Abstract:
A light emitter includes a planar supporting surface, a light source positioned on the spreader region, and an encapsulant positioned on the spreader region to surround the light source. Except where constrained by adhesion to the planar supporting surface, the encapsulant is capable of expanding and contracting in response to a change in temperature so that forces caused by differences in the coefficient of thermal expansion between the different components is minimized. One or more reflective elements can be positioned proximate to the light source to increase the light emitting efficiency of the light emitter. The reflective elements can include a reflective layer on the spreader region and/or a reflective layer on a portion of the encapsulant.

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 60/467,193 filed Apr. 30, 2003. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to light emitters and, more particularly, to light emitter packages with components arranged to withstand thermal stresses. 
     Description of the Related Art 
     Light emitters are an important class of solid-state devices that convert electrical energy to light. One such light emitter is a light emitting diode (LED) which generally includes an active region of semi-conductive material sandwiched between two oppositely doped regions. When a bias is applied across the doped regions, holes and electrons are injected into the active region where they recombine to generate light. The light can be emitted from the active region and through the surfaces of the LED. 
     LEDs are generally divided into classes depending on their power rating. Although there is no standard range for the different classes, low power LEDs typically have a power rating in the range of 0.1 Watts to 0.3 Watts, or lower, and high power LEDs typically have a rating in the range of 0.5 Watts to 1.0 Watt, or higher. 
     Conventional packaging for low power LEDs typically includes a reflector cup with the LED mounted at the bottom of the cup. Cathode and anode leads are electrically coupled to the LED to provide power. The cathode lead can extend through the reflector cup and the anode lead can be wire bonded. The main function of the reflector cup is to redirect light emitted in certain directions in order to control the far-field intensity pattern of the LED. The reflector cup can include a highly reflective surface finish and can be plate stamped or metal plated with a metal such as aluminum (Al) or silver (Ag). 
     The entire structure can be encased in a transparent, hard encapsulant such as a plastic or epoxy. The encapsulant serves a number of functions. One function is to provide a hermetic seal for the LED chip. In another function, light refracts at the encapsulant/air interface, so that the outside shape of the encapsulant can act as a lens to further control the intensity pattern of the LED. 
     One disadvantage of this packaging arrangement, however, is that the LED chip, the reflector cup, and the encapsulant each generally have different coefficients of thermal expansion (CTE). Hence, during operational heating cycles they expand and contract at different rates, which can place a high mechanical stress on the device. In particular, epoxies and silicones typically used for the encapsulant have a CTE that is very different from the CTE of metals or ceramics. The CTE mismatch can also be exacerbated by constraints imposed by the manufacturing flow, such as during epoxy curing. In addition, these packages do not dissipate heat from the LED chip efficiently as they lack good thermal properties. However, because the LED operates at low power, the amount of heat it produces is relatively low so that the differences in CTE do not result in unacceptable failure rates. 
     High power LEDs, however, are generally larger, use larger packaging components, and generate higher amounts of heat. As a result, the CTE mismatch has a much larger impact on reliability and if the low-power LED type packaging is used, the differences in CTE for the packaging components can result in unacceptable failure rates. One of the most common failures is fracturing or cracking of the encapsulant. 
     High power LED packages have been introduced having a heat spreader that serves as a rigid platform for the remainder of the components, and is made of a material with high thermal conductivity such as a metal or ceramic that helps to radiate heat away from the LED chip. A reflector cup is mounted to the platform with the LED chip mounted at the bottom of the cup. The LED chip is contacted by wire bonds from the rigid platform. The reflector cup, LED chip and wire bonds are encased in an optically clear material that provides environmental protection. To compensate for the different coefficients of thermal expansion (CTE) of the package components, the optically clear material can include a soft gel such as silicone. As the different components expand and contract through thermal cycles, the soft gel readily deforms and compensates for the different CTEs. 
     However, soft gel is not as robust as plastics, epoxies, and glass, and cannot be used in some harsh environments without a coating or cover to act as a hermetic seal, which adds complexity to the LED fabrication process. The soft gel also tends to absorb water, which can shorten the LED&#39;s lifespan. It is also more difficult to shape soft gels to control the emission pattern of the LED package. 
     Other high power LED packages have been introduced that utilize a hard epoxy encapsulant, with one such device not utilizing a reflector cup inside the encapsulant. Instead, a second region is included on the heat spreader, with a section of the second region stamped, molded or etched to form a depression that can be coated with a reflective material. The LED chip is then placed at the base of the depression and is contacted. A hard epoxy or silicone fills the depression, covering the LED and any wire bonds. This arrangement reduces, but does not eliminate, the fractures and cracking of the epoxy or silicone encapsulant. This arrangement can also suffer from a different problem of the epoxy or silicone encapsulant delaminating and peeling away from the surfaces of the depression through the LED&#39;s thermal cycles. 
     U.S. Pat. No. 6,274,924 to Carey et al. discloses another high power LED package that includes a heat sinking slug that is inserted into an insert molded leadframe. The slug can include a reflector cup with the LED chip and thermally conductive submount arranged at the base of the cup. Metal leads are electrically and thermally isolated from the slug. An optical lens is added by mounting a thermoplastic lens over the slug. The lens can be molded to leave room for a soft encapsulant between the LED and the inside surface of the lens. This invention claims to operate reliably under high power conditions, but is complex, difficult to manufacture, and expensive. The thermoplastic lens also does not survive high temperatures typically used for the process of soldering LEDs to a printed circuit board. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide LED packages that are particularly adapted to use with high power LEDs and are arranged to reduce the LED package failures due to the differences in the CTE for the package components. The LED packages are also simple, flexible, and rugged. 
     One embodiment of a light emitter according to the present invention comprises a substantially supporting surface, a light source positioned on the supporting surface, and an encapsulant positioned on the supporting surface. The encapsulant surrounds the light source and is capable of expanding and contracting in response to a change in temperature, constrained only by adhesion to said planar support surface. 
     Another embodiment of a light emitter according to the present invention comprises a heat spreader and a light source positioned in thermal contact with a substantially planar surface of the heat spreader. The heat spreader provides support for said light source and an encapsulant is positioned to surround the light source, with the encapsulant capable of expanding and/or contracting in response to a change in temperature constrained only by adhesion to said planar surface. A first reflective element is positioned to reflect light from the light source, the reflective element being integrated with at least one of the heat spreader and the encapsulant. 
     One embodiment of an optical display according to the present invention comprises a heat spreader with a substantially planar surface. A plurality of light emitters are positioned on the planar surface with each light emitter comprising a light source positioned in thermal contact with the heat spreader. An encapsulant is positioned on the heat spreader to surround the light source with the encapsulant being capable of expanding and contracting in response to a change in temperature constrained only by adhesion to said planar surface. Each light emitter comprises at least one reflective element positioned on the heat spreader and/or said encapsulants to increase the light emitting efficiency of the display. 
     One embodiment of a method of fabricating a light emitter includes providing a substantially planar supporting surface and providing a light source positioned on the substantially planar supporting surface. An encapsulant is provided positioned on the supporting surface and over the light source so that the encapsulant can expand and contract with changes in temperature constrained only by adhesion to said planar surface. 
     These and other further features and advantages of the invention would be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified sectional view of a light emitter according to the present invention; 
         FIG. 2  is a simplified sectional view of another embodiment of a light emitter according to the present invention; 
         FIG. 3  is a simplified sectional view of another embodiment of a light emitter according to the present invention having a shaped lens; 
         FIG. 4  is a simplified sectional view of another embodiment of a light emitter according to the present invention having a reflective surface on the shaped lens; 
         FIG. 5  is a simplified sectional view of another embodiment of a light emitter according to the present invention having a bullet shaped lens; 
         FIG. 6  is a simplified sectional view of another embodiment of a light emitter according to the present invention having a concave shaped lens; 
         FIG. 7  is a simplified sectional view of another embodiment of a light emitter according to the present invention having a mushroom shaped lens; 
         FIG. 8  is a simplified sectional view of another embodiment of a light emitter according to the present invention having a circular spherical shaped lens; and 
         FIG. 9  is a simplified flowchart illustrating a method of fabricating a light emitter according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates one embodiment of a light emitter  10  in accordance with the present invention. Emitter  10  includes a heat spreader  12  with a light source  14  positioned on and in thermal contact with the heat spreader region  12 . Spreader region  12  provides a support structure for holding light source  14  and is at least partially made of a high thermal conductivity material to facilitate heat flow away from light source  14 . The preferred heat spreader is made of a high thermal conductive material such as copper (Cu), aluminum (Al), aluminum nitride (AlN), aluminum oxide (AlO), silicon (Si), silicon carbide (SiC) or combinations thereof. 
     Light source  14  comprises an LED, although it can include other light emitters, such as a solid-state laser, a laser diode, or an organic light emitting diode, among others. Power to light source  14  can be provided from first and second wire bonds  16 ,  18  with a bias applied across the light source  14 , and in the embodiment shown the wire bonds apply a bias across oppositely doped layers of the LED light source to cause it to emit light. In other embodiments according to the present invention only one wire bond can be used, with the light source  14  also contacted through the spreader region  12 . In still other embodiments, the light source  14  is contacted only through the spreader region  12 . 
     Emitters according to the present invention can be included in systems designed to emit light either as a single light source or in a display. Emitters according to the invention can also include a single light source or an array of light sources which emit the same or different wavelengths of light. Emitter  10  and the emitters in the following figures are shown with one light source for simplicity and ease of discussion. It is understood, however, that emitters according to the present invention can be arranged in many different ways. 
     A transparent encapsulant  20  is positioned to surround light source  14  and is provided to encapsulate and hermetically seal light source  14  and wire bonds  16 ,  18 . Encapsulant  20  is typically positioned on the top surface of spreader region  12 . Encapsulant  20  can be made of many different hard and optically clear materials such as epoxy, silicone, glass, or plastic, and can be a pre-molded lens or formed directly over light source  14 . Pre-molded encapsulants or lenses can be fabricated using techniques, such as injection molding, and then bonded to heat spreader  12 . 
     The spreader region  12  can also include a reflective layer  22  on the same surface as the light source  14 , with the reflective layer  22  at least covering substantially all of the surface not covered by the light source  14 . In the embodiment shown, the reflective layer  22  covers the entire surface such that part of said reflective layer is sandwiched between the light source  14  and the spreader region  12 . Light source  14  emits light omnidirectionally with light paths  1 ,  2 ,  3 ,  4  and  5  representing a few of the possible light paths from the light source. Light paths  1 ,  2  and  3  extend from light source  14  and through encapsulant  20 . Light can also flow along light paths  4  and  5  which extend from light source  14  to the reflective layer  22  and through encapsulant  20 . Reflective layer  22  can reflect light from light source  14  to increase the optical efficiency of emitter  10 . Reflective layer  22  can comprise many reflective materials reflective at the wavelength of interest, such as aluminum (Al), silver (Ag), or a combination thereof. 
     Emitter  10  has many advantages, one being that it is less complex and, consequently, costs less than conventional devices. The complexity is reduced in one way by combining the reflector layer  22  with spreader region  12  which eliminates the need to have a reflector structure separate from encapsulant  20  and heat spreader  12 , which allows for a simplified manufacturing process. 
     Thermal stresses are also reduced because the reflector function is integrated with other components included in emitter  10 . Hence, there are fewer components expanding and contracting against each other at different rates. As a result, light source  14  can operate more reliably at higher power and, consequently, higher temperature with less risk of having emitter  10  fail. Another cause of failure can be the fracturing or cracking of encapsulant  20  associated with CTE mismatch between the different materials used. However, the probability of this happening is reduced by the arrangement of emitter  10 . The surface between encapsulant  20  and spreader region  12  around the emitter  10  is substantially planar so that at the interface between the encapsulant  20  and the spreader region  12  around the emitter  10 , encapsulant  20  is only constrained at this substantially planar surface. The encapsulant  20  can also be constrained at the surfaces of said emitter. This puts less stress on wire bonds  16  and/or  18  which can cause them to break or loosen and reduce the useful lifetime of emitter  10 . 
     Encapsulant  20  can include hard and high melting point materials, such as glass, to provide a package which is hermetically sealed because the curing process and temperature cycles associated with these materials is no longer a problem. Emitter  10  also provides for greater flexibility in the choice of materials which can be used for encapsulant  20  and spreader region  12  because they can be matched for adhesion. Hence, the probability of encapsulant  20  delaminating and peeling away from spreader region  12  through the emitter&#39;s thermal cycles is reduced. 
     Another advantage is that light emitter  10  has a smaller footprint so that an array of packages can be positioned closer together. This feature is useful in light displays where it is typically desired to position the packages close together in an array to increase resolution and display quality. 
       FIGS. 2 through 8  illustrate additional embodiments of light emitters in accordance with the present invention. It should be noted that the emitters illustrated in the rest of the disclosure include components similar to the components illustrated in  FIG. 1  and similar numbering is used with the understanding that the discussion above in conjunction with emitter  10  applies equally well to the emitters discussed in  FIGS. 2 through 8 . 
       FIG. 2  illustrates another embodiment of a light emitter  30  in accordance with the present invention. Emitter  30  includes spreader region  12  and can include a reflective layer  22 . Light source  14  is positioned on reflective layer  22  and an encapsulant  40  is positioned to encapsulate and seal light source  14 . Encapsulant  40  is shaped around its base to provide an angled surface  42  that reflects sideways directed light emitted from light source  14  by total internal reflection. 
     Light paths  6 ,  7  show two possible light paths from the light source  14 , both of which are incident to surface  42 . Light paths  6  and  7  can be reflected by total internal reflection (TIR) by surface  42  toward the top of encapsulant  40  along respective light paths  8  and  9 . This reduces the light that is emitted out the sides of encapsulant  40  and increases the light emitted out of the top. As a result, emitter  30  can produce more focused light with better light emission efficiency. It should be noted that light emitted from light source  14  can also be reflected from reflective layer  22  and through encapsulant  40 , either directly or indirectly off of surface  42  to further enhance emission efficiency. Light emitter  30  includes all of the features of emitter  10  described above, with the added advantage of more focused light, better optical efficiency. 
       FIG. 3  illustrates another embodiment of a light emitter  50  in accordance with the present invention, which is similar to emitter  30  in  FIG. 2 . Emitter  50  includes spreader region  12  with a reflective layer  22  on the spreader region  12 . A light source  14  is positioned on reflective layer  22  and an encapsulant  60  positioned to surround light source  14  and to provide hermetic sealing. Encapsulant  60  also comprises an angled surface  42  with reflective layer  64  applied to angled surface  42 . Support region  49  is positioned adjacent to second reflective layer  64  and spreader region  22 . 
     Second reflective layer  64  reflects most or all of the light incident on the angled surface  42  including the light that does not experience TIR and would otherwise pass through angled surface  42 . This further focuses the light from light source  14  toward the top of encapsulant  60  and increases the optical efficiency by increasing the amount of emitted light. Second reflective layer  64  can be made of different materials with different reflectivities, such as silver (Ag), aluminum (Al), titanium oxide (TiO), white resin, or combinations thereof. Second reflective layer  64  can be applied using many different methods such as painting, plating, or deposition and can also be applied before or after encapsulant  60  is positioned over light source  14 . An additional advantage of layer  64 , which is opaque to light, is that it allows optional barrier region  49  to be included for mechanical support and environmental protection without degrading the light efficiency of emitter  50 . The material used for region  49  should be chosen so that it does not constrain the encapsulant  60  under thermal cycling. 
       FIG. 4  illustrates another embodiment of light emitter  70  in accordance with the present invention, which is similar to emitter  10  of  FIG. 1 . Emitter  70  includes spreader region  12 , light source  14 , and a reflective layer  22 . Emitter  70  also comprises an encapsulant  80  that is a preformed lens having a cavity  81  in its base. Like the encapsulants described above, lens  80  can be made of an epoxy, silicone, glass, or plastic and can be fabricated using methods such as injection molding. Encapsulant  80  is mounted over light source  14  to the top surface of heat spreader  12  with light source  14  and wire bonds  16 ,  18  arranged in cavity  81 . A bonding material  82  fills the space in cavity  81  and holds lens  80  to heat spreader  12 . Different types of encapsulants can be used provided they are sized to fit on heat spreader  12  while providing a cavity for light source  14 , wire bonds  16 ,  18  and bonding material  82 . 
     Bonding material  82  can include different materials such as an epoxy, glue, or silicone gel. The index of refraction of bonding material  82  is preferably the same as that of encapsulant  80  to minimize reflections between the two materials and can be chosen to obtain a desired light emitting efficiency. Material  82  can be positioned in cavity  81  before encapsulant  80  is positioned over light source  14  or encapsulant  80  can be positioned in place and material  82  can be injected through encapsulant  80  or through a hole (not shown) in heat spreader  12 . The hole can then be sealed with a plug made from resin or a similar material. 
     This arrangement has the advantages of emitter  10  with added flexibility in the type and shape of encapsulant that can be mounted over light source  14  and heat spreader  12 . Different types of lenses can be used provided they are sized on the spreader region  12  while providing a cavity for the light emitter  14 , wire bonds  16 ,  18 , and the bonding material  82 . If silicone gel is used for material  82 , then it can compensate for differences in the CTE of the different materials. 
       FIG. 5  illustrates another embodiment of a light emitter  90  in accordance with the present invention. Emitter  90  includes spreader region  12 , light source  14 , and reflective layer  22 . Emitter  90  also includes a hard “bullet shaped” encapsulant  100 , which can be a pre-molded lens or an epoxy positioned over light source  14  and shaped. The shape of encapsulant  100  is chosen to refract light along light paths  1 ,  3 ,  4 , and  5  toward the top of emitter  90  as the light passes out of encapsulant  100  at a surface  121 . This light refraction helps to focus the light from light source  14 . Light that hits the surface of encapsulant  100  at exactly 90° (i.e. along light path  2 ) will not be refracted. 
       FIG. 6  illustrates another embodiment of a light emitter  110  according to the present invention which also includes a spreader region  12 , light source  14 , and reflective layer  22 . Emitter  110  also includes a “concave” shaped encapsulant  120  that more effectively reflects light internally toward the top of emitter  110  and can also more efficiently refract light passing out of the encapsulant  120  toward the top of the emitter  110 . Encapsulant  120  includes an angled surface  122  which is shaped in such a way to increase the focusing power of encapsulant  120  and the light emitting efficiency of emitter  110 . The angle and shape of surface  122  can be chosen to obtain a desired gain in focusing the light and to decrease any losses from TIR. The angled surface  122  can be straight or substantially straight and can be adjacent to the “concave” portion of the encapsulant  120 . The angled surface  122  and the “concave” portion of the encapsulant  120  can be external surfaces. 
       FIG. 7  illustrates another embodiment of light emitter  130  in accordance with the present invention that comprises spreader region  12 , light source  14 , wire bonds  16  and  18 , and reflective layer  22 . Emitter  130  also comprises a mushroom shaped encapsulant  140  having a dome  142  and angled stem  146 . Stem  146  can be covered by a second reflective layer  147  such that light from light source  14  that strikes stem  146  along light paths  6  and  7  is reflected toward dome  142  along respective light paths  8  and  9 . This arrangement also provides focused light and is more efficient because less light is lost to TIR. 
       FIG. 8  illustrates still another embodiment of a light emitter  150  in accordance with the present invention, which includes a spreader region  12 , light source  14 , and reflective layer  22 . Emitter  150  also includes a spherical shape encapsulant  160  that can also include a reflective region  64  on its lower hemisphere to reflect light along light paths  6  and  7  toward the top of encapsulant  160  along respective light paths  8  and  9 . This arrangement also provides focused light and has less TIR losses because of encapsulant  160  and reflective region  64 . It is also understood that the encapsulant can be many other detailed shapes in accordance with the present invention. 
       FIG. 9  illustrates a flowchart  200  for one embodiment of a method for fabricating a light emitter in accordance with the present invention. The method includes step  201  of providing a spreader region having at least one planar surface with a reflective layer on it, and step  202  includes providing a light source positioned on at least one planar surface. Step  203  comprises providing an encapsulant positioned on the planar surface of the spreader region and over the light source. By being planar the expansion and contraction of the encapsulant with changes in temperature is constrained only at the planar surface. 
     The encapsulant can be positioned so that it hermetically seals the light source, where the hermetic seal remains unbroken with changes in temperature. The encapsulant can be positioned so that the relative position of the encapsulant and light source remains unchanged with changes in temperature. The relative position will remain unchanged if there is nothing (i.e. a 3D reflector structure) for the encapsulant to push against as the temperature changes. 
     An optional step  204  comprises angling the surface of the encapsulant adjacent to the spreader region to increase the efficiency of the emitter by directing TIR light and refracted light toward the top of the emitter. 
     An optional step  205  comprises providing a second reflective element positioned on the angled surfaces to increase the emission efficiency of the emitter. The second reflective element can be formed by using one of painting, plating, and deposition. An optional step  206  can comprise providing a support region position adjacent to the second reflective element and the spreader region. A barrier region may then be positioned adjacent to the supporting surface and a base of the encapsulant. The barrier region can form a better seal for the light source. It should be noted that the steps illustrated in flowchart  200  can be performed in a different order and that different steps can be used in methods according to the present invention. 
     Although the present invention has been described in considerable detail with reference to certain preferred configurations thereof, other versions are possible. The lenses described above can have many different shapes and can be made of many different materials. Each of the light sources described above can further comprise a submount to provide protection from electrostatic discharge (ESD). In each embodiment above, the heat spreader can be etched to provide a hole to house the light source such that the light source does not extend above the top surface of the heat spreader. The encapsulant could then have a flat base to mount to the heat spreader, over the light source. 
     Therefore, the embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims.