U.S. patent application number 10/683489 was filed with the patent office on 2005-04-14 for high power light emitting diode device.
Invention is credited to Ng, Kee Yean, Tan, Cheng Why, Tham, Ji Kin.
Application Number | 20050077616 10/683489 |
Document ID | / |
Family ID | 33160036 |
Filed Date | 2005-04-14 |
United States Patent
Application |
20050077616 |
Kind Code |
A1 |
Ng, Kee Yean ; et
al. |
April 14, 2005 |
High power light emitting diode device
Abstract
A circuit element having a heat-conducting body having top and
bottom surfaces, and a die having an electronic circuit thereon is
disclosed. The die includes first and second contact points for
powering the electronic circuit. The die is in thermal contact with
the heat-conducting body, the die having a bottom surface that is
smaller than the top surface of the heat-conducting body. The first
contact point on the die is connected to a first trace bonded to
the top surface of the heat-conducting body. An encapsulating cap
covers the die. The first trace has a first portion that extends
outside of the encapsulating cap and a second portion that is
covered by the encapsulating cap. The heat-conducting body is
preferably constructed from copper or aluminum and includes a
cavity having an opening on the first surface in which the die is
mounted. The die preferably includes a light-emitting device.
Inventors: |
Ng, Kee Yean; (Prai, MY)
; Tan, Cheng Why; (Bukit Mertajam, MY) ; Tham, Ji
Kin; (Gelugor, MY) |
Correspondence
Address: |
AGILENT TECHNOLOGIES, INC.
Intellectual Property Administration
Legal Department, DL429
P.O. Box 7599
Loveland
CO
80537-0599
US
|
Family ID: |
33160036 |
Appl. No.: |
10/683489 |
Filed: |
October 9, 2003 |
Current U.S.
Class: |
257/707 ;
257/E23.01; 257/E23.08; 257/E23.101; 257/E33.066; 257/E33.075 |
Current CPC
Class: |
H01L 2224/49175
20130101; H01L 2924/12041 20130101; H01L 23/36 20130101; H01L
2924/14 20130101; H01L 2924/00014 20130101; H01L 2924/15311
20130101; H01L 2924/181 20130101; H01L 24/48 20130101; H01L
2224/32257 20130101; H01L 2224/73265 20130101; H01L 2924/00014
20130101; H01L 2924/01068 20130101; H01L 2224/32245 20130101; H01L
2224/73265 20130101; H01L 2924/1532 20130101; H01L 33/62 20130101;
H01L 24/49 20130101; H01L 2924/1815 20130101; H01L 2224/48247
20130101; H01L 2924/207 20130101; H01L 2224/32245 20130101; H01L
2224/45015 20130101; H01L 2924/00 20130101; H01L 2924/00 20130101;
H01L 2224/45099 20130101; H01L 2224/48247 20130101; H01L 2924/00012
20130101; H01L 2924/00014 20130101; H01L 2224/49175 20130101; H01L
2924/00014 20130101; H01L 33/64 20130101; H01L 2224/48091 20130101;
H01L 2224/48247 20130101; H01L 2224/48091 20130101; H01L 2924/181
20130101 |
Class at
Publication: |
257/707 |
International
Class: |
H01L 023/10 |
Claims
What is claimed is:
1. A circuit element comprising: a heat-conducting body having top
and bottom surfaces; a die having an electronic circuit thereon,
said die including first and second contact points for powering
said electronic circuit, said die being in thermal contact with
said heat-conducting body, said die having a bottom surface that is
smaller than said top surface of said heat-conducting body; a first
trace comprising an electrically conducting material bonded to said
top surface of said heat-conducting body and electrically insulated
therefrom; a first electrically conducting path from said first
contact point to said first trace; and an encapsulating cap
covering said die, and said first electrically conducting path,
said first trace having a first portion that extends outside of
said encapsulating cap and a second portion that is covered by said
encapsulating cap.
2. The circuit element of claim 1 wherein said electronic circuit
comprises an LED.
3. The circuit element of claim 1 wherein said first trace
comprises an electrically conducting material on an insulating
substrate, said insulating substrate being bonded to said
heat-conducting body.
4. The circuit element of claim 3 wherein said insulating substrate
comprises an opening, said die being connected to said
heat-conducting body through said opening.
5. The circuit element of claim 1 wherein said heat-conducting body
comprises copper.
6. The circuit element of claim 1 wherein said heat-conducting body
comprises aluminum.
7. The circuit element of claim 1 wherein said heat-conducting body
comprises a cavity having an opening on said first surface and
wherein said die is mounted in said cavity.
8. The circuit element of claim 1 wherein said die comprises a
light-emitting device that emits light in a direction pointing away
from said top surface and wherein said encapsulating cap is
optically transparent to said emitted light.
9. The circuit element of claim 1 wherein said first electrically
conducing path comprises a wire having a first end bonded to said
first contact point and a second end bonded to said first
trace.
10. The circuit element of claim 9 wherein said first trace
comprises a T-shaped strip of copper.
11. The circuit element of claim 9 further comprising a solder ball
on said first portion of said first trace.
12. The circuit element of claim 1 further comprising a second
trace comprising an electrically conducting medium bonded to said
top surface of said heat-conducting body and insulated therefrom,
said second trace being electrically connected to said second
contact point by a second electrically conducting path.
13. The circuit element of claim 12 wherein said second trace
further comprises a solder ball.
14. The circuit element of claim 12 further comprising a third
solder ball positioned on said top surface of said heat conducting
die and positioned non-colinearly with respect to said first and
second solder balls.
15. The circuit element of claim 1 wherein said encapsulating cap
comprises a dam surrounding said die, said dam being filled with a
clear encapsulating material.
16. The circuit element of claim 1 wherein said bottom surface of
said heat-conducting body comprises a surface having a greater
surface area than said top surface of said heat-conducting
body.
17. The circuit element of claim 16 wherein said bottom surface of
said heat-conducting body comprises fins for facilitating heat
transfer from said bottom surface of said heat conducting body.
18. The circuit element of claim 1 further comprising a circuit
board having top and bottom surfaces and a hole therethrough, said
first trace being connected to a conductor on said bottom surface
of said circuit board such that said die is visible from a location
above said top surface of said circuit board.
19. The circuit element of claim 18 wherein said heat conducting
body is connected to said circuit board via the second and third
locations on said bottom surface of said circuit board.
20. The circuit element of claim 19 wherein said connections
between said circuit board, said first trace, and said second and
third locations comprise solder joints.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to packaged integrated
circuits, and more particularly, to high-power LEDs.
BACKGROUND OF THE INVENTION
[0002] Light emitting diodes (LEDs) are fabricated from compound
semiconductor materials, which have the characteristic of emitting
light when biased with a forward current. LEDs are widely used as
indicators or displays in various types of appliances.
Historically, LEDs emitted a relatively low level of light compared
to other light sources and were suitable for indoor applications
only.
[0003] Recent advances in compound semiconductor materials research
have yielded new LEDs, which emit very high levels of light.
Examples of these new LED materials are Aluminum Indium Gallium
Phosphide (AlInGaP) and Indium Gallium Nitride (InGaN). These high
brightness LEDs have given rise to new LED devices suitable for
applications in areas such as outdoor video displays, automotive
signals, traffic signals and illumination.
[0004] The high output achieved with these devices is the result of
efficient semiconductor materials and of driving the LEDs at very
high forward currents. Drive currents in the hundreds or thousands
of milliamperes (mA) are often utilized. Unfortunately, such high
drive currents produce excessive heat. Since the efficiency of an
LED decreases at these high temperatures, light output starts to
drop. In addition, the packaging of the devices starts to break
down due to prolonged exposure to the elevated temperatures. Such
packaging failures limit useful life of the device. A number of
device packages have been proposed; however, none of these provide
sufficient heat dissipation for the current generation of
high-power LEDs.
SUMMARY OF THE INVENTION
[0005] The present invention includes a circuit element having a
heat-conducting body having top and bottom surfaces, and a die
having an electronic circuit thereon. The die includes first and
second contact points for powering the electronic circuit. The die
is in thermal contact with the heat-conducting body, the die having
a bottom surface that is smaller than the top surface of the
heat-conducting body. A first trace constructed from an
electrically conducting material bonded to the top surface of the
heat-conducting body and electrically insulated therefrom is
connected to the first contact point by an electrically conducting
path that is preferably a wire bond. An encapsulating cap covers
the die and the first electrically conducting path. The first trace
has a first portion that extends outside of the encapsulating cap
and a second portion that is covered by the encapsulating cap. The
heat-conducting body is preferably constructed from copper or
aluminum and includes a cavity having an opening on the first
surface in which the die is mounted. The die preferably includes a
light-emitting device that emits light in a direction pointing away
from the top surface, the encapsulating cap being optically
transparent to the emitted light. The encapsulating cap can include
a dam surrounding the die, the dam is filled with a clear
encapsulating material.
[0006] The first trace preferably includes a solder ball on the
first portion thereof. The circuit element may include a second
trace for making the connection to the second contact point on the
die. Alternatively, the second connection can be made through the
heat-conducting die itself. A second solder ball is preferably
placed on the second trace or the heat-conducting body to provide
an electrical connection to the second contact point of the die. A
third solder ball is preferably provided on the top surface of the
heat conducting body at a location that is non-colinear with the
first and second solder balls. The solder balls provide a mechanism
for coupling the circuit element to a printed circuit board as well
as providing power to the die. To further facilitate heat transfer
from the heat-conducting body, the bottom surface of the heat
conducting body may include fins or other features for increasing
the surface area of the bottom surface relative to the top surface
of the heat conducting body.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a cross-sectional view of a packaged LED according
to one prior art design.
[0008] FIG. 2 is a cross-sectional view of the packed LED shown in
FIG. 1 attached to a typical printed circuit board (PCB).
[0009] FIG. 3A is a top view of LED device.
[0010] FIG. 3B is a cross-sectional view through line 341-342 of
LED device shown in FIG. 3A.
[0011] FIG. 3C is a top view of substrate 361 that illustrates the
manner in which an LED device is mounted on a substrate such as a
PCB.
[0012] FIG. 3D is a cross-sectional view through line 351-352 of
the LED device shown in FIG. 3C.
[0013] FIG. 4 is a cross-sectional view of an LED device with a
greater surface area according to another embodiment of the present
invention.
[0014] FIG. 5 is a cross-sectional view of an LED device that
provides a reflector according to another embodiment of the present
invention.
[0015] FIG. 6A is a top view of an LED device.
[0016] FIG. 6B is a cross-sectional view of the LED device shown in
FIG. 6A through line 751-752.
[0017] FIG. 7 is a cross-sectional view of an array of LED devices
that share a single heat sink according to another embodiment of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0018] The manner in which the present invention provides its
advantages can be more easily understood with reference to FIGS. 1
and 2, which illustrate the manner in which one class of prior art
LED provides heat dissipation. Refer now to FIG. 1, which is
cross-sectional view of a packaged LED according to one prior art
design. An LED 100 is mounted in a cavity of a substrate 102 using
a conductive medium 104. A first bond wire 106 electrically
connects one terminal of the LED 100 to one electrical contact 110
while a second bond wire 108 electrically connects a second
terminal of LED 100 to another electrical contact 112. An
encapsulating body 114 substantially encases the LED, the bond
wires, the substrate and the contacts to provide protection for the
LED.
[0019] Refer now to FIG. 2, which is a cross-sectional view of the
packed LED shown in FIG. 1 attached to a typical printed circuit
board (PCB) 116. The base of substrate 102 is mounted on a PCB 116
so that it is in direct contact with PCB 116. One electrical
contact 110 is electrically connected to the trace 118 of the PCB
via an electrically conductive medium 120 while the other
electrical contact 112 is electrically connected to trace 122 of
the PCB via an electrically conductive medium 124. Typically,
solder is used for these connections. The heat generated in LED 100
is conducted to the PCB through substrate 102.
[0020] The LED device in FIG. 1 has many disadvantages. For
instance, the ability of substrate 102 to act as a heat sink and
heat transfer conduit depends on the size of the substrate. Since
the electrical contacts at the sides of the substrate increase the
footprint of the device without providing additional surface area
for heat conduction, these devices cannot incorporate heat sinks as
large as the footprint of the device. That is, the lateral size of
the heat sink will always be smaller than the overall footprint of
the device. Furthermore, there is a limit to how tall or thick the
substrate can be without having to increase device design
complexity. Hence, the ability of the substrate to act as a heat
sink for temporarily absorbing heat from the LED is limited.
[0021] Prior art devices attempt to overcome the limitations of the
substrate size by relying on a secondary heat sink in the form of
the PCB 116 to help conduct the heat away from the LED, and hence,
limit the temperature rise to which the LED is subjected. This
solution moves the heat dissipation problem to the PCB. To provide
adequate heat conduction and sinking, a metal core PCB with some
provision for transferring the heat to the surrounding air is often
needed. Since the cost of such metal core PCBs is significantly
greater than the cost of the more common glass epoxy PCBs, this
solution significantly increases the cost of the final circuit
utilizing the LED. In addition, this solution increases the design
complexity of the final PCB since the PCB must be arranged to
dissipate the heat without subjecting other components on the PCB
to excessive temperatures.
[0022] In addition, these prior art solutions require a good
contact between the PCB and substrate 102. The coplanarity among
the leads 110 & 112 and the substrate 102 can make achieving
adequate thermal contact difficult. Even if a layer of thermal glue
is used to ensure good contact, air gaps or voids may still exist
in between the device and the mounting PCB. Furthermore, such
thermal glue layers can also restrict the flow of heat. Finally,
the thermal glue further increases the cost and complexity of the
assembly of the final PCB.
[0023] The present invention provides a high power LED device,
which has sufficient heat sinking capability to absorb fluctuations
in the heat output of the LED. In addition, the present invention
dissipates heat without relying on secondary heat sinks. Refer now
to FIGS. 3A-D, which illustrate an LED device 300 according to one
embodiment of the present invention. FIG. 3A is a top view of LED
device 300, and FIG. 3B is a cross-sectional view through line
341-342 shown in FIG. 3A. LED device 300 has a body 301 with a
first surface 302 and a second surface 304 on the opposite side. A
circuit trace having electrical contacts 306 and 308 on a thin film
layer 310 is attached to surface 302. The circuit layer has an
opening 312 in the center that provides access to surface 302. An
LED 314 is attached to surface 302 using an adhesive 316.
Electrical connections by way of bond wires 318 and 320 connect the
LED to the electrical contacts 306 and 308. Solder bumps 322 and
324 are then deposited on one portion of the electrical contacts
306 and 308. The LED and bond wires and a portion of the electrical
contacts are encapsulated in an optically clear material 326.
[0024] To facilitate the wire bonding operation, traces 306 and 308
preferably include a T-shaped region as shown at 331 in FIG. 3A.
This enlarged area reduces the precision required in the wire
bonding process.
[0025] Refer now to FIGS. 3C and 3D, which illustrate the manner in
which LED device 300 is mounted on a substrate 361 such as a PCB.
FIG. 3C is a top view of substrate 361, and FIG. 3D is a
cross-sectional view through line 351-352. Substrate 361 includes
an opening 370 through which LED 314 is viewed. Substrate 361 also
includes two traces shown at 371 and 372, which are positioned to
connect to solder bumps 322 and 324.
[0026] LED device 300 is connected to substrate 361 via traces 371
and 372 by any of a number of methods. For example, heat can be
applied to substrate 361 sufficient to cause the solder to reflow
and make the connections between LED device 300 and substrate 361.
In another example, the solder can be deposited on the PCB before
the placement of device 300, and the assembly subsequently
reflowed. Additionally, an electrically conductive adhesive such as
epoxy, silicone or suitable plastic can be used to make the
attachment. Such adhesive can be either cured by heat or other
means, such as exposure to ultraviolet (UV) light.
[0027] Body 301 provides two functions. First, body 301 acts as a
heat sink that buffers thermal fluctuations. Surface 304 dissipates
heat to the surrounding air. Body 301 is preferably made of a metal
such as copper or aluminum to provide a high thermal conductivity.
Since surface 304 is as large as the footprint of the device, this
embodiment of the present invention provides substantially more
heat transfer area than the prior art devices discussed above.
[0028] It should be noted that the heat transfer capability of the
present invention can be enhanced by including a surface having a
greater surface area in place of surface 304. Such an embodiment is
shown in FIG. 4, which is a cross-sectional view of an LED device
400 according to another embodiment of the present invention. In
construction, LED device 400 is similar to LED device 300 discussed
above except for the second surface of the device body. LED device
400 has a body 401 with a first surface 402 and a second surface
404 on the opposite side. A circuit trace consisting of electrical
contacts 406 and 408 on a thin film layer 410 is attached to the
said first surface of the body. The circuit layer has an opening in
the center to provide access to surface 402. An LED 414 is attached
to surface 402 using an adhesive layer. Electrical connections by
way of bond wires 418 and 420 connect the LED to the electrical
contacts 406 and 408. Solder bumps 422 and 424 are then deposited
on one portion of the electrical contacts 406 and 408. The LED and
bond wires and a portion of the electrical contacts are
encapsulated with an optically clear material 426. Instead of a
planar profile, the surface 404 has a fin-like, rib-like or
stub-like shape to enhance heat dissipation. In effect, body 401 is
a heat sink. The fin can be advantageously designed into any shape
such as taper, rectangular, stubs etc. The fins can be molded as
part of a single body as shown in the drawing or attached to
surface 404 discussed above by any mechanism that provides good
heat conduction.
[0029] The above-described embodiments utilize a body having a flat
surface such as surface 302 on which the LED is mounted. However,
the present invention can be implemented by using a body that
includes a cavity having reflective sides that improve light
extraction from the LED by reflecting light leaving the sides of
the LED such that the reflected light becomes part of the output
light from the device. Refer now to FIG. 5, which is a
cross-sectional view of an LED device 600 that provides such a
reflector. In construction, LED device 600 is similar to LED device
300 discussed above except that a recess cavity is provided in the
first surface 602. LED device 600 includes a body 601 having a
first surface 602 and a second surface 604 on the opposite side. A
circuit trace consisting of electrical contacts 606 and 608 on a
thin film layer 610 is attached to surface 602. The circuit layer
has an opening in the center to provide access to surface 602. An
LED 614 is attached to the first surface 602 inside a cavity 603
using an adhesive 616. Electrical connections by way of bond wires
618 and 620 connect the LED to the electrical contacts 606 and 608.
Solder bumps 622 and 624 are then deposited on one portion of the
electrical contacts 606 and 608. The LED and bond wires and a
portion of the electrical contacts are encapsulated in an optically
clear material 626.
[0030] The above-described embodiments of the present invention
utilize an encapsulating layer to protect the LED and bond wires.
Embodiments that utilize a mold ring to aid in this encapsulating
process can also be incorporated. Refer now to FIGS. 6A and 6B,
which illustrate an LED device 700 according to another embodiment
of the present invention. FIG. 6A is a top view of LED device 700,
and FIG. 6B is a cross-sectional view of LED device 700 through
line 751-752. In construction, LED device 700 is similar to LED
device 300 discussed above except that an annular ring 764 is
provided on the first surface 702. LED device 700 has a body 701
having a first surface 702 and a second surface 704 on the opposite
side. An annular shaped ring 764 is attached on the first surface
702 by any known method such as using a thermally conductive
adhesive, solder or just mechanically attached with fasteners. A
circuit trace consisting of electrical contacts 706 and 708 on a
thin film layer 710 is attached to surface 702. The circuit layer
has an opening 712 in the center thereof to provide access to
surface 702. An LED 714 is attached to surface 702 using an
adhesive 716. Electrical connections by way of bond wires 718 and
720 connect the LED to the electrical contacts 706 and 708. Solder
bumps 722 and 724 are then deposited on one portion of the
electrical contacts 706 and 708. The LED and bond wires, and a
portion of the electrical contacts, are encapsulated with optically
clear material 726 by filling the cavity created by annular ring
764.
[0031] The annular-shaped ring 764 can be of any shape such as
circular or polygonal. It acts as a reservoir to contain the
optically clear encapsulant 726. Additionally, an optically clear
lens 765 made of plastic, polymer or glass can be incorporated on
top of the annular-shaped body so as to direct the light in a
desired direction. The lens can be glued to the surface of the
encapsulant or formed in the encapsulant by a molding
operation.
[0032] It should be noted that surface 702 may include additional
solder bumps to provide additional adhesion points for connecting
the LED device to a PCB or the like. Such solder bumps are shown at
771 and 772 in FIG. 6A. These solder bumps may be formed on a
conducting trace that is attached to surface 702 by an appropriate
adhesive or directly on surface 702 if the metal chosen for body
701 is wet by solder. In this regard, copper is the preferred
material for body 701.
[0033] The above-described embodiments utilize bond wires to make
all of the connections between the LED and the solder bumps that
connect to the PCB. However, the body may be used for one of these
connections. If the chip is conductive or the bottom of the chip
having the LED has a contact thereon, and the chip is mounted to
the body by an electrically conducting adhesive, then the body can
be used to connect to that contact. In this case, an appropriately
placed solder bump is formed directly on surface 702.
[0034] The above-described embodiments utilize passive
convection/conduction to move the heat from the bottom surface of
the body, e.g., surface 704 or surface 404, to the surrounding air.
However, embodiments in which a fan is utilized to enhance the
airflow can also be constructed. The fan can be attached to the
bottom surface of the body or provided in the enclosure in which
the LED device is located.
[0035] From the forgoing discussion, it is clear that an LED device
according to the present invention has the body, which spans the
device footprint. Therefore the LED device has a heat sink that
utilizes the full footprint of the device. Additionally, the body
is not encased in any kind of thermally insulative encapsulant, and
therefore, is able to dissipate heat more efficiently. Further, the
problems related to the coplanarity of the leads and the heat sink
in prior art devices have been overcome.
[0036] The bottom surface of the body is exposed to the ambient,
and hence, efficient heat dissipation can be obtained.
Additionally, since the bottom surface does not come in contact
with any other surface, the body can be fabricated such that this
surface extends as long or deep as possible. Hence, it is now
possible to fabricate devices with long or deep heat sinks without
having to increase the lateral dimensions of the devices.
[0037] Furthermore, since an LED device according to the present
invention does not need to conduct heat to the mounting substrate,
the mounting substrate can be constructed from common materials
such as those used in inexpensive PCBs. In addition, the end-user
does not need to provide an additional heat sink, thus simplifying
the design of products that use the LED device.
[0038] The above-described embodiments of the present invention
have been described in terms of transferring the heat generated by
the LED to the air via contact between the air and the second
surface of the body on which the LED is mounted. However, the
present invention can be utilized to construct products having a
number of LEDs on a single PCB which transfer the heat generated in
each of the LEDs to a common heat sink that dissipates the heat.
Refer now to FIG. 7, which is a cross-sectional view of an array
800 of LED devices that share a single heat sink according to
another embodiment of the present invention. Array 800 is
constructed on a PCB 810. A plurality of LED devices according to
the present invention is mounted on PCB 810 in a manner analogous
to that described above. Exemplary LED devices are shown at
801-803. The body of each of the LED devices is in thermal contact
with a common heat sink 821. For example, the individual LED
devices can be connected to heat sink 821 by a layer of heat
conducting adhesive. Heat sink 821 may also include structures,
such as the fins shown at 822 to facilitate the transfer of heat to
the surrounding air. Heat sink 821 can also include a fan 823 to
further enhance the transfer of heat from heat sink 821 to the
surrounding air.
[0039] In the above-described embodiments, the die is mounted on a
heat-conducting body that is preferably made from Aluminum or
Copper. However, other materials such as ceramics and composites
may be utilized for the heat-conducting body.
[0040] Various modifications to the present invention will become
apparent to those skilled in the art from the foregoing description
and accompanying drawings. Accordingly, the present invention is to
be limited solely by the scope of the following claims.
* * * * *