer  25  that is deposited over the active layer. The device is powered from contacts  26  and  27 . Since p-type GaN has a very high resistivity, a current spreading layer  29  is typically deposited on the surface of layer  25 . In the arrangement shown in  FIG. 1 , light is extracted through the top surface of the LED, and hence, the current spreading layer must be transparent. Typically, indium tin oxide (ITO) is used for the current spreading layer. 
     The electrical connections to the LEDs are provided by wire bonds, such as bond  31 , that connect the contacts on the LEDs to corresponding contacts on a printed circuit board. The wire bonds present problems in terms of fabrication cost and reliability, particularly when the light source includes a large number of individual dies. The wire bonds are subject to failure both at the time of initial implementation of the bonds and later due to stresses between the phosphor layer and the encapsulated wire bonds. In addition, the wire bonds block a significant fraction of the light leaving the LEDs, as both the bond pads and the gold wire absorb light. 
     The arrangement shown in  FIG. 1  provides good light capture with respect to the light leaving the sides of the LEDs. However, this aspect requires a more complex mounting substrate having reflective cups. The cost of the substrate increases the cost of the light source. 
     The arrangement shown in  FIG. 1  has the advantage of providing good heat conduction because the entire bottom surface of the LEDs is in contact with the heat-spreading layer  33  of the printed circuit board. Heat removal is an important aspect of high-powered LED light sources, as the efficiency of the LEDs decreases with temperature. In addition, mechanical problems that arise from differences in the thermal coefficient of expansion between phosphor layer  37  and printed circuit board  32  become worse as the operating temperature increases. 
     The problems associated with wire bonds can be reduced by utilizing a flip-chip mounting scheme.  FIG. 2  (prior art) is a cross-sectional view of a portion of another prior art light source that utilizes a flip-chip mounting scheme. Light source  40  includes a plurality of surface mounted LEDs. For the purposes of this discussion, a surface mounted LED is defined to be an LED in which both the p-contact and the n-contact are on one side of the LED, light being emitted primarily through an opposing surface of the LED, although some of the light may be emitted through the side surfaces of the LED. In the case shown in  FIG. 2 , the sapphire substrate  41  faces upward and the LEDs are connected to the mounting substrate by the contacts that are used to power the LEDs. Light is emitted through the sapphire substrate. The p-contact includes a mirror  42  that re-directs light striking the contact such that the light exits through the sapphire substrate or side surfaces of the LED. The mirror can also act as the current spreading layer thereby reducing or eliminating the need to use an ITO layer. While the ITO layer is not needed for current spreading in this arrangement, the ITO layer can still provide an ohmic contact with the p-GaN layer, and hence, a thin ITO layer may be included in the p-contact. Since light does not exit through the p-GaN layer, the p-contact can extend substantially over the entire active layer, and hence the problems of providing current spreading over the highly resistive p-GaN layer are substantially reduced. For the purposes of this discussion, the p-contact will be defined to extend over substantially all of the active layer if the p-contact overlies at least 60 percent of the active layer. 
     The n-contact and p-contact are bonded to corresponding traces  43  and  44 , respectively, on the mounting substrate. These traces are patterned on an insulating layer  45  that overlies the heat-dissipating core region  46  of the printed circuit board. Suitable bonding materials that utilize solder, thermal compression bonding, or asymmetric conducting adhesives are known to the art. Novel asymmetric adhesives will be discussed in more detail below. 
     While the arrangement shown in  FIG. 2  reduces the problems associated with the wire bonds, heat dissipation and the loss of light that exits through the sidewalls of the LEDs remains problematic. If the LEDs are mounted in reflective cups as described above, the cost of the substrate becomes a problem. Furthermore, the bonding process requires that the LEDs be pressed against the printed circuit board during the bonding process, and hence providing a pressure mechanism that can operate on all of the LEDs in a light source at once is problematic if the LEDs are in reflective cups. 
     An LED packaging arrangement is sought that allows light leaving the sides of flip-chip mounted LEDs to be emitted upwards without using reflective cups. 
     SUMMARY 
     The present invention includes a light source and method for making the same. The light source includes a plurality of surface mount LEDs that are bonded to a mounting substrate by a layer of asymmetric conductor. Each LED has surface mount contacts on a first surface thereof and emits light from a second surface thereof that is opposite the first surface. The surface mount contacts include a p-contact and an n-contact for powering that LED. Each LED is characterized by an active layer that generates light of a predetermined wavelength, the p-contact having an area that is greater than or equal to at least half of the active region in the LED. The mounting substrate includes a top surface having a plurality of connection traces. Each connection trace includes an n-trace positioned to underlie a corresponding one of the n-contacts and a p-trace positioned to underlie a corresponding one of the p-contacts, the p-trace having an area greater than the p-contact. The layer of asymmetric conductor is sandwiched between the surface mount contacts and the connection traces. 
     In one aspect of the invention, the LEDs are spaced apart from one another and the LEDs emit light from side surfaces of the LEDs. The asymmetric conductor is present in spaces between the LEDs to a height such that light leaving the side surfaces of the LEDs enters the asymmetric conductor located between the LEDs. The asymmetric conductor includes scattering particles that scatter the light leaving the side surfaces. 
     In another aspect of the invention, a light source includes LED dies that are flip-chip mounted onto a flexible plastic substrate. The flexible substrate is used in a reel-to-reel process to make a strip light source. The dies are attached to the substrate using an asymmetric conductor material (ACF material) in which deformable conducting particles are sandwiched between surface mount contacts on the LED dies and connection traces on the flexible substrate. A diffusively reflective material reflects light that is emitted sideways from the LED dies upwards towards a phosphor layer. The diffusively reflective material is disposed on the top surface of the substrate and contacts the side surfaces of the LED dies. In one embodiment, the diffusively reflective material includes spheres of titanium dioxide suspended in silicone. The light source is manufactured in a reel-to-reel process in which the asymmetric conductor material and the diffusively reflective material are cured at the same time. A silicone layer of molded lenses that has either suspended phosphor particles or a layer of phosphor is also added in the reel-to-reel process over the mounted LED dies. 
     The light scattering particles in the reflective material adjacent to the LED dies provide a means for reflecting light that is emitted from the side surface of the LED dies away from the substrate without placing each LED die in an expensive reflective cup. 
     A method of manufacturing a light source uses a reel-to-reel process to deposit an amount of asymmetric conductor material on a mounting substrate, such as a flexible plastic substrate. The asymmetric conductor material includes deformable conducting particles suspended in a transparent carrier material, such as epoxy or silicone. An LED die is mounted onto the substrate in a flip-chip manner over the deposited amount of asymmetric conductor material. Then a diffusively reflective material is dispensed onto the substrate adjacent to the mounted LED die such that the diffusively reflective material contacts the LED die. The diffusively reflective material includes light scattering particles suspended in the transparent carrier material. The LED die is then pressed against the substrate such that some of the deformable conducting particles deform and form an electrical connection between contacts on the LED die and traces on the substrate. The transparent carrier material is then heated such that both the asymmetric conductor material and the diffusively reflective material cure to a hardened state. A layer of cured transparent carrier material with suspended phosphor particles is then deposited over the LED die and the diffusively reflective material. The layer of cured transparent carrier material includes molded lenses. 
     Further details and embodiments and techniques are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
         FIG. 1  (prior art) is a cross-sectional view of a portion of a prior art, phosphor-converted, LED light source. 
         FIG. 2  (prior art) is a cross-sectional view of a portion of another prior art light source. 
         FIG. 3  is a cross-sectional view of a pair of surfaces that are bonded by an asymmetric conductor. 
         FIG. 4  is a cross-sectional view of a portion of a light source according to one embodiment of the present invention. 
         FIG. 5  is a top view of a portion of a mounting substrate before the LEDs have been bonded to the n-traces and p-traces. 
         FIG. 6  is a cross-sectional view of a portion of another embodiment of a light source according to the present invention. 
         FIGS. 7-9  are cross-sectional views of a portion of a light source according to one embodiment of the present invention at various stages in the fabrication process. 
         FIG. 10  is a cross-sectional view of another embodiment of a light source that has a diffusively reflective material deposited above a layer of asymmetric conductor material. 
         FIG. 11  is a cross-sectional view of yet another embodiment of a light source that has a diffusively reflective material deposited on a flexible plastic substrate. 
         FIG. 12  is a flowchart of steps for making a strip light source by flip-chip mounting LED dies separated by diffusively reflective material. 
         FIG. 13  illustrates a reel-to-reel process for making the strip light source of  FIG. 11 . 
         FIGS. 14A-F  are cross-sectional views of the light source of  FIG. 11  at various stages of the manufacturing method of  FIG. 12 . 
         FIGS. 15A-D  are flowcharts illustrating methods in addition to the method of  FIG. 12  for making light sources that use diffusively reflective material instead of reflective cups to reflect light that is emitted sideways from the LED dies. 
         FIG. 16  is a cross-sectional view of the light source of  FIG. 11  with a lens having a conventional shape. 
         FIG. 17A  is a cross-sectional view of the light source of  FIG. 11  with a lens having a novel shape that improves the light extraction characteristics of the light source. 
         FIG. 17B  shows examples of micro-structures on the surface of the novel lens of  FIG. 17A . 
         FIG. 18  is a perspective view of the strip light source of  FIG. 17A  with the novel dimple-shaped lenses. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     A novel LED packaging arrangement allows light leaving the sides of flip-chip mounted LEDs to be emitted upwards without using reflective cups by making the surface of the mounting substrate between the LEDs reflective and by filling the regions betweens the LEDs with a transparent material that includes scattering particles. The light leaving the sides of the LEDs is scattered until it either leaves the light source in a generally upward direction or is absorbed after multiple reflections. While re-direction of light by scattering is less efficient than embodiments that utilize cups or other reflectors, the reduced cost of fabrication often is more important, as additional LEDs can be added to the array to make up for light losses. While this mode of light re-direction does not require reflective cups, the process does require a separate deposition step in which scattering material is introduced between the LEDs. If the LEDs are in a closely packed array, this injection of material between the LEDs presents challenges. 
     Heat dissipation requires a low thermal resistance path from the LED to the underlying core region and a low thermal resistance path from core region to the structure that finally transfers the heat to the environment, typically at an air interface. If either of these paths presents a large thermal resistance, the LEDs will be forced to operate at elevated temperatures to drive the heat along the resistive path. Typically, GaN LEDs are designed to operate at temperatures below 100° C. or 75° C. above ambient. In one aspect of the invention, the thermal paths from the LEDs to the final heat-radiating structure that transfers the heat to the environment are dimensioned such that the heat generated in the LEDs can be transferred to the environment without requiring the LEDs to be operated at a temperature that is greater than 75° C. above the temperature of the environment in question. 
     The path from the LEDs to the underlying core region has two potential bottlenecks. The first is the connection between the p-contact and the underlying electrical trace, i.e., trace  44  in  FIG. 2 . The second is the thermal path from trace  44  to the underlying core region  46 . Except in the case of an asymmetric conducting adhesive, the first bottleneck is not potentially limiting because solder or direct bonding leads to metal-metal bonds having a thermal conductivity that is greater than that of the LED materials. The second bottleneck presents more significant problems because traces must be insulated from the underlying heat-dissipating core, and the insulating layers used in printed circuit boards also have high thermal resistance. 
     In one aspect of the present invention, an asymmetric conductor that has been modified to provide light scattering as well as vertical connections is utilized. The manner in which asymmetric conductors operate can be more easily understood with reference to  FIG. 3 , which is a cross-sectional view of a pair of surfaces that are bonded by an asymmetric conductor, such as an anisotropic conductive film (ACF). The asymmetric conductor includes a plurality of elastic metal coated spheres  53  that are suspended in a curable epoxy or other insulating carrier material that can be converted to a solid by a curing process. Each of the spheres is coated with a metallic layer  54  that renders the sphere a conductor. When a layer of this material is pressed between two surfaces as shown in  FIG. 3 , the spheres that are trapped between conductors  51  and  52  are deformed as shown at  56 . The deformed spheres make electrical connections between electrodes  51  and  52 . After the carrier medium is cured, the two surfaces are left bonded to one another with opposing conductors on the surfaces being electrically connected to one another. The density of the sphere is chosen to be high enough to assure that any pair of opposing conductors has one or more spheres trapped therebetween but low enough to assure that the spheres do not contact one another in the horizontal direction and form a laterally running conduction path. 
     Asymmetric conductors have been used for bonding arrays of LEDs to underlying substrates having switching circuitry therein for over a decade. For example, U.S. Pat. No. 6,965,361 teaches a display in which a layer that includes an array of organic LEDs is bonded to a substrate having thin film transistors thereon for switching individual LEDs on and off. 
       FIG. 4  shows a cross-sectional view of a portion of a light source according to one embodiment of the present invention. Light source  60  includes a plurality of surface mounted LEDs  61 - 64 . The LEDs are bonded to traces on a mounting substrate  65 . Exemplary traces are labeled at  66  and  67 . The traces are patterned from a metal layer that is deposited on an insulating substrate  68  that is bonded to a heat-dissipating spreading layer  69 . Substrate  68  will be discussed in more detail below. 
     The LEDs are bonded to the traces by an asymmetric adhesive layer of asymmetric conductor material that includes two types of particles suspended in an insulating carrier material. The first type of particle is shown at  72  and consists of a compressible polymer sphere with an outer metal coating that operates in a manner analogous to that described above with reference to spheres  53 . The second class of particles consists of light reflecting particles  71  that scatter light striking the particles. In one aspect of the present invention, these particles are insulating particles such as TiO 2 . The scattering particles have diameters that are significantly less than those of the conducting spheres to assure that the scattering particles do not interfere with the compression of the spheres between the surfaces that are to be connected electrically. For example, the scattering particles can have diameters that are less than the minimum distances between the electrodes of the LEDs and the corresponding traces on layer  68 . It should be noted that the scattering particles preferably have diameters that are greater than the wavelength of light generated by the LEDs. In one aspect of the invention, the light scattering particles have a maximum dimension that is less than half the diameter of the compressible spheres when the compressible spheres are not deformed by being sandwiched between the conductors. 
     In one aspect of the invention, the LEDs are pressed into a layer of asymmetric adhesive material while the carrier material is in a liquid state. The layer of asymmetric adhesive material has a thickness that is sufficient to ensure that when all of the LEDs are pressed into the layer, the excess material will be forced into the spaces between the LEDs to a height that ensures that the edges of the LEDs are covered by an asymmetric conductor as shown at  70  such that light leaving the side walls of the LEDs enters the layer of asymmetric conductor material between the LEDs and is scattered by the scattering particles. For example, the thickness of the pre-cured asymmetric conductor medium can be set such that the height of the asymmetric conductor medium between the LEDs is sufficient to ensure that at least 50, 60, 70, 80, or 90 percent of the light leaving the side surfaces of the LEDs enters the asymmetric conductor medium between the LEDs. 
     In one aspect of the invention, the top surface of layer  68  includes reflective regions, such as region  74 , that reflect any light that is scattered downward back into an upward direction. These reflective regions could be separate reflective areas that are not connected to other structures or reflective extensions of the p-contacts. 
     The conducting spheres can also act as scattering particles provided the metallic coating is chosen from a material that provides a good mirror. In this regard, it should be noted that highly conductive metals such as gold and silver will provide a good mirror surface only if the surface is free of roughness. If the surface is not sufficiently smooth, particles will absorb light via the surface plasmon effect. Prior art particles utilize gold or silver for the outer coating to maximize the conductivity of the coating, and hence do not provide optimal reflective surfaces. In one aspect of the present invention, the conducting particles utilize aluminum as the outer coating to improve the reflectivity of the particles that are trapped between the LEDs. 
     Heat dissipation is an important issue in high-powered light sources based on LEDs for a number of reasons. First, the efficiency of conversion of electricity to light decreases at high temperatures. Second, the lifetimes of the LEDs also decrease with temperature. Third, the differences in thermal coefficient of expansion between the LEDs and the carrier material used in the asymmetric conductor can lead to fractures in the asymmetric conductor and separation of the LEDs from the underlying structure. Hence, maximizing heat transfer from the LEDs to the surrounding environment is an important aspect of any high-power LED light source design. 
     The heat generated by the LEDs must be transferred either to the air above the LEDs or to heat-spreading layer  69 , which is in thermal contact with a heat-dissipating structure that couples the heat to the environment. To transfer the heat to layer  69 , the thermal resistance of layer  68  is preferably much less than the thermal resistance of the asymmetric conductor layer between the contacts on the LED and the corresponding traces on layer  68 . The thermal resistance of layer  68  can be reduced by using a material that has a low thermal resistance while still providing electrical insulation and by increasing the surface area of layer  68  over which heat is transferred to layer  69 , and decreasing the thickness of layer  68 . 
     Because the thickness of the layer of asymmetric conductor between the p-contact on the LED and corresponding trace on layer  68  is very thin, the thermal resistance is determined by the area of contact between the trace and layer  68 . As noted above, it is advantageous to provide a reflecting surface  74  between the LEDs. In one aspect of the present invention, this surface is created by extending the traces opposite the p-contacts on the LEDs, which will be referred to as the p-contact traces. These traces can be coated with an aluminum or other highly reflective coating. The area of the traces can be extended to substantially fill the regions between the LEDs thereby increasing the heat transfer area substantially. The maximum expansion of this area depends on the spacing of the LEDs. In one aspect of the invention, the LEDs are spaced such that the area of the p-traces is at least twice the area of the p-contact on the LEDs. 
       FIG. 5  is a top view of a portion of a mounting substrate before the LEDs have been bonded to the n-traces and p-traces. The expanded p-traces are shown at  81 . The area in which the LEDs bond is shown in phantom at  82 . The n-traces are shown at  83 . The electrical connections to the traces on the surface of insulator  88  are made through vias that are under the traces to conducting planes that are in layers under the heat-spreading layer. 
       FIG. 6  is a cross-sectional view of a portion of another embodiment of a light source according to the present invention. Light source  90  is similar to light source  60  discussed above with reference to  FIG. 4  in that light source  90  includes a plurality of LEDs that are bonded to traces on a mounting substrate by asymmetric conductor material. The traces that connect the p-contacts are enlarged as discussed above with respect to  FIG. 5 . A typical enlarged trace is shown at  99 . The traces are connected to a wiring layer  98  by conducting vias  97  that connect each trace to a corresponding conductor on wiring layer  101 . These vias pass through insulators in heat-spreading layer  91 . Hence, the only high thermal impedance area is the area between the expanded traces on the surface of insulator  102  and heat-spreading layer  91 . 
     Light source  90  also includes a phosphor conversion layer  94  that converts a portion of the light generated by the LEDs to light having a different spectrum that is chosen such that the light leaving layer  94  is perceived to be white light with a predetermined color temperature. The phosphor conversion layer is constructed by suspending phosphor particles  96  in a transparent carrier medium such as an epoxy and then curing the epoxy layer once the suspension has been spread over the light source. Since the areas between the LEDs are filled with the asymmetric conductor material, the phosphor conversion layer can be of a more uniform thickness, and hence, color variations resulting from the blue light from the LEDs passing through different areas of phosphor with differences in thickness of phosphor are reduced. 
     In one aspect of the present invention, phosphor conversion layer  94  is constructed from the same epoxy medium as the asymmetric conductor. In another aspect of the invention, the phosphor conversion layer has a coefficient of thermal expansion that is substantially equal to that of the asymmetric conductor. Here, the two layers will be defined as having substantially equal thermal coefficients of expansion if the difference in thermal coefficients of expansion is less than a difference that would cause the two layers to separate during the thermal cycling of the light source over its design lifetime. This arrangement reduces the problems associated with having different coefficients of thermal expansion associated with different layers. 
     To further improve the thermal conductivity of the asymmetric conductor material and phosphor conversion layer, particles  95  of a high thermal conductivity medium can be included in the layers. For example, particles of diamond, crystalline silicon, or GaN can be included in layer  94  and the asymmetric conductor material. These materials have significantly higher thermal conductivity than the epoxy resins used to construct the asymmetric conductor material and phosphor conversion layer, and hence, their inclusion results in a layer having an average thermal conductivity that is higher than that of the epoxy. These materials are also transparent, and hence do not absorb light. The use of such materials is discussed in detail in co-pending U.S. patent application Ser. No. 12/845,104, filed on Jul. 28, 2010, which is incorporated herein by reference. 
     As noted above, heat-spreading layer  102  moves the heat generated by the LEDs to a region of the light source that has contact with the environment and can include structures such as the fins shown at  93  that help to dissipate the heat to the surroundings. Typically, the heat is dissipated to the air; however, embodiments in which the heat-spreading layer is in contact with other structures that dissipate the heat can also be constructed. 
     In the above-discussed embodiments, the thermal resistance of layer  102  presents the most challenges in terms of removing heat from the LEDs. This layer can be constructed from a thin polymeric layer or a thin layer of an insulating material such as glass. Alternatively, layer  102  can be constructed from an undoped crystalline material that is grown on heat-spreading layer  91 . For example, layers of diamonds can be deposited on a number of substrates at low temperature using chemical vapor deposition or similar techniques. Such coatings are commonly used as scratch resistant coatings on glass or plastics. Similarly, undoped silicon could also be used as the insulator. These crystalline materials have significantly higher thermal conduction than do polymeric layers. 
     In one aspect of the invention, the wiring layer is coupled to a drive circuit  103  that includes a power connector  104  for providing power to the LEDs. The drive circuit can also include switching circuitry that determines the internal connection topography for the array of LEDs. 
     The manner in which a light source according to one embodiment of the present invention is constructed will now be explained with reference to  FIGS. 7-9 , which are cross-sectional views of a portion of a light source according to one embodiment of the present invention at various stages in the fabrication process. Initially, a mounting substrate  115  is covered with a layer  116  of the asymmetric conductor material in a non-cured liquid state as shown in  FIG. 7 . Each LED  117  is positioned such that the contacts on the LED are over the corresponding traces on mounting substrate  115 . The positioned LEDs are then forced against mounting substrate  115  as shown in  FIG. 8 . The LEDs can be forced into the layer  116  of asymmetric conductor material one at a time or attached to a temporary carrier and forced into the asymmetric conductor simultaneously. After the LEDs have been forced into the layer of the asymmetric conductor material, pressure is applied to the LEDs and the asymmetric conductor material is heated to cure the material, and hence render the asymmetric conductor layer solid. As noted above, the depth of the uncured asymmetric conductor material is set such that the asymmetric conductor fills the regions betweens the LEDs when the LEDs are forced into the asymmetric conductor material. After the asymmetric conductor material has cured, the layer  118  of phosphor-containing material is deposited over the cured asymmetric conductor layer and cured as shown in  FIG. 8 . 
     The above description refers to various surfaces in terms of top or bottom surfaces. These are merely labels that express the relationship of the surfaces as seen in the drawings when the drawings are held in a particular orientation. These labels do not imply any relationship with respect to orientation on the earth. 
     The LEDs in the above-described embodiments of the present invention have been described in terms of an active layer that is sandwiched between an n-layer and a p-layer, the various layers being grown on a substrate. However, it is to be understood that each of the layers may include a plurality of sub-layers. Similarly, the substrate may include one or more buffer layers that are deposited prior to depositing the LED layers. 
       FIG. 10  shows a cross-sectional view of a portion of a light source  120  according to another embodiment that exhibits improved light extraction characteristics. Light source  120  includes a surface mounted LED die  121  that is mounted on the top surface  122  of a mounting substrate  123 . LED die  121  has surface mount contact pads  124 - 125  on a first surface  126  and emits light from a second surface  127  and from side surfaces  128 . The light that is emitted from second surface  127  first travels from the active layer through the transparent sapphire substrate. N-contact pad  124  and p-contact pad  125  are electrically coupled to an n-trace  129  and a p-trace  130 , respectively, that are patterned from a metal layer that is deposited on mounting substrate  123 . Mounting substrate  123  is bonded to a heat-dissipating spreading layer  131 . An anisotropic conductive film (ACF) material  132 , also called asymmetric conductor material, is sandwiched between the surface mount contacts  124 - 125  and the connection traces  129 - 130  such that those deformable conducting particles  133  that touch both the contacts  124 - 125  and traces  129 - 130  are deformed and form an electrical connection between the contacts and traces. Except between the contacts and traces, however, the deformable conducting particles  133  are suspended in a transparent carrier material and do not conduct current. Unlike the asymmetric conductor material of  FIG. 4 , asymmetric conductor material  132  includes no smaller light reflecting particles but only the larger deformable conducting particles  133 , which are compressible polymer spheres with a metal outer coating. In one example, asymmetric conductor material  132  is SLP-01 made by Sony Chemicals Corp. The conducting particles  133  have an epoxy center electroplated with nickel/gold and have a diameter of about 5 microns. Other conducting particles  133  have a coating of aluminum. 
     The conducting particles  133  have a relatively low reflectivity of about 60% compared to a reflectivity of about 95% for small spheres of titanium dioxide. In the embodiment of  FIG. 4 , the light that is emitted from the side surfaces of the LED dies collides not only with the smaller light reflecting particles  71  but also with the larger deformable conducting particles  72 . Consequently, about 40% of the light that collides with the larger conducting particles  72  is absorbed and is not reflected up and out of the light source  60 . More light can be extracted from light source  120  by removing the larger conducting particles  72  from the transparent carrier material that is placed adjacent to the side surfaces  128 . In one example, the transparent carrier material in asymmetric conductor material  132  is an epoxy. 
     In the embodiment of  FIG. 10 , only a thin layer  134  of asymmetric conductor material  132  is deposited over top surface  122  of mounting substrate  123 . Layer  134  is sufficiently thick to electrically and mechanically connect LED die  121  to mounting substrate  123  but not so thick that asymmetric conductor material  132  covers the side surfaces  128  of LED die  121 . After LED die  121  has been mounted onto mounting substrate  123  by pressing the surface mount contacts  124 - 125  into the connection traces  129 - 130  such that some conducting particles  133  touch both the contacts  124 - 125  and the traces  129 - 130  and are deformed and form an electrical connection between the contacts and traces, asymmetric conductor material  132  is cured by heating. Then a diffusively reflective material  135  is dispensed over layer  134  adjacent to side surface  128  and between LED die  121  and the next LED die  136 . The diffusively reflective material  135  includes light scattering particles  137  suspended in a transparent carrier material. In one example, the transparent carrier material in reflective material  135  is silicone, and the light scattering particles  137  are spheres of titanium dioxide (TiO 2 ) with a diameter of about two microns. For comparison, the deformable conducting particles  133  have a diameter of about five microns when they are not deformed. In this example, the light scattering particles  137  have a reflectivity of more than 95%. 
     After the diffusively reflective material  135  is dispersed over the cured asymmetric conductor material  132 , the reflective material  135  is also cured by heating. Then a thin layer  138  of silicone is spread over the top of the LED dies  121 ,  136  and the diffusively reflective material  135 . Before silicone layer  138  is cured, and optics layer  139  of cured silicone is placed over silicone layer  138 . Silicone layer  138  acts as an adhesive and attaches optics layer  139  over the top of the LED dies  121 ,  136  and the diffusively reflective material  135 . Silicone layer  138  is then cured by heating. Optics layer  139  has preformed lenses molded from silicone that contains phosphor particles  140 . Alternatively, a layer of phosphor particles  140  can be deposited onto the bottom surface of optics layer  139  before optics layer  139  is attached over silicone layer  138 . In this case, optics layer  139  would have no phosphor particles dispersed throughout the silicone. 
     The light extraction characteristics of light source  120  are improved because light leaving the side surfaces  128  of LED die  121  enters diffusively reflective material  135  and is scattered by the scattering particles  137  without first being absorbed by any conducting particles  133  in material  135 . A higher percentage of the blue light emitted sideways from LED die  121  is eventually reflected upwards and out of light source  120  or collides with phosphor particles  140  in optics layer  139  and is converted to light in the yellow region of the optical spectrum. In addition, the portion of yellow light that is emitted by the phosphor particles  140  in a downwardly direction is not absorbed by any conducting particles  133  in material  135 . The yellow light that is emitted downwardly is more likely to be reflected by the light scattering particles  137  back up through optics layer  139  than if diffusively reflective material  135  contained conducting particles  133 . 
     Some of the light leaving the side surfaces  128  is reflected downwards by the scattering particles  137  and enters layer  134  of asymmetric conductor material  132 . Only about 60% of the light that collides with the deformable conducting particles  133  is reflected. In an alternate embodiment, the scattering particles  137  are added to asymmetric conductor material  132  to improve the reflectivity of layer  134 . In addition, a reflective coating or trace is deposited on top surface  122  of mounting substrate  123  between LED dies  121  and  136 . The reflective coating beneath layer  134  and the scattering particles  137  added to layer  134  increase the amount of light that is reflected back upwards towards optics layer  139 . In yet another alternate embodiment, the diffusively reflective material  135  is replaced with phosphor particles dispersed in a transparent carrier material, such as silicone or epoxy. Instead of reflecting the blue light emitted from the LED dies, the phosphor particles absorb the blue light and isotropically emit yellowish light. The yellowish light that is emitted downwards is reflected back up by a reflective coating on top surface  122  of mounting substrate  123  between the LED dies. 
       FIG. 11  shows another embodiment of a light source  141  that can be inexpensively produced and is suitable for use in strip lighting. For example, light source  141  can be manufactured as a strip of LED dies in the linear format of a T 8  fluorescent lamp that is installed in a troffer. A 1-by-x strip of LED dies are placed on a flexible plastic substrate  142  having the electrical topology for mounting the LED dies. Flexible plastic substrate  142  is made from a polymer, such as polyimide, polyethylene terephthalate (PET), polyethylene naphthalate (PEN) or liquid crystal polymer (LCP). Flexible plastic substrate  142  has a peel-off adhesive backing  143  that can be used to attach a strip of light source  141  to the inside of a troffer. The metal frame of the troffer then acts as a heat sink. In one example, square dies that are about 0.5 mm on a side are placed about 10 mm apart on flexible plastic substrate  142 . ( FIG. 11  is not drawn to scale.) Because plastic substrate  142  is flexible, the LED dies  121 ,  136  can be picked and placed onto flexible plastic substrate  142  in a reel-to-reel process. Even when flexible plastic substrate  142  is wound on a reel, however, the portion of top surface  122  below the 0.5 millimeter length of LED die  121  remains substantially flat. 
     Instead of placing the dies on a layer  134  of asymmetric conductor material  132 , as done for light source  120 , the dies of light source  141  are placed on only a small amount of asymmetric conductor material  132 . Just enough asymmetric conductor material is dispensed to form an electrical and mechanical connection between LED die  121  and substrate  142  without seeping out significantly beyond the lateral boundary  143  of LED die  121  when the die is pressed into the substrate to deform the conducting particles  133 . Diffusively reflective material  135  is then dispensed over top surface  122  of substrate  142  between the dies. Because asymmetric conductor material  132  is present between the dies and substrate  142 , the diffusively reflective material  135  remains outside the lateral boundary  144  of LED die  121 . 
     In one embodiment, asymmetric conductor material  132  and diffusively reflective material  135  are cured in a single heating step. The carrier material should be the same for both asymmetric conductor material  132  and reflective material  135  if a single curing step is used. In this case, the transparent carrier material in asymmetric conductor material  132  is silicone as opposed to epoxy. Uncured silicone should not be placed in contact with uncured epoxy because the epoxy will react with the palladium catalyst in the silicone and degrade the silicone. Several small drops of asymmetric conductor material  132  are first dispensed onto the traces on top surface  122  of flexible substrate  142 . LED dies  121  and  136  are then placed over the appropriate traces. Before asymmetric conductor material  132  is cured, diffusively reflective material  135  is dispensed onto top surface  122  between LED die  121  and LED die  136 . Sufficient reflective material  135  is dispensed to cover the side surfaces  128  of LED dies  121  and  136 . Then LED dies  121  and  136  are pressed into asymmetric conductor material  132  such that the conducting particles  133  deform between the contacts on the dies and the traces on substrate  142 . While the dies are being pressed down onto substrate  142 , asymmetric conductor material  132  and diffusively reflective material  135  are cured together. 
     Then thin layer  138  of silicone is spread over diffusively reflective material  135  and the tops of the LED dies  121 ,  136 . Thin layer  138  acts as an adhesive to bond optics layer  139  over the LED dies. A layer of phosphor particles  140  is sprayed over the bottom surface of optics layer  139  before optics layer  139  is attached to silicone layer  138 . In this low-cost light source  141 , optics layer  139  has no phosphor particles dispersed in the cured silicone that forms the lenslets. Before silicone layer  138  is cured, and optics layer  139  is placed over silicone layer  138 . Optics layer  139  is rolled over the top of layer  138  in a reel-to-reel process. Silicone layer  138  is then cured by heating. 
       FIG. 12  is a flowchart illustrating steps  145 - 152  of a reel-to-reel process by which light source  141  of  FIG. 11  is manufactured. The steps of the method of  FIG. 12  are illustrated in  FIGS. 13 and 14 . 
     In a first step  145 , flexible plastic substrate  142  is unrolled using a reel-to-reel machine before asymmetric conductor material  132  is deposited onto the substrate. 
     In step  146 , a small amount of asymmetric conductor material  132  is deposited on a mounting substrate. Step  146  is illustrated by  FIG. 14A . The mounting substrate is flexible plastic substrate  142  that was unrolled from a reel of flexible substrate in step  145 . The asymmetric conductor material  132  includes deformable conducting particles  133  suspended in a transparent carrier material. Although  FIG. 14A  shows a single drop of asymmetric conductor material  132  being dispensed onto substrate  142 , several smaller dots are deposited in the actual manufacturing process. The asymmetric conductor material  132  is deposited over the traces  129 - 130  on top surface  122  of substrate  142  on which an LED die will be placed. 
     In step  147 , LED die  121  is mounted onto substrate  142  in a flip-chip manner over the deposited amount of asymmetric conductor material  132 . Step  147  is illustrated by  FIG. 14B . 
     In step  148 , diffusively reflective material  135  is dispensed onto the mounting substrate adjacent to the mounted LED dies such that reflective material  135  contacts the side surfaces  128  of the LED dies. Diffusively reflective material  135  includes light scattering particles  137  suspended in the transparent carrier material. The carrier materials of the asymmetric conductor material  132  and the reflective material  135  are the same. In this case, both carrier materials are silicone. In another embodiment, epoxy is the carrier material in both asymmetric conductor material  132  and reflective material  135 . Because the carrier materials are the same, asymmetric conductor material  132  need not be cured before reflective material  135  is dispensed adjacent to the dies and contacting the uncured asymmetric conductor material  132 . Step  148  is illustrated by  FIG. 14C . 
     In step  149 , LED die  121  is pressed against mounting substrate  142  such that some of the deformable conducting particles  133  deform and form an electrical connection between the contact pads on LED die  121  and the traces on substrate  142 . Step  149  is illustrated by  FIG. 14D . 
     In step  150 , the transparent carrier material of both the asymmetric conductor material and the diffusively reflective material is heated such that both asymmetric conductor material  132  and reflective material  135  cure to a hardened state. In one embodiment of the manufacturing method of  FIG. 12 , the curing step  150  and the pressing step  149  are performed concurrently. Step  150  is performed in two substeps. In the first substep, asymmetric conductor material  132  and reflective material  135  are pre-heated to about 80 degrees Celsius for about two minutes. Then in step  149 , LED die  121  is pressed down with two bumps, each having a force of about 0.4 Neutons. So the LED dies are pressed with a total force of 0.8 Neutons to deform the conducting particles  133 . While the LED dies are being pressed down onto substrate  142 , the second curing substep is performed. While the LED dies are being pressed, asymmetric conductor material  132  and reflective material  135  are heated to about 230 degrees Celsius for about thirty seconds to complete the curing. 
     In step  151 , thin layer  138  of transparent carrier material is spread over the top of the LED dies  121 ,  136  and the cured diffusively reflective material  135 . Layer  138  is not cured in step  151 . Step  151  is illustrated by  FIG. 14E . 
     In step  152 , a layer of cured transparent carrier material is deposited over the thin layer  138  of uncured transparent carrier material. Layer  138  acts as an adhesive and attaches the layer of cured transparent carrier material to the top of the LED dies  121 ,  136  and the diffusively reflective material  135 . The entire light source  141  is then heated, and the thin layer  138  of carrier material cures. In one embodiment, the layer of cured transparent carrier material is optics layer  139  in which lenslets have previously been formed using a molding process. Phosphor particles  140  suspended in the cured transparent carrier material and convert the blue light emitted from LED die  121  into yellowish light. Optics layer  139  is unrolled from a reel using a reel-to-reel machine. Step  152  is illustrated by  FIG. 14F . 
     In another embodiment, a layer of phosphor particles  140  is dusted onto the bottom surface of the layer of cured transparent carrier material  139  as shown in  FIG. 13 . The phosphor particles are then embedded into the uncured transparent carrier material  138  before the layer  138  is heated. Applying a layer of phosphor particles  140  to the underside of optics layer  139  can be less expensive than molding optics layer  139  using a carrier material in which phosphor particles are disbursed. 
     The string of LED dies with lenslets of light source  141  can then be rolled up onto a reel. The reel of light source  141  can easily be transported to the installation site, such as a commercial building. For example, at the installation site, a strip of light source  141  can be cut from the reel at a length corresponding to a T8 fluorescent bulb. The protective paper can then be peeled from adhesive backing  143  on the underside of flexible plastic substrate  142  of light source  141 , and the strip of light source  141  can be taped to the frame of a troffer. Traces that extend from upper surface  122  of flexible plastic substrate  142  are then connected to the power lines of the troffer. 
     The method of manufacturing light source  141  shown in  FIG. 12  reduces the number of required curing steps and the number of components in order to achieve a low cost process for making distributed lighting. Modifications of the method of  FIG. 12  can improve the durability and the light extraction of the produced light sources at the expense of added steps and components.  FIGS. 15A-D  are flowcharts illustrating other methods of making light sources that use diffusively reflective material instead of reflective cups to reflect light that is emitted sideways from the LED dies. 
       FIG. 15A  shows the steps  153 - 161  of a first alternate method of manufacturing a strip light source in which epoxy is the transparent carrier material in both asymmetric conductor material  132  and reflective material  135 . LED dies are flip-chip mounted over an epoxy-based asymmetric conductor material that has been dispensed on a mounting substrate. The epoxy-based diffusively reflective material containing TiO 2  spheres is dispensed on the mounting substrate between the LED dies. The carrier material is pre-cure for two minutes at 80 degrees Celsius, which starts the chemical reaction of the curing process. Then “tonnage” (a metal block) is applied to press the LED dies down onto the substrate and thereby deform those conducting particles that happen to be positioned between the contacts on the LED dies and the corresponding traces on the substrate. While the LED dies are being pressed down, the carrier material is heated to 230 degrees Celsius for thirty seconds. Then a thin layer of silicone is applied as glue over the top of the LED dies and the cured reflective material. A lens strip molded from silicone is placed over the thin layer of silicone, and the silicone layer is cured. The epoxy-based carrier material of the light source produced with the method of  FIG. 15A  will likely degrade faster than silicone-based carrier material in the presence of the heat generated by the LED dies. 
       FIG. 15B  shows the steps  162 - 171  of a second alternate method of manufacturing a strip light source in which epoxy is the transparent carrier material for asymmetric conductor material  132 , but silicone is the transparent carrier material for reflective material  135 . Because uncured silicone should not be placed in contact with uncured epoxy, the method of  FIG. 15B  includes an additional curing step for the epoxy before the silicone-based reflective material is dispensed between the LED dies. 
       FIG. 15C  shows the steps  172 - 179  of a third alternate method of manufacturing a strip light source in which the uncured reflective material is used as a glue to attach the lens strip. 
       FIG. 15D  shows the steps  180 - 187  of a fourth alternate method of manufacturing a strip light source in which a single high-temperature cure step is used to cure the asymmetric conductor material, the diffusively reflective material and the thin silicone layer under the lens strip. In the method of  FIG. 15D , silicone is used as the carrier material for all of the asymmetric conductor material, the reflective material, the thin layer that attaches the lens strip and the molded lens strip itself. Alternatively, epoxy can be used as the carrier material for each of these materials and layers. Thus, the lens strip that includes phosphor particles would be constructed from the same epoxy medium as the asymmetric conductor material. 
       FIG. 16  shows a light source  190  in which diffusively reflective material  135  reflects light emitted from the side surfaces  128  of LED die  121  in order to achieve greater light extraction without using a reflective cup. Much of the light emitted from LED die  121 , however, still does not escape light source  190  because it is reflected back from the surface of the conventional lens  191  and is absorbed by LED die  121 . Most of the light emitted by LED die  121  exits through the upper surface as opposed to the side surfaces. The conventional form of lens  191  maximizes the area of the lens surface that is at a right angle to the location from which the most light is being emitted because light is less likely to be reflected if it strikes the lens surface closer to a right (normal) angle. So the lens is designed so that the first order effect of reflection at the lens surface is minimized. 
     Where much of the light does not exit the lens on the first pass, however, the second order effects of whether the reflected light is absorbed should be given a larger influence over the shape of the lens. About 45% of the exiting light that strikes the silicone/air interface of lens  191  at a normal angle is reflected because the index of refraction of the silicone is about 1.41 (the index changes with temperature) and the index of refraction of air is about 1.00. The upper surface of LED die  121  is assumed to act as a Lambertian emitter in which the intensity of the emitted light is at a maximum normal to the upper surface and decreases in proportion to the cosine of the angle away from normal. Thus, because the upper surface of LED die  121  is not a point light source, a majority of the light does not strike the silicone/air interface of lens  191  exactly at a normal angle, and a majority of the light emitted by LED die  121  is reflected back. The conventional form of lens  191  reflects light approximately back to its source. Because the upper surface of LED die  121  has a reflectivity of about 50% as opposed to the 95% reflectivity of reflective material  135 , the conventional form of lens  191  shown in  FIG. 16  does not maximize the light extraction characteristics of light source  190 , which has a thick layer of reflective material  135  adjacent to LED die  121 . 
       FIG. 17A  shows a strip light source  192  with reflective material  135  adjacent to LED die  121  and a novel lens  193  having a dimple above the die. More light is able to exit light source  192  than light source  190  because the light that does not exit lens  193  on the first pass is more likely to be reflected back to the 95% reflective material  135  instead of to the 50% reflective upper surface of LED die  121 . Emitted light that is reflected back off the surface of lens  193  is directed away from the light&#39;s source on the LED die and towards the area adjacent to the die. In addition, light  194  that exits the side surfaces  128  of LED die  121 , is reflected up by material  135 , and then is reflected back down at the silicone/air interface of lens  193  is more likely to strike reflective material  135  than LED die  121 . Dimple shaped lens  193  improves the light extraction characteristics of light sources with large second order effects of light being reflected back off the lens surface where the LED die has a much lower reflectivity than the material surrounding the die. 
       FIG. 17B  shows structures on the surface of lens  193  that improve light extraction. Although the largest plurality of light originates from the center of the upper surface of LED die  121 , the light that strikes lens  193  originates from multiple locations, including the many phosphor particles  140 , the many light scattering particles  137  and many locations over the upper surface of LED die  121 . Thus, light strikes each location of lens  193  from many different angles. The light that strikes a structured surface of lens  193  instead of a smooth surface is more likely to exit lens  193  by finding a normal angle that exhibits a lower total internal reflection (TIR).  FIG. 17B  shows three types of structured surfaces of lens  193  that improve light extraction. The surface of lens  193  can have a small sinusoidal wave structure  195 , a “rectified” wave structure  196  or a saw-tooth structure  197 . In the actual implementation, one micro-structure covers the entire surface of lens  193 . Each of these micro-structures can be two-dimensional or three-dimensional. For example, ridges of the two-dimensional saw-tooth structure  197  can extend laterally across the light source strip. Or a three-dimensional saw-tooth micro-structure  197  would result in pyramids across the surface of lens  193 . The three-dimensional “rectified” wave structure  196  would result in small hemispheres across the surface of lens  193 . 
       FIG. 18  is a perspective view of strip light source  192  with dimple-shaped lenses  193 . When light source  192  is attached to the inside surface of a troffer, optics layer  139  and reflective material  135  is peeled back to expose (at  198 ) the power and ground traces on the flexible plastic substrate  142 . The power and ground traces are then attached to the power and ground wires of the troffer. 
     Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.