U.S. patent application number 10/822191 was filed with the patent office on 2005-10-13 for light emitting diode arrays with improved light extraction.
Invention is credited to Amaya, Edmar, Mazzochette, Joseph.
Application Number | 20050225222 10/822191 |
Document ID | / |
Family ID | 35059909 |
Filed Date | 2005-10-13 |
United States Patent
Application |
20050225222 |
Kind Code |
A1 |
Mazzochette, Joseph ; et
al. |
October 13, 2005 |
Light emitting diode arrays with improved light extraction
Abstract
In accordance with the invention, an illumination device
comprises a highly thermally conductive substrate having a surface,
a plurality of light emitting diodes (LEDs) supported by the
surface and arranged in an array to provide illumination. At least
one reflective barrier at least partially surrounds each LED. The
reflective barrier is shaped to reflect away from the LED light
emitted by other LEDs in the array. Advantageously the substrate
and reflective barrier are thermally coupled to a heat spreader to
dissipate heat generated by the LEDs. The substrate preferably
comprises an LTTC-M heat spreader, and the reflective thermal
barriers preferably comprise metal ridges or cups.
Inventors: |
Mazzochette, Joseph; (Cherry
Hill, NJ) ; Amaya, Edmar; (King of Prussia,
PA) |
Correspondence
Address: |
DOCKET ADMINISTRATOR
LOWENSTEIN SANDLER PC
65 Livingston Avenue
Roseland
NJ
07068-1791
US
|
Family ID: |
35059909 |
Appl. No.: |
10/822191 |
Filed: |
April 9, 2004 |
Current U.S.
Class: |
313/46 ;
257/E33.072; 313/512 |
Current CPC
Class: |
H01L 2224/48472
20130101; H01L 2224/32225 20130101; G09F 9/33 20130101; H01L 33/60
20130101; H01L 2224/48472 20130101; H01L 2924/01079 20130101; H01L
2224/48227 20130101; H01L 2224/32225 20130101; H01L 2224/48227
20130101; H01L 2924/00 20130101; H01L 2924/00 20130101; H01L
2924/00 20130101; H01L 2224/48091 20130101; H01L 2924/00014
20130101; H01L 2924/00 20130101; H01L 2924/01057 20130101; H01L
2224/73265 20130101; H01L 2924/0102 20130101; H01L 2224/48091
20130101; H01L 2224/48472 20130101; H01L 2924/01025 20130101; H01L
2924/09701 20130101; H01L 2924/00 20130101; H01L 2224/48227
20130101; H01L 2224/48091 20130101; H01L 2924/12041 20130101; H01L
2224/48227 20130101; H01L 2924/19041 20130101; H01L 2224/48472
20130101; H01L 33/62 20130101; H01L 2924/01004 20130101; H01L
2224/73265 20130101; F21K 9/68 20160801; H01L 2924/01012
20130101 |
Class at
Publication: |
313/046 ;
313/512 |
International
Class: |
H01J 001/62 |
Claims
What is claimed:
1. An illumination device comprising: a substrate having a surface
and including a highly thermally conductive heat spreader; a
plurality of light emitting diodes (LEDs) supported by the surface,
the LEDs arranged in an array to provide illumination; at least one
reflective barrier at least partially surrounding each LED, the
reflective barrier shaped to reflect away from the LED light
emitted by other LEDs in the array; the LEDs and the reflective
barrier thermally coupled to the heat spreader to dissipate heat
generated by the LEDs and heat produced by light absorption.
2. The device of claim 1 wherein the substrate comprises an LTCC-M
heat spreader.
3. The device of claim 1 wherein the at least one reflective
barrier comprises a periodic array of troughs and reflective
ridges, the ridges shaped to reflect away from an LED light from an
LED in an adjacent trough.
4. The device of claim 1 wherein the at least one reflective
barrier comprises a reflective ridge shaped to reflect away LED
light from an adjacent LED.
5. The device of claim 1 wherein at least one reflective barrier
comprises a cup substantially peripherally surrounding an LED to
reflect light away from adjacent LEDs.
6. The device of claim 4 wherein the at least one reflective
barrier comprises an array of cups, each cup substantially
peripherally surrounding a respective LED to reflect light away
from adjacent LEDs.
7. The device of claim 1 wherein the at least one reflective
barrier comprises a plurality of reflective circular sectors
arranged in a circle, each reflective sector shaped to reflect away
light from other sectors in the array.
8. The device of claim 1 wherein the at least one reflective
barrier comprises a cavity having reflective walls and one or more
smoothly curved reflective edges formed by the cooling of molten
metal.
9. The device of claim 1 wherein the at least one reflective
barrier is shaped to provide directional illumination.
Description
FIELD OF THE INVENTION
[0001] This invention relates to light emitting diode (LED) arrays
and, in particular, to LED arrays with integral reflective barriers
and methods for making same.
BACKGROUND OF THE INVENTION
[0002] Light emitting diodes (LEDs) are being used as light sources
in an increasing variety of applications extending from
communications and instrumentation to household, automotive and
other visual displays. LED arrays comprise a plurality of LEDs
arranged on a common substrate. One problem with LED arrays is the
significant heat generated by dense concentrations of LEDs.
Solutions to the thermal problems associated with LED arrays are
the subject of a related application entitled, "Light Emitting
Diodes Packaged For High Temperature Operation" U.S. patent
application Ser. No. 10/638,579, filed Aug. 11, 2003. The Ser. No.
10/630,579 application is incorporated herein by reference.
[0003] Another problem in LED arrays concerns illumination
efficiency. Illumination efficiency is a measure of the percentage
of generated light that actually leaves an LED package and that can
serve as useable light in the intended application. FIGS. 1 and 2
show a typical LED array 10. LED dies (semiconductor chips) 12
generate light. LED dies 12 are typically box-like in structure
with 6 sides. Since they are almost always mounted on one of the
light surfaces, the other 5 surfaces are capable of emitting light
generated by the device. Some of the light is absorbed by nearby
walls 22 of array package 11, some is reflected back to the
emitting die, and some is absorbed directly by nearby LED die 12 in
the array. The remainder of the light exits the package.
[0004] There is a relationship between illumination efficiency and
the thermal problems of LED arrays. Self-heating by absorption
contributes to thermal problems. Thus, there is a need for an LED
packaging arrangement that can increase the illumination efficiency
of LED array devices and reduce the thermal problems produced by
absorption.
SUMMARY OF THE INVENTION
[0005] In accordance with the invention, an illumination device
comprises a highly thermally conductive substrate having a surface,
a plurality of light emitting diodes (LEDs) supported by the
surface and arranged in an array to provide illumination. At least
one reflective barrier at least partially surrounds each LED. The
reflective barrier is shaped to reflect away from the LED light
emitted by other LEDs in the array. Advantageously the substrate
and reflective barrier are thermally coupled to a heat spreader to
dissipate heat. The substrate preferably comprises an LTTC-M heat
spreader, and the reflective thermal barriers preferably comprise
metal ridges or cups.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The advantages, nature and various additional features of
the invention will appear more fully upon consideration of the
illustrative embodiments now to be described in detail in
connection with the accompanying drawings. In the drawings:
[0007] FIG. 1 shows a typical LED array according to the prior
art;
[0008] FIG. 2 shows a side view of the LED array of FIG. 1;
[0009] FIG. 3 shows an exemplary LED package of the prior art;
[0010] FIG. 4 shows accordion-tapered reflective thermal
barriers;
[0011] FIG. 5 shows a side view of an LED array with integral
tapered thermal barrier reflectors;
[0012] FIG. 6 shows a metal cup tapered reflective barrier;
[0013] FIG. 7 shows a multi-cup tapered reflective barrier; and
[0014] FIG. 8 shows wedge shaped cup tapered barriers arranged to
form a circular array.
[0015] It is to be understood that these drawings are for
illustrating the concepts of the invention and are not to
scale.
DETAILED DESCRIPTION
[0016] This description is divided into two parts. Part I describes
the structure and features of light emitting diodes (LEDs) packaged
in an array for high illumination efficiency in accordance with the
invention and illustrate exemplary embodiments. Part II provides
further details of the LTCC-M packaging technology as applicable to
LED arrays.
[0017] I. LEDs Packaged for High Illumination Efficiency
[0018] FIG. 4 illustrates, a tapered barrier reflector 40
fabricated as a periodic array of troughs 41 and tapered reflective
ridges 42. This accordion-like structure is a particularly cost
effective to manufacture. Metal reflective material can be folded
in an accordion-like manner to form the tapered reflective barriers
42. LED dies 12 can be affixed in the troughs 43 between reflective
barriers 42.
[0019] The barrier reflector 40 can provide a connection to the
anode or cathode of LED dies 12. The barrier 40 also serves a
thermal cooling function. Heat from the LED die 12 can be channeled
by the barrier reflector 40 to associated thermal spreaders and to
a supporting LTCC-M substrate
[0020] FIG. 5 shows an alternative high efficiency LED array 50
comprising discrete reflective barrier structures 52. The
reflective thermal barriers 52 are advantageously shaped as fins
causing the heat to move from the bottom of the LED 12 through the
thermally conductive material (such as solder or silver epoxy 53)
to the top of the fin. The length and angle of the fin can be
modified by those skilled in the art. The thermal resistance of a
LED array package is inversely proportional to the heat dissipating
area. Thus the more and longer reflective thermal barriers 52 are,
the larger the area for heat dissipation. The LED devises are
subsequently encapsulated as by an optically matched clear epoxy 55
formed in a domed shaped in order to increase light extraction and
to minimize total internal reflection (TIR).
[0021] LED dies 12 are disposed in an array pattern overlying
substrate 54. Tapered discrete reflective barriers 52 cause light
that would have been absorbed by other die or walls of the package
to reflect out of the array package, thus increasing the
illumination efficiency. LED dies 12 can be affixed to substrate 54
by solder or epoxy 53.
[0022] Substrate 54 may be non-conducting or conducting. In the
case where substrate 54 is conducting, or has overlaying conductive
patterns and traces (not shown), an electrical connection can be
made on the mounting surface to either the anode or cathode of LED
dies 12. In the case of conductive traces, both the anode and
cathode connections can be made on the mounting surfaces of dies
12.
[0023] FIG. 3 shows further detail of possible electrical
connections to die 12. The electrical connections can be made via
wire bonds 32 to the LED anode and cathode. Alternatively either
the anode or cathode can make electrical contact with a conductor
on an insulating substrate, or a conductive substrate. In this
case, the remaining terminal can then be connected to dies 12 by a
single wire bond. LED die can be soldered (to a metal substrate 36
or overlying conductor 35) or they can be epoxied to substrate 36.
A typically translucent or transparent package wall 34 can support
a translucent or transparent LED encapsulate 31.
[0024] Turning back to FIG. 5, it can be seen that liquid epoxy 53
can be deposited on a metal conductor 52 and conductor 52 can be
attached to substrate 54. Conductor 52 can be thick film, thin
film, electro-deposited, a metal laminate, or other suitable
electrical and thermal conductor. If no electrical contact is
required conductor 52 can be omitted; however additional heat
spreading from the die can be accomplished if conductor 52 is used.
Substrate 54 can be a ceramic, multilayer printed wire board, low
temperature cofired ceramic (LTCC), LTCC on metal (LTCC-M), high
temperature cofired ceramic (HTCC), or other suitable electrical
insulator and thermal conductor. Substrate 54 can be an
electrically conducting material if electrical contact to die 12 is
desirable, or it can be an electrically insulating layer formed
between the substrate and die 12.
[0025] FIG. 6 is an exploded view that shows a device 60 where the
tapered barrier is a reflective cup 61. LED dies 12 can then be
affixed within each cup 61. Each cup 61 is affixed to substrate 54
by solder or epoxy. A small hole 62, with a diameter slightly
smaller than the width of the die, can be formed in cup 61. The
hole 62 allows some of the liquid epoxy or solder to seep into the
cup. Die 12 is then placed into the cup on top of the epoxy or
solder. Similarly, LED die 12 can be placed in cup 61 and bonded
directly through hole 62 to substrate 54 by epoxy or solder. While
hole 62 is not required in assembly 60, the hole facilitates
fabrication because the additional step of adding epoxy or solder
in the cup for the die can be avoided.
[0026] Cup 61 can be fabricated from aluminum, stainless steel,
tin, nickel, or other reflective material. The cups can be formed
by stamping, etching, coining, machining, or other manufacturing
method. Assembly of an array of LED die in reflective cup tapered
barriers can be accomplished using inexpensive pick-and-place
assembly.
[0027] FIG. 7 illustrates an advantageous embodiment 70 using an
array of cups 71. This multi-cup assembly can be pressed, stamped,
or otherwise formed from a sheet of metal 72. Cups 71 need not be
round and can be elliptical, square, or rectangular in shape. The
metal can gauge from 0.002" to 0.030", and it can comprise brass,
copper, phosphorous bronze, beryllium copper, stainless steel,
titanium, Inconel, carbon or alloy steel or precious metal.
[0028] FIG. 8 shows a circular array 80 created from wedge-shaped
sector cups 81. Each wedge 81 can include an LED die 12, and the
sector cups can be arranged in the form of a circle.
[0029] Referring to FIGS. 9, 10 and 11, yet another method to
create a reflecting surface on an LED cavity wall is to metallize
the wall of the cavity with a thick or thin reflective metal film
101, such as silver. Then a molten reflecting metal 92 such as
solder is applied to use the cavity edge as a capillary holder
(FIG. 9). The molten metal will conform to the edge capillary and
will shape the reflector cup wall into a parabola shape as seen in
assemblies 100 and 110. The middle of the cup can have a metal
insert such as a high temperature solder ball 91 (FIG. 10) or a
metal/solder column 111 (FIG. 11).
[0030] In all of the above mentioned embodiments, the materials
used to form the reflective barriers should have low absorption
characteristics in the 300 to 800 nm wavelength range. The barrier
surfaces can be dispersive or non-dispersive depending on the
application.
[0031] It is sometimes desirable to direct light that comes out of
the LED on to a certain target. FIG. 12 shows a conventional
assembly 120 composed of multiple subassemblies 121 illuminating
from the periphery of a mushroom-like heat sink 122. The target
illumination profile of this subassembly is 360 degrees at an
inclination angle .theta.. In order to direct light at this angle,
a truncated cone shape heat sink would have to be machined. Placing
the subassemblies on the angled surface of the heat sink is
difficult. Standard surface mount (SMT) technology can not be used.
Thus fabrication becomes a complex, laborious and expensive
process.
[0032] In accordance with this aspect of the invention, reflector
walls are used as guides for reflecting light directionally from an
array. The barrier surfaces can be shaped as light guides to
reflect the light at a particular target angle. FIG. 13 depicts the
outline of a stamped reflector cup 131 adapted for a directional
lighting array. Direction at an angle .theta. is accomplished by
shaping the cup. The die will sit at an angle due to the slope of
the cup. The LED die will be connected with the substrate using
conductive epoxy and wired using a wirebond though hole 132. The
die/cup assemblies can be mounted on a planar substrate by SMT
assembly.
[0033] Another advantageous embodiment is depicted in FIG. 14. The
LED 12 can be placed using standard SMT equipment parallel with the
thermally conductive substrate. A glob of conductive epoxy 142 is
dispensed on the bottom of the cup 141, attaching to the board both
the cup and the LED die. The cup 141 is shaped so that the majority
of the rays come out at angle .theta.. The electrical connection
can be made with a gold wirebond 32 through a hole 132 in the cup.
This embodiment is advantageous due to its simplicity and ease of
volume manufacturing. Cup 141 can be fabricated from aluminum,
stainless steel, tin, nickel, or other reflective material. It can
be formed by stamping, etching, coining, machining, or other
manufacturing method.
[0034] II. LTCC-M Packaging
[0035] LTCC-M packaging is particularly suitable for dispensing
heat generated by densely packed arrays of LED die. This section
highlights some of the important aspects of LTCC-M packaging
applicable to fabricating LED arrays with reflective barriers.
[0036] Multilayer ceramic circuit boards are made from layers of
green ceramic tapes. A green tape is made from particular glass
compositions and optional ceramic powders, which are mixed with
organic binders and a solvent, cast and cut to form the tape.
Wiring patterns can be screen printed onto the tape layers to carry
out various functions. Vias are then punched in the tape and are
filled with a conductor ink to connect the wiring on one green tape
to wiring on another green tape. The tapes are then aligned,
laminated, and fired to remove the organic materials, to sinter the
metal patterns and to crystallize the glasses. This is generally
carried out at temperatures below about 1000.degree. C., and
preferably from about 750-950.degree. C. The composition of the
glasses determines the coefficient of thermal expansion, the
dielectric constant and the compatibility of the multilayer ceramic
circuit boards to various electronic components. Exemplary
crystallizing glasses with inorganic fillers that sinter in the
temperature range 700 to 1000.degree. C. are Magnesium
Alumino-Silicate, Calcium Boro-Silicate, Lead Boro-Silicate, and
Calcium Alumino-Boricate.
[0037] More recently, metal support substrates (metal boards) have
been used to support the green tapes. The metal boards lend
strength to the glass layers. Moreover since the green tape layers
can be mounted on both sides of a metal board and can be adhered to
a metal board with suitable bonding glasses, the metal boards
permit increased complexity and density of circuits and devices. In
addition, passive and active components, such as resistors,
inductors, and capacitors can be incorporated into the circuit
boards for additional functionality. Where optical components, such
as LEDs are installed, the walls of the ceramic layers can be
shaped and/or coated to enhance the reflective optical properties
of the package, or reflective barriers as described herein in Part
I, can be used to further improve both the illumination and thermal
efficiency of the LED array package.
[0038] This system, known as low temperature cofired ceramic-metal
support boards, or LTCC-M, has proven to be a means for high
integration of various devices and circuitry in a single package.
The system can be tailored to be compatible with devices including
silicon-based devices, indium phosphide-based devices and gallium
arsenide-based devices, for example, by proper choice of the metal
for the support board and of the glasses in the green tapes.
[0039] The ceramic layers of the LTCC-M structure must be matched
to the thermal coefficient of expansion of the metal support board.
Glass ceramic compositions are known that match the thermal
expansion properties of various metal or metal matrix composites.
The LTCC-M structure and materials are described in U.S. Pat. No.
6,455,930, "Integrated heat sinking packages using low temperature
co-fired ceramic metal circuit board technology", issued Sep. 24,
2002 to Ponnuswamy, et al and assigned to Lamina Ceramics. U.S.
Pat. No. 6,455,930 is incorporated by reference herein. The LTCC-M
structure is further described in U.S. Pat. Nos. 5,581,876,
5,725,808, 5,953,203, and 6,518,502, all of which are assigned to
Lamina Ceramics and also incorporated by reference herein.
[0040] The metal support boards used for LTCC-M technology do have
a high thermal conductivity, but some metal boards have a high
thermal coefficient of expansion, and thus a bare die cannot always
be directly mounted to such metal support boards. However, some
metal support boards are known that can be used for such purposes,
such as metal composites of copper and molybdenum (including from
10-25% by weight of copper) or copper and tungsten (including
10-25% by weight of copper), made using powder metallurgical
techniques. Copper clad Kovar.RTM., a metal alloy of iron, nickel,
cobalt and manganese, a trademark of Carpenter Technology, is a
very useful support board. AlSiC is another material that can be
used for direct attachment, as can aluminum or copper graphite
composites.
[0041] Another instance wherein good cooling is required is for
thermal management of flip chip packaging. Densely packed
microcircuitry, and devices such as decoder/drivers, amplifiers,
oscillators and the like which generate large amounts of heat, can
also use LTCC-M techniques advantageously. Metallization on the top
layers of an integrated circuit bring input/output lines to the
edge of the chip so as to be able to wire bond to the package or
module that contains the chip. Thus the length of the wirebond wire
becomes an issue; too long a wire leads to parasitics. The cost of
very high integration chips may be determined by the arrangement of
the bond pads, rather than by the area of silicon needed to create
the circuitry. Flip chip packaging overcomes at least some of these
problems by using solder bumps rather than wirebond pads to make
connections. These solder bumps are smaller than wire bond pads
and, when the chip is turned upside down, or flipped, solder reflow
can be used to attach the chip to the package. Since the solder
bumps are small, the chip can contain input/output connections
within its interior if multilayer packaging is used. Thus the
number of die in it, rather than the number and size of bond pads
will determine the chip size.
[0042] However, increased density and integration of functions on a
single chip leads to higher temperatures on the chip, which may
prevent full utilization of optimal circuit density. The only heat
sinks are the small solder bumps that connect the chip to the
package. If this is insufficient, small active or passive heat
sinks must be added on top of the flip chip. Such additional heat
sinks increase assembly costs, increase the number of parts
required, and increase the package costs. Particularly if the heat
sinks have a small thermal mass, they have limited effectiveness as
well.
[0043] In the simplest form of the present invention, LTCC-M
technology is used to provide an integrated package for a
semiconductor component and accompanying circuitry, wherein the
conductive metal support board provides a heat sink for the
component. A bare semiconductor die, for example, can be mounted
directly onto a metal base of the LTCC-M system having high thermal
conductivity to cool the semiconductor component. In such case, the
electrical signals to operate the component must be connected to
the component from the ceramic.
[0044] Indirect attachment to the metal support board can also be
used. In this package, all of the required components are thermally
coupled to a metal support board, that can also incorporate
embedded passive components such as conductors and resistors into
the multilayer ceramic portion, to connect the various components,
i.e., semiconductor components, circuits, heat sink and the like,
in an integrated package. In the case of LED arrays, where
electrical circuit considerations would dictate an insulating
material be used, thermal conduction can be problematic. Here the
inventive reflective barriers further serve as thermal spreading
devices to help transfer heat received by conduction and radiation
through the insulating layer to the metal base.
[0045] For a more complex structure having improved heat sinking,
the integrated package of the invention combines a first and a
second LTCC-M substrate. The first substrate can have mounted
thereon a semiconductor device, and a multilayer ceramic circuit
board with embedded circuitry for operating the component; the
second substrate has a heat sink or conductive heat spreader
mounted thereon. Thermoelectric (TEC) plates (Peltier devices) and
temperature control circuitry are mounted between the first and
second substrates to provide improved temperature control of
semiconductor devices. A hermetic enclosure can be adhered to the
metal support board.
[0046] The use of LTCC-M technology can also utilize the advantages
of flip chip packaging together with integrated heat sinking. The
packages of the invention can be made smaller, cheaper and more
efficient than existing present-day packaging. The metal substrate
serves as a heat spreader or heat sink. The flip chip can be
mounted directly on the metal substrate, which is an integral part
of the package, eliminating the need for additional heat sinking. A
flexible circuit can be mounted over the bumps on the flip chip.
The use of multilayer ceramic layers can also accomplish a fan-out
and routing of traces to the periphery of the package, further
improving heat sinking. High power integrated circuits and devices
that have high thermal management needs can be used with this new
LTCC-M technology.
[0047] It is understood that the above-described embodiments are
illustrative of only a few of the many possible specific
embodiments, which can represent applications of the invention.
Numerous and varied other arrangements can be made by those skilled
in the art without departing from the spirit and scope of the
invention.
* * * * *