US007968899B2
`
`(12) United States Patent
`Karim et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7968,899 B2
`Jun. 28, 2011
`
`(54)
`
`(75)
`
`(73)
`
`LED LIGHT SOURCE HAVING IMPROVED
`RESISTANCE TO THERMAL CYCLING
`
`Inventors: Norfidathul Aizar Abdul Karim,
`Seberang Prai (MY); Siew It Pang,
`Bayan Lepas (MY); Kheng Leng Tan,
`Bayan Lepas (MY); Tong Fatt Chew,
`Bayan Lepas (MY)
`Assignee: Avago Technologies ECBU IP
`(Singapore) Pte. Ltd., Singapore (SG)
`
`(*)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 410 days.
`
`(21)
`(22)
`(65)
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`(51)
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`(52)
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`(58)
`
`Appl. No.: 11/845,501
`Filed:
`Aug. 27, 2007
`
`Prior Publication Data
`US 2009/0057708 A1
`Mar. 5, 2009
`
`Int. C.
`(2010.01)
`HOIL 33/00
`U.S. Cl. ................ 257/98: 257/96; 257/97; 257/99;
`257/100; 257/103; 257/E51.018: 257/E51.022;
`257/E33.001; 257/E33.077; 257/E33.057;
`438/22:438/46; 438/47
`Field of Classification Search .................... 257/96,
`257/97, 98, 99, 100, 101, 102, 103, E51.018,
`257/E51.022, E33.001, E33.077, E33.057,
`257/E25.028, E25.032; 438/22, 46, 47
`See application file for complete search history.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`2004/0041222 A1
`3/2004 Loh ............................... 257/433
`2005, 0145991 A1* 7, 2005 Sakamoto et al. .
`... 257/604
`2005, 0162069 A1* 7, 2005 Ota et al. ...............
`... 313,501
`2005/0269.591 A1* 12/2005 Hsin Chen et al. ............. 257/99
`2006/017571.6 A1* 8, 2006 Nakashima ........
`257/787
`2006/0261366 A1* 1 1/2006 Yang ..................
`... 257/100
`2007/0096 128 A1* 5/2007 Fukudome et al. ............. 257/98
`2007/0262328 A1* 11/2007 Bando ............................. 257/79
`2008/0073662 A1* 3/2008 Wang et al. ..................... 257/99
`FOREIGN PATENT DOCUMENTS
`2005/259972
`9, 2005
`JP
`2007/070445
`3, 2007
`JP
`* cited by examiner
`Primary Examiner — Lynne A Gurley
`Assistant Examiner — Yosef Gebreyesus
`(57)
`ABSTRACT
`A light Source and method for making the same are disclosed.
`The light Source includes a Substrate, a die, and a cup. The
`substrate has a plurality of electrical traces thereon and the die
`includes an LED that is connected to two of the traces. The
`cup overlies the substrate and is filled with an encapsulant
`material. The die is located within the cup and is encapsulated
`by the Substrate and the encapsulant material. The cup and
`encapsulant material have Substantially the same coefficient
`of thermal expansion. The cup can include reflective side
`walls positioned to reflect light leaving the die. The cup,
`encapsulant and Substrate can be constructed from the same
`material.
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`21 Claims, 2 Drawing Sheets
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`25 26 27
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`Nichia Exhibit 1013
`Page 1
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`U.S. Patent
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`Jun. 28, 2011
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`Sheet 1 of 2
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`US 7968,899 B2
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`FIGURE 1
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`FIGURE 3
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`46 49 47
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`46 43
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`Nichia Exhibit 1013
`Page 2
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`U.S. Patent
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`Jun. 28, 2011
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`Sheet 2 of 2
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`US 7968,899 B2
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`FIGURE 4
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`FIGURE 5
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`Nichia Exhibit 1013
`Page 3
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`

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`1.
`LED LIGHT SOURCE HAVING IMPROVED
`RESISTANCE TO THERMAL CYCLING
`
`2
`Substrate includes a lead frame that is encapsulated in a body
`and the cup is provided as a recess in that body.
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`US 7,968,899 B2
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`BACKGROUND OF THE INVENTION
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`Light-emitting diodes (LEDs) are good candidates to
`replace incandescent and other light sources. LEDs have
`higher power to light conversion efficiencies than incandes
`cent lamps and longer lifetimes. In addition, LEDs operate at
`relatively low Voltages, and hence, are better adapted for use
`in many battery-powered devices. Furthermore, LEDs are a
`better approximation to point Sources than a fluorescent
`Source, and hence, are better adapted than fluorescent sources
`for lighting systems in which a point light source that is
`collimated or focused by an optical system is required.
`An LED can be viewed as a three-layer structure in which
`an active layer is sandwiched between p-type and n-type
`layers. Holes and electrons from the outer layers recombine in
`the active layer to produce light. Part of this light exits through
`the upper horizontal surface of the layered structure. Unfor
`tunately, the materials from which the outer layers are con
`structed have relatively high indices of refraction compared to
`air or the plastic encapsulants used to protect the LEDs. As a
`result, a considerable portion of the light is trapped within the
`LED due to internal reflection between the outer boundaries
`of the LED. This light exits the LED through the side surfaces.
`To capture this light, the LEDs are often mounted in a reflect
`ing cup whose sidewalls redirect the light from the sides of the
`LED into the forward direction. In addition, the cups are often
`filled with a clear encapsulant that protects the LED die and
`can provide additional optical functions such as having a
`surface that is molded to form a lens.
`Unfortunately, the packages must be able to withstand
`relatively high processing temperatures. AuSn eutectic die
`attachment can Subject the package to temperatures as high as
`320 degrees centigrade. In addition, LEDs designed for high
`power applications generate significant amounts of heat that
`result in further temperature cycling of the package when the
`LEDs are turned on and off. As noted above, the cups are
`typically filled with an encapsulant. The encapsulant material
`is different from the material from which the reflector is
`formed. As a result, the encapsulant material and the material
`from which the reflector is formed typically have different
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`coefficients of thermal expansion. In addition, the adhesion of
`the encapsulant to the reflector is often less than ideal. As a
`result, the encapsulant tends to delaminate from the cup after
`multiple thermal expansion cycles.
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`SUMMARY OF THE INVENTION
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`The present invention includes a light source and method
`for making the same. The light source includes a Substrate, a
`die, and a cup. The Substrate has a plurality of electrical traces
`thereon and the die includes an LED that is connected to two
`of the traces. The cup overlies the substrate and is filled with
`an encapsulant material. The die is located within the cup and
`is encapsulated by the Substrate and the encapsulant material.
`The cup and encapsulant material are made of the same mate
`rial base and have substantially the same coefficient of ther
`mal expansion. The cup has reflective sidewalls positioned to
`reflect light leaving the die. The cup and the encapsulant can
`be constructed from the same material. In one aspect of the
`invention, the Substrate is also constructed from the same
`encapsulating material. In another aspect of the invention, the
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`FIG. 1 is a cross-sectional view of one embodiment of a
`light Source according to the present invention.
`FIG. 2 is a cross-sectional view of a multi-LED package.
`FIG. 3 is a top view of the multi-LED package shown in
`FIG 2.
`FIG. 4 is a cross-sectional view of a portion of a light
`Source according to one embodiment of the present invention
`prior to the attachment of the reflective layer.
`FIG. 5 is a cross-sectional view of a portion of a light
`Source according to another embodiment of the present inven
`tion.
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`DETAILED DESCRIPTION OF EMBODIMENTS
`OF THE INVENTION
`
`The manner in which the present invention provides its
`advantages can be more easily understood with respect to
`FIG.1, which is a cross-sectional view of one embodiment of
`a light source according to the present invention. Light Source
`20 includes a die 25 having an LED fabricated thereon. The
`package includes a lead frame having leads 21 and 22 that are
`encapsulated in a molded body 23. Die 25 has two contacts for
`powering the LED. One of the contacts is on the bottom
`surface of die 25, and the other contact is on the top surface of
`die 25. Die 25 is connected electrically and thermally to lead
`21 which provides the connection to the bottom contact as
`well as aheat path for the removal of heat generated indie 25.
`The contact on the top surface of die 25 is connected to lead
`22 by wire bond 26.
`The package includes a cup 24 having reflective walls
`which serve two functions. First, cup 24 redirects light leav
`ing the side surfaces of die 25 so that the light leaves the light
`Source within a cone of angles that includes the light that
`leaves the top surface of die 25. The material from which the
`cup is constructed can include particles such as TiO, that
`render the walls of cup 24 “white', and hence, provide a
`diffuse reflector.
`Second, cup 24 provides a “mold that is filled with a
`transparent medium 27 that, together with the lead frame
`encapsulates die 25 and wire bond 26. The encapsulant pro
`tects die 25 and wire bond 26 from moisture and other envi
`ronmental attacks. In addition, the encapsulant improves the
`efficiency with which light is extracted from die 25. The
`material from which die 25 is constructed typically has a very
`large index of refraction. As a result, the difference in index of
`refraction between this material and air results in a significant
`fraction of the light generated in die 25 being trapped by
`internal reflection. Some of this trapped light eventually
`escapes through the side walls of the die after a number of
`reflections. However, a significant fraction is absorbed in die
`25. By providing a medium having an index of refraction that
`is intermediate between that of air and the index of refraction
`of the materials from which die 25 is constructed, the encap
`Sulant reduces the amount of light that is trapped by internal
`reflection. The encapsulant can be molded such that the top
`surface is curved. The curved surface further reduces the
`amount of light that is trapped within the light Source due to
`differences in the index of refraction of the die and that of the
`Surrounding air.
`Finally, the encapsulant provides a carrier that can be used
`to provide a layer of wavelength converting material that
`alters the spectrum of light emitted by die 25 to provide a light
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`Nichia Exhibit 1013
`Page 4
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`US 7,968,899 B2
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`Source having a more desirable output spectrum. LEDs emit
`light in relatively narrow bands of wavelengths. While this
`narrow band emission is useful in applications in which a
`light source having a single color is required Such as a red
`light source for the tail light of an automobile, in many appli
`cations of interest, a broadband light source is required. One
`method for broadening the output spectrum is to provide a
`phosphor that converts a portion of the light generated by the
`LED to light of a different color. The light source then has an
`output spectrum that is the sum of the phosphor emission
`spectrum and that of the LED itself. For example, a “white'
`LED can be constructed by providing a layer of phosphor that
`emits yellow light over an LED that emits blue light. The
`combination of blue and yellow light is perceived to be
`“white' light by a human observer. The phosphorparticles are
`Suspended in the encapsulant material while the material is in
`a liquid state. This slurry is then dispensed into cup 24 and
`cured in a time that is sufficiently short to prevent the settling
`of the particles out of the material.
`Prior art light sources of the configuration shown in FIG. 1
`utilize a base encapsulant material that is different from the
`material from which cup 24 is formed. The preferred encap
`Sulants are epoxies or silicones. The cup is typically formed
`from a metal or some other plastic Such as the plastic from
`which the body of the light source is molded. For example, the
`cup may be made of a liquid crystal polymer, while a first
`encapsulant layer covering the LED comprises a phosphor
`filled epoxy while a second encapsulant layer comprises a
`transparent epoxy positioned over the first layer. The different
`materials have different coefficients of thermal expansion and
`different degrees of adhesion due to major differences in
`chemical composition. Hence, stress is induced at the inter
`face of the two materials on the cup walls when the light
`Source is subjected to heating and cooling. Such thermal
`cycling can result from the Subsequent bonding of the com
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`pleted light source to a Substrate in a larger product that
`includes the light source. For example, infrared reflow sol
`dering can Subject the light Source to high temperatures for
`short periods of time. In addition, as the power outputs of
`LED light sources are increased to provide power levels con
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`sistent with space lighting applications, the heat generated in
`the die can result in the light source reaching high enough
`temperatures that the light source is thermally cycled each
`time the source is turned on and off. As a result of such
`thermally induced stress, the encapsulant can separate from
`the walls of cup 24 and subject die 25 and wire bond 26 to
`forces that cause the light source to fail. In addition, Such
`separation can lead to openings that allow moisture to reach
`die 25.
`The present invention overcomes the problems caused by
`the thermal cycling by utilizing an encapsulant that has Sub
`stantially the same coefficient of thermal expansion as cup 24.
`In addition, the delamination problems are substantially
`reduced by an encapsulant layer that is constructed from the
`same basematerial as the cup. Here, two materials will be said
`to have the same base material if the materials belong to a
`similar family of compounds or polymers and have similar
`chemical composition and thermal coefficients of expansion.
`Small amount of additives may be added to the base material
`composition Such as diffusing particles, fillers, stabilizers and
`phosphors. For example, a cup constructed from a molded
`epoxy is filled with an epoxy encapsulantin one embodiment.
`Similarly, a cup constructed from a silicone material is filled
`with a silicone encapsulant in another embodiment. In the
`case of an encapsulant that is augmented with phosphor par
`ticles or the like, the thermal coefficient of expansion of the
`encapsulant layer may not exactly match the thermal coeffi
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`cient of expansion of the cup. Similarly, the cup material can
`include particulates or other materials to render the cup
`reflective, and hence, the cup may also have a somewhat
`different thermal coefficient of expansion relative to the
`encapsulant layer. However, the differences in coefficients of
`thermal expansion are substantially less than those encoun
`tered when different base materials are utilized for the two
`components, and hence, the present invention provides a sig
`nificant improvement.
`The embodiments discussed above utilize a lead frame
`system. However, embodiments based on other forms of
`packaging can also be utilized. Refer now to FIGS. 2 and 3.
`FIG. 2 is a cross-sectional view of a multi-LED package, and
`FIG. 3 is a top view of that multi-LED package. Package 40
`includes three LEDs shown at 41-43 that are attached to a
`Substrate 44. Substrate 44 is an insulating Substrate having a
`plurality of conducting traces that terminate in pads 51 for
`providing connections between the LEDs and external circuit
`driving circuits. The number of such pads and traces depends
`on the particular circuit configuration, the number of LEDs,
`and other design criteria. The LEDs are connected to the
`conducting traces by wire bonds 47 and/or conducting pads
`on the bottom of the LED dies. The LEDs are located in
`reflecting cups such as cups 48-50 formed in layer 46 having
`an inner Surface that is typically coated with a highly reflec
`tive material such as A1 or layer 46 can include white particles
`that render the walls white thus providing a reflector with a
`matte finish.
`As discussed above, the interior of the cup is typically filled
`with an encapsulating material that protects the LEDs and any
`wire bonds. Layer 46 is constructed from the same base
`material as the encapsulant. For example, layer 46 can be
`constructed from a sheet of silicone in which holes have been
`molded to provide the reflector walls. The cups are then filled
`with silicone after the dies have been attached. In another
`embodiment, layer 46 is constructed from epoxy and the cups
`are filled with an epoxy that is cured in place.
`For many applications, the preferred base material is a
`flexible silicone. Since the cup layer and encapsulant will
`have substantially the same coefficients of thermal expansion
`in this case, only the differences between the thermal expan
`sion coefficient of the underlying carrier and the silicone
`components need be accommodated. Since layer 46 and the
`encapsulant material are flexible, these differences in thermal
`coefficient of expansion can be accommodated by Small
`deformations in layer 46 and the encapsulant layer.
`The reflector layer can be applied to the carrier as a separate
`component or molded in place on the carrier. Refer now to
`FIG.4, which is a cross-sectional view of a portion of a light
`Source according to one embodiment of the present invention
`prior to the attachment of the reflective layer. Light source 60
`is constructed from a reflector layer 61 that is molded sepa
`rately and then attached to circuit carrier 62. Reflector layer
`61 is molded from a flexible compound such as silicone and
`includes holes such as hole 65 having reflective walls 66. The
`LED dies 63 can be attached to circuit carrier 62 and electri
`cally connected to circuit carrier 62 prior to the attachment of
`reflector layer 61. In the example shown in FIG. 4, the LEDs
`are connected to one trace that is under die 63 and one trace
`that is connected to the die by a wire bond such as wire bond
`64. The reflector layer could be bonded to the circuit carrier
`by a silicone-based cement in this embodiment. After the
`reflector layer is bonded to circuit carrier 62, the reflective
`cups can be filled with the appropriate encapsulant.
`In another embodiment, the reflector layer is connected to
`the circuit carrier prior to the attachment of the die to the
`circuit carrier. The reflective cups have sufficient space to
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`US 7,968,899 B2
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`allow the dies to be attached and wire bonded by accessing the
`circuit carrier through the reflective cup. After the dies have
`been connected, the reflective cups are filled with the corre
`sponding encapsulant as discussed above.
`Alternatively, the reflector layer could be molded in place
`over the carrier. Refer now to FIG. 5, which is a cross-sec
`tional view of a portion of a light source according to another
`embodiment of the present invention. Light source 70 is simi
`lar to light source 60 discussed above. However, light source
`70 includes a reflector layer 71 that is molded onto circuit
`carrier 62. The layer can be molded either before or after the
`LEDs are attached and connected to circuit carrier 62.
`A light source according to the present invention can be
`constructed in a manner analogous to construction of a multi
`LED light source Such as the light Sources discussed above
`with reference to FIGS. 2 and 3. After a sheet of light sources
`has been fabricated, the individual light Sources containing a
`predetermined number of LEDs are then separated from the
`sheet by cutting the sheet between the sources.
`The above-described embodiments of the present inven
`tion utilize reflectors with reflective walls. For the purposes of
`this discussion, a reflector wall is defined as being reflective if
`that wall reflects more than 80 percent of the light generated
`in said LED and any luminescent conversion material that
`strikes that wall.
`The above-described embodiments utilize reflectors made
`from flexible materials. For the purposes of this discussion,
`the layer of material having the cavities that become the
`reflectors will be defined as being flexible if the material
`distorts sufficiently to accommodate differences in the ther
`mal coefficient of expansion between the underlying circuit
`carrier and the reflector and the encapsulant layer and the
`reflector without distorting the encapsulant or causing the
`encapsulant and reflector to separate from one another.
`Various modifications to the present invention will become
`apparent to those skilled in the art from the foregoing descrip
`tion and accompanying drawings. Accordingly, the present
`invention is to be limited solely by the scope of the following
`claims.
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`6. The light source of claim 1 wherein the cup comprises
`white side walls.
`7. The light source of claim 1 wherein the encapsulant
`comprises a curved top Surface.
`8. A light source comprising:
`a molded body;
`a lead frame at least partially encapsulated in the molded
`body;
`a die comprising an LED, the LED being electrically and
`thermally connected to the lead frame, wherein the lead
`frame is configured to provide power and heat dissipa
`tion to the die;
`a cup comprising a recess in the molded body overlying the
`lead frame and filled with an encapsulant material, the
`die being located within the cup and being encapsulated
`by the lead frame and the encapsulant material; and
`wherein the molded body and the encapsulant material
`comprise a flexible base material, the flexible base mate
`rial being the same for both the molded body and the
`encapsulant material, wherein the molded body has Sub
`stantially the same coefficient of thermal expansion as
`the encapsulant material, and wherein the cup comprises
`reflective sidewalls positioned to reflect light leaving the
`die.
`9. The light source of claim 8, wherein the cup comprises
`white sidewalls positioned to reflect light leaving the die.
`10. The light source of claim 8, wherein the cup comprises
`particulates that render the cup reflective.
`11. The light source of claim 8, wherein the cup comprises
`a reflective, flexible material.
`12. The light source of claim 8, wherein the cup comprises
`an inner surface that includes white particles that render the
`sidewalls of the cup reflective.
`13. The light source of claim 8, wherein the cup comprises
`an inner surface coated with TiO.
`14. The light source of claim 8, wherein the encapsulant
`comprises wavelength converting material that alters the
`spectrum of light emitted by the LED.
`15. The light source of claim 8, wherein the encapsulant
`comprises Suspended wavelength converting particles that
`convert a portion of the light emitted from the LED to light of
`a different color.
`16. A light source comprising:
`a Substrate,
`a body on the substrate;
`a lead frame at least partially encapsulated by the body and
`the substrate;
`a die comprising an LED, the LED being electrically and
`thermally connected to the lead frame, wherein the lead
`frame is configured to provide power and heat dissipa
`tion to the die; and
`a cup comprising a recess in the body overlying the lead
`frame and filled with an encapsulant material, the die
`being located within the cup and being encapsulated
`by the lead frame and the encapsulant material,
`wherein the cup comprises reflective sidewalls posi
`tioned to reflect light leaving the LED;
`wherein the body, the substrate and the encapsulant
`material comprise a flexible base material, the flex
`ible base material being the same for the body, the
`Substrate and the encapsulant material, wherein the
`body and the substrate have substantially the same
`coefficient of thermal expansion as the encapsulant
`material, wherein the encapsulant material com
`prises a wavelength converting material that alters
`
`What is claimed is:
`1. A light source comprising:
`a body;
`a lead frame at least partially encapsulated in the body;
`a die comprising an LED, the LED being electrically and
`thermally connected to the lead frame; and
`a cup comprising a recess in the body overlying the lead
`frame and filled with an encapsulant material, the die
`being located within the cup and being encapsulated by
`the lead frame and the encapsulant material;
`wherein the cup and the encapsulant material comprise a
`flexible base material, the flexible base material being
`the same for both the cup and the encapsulant material,
`and wherein the cup comprises reflective sidewalls posi
`tioned to reflect light leaving the die.
`2. The light source of claim 1 wherein the cup, the encap
`Sulant, and the body comprise a flexible base material having
`Substantially the same coefficient of thermal expansion.
`3. The light source of claim 1, wherein the cup has sub
`stantially the same coefficient of thermal expansion as the
`encapsulant material.
`4. The light source of claim 1 wherein the encapsulant
`material comprises a medium having an index of refraction
`that is intermediate between that of air and the index of
`refraction of the die.
`5. The light source of claim 1 wherein the flexible base
`material comprises a flexible silicone.
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`the spectrum of light emitted by the LED, and
`wherein the encapsulant material comprises a
`curved top Surface.
`17. The light source of claim 16, wherein the cup comprises
`white sidewalls positioned to reflect light leaving the LED.
`18. The light source of claim 16, wherein the cup comprises
`particulates that render the cup reflective.
`19. The light source of claim 16, wherein the cup comprises
`a reflective, flexible material.
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`20. The light source of claim 16, wherein the cup comprises
`an inner surface that includes white particles that render the
`white sidewalls of the cup reflective.
`21. The light source of claim 16, wherein the cup comprises
`an inner surface coated with TiO.
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`Nichia Exhibit 1013
`Page 7
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