U.S. patent application number 15/279043 was filed with the patent office on 2017-01-19 for jetting a highly reflective layer onto an led assembly.
The applicant listed for this patent is Bridgelux, Inc.. Invention is credited to R. Scott West.
Application Number | 20170018539 15/279043 |
Document ID | / |
Family ID | 48168408 |
Filed Date | 2017-01-19 |
United States Patent
Application |
20170018539 |
Kind Code |
A1 |
West; R. Scott |
January 19, 2017 |
Jetting a Highly Reflective Layer Onto an LED Assembly
Abstract
A layer of Highly Reflective (HR) material is deposited by
jetting microdots of the HR material in liquid form onto a
substrate and then allowing the HR material to harden. In one
example, the HR layer is the HR layer of a white LED assembly. The
HR layer is jetted onto the substrate around LED dice of the
assembly after die attach and wire bonding have been completed. The
HR material can be made to flow laterally so that areas of the
substrate under wire bonds are coated with HR material, so that HR
material contacts side edges of the LED dice, and so that HR
material contacts the inside side edge of a retaining ring. By
jetting the HR material in this way, the amount of substrate that
is not covered with HR material is reduced, thereby improving the
light efficiency of the resulting LED assembly.
Inventors: |
West; R. Scott; (Pleasanton,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bridgelux, Inc. |
Livermore |
CA |
US |
|
|
Family ID: |
48168408 |
Appl. No.: |
15/279043 |
Filed: |
September 28, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13284835 |
Oct 28, 2011 |
9461023 |
|
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15279043 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 25/0753 20130101;
H01L 2224/45144 20130101; H01L 2224/73265 20130101; H01L 33/60
20130101; H01L 2933/0025 20130101; H01L 2224/48227 20130101; H01L
2224/73265 20130101; H01L 33/502 20130101; H01L 2224/48137
20130101; H01L 33/62 20130101; H01L 2224/8592 20130101; H01L
2924/181 20130101; H01L 2224/45144 20130101; H01L 2933/0058
20130101; H01L 2224/32225 20130101; H01L 2933/0066 20130101; H01L
2224/32225 20130101; H01L 2924/181 20130101; H01L 33/46 20130101;
H01L 2924/00012 20130101; H01L 2924/00 20130101; H01L 2924/00
20130101; H01L 2224/48227 20130101 |
International
Class: |
H01L 25/075 20060101
H01L025/075; H01L 33/62 20060101 H01L033/62; H01L 33/50 20060101
H01L033/50; H01L 33/46 20060101 H01L033/46; H01L 33/60 20060101
H01L033/60 |
Claims
1-25 (canceled)
26. An apparatus comprising: a substrate having an upper surface; a
Light Emitting Diode (LED) die having a plurality of side edges,
wherein the LED die is disposed on a first part of the upper
surface of substrate; and a layer of highly reflective (HR)
material disposed on a second part of the upper surface of the
substrate such that the layer does not extend under the LED die and
does not extend over the LED die but such that the layer contacts
at least one of the side edges of the LED die.
27. The apparatus of claim 26, further comprising: a bond wire
attached to the LED die, wherein the layer of HR material extends
under the bond wire between the bond wire and the substrate.
28. The apparatus of claim 26, wherein the layer of HR material is
more than 10 microns thick, and wherein the HR material comprises
titanium.
29. The apparatus of claim 26, wherein the HR material has a
reflectivity of more than 85 percent.
30. The apparatus of claim 26, wherein the upper surface of the
substrate forms a well, wherein the LED die is disposed in the
well, and wherein at least a portion of the layer of HR material is
disposed in the well.
31. The apparatus of claim 26, wherein the upper surface of the
substrate has a nonplanar shape, and wherein the layer of HR
material is substantially conformal to the upper surface but does
not cover the LED die.
32. The apparatus of claim 26, wherein the layer of HR material
comprises titanium dioxide and silicone.
33. The apparatus of claim 26, wherein the HR material has a
viscosity less than 1100 centipois (cP) at room temperature when
deposited on the second part of the upper surface of the
substrate.
34. The apparatus of claim 26, wherein the layer of HR material is
substantially planar, wherein there is a window in the layer of HR
material, and wherein the LED die is disposed in the window.
35. The apparatus of claim 26, wherein the substrate is a printed
circuit board.
36. An apparatus comprising: a substrate having an upper surface; a
Light Emitting Diode (LED) die that is disposed on a first part of
the upper surface of substrate; and a layer of highly reflective
(HR) material disposed on a second part of the upper surface of the
substrate such that the layer of HR material does not extend under
the LED die and does not extend over the LED die, wherein the layer
of HR material has a reflectivity of at least 85 percent.
37. The apparatus of claim 36, wherein the LED die has a plurality
of side edges, and wherein the layer of HR material contacts at
least one of the side edges of the LED die.
38. The apparatus of claim 36, wherein none of the upper surface of
the substrate is not covered by the HR material between the LED die
and the layer of HR material.
39. The apparatus of claim 36, further comprising: a bond wire
attached to the LED die, wherein the layer of HR material extends
under the bond wire between the bond wire and the substrate.
40. The apparatus of claim 36, wherein the layer of HR material is
more than 10 microns thick.
41. The apparatus of claim 36, wherein the layer of HR material
comprises titanium dioxide and silicone.
42. The apparatus of claim 36, wherein the upper surface of the
substrate forms a well with sidewalls, wherein the LED die is
disposed in the well, and wherein at least a portion of the layer
of HR material is disposed on the sidewalls of the well.
43. The apparatus of claim 36, wherein the upper surface of the
substrate has a nonplanar shape, and wherein the layer of HR
material is substantially conformal to the upper surface but does
not cover the LED die.
44. The apparatus of claim 36, wherein the HR material has a
viscosity less than 1100 centipois (cP) at room temperature when
deposited on the second part of the upper surface of the
substrate.
45. The apparatus of claim 36, wherein the layer of HR material is
substantially planar, wherein there is a window in the layer of HR
material, and wherein the LED die is disposed in the window.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of, and claims priority
under 35 U.S.C. .sctn.120 from, nonprovisional U.S. patent
application Ser. No. 13/284,835 entitled "Jetting a Highly
Reflective Layer onto an LED Assembly," now U.S. Pat. No.
9,461,023, filed on Oct. 28, 2011, the subject matter of which is
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present disclosure relates generally to Light Emitting
Diode (LED) devices and assemblies and related methods.
BACKGROUND INFORMATION
[0003] There are many different types of Light Emitting Diode (LED)
assemblies. FIG. 1 (Prior Art) is a top-down diagram of one such
LED assembly 1. LED assembly 1 includes four laterally-contacted
LED dices 2-5 that are mounted on a metal core substrate 6.
Substrate 6 in this case is a Metal Core Printed Circuit Board
(MCPCB). Areas 7-10 illustrated in dashed lines represent portions
of a metal layer that is disposed underneath a solder mask layer 11
(see FIG. 2). Reference numeral 12 identifies a portion of metal
portion 7 that is exposed through a first opening in the solder
mask layer 11. Similarly, reference numeral 13 identifies a portion
of metal portion 8 that is exposed through a second opening in
solder mask layer 11. These exposed portions 12 and 13 serve as
bond pads. Ring structure 14 is a retaining ring of silicone. An
amount of a material often referred to as phosphor 15 is disposed
within the ring structure 14 over the LED dice. This phosphor
actually comprises silicone and particles of phosphor that area
embedded in the silicone.
[0004] FIG. 2 (Prior Art) is a simplified cross-sectional diagram
of LED assembly 1 of FIG. 1. MCPCB 6 includes an aluminum layer 16,
a global dielectric layer 17, a layer 18 of metallization of which
metal portions 7-10 are parts, and solder mask layer 11. Layer 18
of metal may involve multiple sublayers of metal including an upper
layer of a very reflective metal such as silver. Metal portion 10
is a square pad of metal upon which the LED dice 2-5 are mounted.
The LED dice 2-5 are fixed to pad 10 by associated amounts of
silver epoxy. Amount 19 of silver epoxy is shown fixing LED die 4
to pad 10. Amount 20 of silver epoxy is shown fixing LED die 5 of
pad 10. Reference numerals 21-23 identify wire bonds.
[0005] A layer 24 of a Highly Reflective (HR) material is disposed
within ring 14 between and around the dice 2-5 and wire bonds 21-23
as illustrated. The diagram is simplified in that the regions of
the HR material have smooth and rounded edges. Some of the light
emitted by LED dice 2-5 may be absorbed by phosphor particles in
phosphor 15. These particles may then fluoresce and re-emit light
such that this light is directed downward, rather than upward as is
desired. Reference numeral 35 identifies one such particle of
phosphor. A light ray 36 is emitted from the top of LED die 4 and
travels up and is absorbed by particle 35. A second light ray 37 is
then emitted from particle 35 and this second light ray travels
back downward as shown. HR material 24 is provided so that this
light ray will be reflected so that it can pass upward and out of
the assembly as light ray 38. Particle 35 is but one such particle.
There are numerous particles dispersed throughout the silicone
material of phosphor 15. Light emitted from the LED dice 2-5 can be
emitted in various different directions including out of the sides
of the LED dice. Similarly, a light ray emitted from a phosphor
particle can travel away from the particle any direction. The
illustration of particle 35, of the direction of light emission
from particle 35, and of the associated light rays 36, 37 and 38 in
FIG. 2 are only representative of one such particle and its
associated light rays. An example of an HR material is a silicone
material that is commercially available from ShinEtsu Chemical Co.
Ltd. of Tokyo, Japan.
[0006] FIGS. 3-10 (Prior Art) illustrate a prior art method of
manufacturing the LED assembly 1 of FIG. 1. FIG. 3 (Prior Art) is a
top-down diagram of a panel 25 of MCPCBs. MCPCB 6 is one of the
MCPCBs of the panel. FIG. 4 (Prior Art) is a top-down diagram of
the pad portion 10 of the MCPCB portion 6 of panel 25. This pad
portion 10 is exposed through an opening in the solder mask layer
11. FIG. 5 (Prior Art) is an illustration of a screen printing mask
26 used in the next step of forming the layer 24 of Highly
Reflective (HR) material. FIG. 6 (Prior Art) is a diagram that
shows the result of using the screen printing mask 26 of FIG. 5 to
deposit the HR layer 24 onto panel 25. HR material of layer 24 is
deposited in the shaded circular region. This circular region is in
the center of MCPCB 6. As illustrated, there are eight windows
27-34 in the circular HR layer 24. FIG. 7 (Prior Art) is a diagram
that shows the result of a next die attach step. Each of the four
dice 2-5 is attached by an amount of silver epoxy in a
corresponding one of the four center windows 27-30 in the HR layer
24. Each of the openings 27-30 in the HR layer is slightly larger
than its associated die in order to accommodate variations in
physical dimensions and inaccuracies of the placement of the dice
and wire bonds. FIG. 8 (Prior Art) is a diagram that shows the
result of a next step of attaching wire bonds. Only three of the
wire bonds 21-23 are identified in the diagram with reference
numerals. Some of the wire bonds extend between dice. Others of the
wire bonds extend from a die to a conductive upper layer of the
substrate. FIG. 9 (Prior Art) shows the result of a next step of
forming retaining ring 14. Retaining ring 14 is formed so that it
encircles the circular layer 24 of HR material as illustrated. FIG.
10 (Prior Art) shows the result of a next step of placing the
phosphor 15 over the LED dice 2-5 in the area bounded by retaining
ring 14. After the phosphor 15 has cured, the panel 25 is
singulated to form multiple LED assemblies of which LED assembly 1
is one.
SUMMARY
[0007] After any die attach and wire bonding steps in the
manufacturing of an array-based LED assembly, a layer of Highly
Reflective (HR) material is deposited around the LED dice to coat
the upper surface of the substrate. In one example, the HR material
is deposited with precision by jetting microdots of the HR material
in liquid form onto selected portions of the upper surface of the
substrate, thereby forming a layer of HR material that is thick
enough (at least 10 microns thick) to have a reflectivity of at
least 85 percent.
[0008] Limits on mechanical tolerances can lead to physical
differences between LED assemblies being manufactured. LED dice may
differ slightly in size, and LED dice may be placed in slightly
different locations from one LED assembly to the next. In
accordance with one novel aspect, machine imaging is usable to
detect such physical differences from LED assembly to LED assembly
and to control the jetting process to adjust for such physical
differences so that in each LED assembly being manufactured
substantially all of the upper substrate surface that is not
covered with an LED die is coated with HR material.
[0009] In one example, each microdot of HR material has a diameter
of less than 100 microns and is typically 50-80 microns in
diameter. The HR material has an adequately low viscosity (less
than 1100 cP) that once reaching the substrate surface the HR
material flows laterally to some degree. Due to the lateral flow of
the HR material, the HR material can be made to flow under bridging
wire bonds and to coat the substrate underneath the wire bonds. Due
to this lateral flow, the HR material flows can be made to flow
laterally and to reach and to wet side edges of the LED dice. Due
to this lateral flow, the HR material can be made to flow laterally
and to reach and to wet the inside side edge of a phosphor
retaining ring. In one example, the area of substrate between LED
dice is not coated with HR material in order to reduce
manufacturing time. Because the HR material is only deposited after
die attach and after wire bonding, fiducial markers on the upper
surface of the substrate (that would otherwise be covered and
obscured by HR material were conventional screen printing used to
deposit the HR material) are observable and usable during die
attach and wire bonding. The depositing of the HR layer by jetting
microdots of HR material results in a reduction in the amount of
exposed substrate area that is not covered with HR material.
Reducing the amount of exposed substrate area that is not covered
with HR material serves to improve the light efficiency of the
resulting LED assembly.
[0010] Further details and embodiments and techniques are described
in the detailed description below. This summary does not purport to
define the invention. The invention is defined by the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, where like numerals indicate like
components, illustrate embodiments of the invention.
[0012] FIG. 1 (Prior Art) is top-down diagram of one type of
conventional LED assembly.
[0013] FIG. 2 (Prior Art) is a simplified cross-sectional side view
of the LED assembly of FIG. 1.
[0014] FIG. 3 (Prior Art) is a top-down diagram of a panel of
MCPCBs, including MCPCB 6.
[0015] FIG. 4 (Prior Art) is a top-down diagram of the die
placement area of MCPCB 6 before die placement.
[0016] FIG. 5 (Prior Art) is a diagram of a screen printing mask
used to apply a Highly Reflective (HR) material onto the die
placement area of FIG. 4.
[0017] FIG. 6 (Prior Art) is a top-down diagram of the die
placement area of FIG. 4 after deposition of the HR material.
[0018] FIG. 7 (Prior Art) is a top-down diagram of the die
placement area of FIG. 6 after die attach has been completed.
[0019] FIG. 8 (Prior Art) is a top-down diagram of the die
placement area of FIG. 7 after wire bonding has been completed.
[0020] FIG. 9 (Prior Art) is a top-down diagram of the die
placement area of FIG. 8 after formation of a phosphor retaining
ring.
[0021] FIG. 10 (Prior Art) is a top-down diagram of the die
placement area of FIG. 9 after placement of phosphor within the
retaining ring.
[0022] FIG. 11 is a top-down diagram of a white LED assembly in
accordance with one novel aspect.
[0023] FIG. 12 is a simplified cross-sectional side view of the
white LED assembly of FIG. 11.
[0024] FIG. 13 is a top-down diagram of a panel of MCPCBs of which
the MCPCB 56 of FIG. 12 is one.
[0025] FIG. 14 is a top-down diagram of the die placement area of
MCPCB 56.
[0026] FIG. 15 is a top-down diagram of the placement area of FIG.
14 after die attach has been completed.
[0027] FIG. 16 is a top-down diagram of the placement area of FIG.
15 after wire bonding has been completed.
[0028] FIG. 17 is a top-down diagram of the placement area of FIG.
16 after formation of the phosphor retaining ring.
[0029] FIG. 18 is a simplified cross-sectional diagram that shows
the deposition of an HR layer by jetting microdots of HR material
onto the substrate around and between the LED dice of the LED
assembly.
[0030] FIG. 19 is a simplified top-down diagram of the die
placement area after the jetting of the HR material has been
completed.
[0031] FIG. 20 is a simplified top-down diagram of the die
placement area after phosphor has been placed over the LED dice
within the confines of the retaining ring.
[0032] FIG. 21 is a simplified cross-sectional diagram of a white
LED assembly where the LED dice are disposed in a well.
[0033] FIG. 22 is a simplified cross-sectional diagram of a white
LED assembly having a ceramic substrate.
[0034] FIG. 23 is a simplified cross-sectional diagram of a white
LED assembly having a ceramic substrate, where the HR material does
not touch a side edge of any of the LED dice, and is not disposed
between the LED dice.
[0035] FIG. 24 is a flowchart of a method in accordance with a
novel aspect. In a first novel aspect, an HR layer is deposited
onto the substrate of an LED assembly after die attach and after
wire bonding. In a second novel aspect, an HR layer is deposited by
jetting microdots of HR material onto a substrate of the LED
assembly.
DETAILED DESCRIPTION
[0036] Reference will now be made in detail to some embodiments of
the invention, examples of which are illustrated in the
accompanying drawings.
[0037] FIG. 11 is a simplified top-down diagram of a white Light
Emitting Diode (LED) assembly 50 in accordance with one novel
aspect. LED assembly 50 includes four laterally-contacted LED dice
52-55 that are mounted on a substrate 56. In the present example,
the substrate is a Metal Core Printed Circuit Board (MCPCB). Areas
57-60 illustrated in dashed lines represent portions of a metal
layer 68 that is disposed underneath a solder mask layer 61 (see
FIG. 12). Reference numeral 62 identifies a portion of metal
portion 57 that is exposed through a first opening in solder mask
layer 61. Reference numeral 63 identifies a portion of metal
portion 58 that is exposed through a second opening in solder mask
layer 61. These exposed portions 62 and 63 serve as bond pads. Ring
structure 64 is a retaining ring of silicone. An amount of phosphor
65 is disposed within the ring structure 64 over the LED dice. This
phosphor actually comprises silicone and particles of phosphor that
are embedded in the silicone.
[0038] FIG. 12 is a simplified cross-sectional side view of the LED
assembly 50 of FIG. 11. MCPCB 56 includes an aluminum layer 66, a
global dielectric layer 67, a layer 68 of metallization, and solder
mask layer 61. Metal portions 57-60 are parts of layer 68. Layer 68
of metal involves multiple sublayers of metal including a lower
layer of copper, a middle layer of nickel, and an upper layer of a
very reflective metal such as silver. Metal portion 60 is a square
pad of metal upon which the LED dice 52-55 are mounted. The LED
dice are laterally-contacted blue LED devices whose epitaxial
layers are fabricated on an insulative sapphire substrate. LED dice
52-55 are fixed to pad 60 by associated amounts of silver epoxy.
Amount 69 of silver epoxy is shown fixing LED die 54 to pad 60.
Amount 70 of silver epoxy is shown fixing LED die 55 of pad 60.
Reference numerals 71-73 identify three of the wire bonds seen in
top-down perspective in FIG. 11.
[0039] A layer 74 of a Highly Reflective (HR) material is disposed
within ring 64 between and around the dice and the wire bonds as
illustrated. In the example of FIG. 12, the layer 74 contacts the
retaining ring 64 and also contacts the side edges of the LED dice
52-55.
[0040] FIGS. 13-20 illustrate a method of manufacturing the LED
assembly 50 of FIG. 11.
[0041] FIG. 13 is a top-down diagram of a panel 75 of MCPCBs. MCPCB
56 is one of the MCPCBs of the panel.
[0042] FIG. 14 is a top-down diagram of the pad portion 60 of the
MCPCB 56 of panel 75. This pad portion 60 is exposed through an
opening in the solder mask layer 61. The metal surfaces of the
panel are plasma cleaned. The corners 60A-60D serve as fiducial
markers used in later assembly steps.
[0043] FIG. 15 shows the result of the next step of the method. LED
dice 52-55 are placed and bonded to pad portion 60 as illustrated.
Each die is bonded to pad portion 60 by an associated amount of
silver epoxy. The bond line thickness (distance between the bottom
of the die and the top of the substrate surface) is less than 12
microns, and it is typically about 8 microns.
[0044] FIG. 16 shows the result of the next step of the method.
Wire bonds are attached. Some of these wire bonds extend between
dice. Others of the wire bonds extend from a die to a conductive
upper layer of the substrate. Reference numerals 71-73 identify
three of the wire bonds. The wire bonds may be sections of 1 mil
diameter gold wire.
[0045] FIG. 17 shows the result of the next step of the method.
Retaining ring 64 is formed on the structure as shown.
[0046] FIG. 18 illustrates a next step in the method in which layer
74 of HR material is deposited. In one example, layer 74 of HR
material is deposited using a jetting process. Microdots of HR
material are jetted out of a jet head 76 so that the microdots
travel toward the substrate 56 (MCPCB) and impact the substrate,
thereby effectively painting the surface of the substrate with HR
material. The liquid HR material does not flow under the LED dice
due to the silver epoxy bonding material occupying this space. The
jet head 76 is moved across the surface of the assembly of FIG. 17
as microdots of HR material are shot at the substrate so that areas
of the surface of the substrate around the dice, and between the
dice, and within the confines of circular retaining ring 64 are
painted with HR material, but such that the top surfaces of the
dice and the top surfaces of the wire bonds are not painted. One of
these microdots is identified with reference numeral 77 in FIG. 18.
Arrow 78 indicates the path of its travel from jet head 76 toward
the surface of the substrate. In one example, each microdot has a
diameter of less than 100 microns and is typically 50-80 microns in
diameter. The layer 74 is deposited to be at least 10 microns
thick. Arrows 82 identify this thickness. In the illustrated
example, layer 74 is 50 microns thick. The distance 80 between the
bottom of the jet head 76 and the upper extent of the wire bonds is
approximately 500 microns. In this example, the distance 79 between
the bottom of the jet head 76 and the upper surface of metal layer
68 (including pad 60) is approximately 1000 microns. In this
example, the distance 81 between the bottom of the jet head 76 and
the upper surface of retaining ring 64 is approximately 500
microns.
[0047] The HR material being jetted is made to have a predetermined
and controlled viscosity such that the liquid HR material will flow
laterally somewhat across the surface being painted before the HR
material cures and solidifies. Due to this flowing action,
microdots of liquid HR material are fired onto the substrate
surface near to a wire bond. The liquid HR material once on the
substrate surface then flows laterally underneath the wire bond so
that after the step of depositing the HR material is completed the
HR layer 74 coats the surfaces of substrate 56 (MCPCB) that are
directly underneath wire bonds. At an end of a wire bond where the
wire bond contacts the substrate, the entire circular circumference
of the wire is contacting HR material. Similarly, due to the
predetermined viscosity of the liquid HR material, the HR material
flows laterally such that it reaches and wets the side edges of the
LED dice 52-55 as illustrated. Reference numeral 83 identifies a
side edge of LED die 54. In this example, only the bottom sapphire
portion of the side edge 83 is wetted. The upper epitaxial portion
of the side edge 83 is not wetted. Similarly, the HR material is
made to flow laterally and to wet the inside side edge of the
retaining ring 64 as illustrated. Reference numeral 84 identifies
the inside side edge of retaining ring 64. The HR material is
deposited with such a thickness that once it has cured and
solidified it has a reflectivity of at least 85 percent (for
example, 94 percent).
[0048] In one example, the HR material is the material KER-2010-DAM
or material KER-2020 that is commercially available from ShinEtsu
Chemical Co. Ltd. of Tokyo, Japan. The HR material may comprise
silicone and a titanium dioxide powder, where the titanium dioxide
powder is suspended in the silicone. The HR material is made
jettable by cutting it with a solvent. In one example, the solvent
is an oil-based solvent such as dimethylformamide (DMF)
commercially available from ShinEtsu as DMF0.65CS. The HR material
after being appropriately cut with the solvent has a viscosity less
than 1100 centipois (cP) at room temperature and in this example
has a viscosity of 1000 cP at room temperature. In one example, the
jetting equipment used to jet the HR material is an Asymtek X1020
jetting machine available from Hordson Asmtek of 2747 Loker Avenue
West, Carlsbad, Calif. 92010. The jetting machine has two jet
heads. The first jet head is used to apply HR material with a first
viscosity, whereas the second jet head is used to apply HR material
with a second viscosity.
[0049] FIG. 19 shows the result of the step of depositing the HR
material. Layer 74 of HR material covers substantially all the area
within the confines of the retaining ring 64 other than the top
surfaces of LED dice 52-55. Layer 74 coats the upper surface of the
substrate underneath the bridging bond wires. Whereas in the prior
art example of FIG. 9 there exists a peripheral strip of the
substrate around each LED die that is not covered with HR material,
in the structure illustrated in FIG. 19 there is no such uncovered
peripheral strip. Whereas in the prior art example of FIG. 9 there
are uncovered areas of the substrate in the areas where wire bonds
attach to the substrate, in the structure illustrated in FIG. 19
there are no such uncovered areas. The HR material is made to coat
the upper surface of the substrate right up location where the wire
bond makes contact with the substrate. The HR material is also made
to coat the upper surface of the substrate right up to the side
edges of the LED dice. The HR material is made to coat the upper
surface of the substrate right up to the inside side edge of the
retaining ring 64.
[0050] FIG. 20 shows the result of the next step in the method.
Phosphor 65 is deposited into the circular area bounded by the
retaining ring 64 so that the phosphor 65 covers the LED dice as
illustrated in FIG. 12. The phosphor is then allowed to cure and
harden. Once the phosphor 65 has been deposited, the panel of
MCPCBs is singulated, thereby forming a plurality of LED
assemblies. The LED assembly structure 50 illustrated in FIG. 11 is
one of these LED assemblies.
[0051] The method set forth above in connection with FIGS. 11-20
has several advantageous aspects in comparison with the prior art
method set forth above in connection with FIGS. 1-10. First, the
amount of the upper surface of the substrate that is left uncovered
by HR material is reduced in comparison with the prior art screen
printing method. Parts of the substrate that are not covered by HR
material may and typically do absorb light or otherwise do not
reflect light well, thereby reducing the light efficiency of the
LED assembly. By covering more of the surface of the substrate with
HR material using the jetting process, more light is reflected from
the LED assembly and the light efficiency of the LED assembly is
increased. In the prior art screen printing process used to deposit
HR material, variations in physical sizes and imperfections in die
attach and wire bonding processes required the windows in the HR
layer to be so large that after die attach and wire bonding
substantial areas of exposed substrate remained uncovered by HR
material. In the jetting process, the HR material is applied after
die placement and wire bonding, and machine vision and control
techniques are used to control the jetting process so that the
substrate is coated up to the edges of structures (the LED dice and
the retaining ring) even if the structures are in slightly
different places, from one assembly to the next. The use of
laterally flowing HR material reduces the need to account for
differences in die placement and wire bond locations from assembly
to assembly. The HR material naturally flows laterally up to the
proper structures even if the structures are not always disposed in
the same location from assembly to assembly.
[0052] Second, the HR layer is deposited after the sensitive die
attach and wire bonding process steps. In the prior art screen
printing method of depositing HR material, on the other hand, the
HR material is screen printed onto the substrate prior to die
attach and wire bonding. The HR material is an organic material. If
die attach and wire bonding are performed when organic residue is
present on the substrate, then errors in die attach and wire
bonding can occur and such error reduce LED assembly manufacturing
yield. Accordingly, plasma cleaning is often conventionally done
after the HR screen printing step in an attempt to remove all such
organic residue prior to die attach and wire bonding. This plasma
cleaning is, however, difficult to perform as compared to
performing die attach on a plasma cleaned surface that has never
been exposed to organics. Accordingly, defects due to performing
die attach and wire bonding on surfaces having organic residues are
reduced or eliminated using the jetting process.
[0053] Third, the jetted HR layer can be made to coat surfaces with
relatively large steps and with different levels and sloped
surfaces. In the prior art screen printing method, on the other
hand, the surfaces to which the HR material is being applied must
be more planar. In one example of the novel jetting process, a
first HR material with less viscosity is applied to certain areas
of the substrate that are relatively flat and planar so that the HR
material will flow under wire bonds and will flow up to the edges
of dice, whereas a second HR material with more viscosity is
applied to other portions of the surface of the substrate that are
more inclined or more stepped. The first HR material is applied
with a first jet head of the jetting machine, whereas the second HR
material is applied with a second jet head of the jetting
machine.
[0054] Fourth, the production rate of LED assemblies is increased
by not coating certain parts of the substrate with HR material in
certain situations. In some examples, the area of the substrate
between LED dice is small. It has been found that the benefit of
coating this small inter-dice area is only slight. Accordingly, the
HR material is not jetted into the inter-dice areas in order to
save manufacturing time.
[0055] Fifth, it is generally desirable to be able to place
fiducial markers on the substrate surface and to have the imaging
systems of the die attach and wire bonding equipment use these
fiducial markers during die attach and wire bonding processing. In
the prior art screen printing process where the HR layer has been
deposited prior to die attach and wire bonding, there is limited
exposed substrate area available for placement of appropriate
fiducial markers. Most of the upper surface of the substrate has
been covered by HR material. In the novel jetting method of
applying HR material, on the other hand, die attach and wire
bonding occur prior to the depositing of the HR layer. Accordingly,
fiducial markers (for example, 60A-60D) that will later be covered
over by HR material are nevertheless usable at die attach and wire
bonding time by die attach and wire bonding imaging systems.
[0056] The deposition of an HR layer using jetting is not limited
to the particular LED assembly set forth FIGS. 12. FIG. 21 is a
diagram of another type of LED assembly 100. In the diagrams of
FIG. 21 and FIG. 12, the same reference numerals are used to denote
the same or similar structures. In the LED assembly of FIG. 21, the
substrate 56 forms a well 101. The upper surface of the substrate
has a nonplanar shape. The four LED dice 52-55 are mounted to metal
pad 60 at the bottom of the well 101 as illustrated. Jetting is
used to coat the sidewalls of this well with HR material. In the
specific example illustrated, substantially all of the upper
surface of the substrate within the circular confines of retaining
ring 64 but for the LED dice 52-55 is coated with HR material. The
liquid HR material that is painted onto the sidewalls of the well
can be a liquid HR material with a relatively higher viscosity as
compared with the viscosity of the liquid HR material that is
painted onto the remainder of the surface of the substrate. The
resulting HR layer is conformal to the nonplanar upper surface of
the substrate over the various edges and sloping surfaces of the
substrate.
[0057] FIG. 22 is a diagram of another type of LED assembly 200.
The substrate 56 in this case includes a ceramic portion 202. A
first electrode 203 (P+ electrode), a second electrode 204 (N-
electrode), and a thermal pad 205 of metal are disposed on the
bottom surface of the ceramic portion 202. A conductive via 206
couples the P+ electrode 203 to metal portion 59 on the upper
surface of the ceramic portion 202. Similarly, a conductive via 207
couples the N- electrode 204 to metal portion 57 on the upper
surface of the ceramic portion 202. The thickness of the metal
layers on the top and bottom of the substrate may be large, such as
81 microns, and this large thickness makes screen printing the HR
material difficult. The HR layer 74 contacts substantially all of
at least one side edge of each LED die as pictured. In the
illustrated example, the surface area of the substrate 56 between
LED dice 52-55 is not covered with HR material as described above
in order to reduce production times. The inter-dice distance
between the LED dice 52-55 is less than 300 microns and the
inter-dice area is not jetted with HR material. In other examples,
this inter-dice area is coated with HR material. In an example
where a retaining ring is provided (not shown), the HR layer 74 may
or may not extend outward all the way to the retaining ring. The HR
layer 74 may contact the inside side edge of such a retaining ring,
or may stop short of the retaining ring such that the HR layer 74
does not touch the inside side edge of the retaining ring.
[0058] FIG. 23 is a diagram of an LED assembly 300 where the
substrate 56 involves a ceramic portion 202 as in FIG. 22, but the
HR layer 74 does not contact a side edge of any of the LED dice
52-55. The HR layer 74 is deposited to stop short of the LED dice
so that the HR layer 74 does not contact any side edge of any LED
die. In the final assembly, the LED dice appear disposed in a
central window in the HR layer 74. As compared the screen printing
conventional method of applying HR material, however, the amount of
exposed substrate (substrate under the phosphor 65 that is not
covered by either an LED die or HR material) is much reduced in the
structures of both FIG. 22 and FIG. 23.
[0059] FIG. 24 is a flowchart of a method 400. Initially, a
substrate is cleaned (step 401) as necessary. In one example, the
substrate 56 is part of the panel 75 of FIG. 13. Panel 75 is plasma
cleaned to remove any organic materials from its surface. Next
(step 402), a plurality of LED dice is attached to the substrate.
In one example, the LED dice are the dice 52-55 that are attached
using silver epoxy to the substrate 56. FIG. 15 shows the result of
this die attach step. Next (step 403), wire bonding is performed as
necessary. In some cases, wire bonding is not used and the die is
electrically connected to the substrate without wire bonding. In an
example where wire bonding is performed, the result of the wire
bonding step is as shown in FIG. 16. Next (step 404), a retaining
ring is formed around the LED dice as necessary. In one example
where a retaining ring 64 is used, the result of the step of
forming the retaining ring is as illustrated in FIG. 17. Next (step
405), a layer of an HR material is deposited onto the substrate 56
such that the HR material does not cover the LED dice. FIG. 18
shows one example of how this HR material might be deposited in a
jetting process. The HR material is jetted onto exposed portions of
the upper surface of the substrate around the dice 52-55, and the
liquid HR material is allowed to cure and harden. Next (step 406),
an amount of liquid phosphor (actually silicone bearing phosphor
particles) is placed over the LED dice and allowed to cure. In one
example, the result of this step is illustrated in FIG. 20. The
resulting panel of LED assemblies is then singulated to form a
plurality of separate LED assemblies. In one example, FIG. 11 is a
top-down diagram of one of these separate LED assemblies. In a
first novel aspect, the HR layer of the LED assembly is deposited
after the die attach step and after the wire bonding step in the
LED assembly process. In a second novel aspect, the HR layer of an
LED assembly is deposited by jetting microdots of liquid HR
material onto a substrate of the LED assembly.
[0060] Although certain specific embodiments are described above
for instructional purposes, the teachings of this patent document
have general applicability and are not limited to the specific
embodiments described above. The material being jetted need not be
HR material. In some examples, different types of HR material are
jetted onto different parts of the LED assembly. Accordingly,
various modifications, adaptations, and combinations of various
features of the described embodiments can be practiced without
departing from the scope of the invention as set forth in the
claims.
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