I lllll llllllll II llllll lllll lllll lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US 20080012036Al
`
`c19) United States
`c12) Patent Application Publication
`Loh et al.
`
`c10) Pub. No.: US 2008/0012036 Al
`Jan. 17, 2008
`(43) Pub. Date:
`
`(54) LEADFRAME-BASED PACKAGES FOR
`SOLID STATE LIGHT EMITTING DEVICES
`AND METHODS OF FORMING
`LEADFRAME-BASED PACKAGES FOR
`SOLID STATE LIGHT EMITTING DEVICES
`
`(76)
`
`Inventors: Ban P. Loh, Durham, NC (US);
`Nicholas W. Medendorp JR., Raleigh,
`NC (US); Eric Tarsa, Goleta, CA (US);
`Bernd Keller, Santa Barbara, CA (US)
`
`Correspondence Address:
`MYERS BIGEL SIBLEY & SAJOVEC, P.A.
`P.O. BOX 37428
`RALEIGH, NC 27627 (US)
`
`(21) Appl. No.:
`
`111657,347
`
`(22) Filed:
`
`Jan.24, 2007
`
`Related U.S. Application Data
`
`(63) Continuation-in-part of application No. 11/486,244,
`filed on Jul. 13, 2006.
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`(2006.01)
`HOJL 33100
`(2006.01)
`HOJL 21100
`(52) U.S. Cl. ............................. 257/99; 438/26; 257/E33;
`257/E21
`
`(57)
`
`ABSTRACT
`
`A modular package for a light emitting device includes a
`leadframe including a first region having a top surface, a
`bottom surface and a first thickness and a second region
`having a top surface, a bottom surface and a second thick(cid:173)
`ness that is less than the first thickness. The leadframe
`further includes an electrical lead extending laterally away
`from the second region, and the package further includes a
`thermoset package body on the leadframe and surrounding
`the first region. The thermo set package body may be on both
`the top and bottom surfaces of the second region. A leak
`barrier may be on the leadframe, and the package body may
`be on the leak barrier. Methods of forming modular pack(cid:173)
`ages including thermoset package bodies on leadframes are
`also disclosed.
`
`360 \
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`Patent Application Publication Jan. 17, 2008 Sheet 1 of 10
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`US 2008/0012036 Al
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`15A
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`158
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`FIG. 1A
`PRIOR ART
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`/20
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`21
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`25A
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`258
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`23
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`FIGURE1B
`PRIOR ART
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`104
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`A
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`FIGURE2A
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`FIGURE2B
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`Patent Application Publication Jan. 17, 2008 Sheet 3 of 10
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`160
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`\
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`106
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`106b
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`130b
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`102
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`FIGURE 3A
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`130
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`104b
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`104
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`\
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`102
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`FIGURE 38
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`Patent Application Publication Jan. 17, 2008 Sheet 4 of 10
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`100'
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`106
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`315 t
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`310
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`104
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`320
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`102'
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`FIGURE4A
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`100
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`102
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`FIGURE4B
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`Patent Application Publication Jan. 17, 2008 Sheet 5 of 10
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`\
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`206
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`204
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`200b
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`226
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`202b
`(FIGURE 5
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`201
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`202b 202a
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`206a
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`206b
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`206c
`206d
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`D
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`204a
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`204b
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`204c
`204d
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`202c 202d
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`00'
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`FIGURE6
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`Patent Application Publication Jan. 17, 2008 Sheet 6 of 10
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`FIGURE7
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`360 \
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`335
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`206
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`FIGURES
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`Patent Application Publication Jan. 17, 2008 Sheet 7 of 10
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`US 2008/0012036 Al
`
`START
`
`p
`
`PROVIDE DUAL-GAUGE
`LEADFRAME
`910
`, '
`PLACE DUAL-GAUGE
`LEADFRAME INTO MOLD
`920
`
`DISPENSE THERMOSET
`PLASTIC INTO MOLD
`930
`
`, r
`
`COMPRESS MOLD
`940
`,,
`
`CURE THERMOSET PLASTIC
`950
`,,
`END
`
`FIGURE 9
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`Patent Application Publication Jan. 17, 2008 Sheet 8 of 10
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`US 2008/0012036 Al
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`FIGURE 10A
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`FIGURE 108
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`402
`
`FIGURE 10C
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`Patent Application Publication Jan. 17, 2008 Sheet 9 of 10
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`US 2008/0012036 Al
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`420
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`FIGURE 11A
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`FIGURE 118
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`Patent Application Publication Jan. 17, 2008 Sheet 10 of 10
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`420'
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`FIGURE 12
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`406
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`FIGURE 13
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`US 2008/0012036 Al
`
`Jan. 17, 2008
`
`1
`
`LEADFRAME-BASED PACKAGES FOR SOLID
`STATE LIGHT EMITTING DEVICES AND
`METHODS OF FORMING LEADFRAME-BASED
`PACKAGES FOR SOLID STATE LIGHT EMITTING
`DEVICES
`
`CLAIM OF PRIORITY
`
`[0001] The present application is a continuation-in-part of
`U.S. patent application Ser. No. 11/486,244, filed on Jul. 13,
`2006, the disclosure of which is hereby incorporated by
`reference herein as if set forth in its entirety.
`
`FIELD OF THE INVENTION
`
`[0002] This invention relates to solid state light emitting
`devices, and more particularly to packages for solid state
`light emitting devices and methods of forming packages for
`solid state light emitting devices.
`
`BACKGROUND
`
`[0003]
`It is known to mount solid state light sources, such
`as semiconductor light emitting devices, in packages that
`may provide protection, color selection, focusing, and the
`like, for light emitted by the light emitting device. A solid
`state light emitting device may be, for example, an organic
`or inorganic light emitting diode ("LED"). Some packages
`for light emitting diodes are described in U.S. Pre-grant
`Publication Nos. 2004/0079957, 2004/0126913, and 2005/
`0269587 which are assigned to the assignee of the present
`invention, and which are incorporated herein by reference as
`if set forth fully herein.
`
`[0004] Packages as described in the above referenced
`publications may be suitable for high power, solid state
`illumination applications. However, notwithstanding the
`advances described therein, there remains a need for
`improved packages in which multiple LEDs may be
`mounted. In particular, in some general lighting applications
`it may be desirable for an LED package to include multiple
`LEDs emitting in different regions of the visible spectrum.
`Light emitted by the LEDs may combine to produce a
`desired intensity and/or color of light, such as white light or
`any other desired color. In that case, it may be desirable for
`the LEDs in the package to be mounted relatively closely
`together.
`
`[0005] A typical leadframe-based LED package includes
`electrical leads, contacts or traces for electrically connecting
`the LED package to an external circuit. In a typical LED
`package 10 illustrated in FIG. lA, an LED chip 12 is
`mounted on a reflective cup 13 by means of a solder bond
`or conductive epoxy. One or more wirebonds 11 connect the
`ohmic contacts of the LED chip 12 to leads 15Aand/or 15B,
`which may be attached to or integral with the reflective cup
`13. The reflective cup 13 may be filled with an encapsulant
`material 16 containing a wavelength conversion material
`such as a phosphor. Light emitted by the LED at a first
`wavelength may be absorbed by the phosphor, which may
`responsively emit light at a second wavelength. The entire
`assembly is then encapsulated in a clear protective resin 14,
`which may be molded in the shape of a lens to collimate the
`light emitted from the LED chip 12. However, heat retention
`may be an issue for a package such as the package 10 shown
`in FIG. lA, since it may be difficult to extract heat through
`the leads 15A, 15B, as both leads 15A, 15B cannot be
`
`connected to a heatsink, or they will be electrically shorted.
`Furthermore, both leads 15A, 15B are usually made of thin
`sheet metal of0.50 mm maximal thickness, beyond which it
`is difficult to be manufactured and handled.
`
`[0006] A conventional surface-mountable
`leadframe(cid:173)
`based package 20 is illustrated in FIG. lB. The package 20
`includes an LED chip 22 mounted on a reflective cup 23.
`One or more wirebonds 21 connect the ohmic contacts of the
`LED chip 22 to leads 25A and/or 25B, which may be
`attached to or integral with the reflective cup 23. A clear
`protective resin 24 is cast around the assembly. The reflec(cid:173)
`tive cup 23 may be formed by stamping a thin sheet of metal
`when the leadframe is formed. Stamping the reflective cup
`23 may result in thinning of the base and/or sidewalls of the
`cup 23, thus reducing its thermal spreading properties and
`capacity to dissipate heat generated by the semiconductor
`chips mounted at the bottom of the cup. Hence, less heat is
`extracted out of the package through the leads 25A, 25B,
`resulting in higher thermal resistance that limits the perfor(cid:173)
`mance of the device. This type of package usually can
`handle a maximum power up to only about 0.5 Watt.
`
`SUMMARY
`
`[0007] Embodiments of the invention provide a modular
`package for a light emitting device. The modular package
`includes a leadframe including a first region having a top
`surface, a bottom surface and a first thickness and a second
`region having a top surface, a bottom surface and a second
`thickness that is less than the first thickness. The leadframe
`further includes an electrical lead extending laterally away
`from the second region, and the package further includes a
`thermoset package body on the leadframe and surrounding
`the first region. The thermo set package body may be on both
`the top and bottom surfaces of the second region.
`
`[0008] The second region may include a recess in the
`leadframe, and the thermoset package body may at least
`partially fill the recess.
`
`[0009] The first region may include a die mounting region
`that is isolated from the electrical lead, and the thermoset
`package body may include upper sidewalls that define an
`optical cavity above the die mounting region. The upper
`sidewalls may include oblique inner surfaces that define a
`reflector cup surrounding the die mounting region, and the
`package may further include an encapsulant in the reflector
`cup.
`
`[0010]
`In some embodiments, at least a portion of the
`thermoset package body may extend through the leadframe.
`
`[0011] The first region may include a reflector cup therein
`including oblique sidewalls extending from an upper corner
`of the reflector cup to a base of the reflector cup. A third
`thickness between the base of the reflector cup and the
`bottom surface of the first region is greater than the second
`thickness.
`
`[0012] A width of the first region may be greater than a
`width of the base of the reflector cup. Furthermore, a width
`of the first region may be greater than or equal to a width of
`the reflector cup at the upper corner thereof.
`
`[0013] The modular package may further include a sub(cid:173)
`mount on the base of the reflector cup, a solid state light
`
`VIZIO Ex. 1004 Page 0012
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`

`US 2008/0012036 Al
`
`Jan. 17, 2008
`
`2
`
`emitting device on the submount, and a wirebond connec(cid:173)
`tion from the solid state light emitting device to the electrical
`lead.
`
`[0014] The thermoset package body may include upper
`sidewalls that define an optical cavity above the reflector
`cup. In particular, the reflector cup may include a first
`reflector cup and the upper sidewalls may include oblique
`inner surfaces that define a second reflector cup surrounding
`the first reflector cup.
`
`[0015] The thermoset package body may have a bottom
`surface that is substantially coplanar with the bottom surface
`of the first region.
`
`[0016] The modular package may further include a plu(cid:173)
`rality of electrical leads, and the first region may include a
`plurality of die mounting pads that are electrically connected
`to respective ones of the plurality of electrical leads and that
`are configured to receive a light emitting device.
`
`[0017] Methods of forming a package for a solid state light
`emitting device according to some embodiments of the
`invention include providing a leadframe including a first
`region having a top surface, a bottom surface and a first
`thickness, a second region having a top surface, a bottom
`surface and a second thickness that is less than the first
`thickness, and an electrical lead extending laterally away
`from the second region. The leadframe is placed into a mold
`having a mold cavity, and a thermoset precursor material is
`dispensed into the mold cavity. Pressure is applied to the
`mold, and the thermoset precursor material is cured to form
`a thermoset package body on the leadframe.
`
`[0018] The thermoset package body may expose the bot(cid:173)
`tom surface of the first region, and the thermoset package
`body may be at least partially formed beneath a bottom
`surface of the electrical lead.
`
`[0019] The first region may include a die mounting region,
`and the thermoset package body may include upper side(cid:173)
`walls that define an optical cavity above the die mounting
`region and that include oblique inner surfaces that define a
`reflector cup surrounding the die mounting region, the
`method may further include dispensing an encapsulant in the
`reflector cup.
`
`[0020] The thermoset package body may further include a
`circumferential rim surrounding the first region, and posi(cid:173)
`tioning the lens above the reflector cup may include bringing
`the lens into contact with the circumferential rim.
`
`[0021] The first region may include a reflector cup therein
`including oblique sidewalls extending from an upper corner
`of the reflector cup to a base of the reflector cup. A third
`thickness between the base of the reflector cup and the
`bottom surface of the first region may be greater than the
`second thickness. The methods may further include posi(cid:173)
`tioning a submount on the base of the reflector cup, posi(cid:173)
`tioning a solid state light emitting device on the submount,
`and forming a wirebond connection from the solid state light
`emitting device to the electrical lead.
`
`[0022] Forming the thermoset package body may include
`forming the thermoset package body to expose a bottom
`surface of the first region of the leadframe.
`
`[0023]
`In some embodiments, providing the leadframe
`may include providing a leadframe blank having a top
`
`surface, a first region having a bottom surface and having a
`first thickness between the top surface of the leadframe
`blank and the bottom surface of the first region, and a portion
`extending laterally away from the first region, the portion
`extending laterally away from the first region having a
`bottom surface and a second thickness less than the first
`thickness adjacent the first region from the top surface of the
`leadframe to the bottom surface of the portion extending
`away from the first region. A reflector cup is stamped into the
`first region.
`[0024] Stamping the reflector cup into the first region may
`include bringing a stamp including a protrusion having a
`shape defining a desired shape of the reflector cup into
`contact with the upper surface of the leadframe blank above
`the first region, and applying sufficient energy to the stamp
`to impress an image of the protrusion into the first region of
`the leadframe blank.
`[0025] The methods may further include trimming excess
`material squeezed out while stamping the reflector cup from
`the leadframe blank.
`[0026] The reflector cup may include oblique sidewalls
`extending from an upper corner of the reflector cup to a base
`of the reflector cup, and a third thickness between the base
`of the reflector cup and the bottom surface of the first region
`may be greater than the second thickness.
`[0027]
`In some embodiments, providing the leadframe
`may include providing a leadframe blank having a top
`surface and a bottom surface, and selectively etching, rolling
`and/or milling the leadframe blank to provide a first region
`having a bottom surface and having a first thickness between
`the top surface of the leadframe blank and the bottom
`surface of the region, and a second region having a bottom
`surface and a second thickness less than the first thickness
`from the top surface of the leadframe to the bottom surface
`of the second region. Selectively etching, rolling and/or
`milling the leadframe blank may include selectively etching,
`rolling and/or milling the leadframe blank to form a recess
`in the leadframe.
`[0028] Some embodiments of the invention provide a
`modular package for a light emitting device. The package
`includes a leadframe including a first region having a top
`surface, a bottom surface and a first thickness and a second
`region having a top surface, a bottom surface and a second
`thickness that is less than the first thickness. The leadframe
`may further include an electrical lead extending laterally
`away from the second region and a leak barrier on a surface
`of the first region or a surface of the second region. A
`package body is on the leadframe and on the first region. The
`package body is on the top and bottom surfaces of the
`second region, and is also on the leak barrier.
`[0029] The leak barrier may include a notch or groove in
`the leadframe, and the package body may be at least partially
`within the notch or groove. In some embodiments, the leak
`barrier may include a protrusion on the leadframe. The
`package body may be on the protrusion, such that the
`protrusion becomes molded inside the package body.
`[0030] The package body may include a thermoset, such
`as a thermoset plastic.
`[0031] The second region may include a recess in the
`leadframe, and the thermoset package body may at least
`partially fill the recess.
`
`VIZIO Ex. 1004 Page 0013
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`

`US 2008/0012036 Al
`
`Jan. 17, 2008
`
`3
`
`[0032] The first region may include a mounting region on
`the top surface thereof. The leak barrier may be on the top
`surface of the first region outside the mounting region, and
`the package body may be on the leak barrier.
`
`[0033] The package body may include upper sidewalls
`that define an optical cavity above the mounting region, and
`the upper sidewalls may include oblique inner surfaces that
`define a reflector cup surrounding the mounting region. The
`package may further include an encapsulant in the reflector
`cup.
`
`[0034] The leadframe may include a support lead, and the
`leak barrier may be on the support lead and/or on the
`electrical lead.
`
`[0035] The first region may include a sidewall between the
`top surface and the bottom surface thereof. The leak barrier
`may be on the sidewall. In particular, the leak barrier may
`include a protrusion from the sidewall.
`
`[0036] The package may further include a peripheral notch
`on a corner of the first region adjacent the bottom surface
`thereof. The package body may at least partially fill the
`peripheral notch and expose the bottom surface of the first
`region.
`
`[0037] Methods of forming a package for a solid state light
`emitting device according to some embodiments of the
`invention include providing a leadframe including a first
`region having a top surface, a bottom surface and a first
`thickness and a second region having a top surface, a bottom
`surface and a second thickness that is less than the first
`thickness. The leadframe may further include an electrical
`lead extending laterally away from the second region and a
`leak barrier on a surface of the first region or the second
`region. The methods may further include placing the lead(cid:173)
`frame into a mold having a mold cavity, dispensing a
`precursor material into the mold cavity, applying pressure to
`the mold, and curing the precursor material to form a
`package body on the leadframe. The package body may be
`formed on the leak barrier.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`[0038] The accompanying drawings, which are included
`to provide a further understanding of the invention and are
`incorporated in and constitute a part of this application,
`illustrate certain embodiment(s) of the invention. In the
`drawings:
`
`[0039] FIGS. lA and lB are cross-sectional side views
`illustrating conventional packages for light emitting devices.
`
`[0040] FIG. 2A is a top view and FIGS. 2B and 2C are a
`cross-sectional side views illustrating a leadframe for one or
`more light emitting devices according to some embodiments
`of the present invention;
`
`[0041] FIG. 3A is a side view and FIG. 3B is a cross
`sectional side view of a package for one or more light
`emitting devices according to some embodiments of the
`invention;
`[0042] FIGS. 4A and 4B are schematic diagrams illustrat(cid:173)
`ing the formation of a lead frame according to some embodi(cid:173)
`ments of the invention;
`[0043] FIG. 5 is a cross sectional side view of a package
`for one or more light emitting devices according to further
`embodiments of the invention;
`
`[0044] FIG. 6 is a top view of a leadframe configured for
`use in a package according to embodiments of the invention;
`
`[0045] FIG. 7 is a cutaway view of a package for one or
`more light emitting devices according to embodiments of the
`invention; and
`
`[0046] FIG. 8 is a cross sectional side view of a package
`for one or more light emitting devices according to still
`further embodiments of the invention.
`
`[0047] FIG. 9 is a flowchart illustrating operations accord(cid:173)
`ing to some embodiments of the invention.
`
`[0048] FIGS. lOA-lOC are front and back isometric pro(cid:173)
`jections and a top view, respectively, of a dual-gauge lead(cid:173)
`frame according to some embodiments of the invention.
`
`[0049] FIGS. llA-B are front and back isometric projec(cid:173)
`tions of a leadframe/body assembly according to some
`embodiments of the invention.
`
`[0050] FIG. 12 is a cross-sectional illustration of a lead(cid:173)
`frame/body assembly according to some embodiments of the
`invention.
`
`[0051] FIG. 13 is a detail cross-sectional illustration of an
`interface between a leadframe and a package body according
`to some embodiments of the invention.
`
`DETAILED DESCRIPTION OF EMBODIMENTS
`OF THE INVENTION
`
`[0052] The present invention now will be described more
`fully hereinafter with reference to the accompanying draw(cid:173)
`ings, in which embodiments of the invention are shown. This
`invention may, however, be embodied in many different
`forms and should not be construed as limited to the embodi(cid:173)
`ments set forth herein. Rather, these embodiments are pro(cid:173)
`vided so that this disclosure will be thorough and complete,
`and will fully convey the scope of the invention to those
`skilled in the art. In the drawings, the size and relative sizes
`of layers and regions may be exaggerated for clarity. Like
`numbers refer to like elements throughout.
`
`[0053]
`It will be understood that when an element such as
`a layer, region or substrate is referred to as being "on" or
`extending "onto" another element, it can be directly on or
`extend directly onto the other element or intervening ele(cid:173)
`ments may also be present. In contrast, when an element is
`referred to as being "directly on" or extending "directly
`onto" another element, there are no intervening elements
`present. It will also be understood that when an element is
`referred to as being "connected" or "coupled" to another
`element, it can be directly connected or coupled to the other
`element or intervening elements may be present. In contrast,
`when an element is referred to as being "directly connected"
`or "directly coupled" to another element, there are no
`intervening elements present.
`[0054] Relative terms such as "below" or "above" or
`"upper" or "lower" or "horizontal" or "lateral" or "vertical"
`may be used herein to describe a relationship of one element,
`layer or region to another element, layer or region as
`illustrated in the figures. It will be understood that these
`terms are intended to encompass different orientations of the
`device in addition to the orientation depicted in the figures.
`
`[0055]
`It will be understood that, although the terms first,
`second, etc. may be used herein to describe various ele-
`
`VIZIO Ex. 1004 Page 0014
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`Jan. 17, 2008
`
`4
`
`ments, components, regions, layers and/or sections, these
`elements, components, regions,
`layers and/or sections
`should not be limited by these terms. These terms are only
`used to distinguish one element, component, region, layer or
`section from another region, layer or section. Thus, a first
`element, component, region, layer or section discussed
`below could be termed a second element, component,
`region, layer or section without departing from the teachings
`of the present invention.
`
`[0056] Unless otherwise defined, all terms (including tech(cid:173)
`nical and scientific terms) used herein have the same mean(cid:173)
`ing as commonly understood by one of ordinary skill in the
`art to which this invention belongs. It will be further
`understood that terms used herein should be interpreted as
`having a meaning that is consistent with their meaning in the
`context of this specification and the relevant art and will not
`be interpreted in an idealized or overly formal sense unless
`expressly so defined herein.
`
`[0057] Embodiments of the invention are described herein
`with reference to cross-section illustrations that are sche(cid:173)
`matic illustrations of idealized embodiments (and interme(cid:173)
`diate structures) of the invention. The thickness oflayers and
`regions in the drawings may be exaggerated for clarity.
`Additionally, variations from the shapes of the illustrations
`as a result, for example, of manufacturing techniques and/or
`tolerances, are to be expected. Thus, embodiments of the
`invention should not be construed as limited to the particular
`shapes of regions illustrated herein but are to include devia(cid:173)
`tions in shapes that result, for example, from manufacturing.
`
`[0058] As used herein, the term semiconductor light emit(cid:173)
`ting device may include a light emitting diode, laser diode
`and/or other semiconductor device which includes one or
`more semiconductor layers, which may include silicon,
`silicon carbide, gallium nitride and/or other semiconductor
`materials, a substrate which may include sapphire, silicon,
`silicon carbide and/or other microelectronic substrates, and
`one or more contact layers which may include metal and/or
`other conductive layers. In some embodiments, ultraviolet,
`blue and/or green light emitting diodes ("LEDs") may be
`provided. Red and/or amber LEDs may also be provided.
`The design and fabrication of semiconductor light emitting
`devices are well known to those having skill in the art and
`need not be described in detail herein.
`
`[0059] For example, the semiconductor light emitting
`devices packaged in accordance with embodiments of the
`invention may be gallium nitride-based LEDs or lasers
`fabricated on a silicon carbide substrate such as those
`devices manufactured and sold by Cree, Inc. of Durham,
`N.C. The LEDs and/or lasers may be configured to operate
`such that light emission occurs through the substrate in a
`so-called "flip chip" orientation.
`
`[0060] Referring now to FIGS. 2A-2C, a leadframe 100
`according to some embodiments of the invention is illus(cid:173)
`trated. FIG. 2A is a top view of the leadframe 100, while
`FIGS. 2B and 2C are cross sections taken along line A-A of
`FIG. 2A. The leadframe 100 includes a central region 102
`and a plurality of leads 104, 106 extending away from the
`central region 102. The electrical leads 104, 106 may be
`electrically isolated from one another and/or from the central
`region 102 of the leadframe 100. The leads may be arranged
`such that leads of opposite polarity type (e.g. anodes or
`
`cathodes) are provided on opposite sides of the leadframe
`100, which may facilitate the connection of packages using
`the leadframes 100 in series.
`
`[0061] As shown in FIG. 2A, the leadframe 100 further
`has an upper surface lOOa. The central region 102 of the
`leadframe 100 has a substantially flat lower surface 102b
`that is spaced apart from lower surfaces 104b, 104c of the
`leads 104, 106 by sidewalls 102c. The central region 102 has
`a first thickness (i.e. the distance between the upper surface
`lOOa of the leadframe 100 and the lower surface 102b of the
`central region 102), and the electrical leads 104, 106 have a
`second thickness (i.e. the distance between the upper surface
`lOOa of the leadframe 100 and the lower surface 104b, 106b
`of the respective leads 104, 106) that is less than the first
`thickness.
`
`[0062] A reflector cup 120 is formed in the central region
`102. The reflector cup 124 includes an oblique sidewall that
`extends from the upper surface lOOa of the leadframe 100 to
`a base 124b located within the central region 102. The
`reflector cup 124 may have an arbitrary peripheral shape.
`However, in the embodiments illustrated in FIGS. 2A-2C,
`the reflector cup 124 has a generally circular peripheral
`shape. Thus, the oblique sidewall of the reflector cup 124
`may form a generally circular upper lip 124a where the
`reflector cup 124 intersects the upper surface lOOa of the
`leadframe 100. The sidewall of the reflector cup 124 shown
`in FIGS. 2A-C has the shape of a conic section (e.g. a
`frustum). However, the sidewall of the reflector cup 124 may
`form other shapes, for example, a solid parabolic section.
`
`[0063] The base 124b of the reflector cup 124 has a
`diameter that is less than a width of the central region 102
`(i.e. a distance between the sidewalls 102c of the central
`region 102). Furthermore, the upper lip 124a of the reflector
`cup 124 has a diameter that may be less than or equal to the
`width of the central region 102. Moreover, the thickness of
`the central region 102 between the base 124b of the reflector
`cup 124 and the lower surface 102b of the central region 102
`may be thicker than electrical leads 104, 106. As will be
`explained in greater detail below, a package for a solid state
`light emitting device may dissipate heat through the central
`region 102 of the leadframe 100, rather than through the
`leads 104, 106. Thus, the relative physical dimensions of the
`central region 102 may improve the heat dissipation prop(cid:173)
`erties of the package by reducing the thermal resistance of
`the package.
`
`[0064]
`In general, thermal resistance is inversely propor(cid:173)
`tional to the surface area through which heat is conducted.
`That is, in a simplified model, thermal resistance is defined
`by the equation
`
`(1)
`where k is the coefficient of thermal conductivity, L is the
`length of the material through which heat is to be dissipated,
`and A represents the area through which heat is to be
`dissipated.
`
`In semiconductor packages, heat flows from a
`[0065]
`relatively small chip to a much larger area of a die-attach
`pad. Thus, thermal spreading and conduction may not be
`adequately modeled by a simple one-dimensional formula
`such as Equation (1 ). Rather, thermal resistance of a package
`device can be more accurately modeled using a thermal
`spreading resistance factor which takes into account the
`
`VIZIO Ex. 1004 Page 0015
`
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`US 2008/0012036 Al
`
`Jan. 17, 2008
`
`5
`
`three-dimensional geometries of the chip and die-attach pad
`and their boundary conditions. According to this type of
`analysis, aside from the thermal conductivity of the die(cid:173)
`attach pad, i.e. the heatspreader (such as the central region
`102 in FIGS. 2C through 3B), the surface area around and
`underneath the chip and its thickness are the two most
`important parameters to give good thermal spreading before
`being conducted away through another interface, such as a
`solder joint between the heatspreader and an external heat(cid:173)
`sink-which may be a metal-core PCB (Printed Circuit
`Board) or a housing. Hence, in the design of a dual gauge
`leadframe, the central region 102 should be large and thick
`enough to achieve an efficient thermal spreading, which may
`result in good thermal performance for the entire package, in
`order to take better advantage of the relatively large surface
`area of the central region 102.
`
`[0066] Through computer modeling, e.g. computer mod(cid:173)
`eling based on Kennedy's thermal spreading resistance
`graphs, it has been found that a package having a low
`thermal resistance can be designed in a practical and cost(cid:173)
`effective manner by having the bottom side of the semicon(cid:173)
`ductor chip (which acts as a heat source) and solder pad
`(which acts as a heat sink) on two opposite sides of a copper
`substrate having a particular thickness. Thus, according to
`some embodiments of the invention, a dual gauge (thick(cid:173)
`ness) copper alloy sheet may be provided. A thinner section
`of the sheet may be stamped into electrical leads, while the
`thicker portion of the sheet may be stamped to form a die
`attach pad on its front face and a solder pad on its back face.
`In particular embodiments of the invention, electrical cur(cid:173)
`rent can be adequately conducted by thinner leads, while the
`heat energy may be effectively spread by the thicker section
`in which the die-attach pad is formed and that has a surface
`area that is a few times that of the chip footprint. In some
`embodimen