Abstract:
A display segment includes a carrier, a first light-emitting diode attached to the carrier, and a second light-emitting diode attached to the carrier. An intermediate reflector structure is disposed on the carrier between the first light-emitting diode and the second light-emitting diode. The intermediate reflector structure has a first intermediate reflector wall proximate to the first light-emitting diode and a second intermediate reflector wall proximate to the second light-emitting diode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       REFERENCE TO MICROFICHE APPENDIX  
       [0003]     Not applicable.  
       BACKGROUND OF THE INVENTION  
       [0004]     Light-emitting devices, such as light-emitting diodes (“LEDs”) used in LED displays, are packaged to facilitate handling and incorporation into electrical devices. LED dice are generally attached to a carrier, such as a printed circuit board, leadframe, or a flexible circuit, and electrically coupled the carrier with wire bonds. In some instances, the die is electrically and mechanically attached to the carrier, such as by soldering, conductive epoxy, or eutectic die attach, and a single wire bond from the top of the die to a trace on the carrier completes the electrical connection between the carrier and the LED die.  
         [0005]     A plastic housing with a cavity that confines and directs the light output from the LED dice is typically placed on the carrier, with a number of LEDs being inside the cavity. The plastic housing is commonly called a “reflector” because it has reflective areas that direct the light from the LEDs in a desired direction, i.e. away from the carrier and dice. The cavity is usually filled or partially filled with optical-grade epoxy resin. Each cavity is commonly called a “segment” of the display.  
         [0006]     Multiple dice are placed within a single cavity when a physically larger segment is desired. For example, a segment of a display for a wrist watch may be relatively small, while a segment of a display for a microwave oven or other household appliance is typically larger. Adding additional LEDs to a segment increases the total amount of light output by the segment, which is desirable for making brighter, large-format displays.  
         [0007]     However, when two, three, or more LED dice are placed within a single cavity, the light emitted by that segment is often not uniform. Brightness is highly concentrated in certain areas, and appears as uneven brightness through out the segment. An area of a segment of an LED display with greater brightness is called a “hot spot.”  FIG. 1A  is a cross section of a conventional display segment  100  with two LED dice  102 ,  104 . The LED dice  102 ,  104  are mounted on a carrier  106 , and the tops of the LED dice  102 ,  104  are electrically connected to wire traces (not shown) on the carrier  106  with wire bonds  108 ,  110 . The LED dice  102 ,  104  sit within a cavity  111  formed by a reflector  112  that is made of plastic and has reflective surfaces  114 ,  116  around the perimeter of the cavity  111 . The remainder of the cavity is filled with an encapsulant  118 , such as optical epoxy, silicon other organic or inorganic material. Alternatively, the cavity is not filled with a solid material, but is left as an air gap.  
         [0008]      FIG. 1B  shows the relative light intensity across the cavity of the display segment of  FIG. 1A  according to a modeled simulation. Hot spots  120 ,  122  occur essentially where the LED dice are located. A region of relatively low intensity  124  occurs between the hot spots  120 ,  122 . For some display applications, such hot spots are unacceptable.  
         [0009]      FIG. 2A  is a cross section of a display segment  130  with an additional LED die  132  in the cavity  111 ′, which was one approach that was tried to reduce the hot spot regions of  FIG. 1B .  FIG. 2B  shows the relative light intensity across the cavity of the display segment of  FIG. 2A . The extra LED die fills in the low intensity region and reduces the perceived appearance of hot spots in the segment, but increases electrical power requirements for the segment, heat sinking, and cost by adding the additional relatively expensive LED die and associated die attach and wire bond.  
         [0010]     Another approach to reducing hot spots is to include a diffusant in the epoxy used to encapsulate the LED dice and bond wires. Diffusants are made up of small particles that internally reflect and scatter incident light rays. This diffuses the light from hot spots, resulting in a more uniform intensity across the cavity of the segment. However, significant loss and absorption of light by the diffusant particles results in less light exiting from the segment.  
         [0011]      FIG. 3  is a cross section of a display segment  140  that illustrates how light rays, represented by arrows from the LED die  102 , travel through diffusant-loaded encapsulant  118 ′. Some of the light rays, such as the light ray  142 , have to travel a relatively long way before it exits the diffusant-loaded encapsulant  118 ′. This results in loss of light intensity from the display segment  140 .  
         [0012]     Another approach to reducing hot spots is to deepen the cavity so that beam spreading from the LED dice produces a more uniform intensity across the segment. However, deepening the cavity results in a bigger packaged display, more material costs for the reflector, and more material costs for the encapsulant. Additionally, a deeper cavity means that all light from the LED dice has to travel through more encapsulant than a similar, shallower, cavity. This results in more light being absorbed and/or backscattered, particularly if the encapsulant includes diffusant, and less light being provided by the display segment.  
         [0013]     It is desirable to reduce the formation of hot spots in LED display segments having multiple LEDs, and is further desirable to provide display segments having improved brightness.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     A display segment includes a carrier, a first light-emitting diode attached to the carrier, and a second light-emitting diode attached to the carrier. An intermediate reflector structure is disposed on the carrier between the first light-emitting diode and the second light-emitting diode. The intermediate reflector structure has a first intermediate reflector wall proximate to the first light-emitting diode and a second intermediate reflector wall proximate to the second light-emitting diode. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1A  is a cross section of a conventional display segment with two LED dice.  
         [0016]      FIG. 1B  shows the relative light intensity across the cavity of the display segment of  FIG. 1A .  
         [0017]      FIG. 2A  is a cross section of a display segment with an additional LED die in the cavity.  
         [0018]      FIG. 2B  shows the relative light intensity across the cavity of the display segment of  FIG. 2A .  
         [0019]      FIG. 3  is a cross section of a display segment that illustrates how light rays travel through diffusant-loaded encapsulant.  
         [0020]      FIG. 4A  is a cross section of a display segment for use in an LED display according to an embodiment of the invention.  
         [0021]      FIG. 4B  shows the relative light intensity across the cavity of the display segment of  FIG. 4A .  
         [0022]      FIG. 4C  is a cross section of the display segment of  FIG. 4A  showing paths of light rays from the LED die.  
         [0023]      FIG. 5A  is a plan view of a display segment according to an embodiment of the invention.  
         [0024]      FIG. 5B  is an isometric view of the display segment of  FIG. 5A  showing the common carrier.  
         [0025]      FIG. 6  is a plan view of a display segment according to another embodiment of the invention.  
         [0026]      FIGS. 7A-7H  are cross sections of embodiments of intermediate reflector structures. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0027]      FIG. 4A  is a cross section of a display segment  400  for use in an LED display according to an embodiment of the invention. The display segment  400  includes two LED diodes  102 ,  104 , which in this embodiment are chips, also known as dice, mounted on a carrier  106 . The carrier  106  is a ceramic base, printed circuit board, or a lead frame, for example. The LED dice  102 ,  104  sit within perimeter reflector walls  412 ,  414 . An intermediate reflector structure  416  transects the cavity  411  between the LED dice  102 ,  104 , forming two reflective compartments, one for each LED dice. The intermediate reflector structure  416  has intermediate reflector walls  418 ,  420  that cooperate with the perimeter reflector walls  412 ,  414 , respectively, to increase to light output from the display segment  400  and to decrease hot spots (see  FIG. 4C , below). The intermediate reflector walls  418 ,  420  are painted with a white light reflecting paint, such as paint having titanium oxide (TiO 2 ) pigment, to reflect those light rays less than the critical angle that otherwise might escape an unpainted wall. The paint layer is relatively thin, and does not appear separately in this view. The substrate is made of ceramic, printed-circuit board (“PCB”) material, or is a lead frame. In some embodiments, the reflectors are made of polycarbonate material. The reflector structure is coated or plated with aluminum, silver or nickel, for example, or is painted with a white or metallic paint. Alternatively, the material of the reflector is a reflective material, or is loaded with a reflective material, such as polycarbonate loaded with titanium oxide.  
         [0028]     In some embodiments, the height of the intermediate reflector structure  416  (i.e. the maximum height as measured from the surface of the carrier that the LED dice are mounted on) is less than the height of the perimeter reflector walls. This provides better light uniformity from adjacent LED dice in certain applications. Having a lowered intermediate reflector structure also makes it less apparent to the end user, thus enhancing the cosmetic appearance of a display segment according to the invention when used with conventional display segments. Having a lowered intermediate reflector structure also allows filling the remainder of the cavity (i.e. that portion not occupied by the LED dice, intermediate reflector structure and wire bonds) with a single application of encapsulant.  
         [0029]     In some embodiments, the intermediate reflector structure  416  is integrated with a reflector  422 , which is made of plastic and then metalized or painted to form the reflective walls. In a particular embodiment the intermediate reflector structure and the perimeter reflector walls are injection-molded together. Alternatively, an intermediate reflector structure and a perimeter reflector are two components that are assembled on the carrier, which allows adding an intermediate reflector structure to conventional display cavities to result in a display segment with improved intensity and reduced hot spots.  
         [0030]      FIG. 4B  shows the expected relative light intensity across the cavity  411  of the display segment  400  of  FIG. 4A . The light intensity is substantially similar to that of  FIG. 1B  except for the region between the LED dice. The light intensity of the conventional display segment between the LEDs of the display segment of  FIG. 1A  is shown in a dashed line  430 . The total light produced by the display segment  400  of  FIG. 4A  is the area under the intensity curve  432 . Thus, the light from the display segment  400  of  FIG. 4A  is greater than the light from the conventional display segment  100  of  FIG. 1A  by the area  434  between the curves  430 ,  432 . Similarly, the light intensity is more uniform between the LED dice of the embodiment of  FIG. 4A , essentially eliminating the hot spots (see  FIG. 1B , ref. nums.  120 ,  122 ) in the cavity of the segment.  
         [0031]      FIG. 4C  is a cross section of the display segment  400  of  FIG. 4A  showing paths of light rays, represented by arrows, from the LED die  102 . The light ray  442  travels a much shorter path through the encapsulant  118  than a similar ray  442 ′ would travel if the intermediate reflector structure  416  were missing. A shorter path through the encapsulant  118  means that less light from the LED die  102  is absorbed and/or scattered. The angle and shape of the reflector structure are chosen according to the type of light source (e.g. LED) used, the cavity size, the placement of the die in the cavity, the encapsulant type, and the application of the display segment.  
         [0032]      FIG. 5A  is a plan view of a display segment  500  according to an embodiment of the invention. An intermediate reflector structure  502  extends between each of the LED diodes  504 ,  506 ,  508 ,  510 , which are mounted on a common substrate (not shown in this view). The intermediate reflector structure is integrated with a perimeter reflector structure  512  to surround individual LEDs with reflective walls.  
         [0033]      FIG. 5B  is an isometric view of the display segment  500  of  FIG. 5A  showing the common carrier  514 . In other words, each of the LED diodes in the display segment  500  is mounted on the same carrier  514 . Electrical leads (not shown) extend from the bottom and/or sides of the carrier. In one embodiment, each LED is independently controllable to allow setting the light output of each LED to a desired level. Alternatively, two or more of the LEDs in a segment share an electrical connection. In a particular embodiment, all of the LEDs in a segment share an electrical connection. The display segment  500  is an electrical component and several display segments are typically assembled to create a display.  
         [0034]      FIG. 6  is a plan view of a display segment  600  according to another embodiment of the invention. Three LED diodes  602 ,  604 ,  606  are mounted on a carrier (not shown in this view). A reflector  608  includes a perimeter reflective wall  609 , and an intermediate reflector structure  610  that separates each LED dice from each other, and operates in conjunction the perimeter reflective wall  609  to surround each of the LEDs mounted on the carrier with reflective walls. The height of the intermediate reflector structure  610  is the same as the height of the perimeter reflective wall  609 . Alternatively, the height of the intermediate reflector structure  610  is less than the perimeter reflective wall  609 .  
         [0035]      FIGS. 7A-7H  are cross sections of embodiments of intermediate reflector structures.  FIG. 7A  shows an intermediate reflector structure  700  with straight walls  702 ,  704  that meet at an apex  706 . The angle of the walls is selected by width of the base  708  according to the available space between LEDs in the cavity (see  FIG. 4A ).  FIG. 7B  shows an intermediate reflector structure  710  with straight walls  702 ′,  704 ′ and a truncated end  706 ′. An intermediate reflector structure in accordance with  FIG. 7B  was modeled to obtain the simulation results shown in  FIG. 4B .  FIG. 7C  shows an intermediate reflector structure  720  with concave reflective sidewalls  722 ,  724  that meet at an apex  726 . The concave reflective sidewalls are shaped as a portion of a circle, ellipse, parabola, or hyperbola, for example. In one embodiment, each sidewall is similarly curved. Alternatively, one sidewall is curved differently from the other, either by having a different radius, arc, or shape. In a particular embodiment, one sidewall is convex, and the other is concave.  
         [0036]      FIG. 7D  shows an intermediate reflector structure  720 ′ having concave reflective sidewalls  722 ′,  724 ′ that do not meet. The top  726 ′ of the intermediate reflector structure is truncated, similarly to  FIG. 7B .  FIGS. 7E and 7F  show intermediate reflector structures  730 ,  730 ′ with convex reflective sidewalls  732 ,  734 ,  732 ′,  734 ′.  FIG. 7G  is an intermediate reflector structure  740  with a hemi-spherical reflective wall  742 .  FIG. 7H  is an intermediate reflector structure  750  with a half-ellipsoid reflective wall  752 .  
         [0037]     Different shapes and cross-sections of intermediate reflector structures are used in different applications. For example, concave reflective walls, such as are shown in  FIGS. 7C and 7D  are desirable when high brightness of the segment, as viewed from the front, is desired. In a particular embodiment, the concave reflective walls are essentially parabolic. In other applications, such as when good brightness is desired over a wide viewing angle, a convex reflective wall may be more desirable.  
         [0038]     While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to these embodiments might occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.