WO2013104963A1

WO2013104963A1 - Light emitting array with improved light output efficiency

Info

Publication number: WO2013104963A1
Application number: PCT/IB2012/057514
Authority: WO
Inventors: Qingwei Mo
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-01-10
Filing date: 2012-12-20
Publication date: 2013-07-18
Also published as: TW201334238A

Abstract

In an array of LEDs on a substrate, a reflective layer is disposed in the regions of the substrate surrounding the individual LEDs. This reflective layer serves to reflect light that is internally reflected from the optics and other surfaces that are common to the array. This reflective layer may also serve as an under-fill to provide structural support to each of the individual LEDs. The reflective layer may also be shaped to provide or enhance a particular output radiation pattern. Likewise a portion of the reflective surface may be arranged to diffuse or mix light from the array.

Description

LIGHT EMITTING ARRAY WITH IMPROVED LIGHT OUTPUT EFFICIENCY

FIELD OF THE INVENTION

This invention relates to the field of light emitting devices, and in particular to a reflective underfill that reduces optical loss in an array of light emitting devices. BACKGROUND OF THE INVENTION

The increased use of solid-state light emitting devices (LEDs) for lighting

applications has created a highly competitive market in which cost and lighting efficiency are predominant factors. Techniques that reduce the cost of the device, as well as techniques that reduce the subsequent manufacturing and assembly costs are highly desirable, as are techniques that improve the light output efficiency of the devices and/or the lighting assembly.

Arrays of light emitting devices are commonly used to provide a mix of colors, to increase the output luminance, or both. FIGs. 1A-1B illustrate views of an example lighting device that includes an array of light emitting devices 110A-D on a submount 150. Each light emitting device 110A-D may include an individual optical lens 120A-C, and the entire array may be covered by a lens 140 that is common to the array.

The substrate 150 includes a patterned electrode layer 160 that may include segments 165, 166, 167 that serve to interconnect the LEDs 110A-D, and pads 162 A, 162B that facilitate external power connections to the array. In this example, the LEDs 110A-D are coupled in series between two pads 162A-B, although more complex arrangements are also common. Each of the LEDs 110A-D include contacts 115A-116A, 115B-116B for coupling to the segments of the electrode layer 160, as illustrated in FIG. IB. Although the example of FIGs. 1A-1B includes circuit traces on the substrate to couple the LEDs to the electrode layer 160 and to each other, one of skill in the art will recognize that other connection techniques, such as wire bonding, within-substrate routing, and so on, may be used.

Optionally, the region beneath each LED 110A-D may be filled with a non- conductive under- fill material 130 that provides structural support to the LED 110A-D in the regions surrounding the contacts 115A-116 A, 115B-116B. SUMMARY OF THE INVENTION

Although each of the individual LEDs 110A-D of an array may be structured to provide an optimal output luminance, the presence of the common lens 140 may result in a total light output that is significantly less than the sum of the individual LED outputs. As illustrated in FIG. 2, most of the light output 210 is transmitted 215 through the lens 140. However, some of the light 220 is reflected 225 from the lens 140, possibly as a result of internal reflection at the interface of lens 140 and the surrounding air, and does not exit the lens 140.

It would be advantageous to improve the light output efficiency of an array of light emitting devices with a common array lens.

To better address one or more of these concerns, in an embodiment of this invention, in an array of LEDs on a substrate, a reflective layer is disposed in the regions of the substrate surrounding the individual LEDs. This reflective layer serves to reflect light that is internally reflected from the optics, e.g. lens 140, and other surfaces that are common to the array. This reflective layer may also serve as an under- fill to provide structural support to each of the individual LEDs. The reflective layer may also be shaped to provide or enhance a particular output radiation pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein:

FIGs. 1A-1B illustrate an example prior art lighting device comprising an array of LEDs with a lens that is common to the array.

FIG. 2 illustrates an example radiation pattern from an array of LEDs with a common lens. FIGs. 3A-3B illustrate example arrays of LEDs with a reflective coating that enhances the light output from the common lens.

FIGs. 4A-4E illustrate an example flow for creating an array of LEDs with a reflective coating that enhances the light output from the common lens.

Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention. DETAILED DESCRIPTION

In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

FIGs. 3A-3B illustrate example arrays of LEDs 1 10 with a reflective coating 330 that enhances the light output from the common lens 140. For the purposes of this invention, the term 'array' is used herein to indicate a plurality of LEDs that are arranged in a particular configuration to achieve a desired light output. The term 'array' is not intended to limit the term to any particular arrangement of the devices 110 on the substrate 150.

The coating 330 is configured to reflect light 320 that is reflected from the lens 140, thereby allowing the re-reflected light 325 to exit 327 the lens 140 as light 327. Other light exits the array without reflecting from coating 330.

In FIG. 3A, a uniformly thick layer of reflective material 330 occupies regions on the substrate surrounding each light emitting device 110. In a preferred embodiment, the material 330 is selected for providing sufficient structural support beneath each LED 110, obviating the need for a separate under-fill process. Optionally, the material 330 may include a conventional under-fill material that is tinted or otherwise processed to be reflective. For example, a white powder material, such as Ti0₂, may be mixed with a conventional under-fill material before application, or applied as a top layer while the under-fill material on the substrate is still tacky. Other techniques for forming the reflective layer 330 may be used as well. Likewise, layer 330 may not be thick, for example a thin coating may be applied to existing support structures such as an under-fill material. In FIG. 3B, the reflective material 330 is shaped to form a bowl-like structure that surrounds the array of LEDs 1 10. The walls 335 of the bowl- like structure extend above the level of the LEDs 110, and serve to reflect light 310 that hit the walls 335 toward 325 the common lens 140, increasing light output 317 in a desired direction. As illustrated in FIG. 3B, such a structure will provide a more collimated light output by redirecting the light output that would otherwise be emitted from the sides of the lens 140 in FIG. 3 A.

The walls 335 may be formed by a conventional molding process, and may be formed when the material 330 is placed in the regions surrounding the individual LEDs 110, or formed as a separate process, after the initial layer of material 330 is formed on the substrate. In like manner, the walls 335 may be pre-formed on the substrate 150 before the LEDs 110 are placed on the substrate. In such an arrangement, the regions surrounding the LEDs may also be pre-formed on the substrate, producing cavities in which the LEDs 110 may be placed. Alternatively, the LEDs 110 may be placed on the substrate 150 within the preformed walls 335, followed by forming the material 330 in the regions surrounding the LEDs within the pre- formed walls 335.

In the example of FIG. 3B, the common lens 140 is illustrated as extending between the peaks of the wall 335 of the structure. In this embodiment, the walls 335 will facilitate the formation of the lens 140 by forming the bowl that will form the sides of the lens 140. One of skill in the art will recognize, however, that the lens 140 may be any of a variety of forms, and may, for example, extend to the extremes of the coating 330, encompassing the entirety of the walls 335.

One of skill in the art will recognize other techniques that may be used to form a reflective structure around an array of LEDs in view of this disclosure. In like manner, one of skill in the art will recognize that other structures may be formed to achieve a desired output radiation pattern, including sub-structures within the regions between the LEDs.

Although a bowl structure is shown, other arrangements of the a reflective surround such as angled walls, diffusing reflective surfaces and light mixing surfaces alone or in combination with each other or a bowl shape, are contemplated and are included within the scope of this invention. In the above examples, the reflective layer 330 is formed in the regions surrounding fully-formed light emitting devices 110. Alternatively, the forming of the reflective layer 330 may be included within an integrated process for forming an array of devices on a substrate, as illustrated in FIGs. 4A-4E. Of particular note, by integrating the formation of the reflective layer 330 into this process, the layer 330 may be formed before the formation of the individual lenses 120, simplifying the formation of the layer 330.

In a conventional 'flip-chip' process, the light emitting device is formed/grown on a growth substrate, with the contacts to the light emitting device on the upper surface of the device. The chip comprising the growth substrate and the light emitting device is connected, upper-surface down, to the substrate, and then the growth surface is removed, exposing the light emitting surface of the device.

FIG. 4A illustrates light emitting devices 410 formed on growth substrates 450, and bonded to the segments of the electrode layer 160 on the substrate 150. A layer of material 330 is also illustrated as covering these structures 410-450, although the material 330 could be selectively applied to the areas surrounding each structure 410-450.

At FIG. 4B, the material 330 is shaped to its desired form, in this example, to a plane that is substantially coincident with the plane of the upper surfaces of the light emitting devices 410.

The growth substrate 450 is subsequently removed. One of skill in the art will recognize that the shaping of the material 330 and the removal of the substrate 450 may be part of a common process. For example, the material 330 may be formed to a desired shape, using, for example, a molding process that accommodates for the presence of the substrate 450, followed by removal of the growth substrate 450. If required, the surfaces of the material 330 may then be planed to a common level with the surfaces of the devices 410.

The light emitting surfaces of the devices 410 may also be processed to improve the light extraction efficiency, typically be roughening the surface. It is also significant to note that because the reflective material 330 extends up the sides of the devices 410 to the light emitting layer, it also serves to prevent emission of light from the sides of the devices 410, further improving the light output efficiency.

Optionally, at FIG. 4C, an additional layer 460 of material, such as a wavelength conversion material, may be added to one or more of the light emitting devices 410 to achieve a particular mix of color from the device 410. At FIG. 4D, the individual lenses 420 are formed above the light emitting devices 410. Although illustrated as a hemispherical dome above each device 410, the lenses 420 may be of any desired shape, and different shaped lenses may be applied to different devices 410 to achieve a desired light output pattern from each device 410. In this example, the lenses 420 extend beyond the area of the light emitting device 410, and the reflective material 330 will serve to also reflect light that is internally reflected within the lens 420.

At FIG. 4E, the common lens 140 is formed above the array of devices 110. As noted above, because the surface of the material 330 is reflective, light that is internally reflected from the common lens 140 and strikes the surface of the material 330 will be reflected back toward the common lens 140, thereby improving the light output efficiency of the array of LEDs 410.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

For example, in the above examples, the array structures are illustrated as being formed on a single substrate 150. One of skill in the art will recognize that an embodiment of the invention may be applied to a larger substrate that is subsequently partitioned

('singulated') into individual substrates 150, each containing an array of light emitting devices. That is, the larger substrate may include multiple copies of the electrode patterns 160, and an array of LEDs may be connected to each copy. The reflective material and lenses may be formed on this larger substrate, then each copy, with attached array, lenses, and reflective material, may be singulated to form an individual lighting device.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A lighting device comprising:

a substrate that includes an electrode layer,

an array of light emitting devices coupled to the electrode layer on the substrate, a lens that is common to the array of light emitting devices, and

a coating that occupies regions surrounding each light emitting device on the substrate and includes a reflective surface that serves to reflect light toward the lens.

2. The device of claim 1, wherein the coating includes a material that extends beneath each light emitting device and serves to support the light emitting device.

3. The device of claim 1, wherein the electrode layer includes one or more pads that facilitate coupling the array to an external power source.

4. The device of claim 1, wherein the reflective surface is at a plane that is substantially coincident with light emitting surfaces of the array of light emitting devices.

5. The device of claim 1, wherein at least a portion of the reflective surface extends above a plane of light emitting surfaces of the array of light emitting devices.

6. The device of claim 1, wherein the coating includes a wall that surrounds the array and extends above a plane of light emitting surfaces of the array of light emitting devices.

7. The device of claim 1, including a wavelength conversion material that is configured to convert light at a first wavelength from at least one of the light emitting devices to light at a second wavelength.

8. A method comprising:

providing a substrate with an electrode layer,

connecting each light emitting device of an array of light emitting devices to the electrode layer on the substrate,

applying a coating to the substrate to occupy regions surrounding each of the light emitting devices, the coating providing a reflective surface between the light emitting devices of the array, and

forming a lens that encompasses the array of light emitting devices.

9. The method of claim 8, including forming an individual lens for each of the light emitting devices of the array.

10. The method of claim 8, wherein the coating extends beneath each light emitting device and serves to support the light emitting device.

11. The method of claim 8, wherein the electrode layer includes one or more pads that facilitate coupling the array to an external power source.

12. The method of claim 8, wherein the reflective surface is at a plane that is substantially coincident with light emitting surfaces of the array of light emitting devices.

13. The method of claim 8, wherein the refiective surface extends above a plane of light emitting surfaces of the array of light emitting devices.

14. The method of claim 8, wherein the coating includes a wall that surrounds the array and extends above a plane of light emitting surfaces of the array of light emitting devices.

15. The method of claim 8, including applying a wavelength conversion material that is configured to convert light at a first wavelength from at least one of the light emitting devices to light at a second wavelength.

16. The method of claim 8, wherein applying the coating includes molding the coating.

17. The method of claim 8, wherein applying the coating is performed prior to connecting the light emitting devices to the electrode layer.