Note: Descriptions are shown in the official language in which they were submitted.
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RADIATION EMITTER DEVICES AND
METHOD OF MAKING THE SAME
BACKGROUND OF THE INVENTION
p001] The present invention generally relates to radiation emitter devices
such as, for
example, light emitting diode (LED) packages, to methods of making radiation
emitter
devices, and to opto-electronic emitter assemblies incorporating optical
radiation emitter
devices.
[0002] As used herein, the term "discrete opto-electronic emitter assembly"
means
packaged radiation emitter devices that emit ultraviale.t (UV), visible, or
infrared (IR)
radiation upon application of electrical power. Such discrete o.pto-electronic
emitter
assemblies include one or more radiation emitters. Radiation emitters,=
particularly
optical radiation emitters, are used in a wide variety of commercial and
industrial
products and systems, and accordingly come in many forms and packages. As used
herein, the term "optical radiation emitter" includes all emitter devices that
emit visible
light, near IR radiation, and UV radiation. Such optical radiation emitters
may be
photoluminescent, electroluminescent, or another type of solid state emitter.
Photoluminescent sources include phosphorescent and fluorescent sources.
Fluorescent
sources include phosphors and fluorescent dyes, pigments, crystals,
substrates, coatings,
and other materials.
[0003] Electroluminescent sources include semiconductor optical radiation
emitters and
other devices that emit optical radiation in response to electrical
excitation.
Semiconductor optical radiation emitters include light emitting diode (LED)
cllips, light
emitting polymers (LEPs), organic light emitting devices (OLEDs), polymer
light
emitting devices (PLEDs), etc.
[0004] Semiconductor optical emitter components, particularly LED devices,
have
become commonplace in a wide variety of consumer and industrial opto-
electronic
applications. Other types of semicondugtor optical tmitter components,
including
OLEDs, LEPs, and the like, may also be packaged in discrete components
suitable as
substitutes for conventional inorganic LEDs in many of these applications.
[0005] Visible LED components of all colors are used alone or in small
clusters as status
indicators on such products as computer monitors, coffee makers, stereo
receivers, CD
players, VCRs, and the like. Such indicators are also found in a diversity of
systems
such as instrument panels in aircraft, trains, ships, cars, trucks, minivans
and sport
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utility vehicles, etc. Addressable arrays containing hundreds or thousands of
visible
LED components are found in moving-message displays such as those found in
many
airports and stock market trading centers and also as high brightness large-
area outdoor
television screens found in many sports complexes and in some urban
billboards.
[0006] Amber, red, and red-orange emitting visible LEDs are used in arrays of
up to
100 components in visual signaling systems such as vehicle center high mounted
stop
lamps (CHMSLs), brake lamps, exterior turn signals and hazard flashers,
exterior
signaling mirrors, and for roadway construction hazard markers. Amber, red,
and blue-
green emitting visible LEDs are increasingly being used in much larger arrays
of up to
400 components as stop/slow/go lights at intersections in urban and suburban
intersections.
[0007] Multi-color combinations of pluralities of visible colored LEDs are
being used as
the source of projected white light for illumination in binary-complementary
and ternary
RGB illuminators. Such illuminators are useful as vehicle or aircraft
maplights, for
example, or as vehicle or aircraft reading or courtesy lights, cargo lights,
license plate
illuminators, backup lights, and exterior mirror puddle lights. Other
pertinent uses
include portable flashlights and other illuminator applications where rugged,
compact,
lightweight, high efficiency, long-life, low voltage sources of white
illumination are
needed. Phosphor-enhanced "white" LEDs may also be used in some of these
instances
as illuminators.
[0008] IR emitting LEDs are being used for remote control and communication in
such
devices as VCR, TV, CD and other audio-visual remote control units. Similarly,
high
intensity IR-emitting LEDs are being used for communication between IRDA
devices
such as desktop, laptop, and palmtop computers; PDAs (personal digital
assistants); and
computer peripherals such as printers, network adapters, pointing devices
("mice,"
trackballs, etc.), keyboards and other computers. IR LED emitters and IR
receivers also
serve as sensors for proxiunity or presence in industrial control systems, for
location or
orientation within such opto-electronic devices such as pointing devices and
optical
encoders, and as read heads in such systems as barcode scanners. IR LED
emitters may
also be used in a night vision system for automobiles.
[0009] Blue, violet, and UV emitting LEDs and LED lasers are being used
extensively
for data storage and retrieval applications such as reading and writing to
high-density
optical storage disks.
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[0010] For discrete LED devices and other discrete ("packaged") opto-
electronic
emitters, increased performance is dependent substantially upon increased
reliable
package power capacity, reduced package thermal resistance, and reduced
susceptibility
of the package to damage during auto-insertion, soldering and other circuit or
system
manufacturing operations.
[0011] Keeping discrete opto-electronic emitters cool during operation
enhances
performance in several ways. The efficiency of the emitter usually decreases
in relation
to increased operating temperature and increases in relation to reduced
operating
temperature. Conversely, emitter efficiency typically increases in relation to
reduced
internal operating temperature. The reliability of the emitter and life of the
materials
and sub-components comprising it usually improves in relation to decreased
operating
temperature. The consistency of the emitter's emission spectra is usually
improved in
relation to decreased or more consistent operating temperature. The decay life
of the
emitter is usually improved in relation to reduced operating temperature. For
these and
other reasons, it is clearly beneficial to employ novel mechanisms for
reducing the
operating temperature of discrete opto-electronic emitters.
[0012] While the ambient environmental temperature is an external factor that
cannot
always be controlled, the temperature rise of the device above the ambient
temperature is
determined mainly by the device's thermal resistance and operating power.
[0013] Unfortunately, most discrete opto-electronic emitters exhibit a
characteristic
contravening to the goal of reduced internal operating temperature. In short,
these types
of devices usually emit greater amounts of useful radiation in proportion to
increased
power up to some practical limit of the package or constituent materials or
subcomponents. Thus, for applications where more radiation is useful (i. e. ,
almost all
applications known), it is beneficial to drive the device at the highest power
consistent
with device and system reliability and consistent with the power-radiation
characteristics
of the device. However, increased power in devices with finite (positive, non-
zero)
thermal resistance results in elevated internal operating temperatures.
[0014] It would be advantageous then to reduce internal operating temperature
without
having to reduce device power, or alternately to maintain internal operating
temperature
while increasing device power. This can be accomplished by reducing the device
thermal resistance.
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[0015] Billions of LED components are used in applications such as those cited
above,
in part because relatively few standardized LED configurations prevail and due
to the
fact that these configurations are readily processed by the automated
processing
equipment iused almost universally by the world's electronic assembly
industries.
Automated processing via mainstream equipment and procedures contributes to
low
capital cost, low defect rates, low labor cost, high throughput, high
precision, high
repeatability and flexible manufacturing practices. Without these attributes,
the use of
LEDs becomes cost prohibitive or otherwise unattractive from a quality
standpoint for
most high-volume applications.
[0016] Two of the most important steps in modern electronic assembly processes
are
high-speed automated insertion and mass-automated soldering. Compatibility
with
automatic insertion or placement machines and one or more common mass-
soldering
process are critical to large-scale commercial viability of discrete
semiconductor optical
emitters (including LEDs).
[0017] Thus, the vast majority of LEDs used take the form of discrete-packaged
THD
(Through Hole Device) or SMD (Surface Mount Device) components. These
configurations primarily include radial-lead THD configurations known as "5
mm," "T-
1," and "T-1 3/4" or similar devices with rectangular shapes, all of which are
readily
adapted onto tape-and-reel, tape-and-ammo, or other standardized packaging for
convenient shipment, handling, and high-speed automated insertion into printed
circuit
boards on radial inserters. Other common discrete THD LED packages include
axial
components such as the "polyLED," which are readily adapted onto tape and reel
for
convenient shipment, handling, and high-speed automated insertion into printed
circuit
boards on axial inserters. Common SMD LED components such as the "TOPLED "
and Pixar are similarly popular as they are readily adapted into blister-pack
reels for
convenient shipment, handling, and high-speed automated placement onto printed
circuit
boards with chip shooters.
[0018] Soldering is a process central to the manufacture of most conventional
circuit
assemblies using standardized discrete electronic devices, whether THD or SMD.
By
soldering the leads or contacts of a discrete electronic component such as an
LED to a
printed circuit board, the component becomes electrically connected to
electrically
conductive traces on the PCB and also to other proximal or remote electronic
devices
used for supplying power to, controlling or otherwise interacting
electronically with, the
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discrete electronic device. Soldering is generally accomplished by wave
solder, IR
reflow solder, convective IR reflow solder, vapor phase reflow solder, or hand
soldering. Each of these approaches differ from one another, but they all
produce
substantially the same end effect - inexpensive electrical connection of
discrete
electronic devices to a printed circuit board by virtue of a metallic or inter-
metallic
bond. Wave and reflow solder processes are known for their ability to solder a
huge
number of discrete devices en masse, achieving very high throughput and low
cost,
along with superior solder bond quality and consistency.
[0019] Widely available cost-effective alternatives to wave solder and reflow
solder
processes for mass production do not presently exist. Hand soldering suffers
from
inconsistency and high cost. Mechanical connection schemes are expensive,
bulky, and
generally ill-suited for large numbers of electrical connections in many
circuits.
Conductive adhesives such as silver-laden epoxies may be used to establish
electrical
connections on some circuit assemblies, but these materials are more costly
and
expensive to apply than solder. Spot soldering with lasers and other selective-
solder
techniques are highly specialized for specific configurations and applications
and may
disrupt flexible manufacturing procedures preferred in automated electronic
circuit
assembly operations. Thus, compatibility with wave solder or reflow solder
processes
are desirable properties of a semiconductor optical emitter component. The
impact of
this property is far reaching, because these solder operations can introduce
large thermal
stresses into an electronic component sufficient to degrade or destroy the
component.
Thus an effective semiconductor optical emitter component must be constructed
in such a
fashion as to protect the device's encapsulation and encapsulated wire bonds,
die attach
and chip from transient heat exposure during soldering.
[0020] Conventional solder processes require that the ends of the leads of the
device
(below any standoff or at a point where the leads touch designated pads on the
PCB) be
heated to the melting point of the solder for a sustained period. This profile
can include
temperature excursions at the device leads as high as 230-300 degrees C for as
long as
15 seconds. Given that the leads of the device are normally constructed of
plated metals
or alloys such as copper or steel, this high temperature transient poses no
problems for
the leads themselves. The problem instead is the ability of these leads to
conduct heat
along their length into the encapsulated body of the device. Since these
heated leads are
in contact with the interior of the body of the device, they temporarily raise
the local
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internal temperature of the device during solder processing. This can harm the
somewhat delicate encapsulation, encapsulated wire bonds, die attach and chip.
This
phenomenon represents one of the fundamental limitations of low-cost, opto-
electronic
semiconductor devices today.
[0021] Keeping the body of an electronic component from rising excessively
above the
glass transition temperature of its encapsulating material during solder
processing is
critical, since the Coefficient of Thermal Expansion of polymer encapsulating
materials
rises dramatically above their glass transition points, typically by a factor
of two or
more. Polymers will increasingly soften, expand and plastically deform above
their
glass transition points. This deformation from polymer phase transition and
thermal
expansion in encapsulants can generate mechanical stress and cumulative
fatigue severe
enough to damage a discrete semiconductor device, resulting in poor
performance of the
device and a latent predisposition toward premature field failure. Such damage
typically
consists of: 1) fatigue or fracture of electrical wire bonds (at the chip bond
pads or at
the lead-frame); 2) partial delamination or decomposition of die-attach
adhesive; 3)
micro-fracture of the chip itself; and 4) degradation of the device
encapsulant, especially
near the entry points of the leads into the encapsulant, and a conipromised
ability to seal
out environmental water vapor, oxygen, or other damaging agents.
[0022] Witli regard to such thermal vulnerability, a crucial difference must
be
recognized between encapsulating materials suitable for non-optical electronic
devices
and those suitable for optical devices. The encapsulants used for non-optical
devices
may be opaque, whereas those used in constructing opto-electronic emitters and
receivers must be substantially transparent in the operating wavelength band
of the
device. The side effects of this distinction are subtle and far ranging.
[0023] Since there is no need for transparency in non-optical devices,
encapsulating
materials for non-optical semiconductor devices may include a wide range of
compositions containing a variety of opaque polymer binders, cross-linking
agents,
fillers, stabilizers and the like. Compositions of this type, such as heavily
filled epoxy,
may possess high glass transition temperatures (Tg), low thermal expansion
coefficients
(Cte), and/or elevated thermal conductivity such that they are suitable for
transient
exposures up to 175 degrees C. Opaque ceramic compositions may be thermally
stable
up to several hundred degrees C, with no significant phase transition
temperatures to
worry about, extremely low Cte and elevated thermal conductivity. For these
reasons,
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exposure of conventional, opaque encapsulation materials for non-optical
devices to
electrical leads heated to 130 degrees C or more for 10 seconds or so (by a
solder wave
at 230-300 degrees C) is not normally a problem.
[0024] However, the need for optical transparency in encapsulants for opto-
electronic
emitters and receivers obviates use of most high-performance polymer-filler
blends,
ceramics and composites that are suitable for non-optical semiconductors.
Without the
presence of inorganic fillers, cross-linking agents or other opaque additives,
the clear
polymer materials used to encapsulate most opto-electronic devices are
varieties of
epoxies having relatively low Tg values, greater Cce, and low thermal
conductivity. As
such, they are not suitable for exposure to transient temperature extremes
greater than
about 130 degrees C, such as occurs during normal soldering.
[0025] To coinpensate for the potentially severe effects of damage from solder
processing, prior art opto-electronic devices have undertaken a variety of
improvements
and compromises. The most notable improvement has been the relatively recent
introduction of clear epoxies for encapsulation capable of enduring
temperatures 10 to 20
degrees C higher than those previously available (up to 130 degrees C now
versus the
previous 110 degrees C). While useful, this has only partially alleviated the
problems
noted - the newest materials in use still fall 50 degrees C or more short of
parity with
conventional non-optical semiconductor encapsulation materials.
[0026] The most common compromise used to get around the transient temperature
rise
problem associated with soldering is to simply increase the thermal resistance
of the
electrical leads used in the device construction. By increasing the thermal
resistance of
these solderable leads, the heat transient experienced within the device body
during
soldering is minimized. Such an increase in thermal resistance can typically
be
accomplished in the following manner without appreciably affecting the
electrical
performance of the leads: 1) using a lead material with lower thermal
conductivity (such
as steel); 2) increasing the stand-off length of the leads (distance between
solder contact
and the device body); or 3) decreasing the cross-sectional area of the leads.
[0027] Using these three techniques, prior art devices have been implemented
with
elevated thermal resistance of the electrical leads to provide the desired
protection from
the solder process.
[0028] While effective at protecting prior art devices from thermal transients
associated
with soldering, there are limits to this approach, particularly in the
application of high
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power semiconductor opto-electronic emitters. Increased lead thermal
resistance results
in elevated internal operating temperatures in prior art devices, severely
compromising
operational performance and reliability of these devices. The soldered
electrical leads of
most prior art LED devices conduct power to the device and serve as the
primary
thermal dissipation path for heat created within the device during operation.
Thus the
electrical leads in prior art devices must be configured to possess thermal
resistance as
low as possible to facilitate heat extraction during normal operation.
Radiation and
natural convection from prior art devices play only a minor role in
transferring internal
heat to ambient, and thermal conduction through their encapsulating media is
severely
impeded by the low thermal conductivity of the optical materials used.
Therefore, the
electrically and thermally conductive metal leads must extract a majority of
the heat to
ambient by the mechanism of conduction. Greater thermal resistance in the
solderable
pins of these devices, necessary to protect the device from the transient
thermal effects
of soldering operations, therefore causes a higher internal temperature rise
within the
encapsulated device body during operation.
[0029] The maximum temperature rise of a portion of the device body in-contact
with
the semiconductor emitter under steady state is approximately equal to the
product of the
power dissipation of the emitter and the thermal resistance between the
emitter and the
ambient environment.
[0030] As previously discusged, severe consequences will result if the device
internal
temperature rises substantially above the encapsulant Tg value. Above this
temperature,
the Cte of the encapsulant typically increases very rapidly, producing great
thermo-
mechanical stress and cumulative fatigue at the LED wire bond and die attach.
For most
mobile applications such as automobiles, aircraft and the like, ambient
temperatures
commonly reach 80 degrees C. With encapsulation maximum operating temperatures
in
the range of 130 degrees C, an opto-electronic emitter for these applications
must
therefore limit its operational dT to an absolute maximum of about 50 degrees
C. This
limits the power that can be dissipated in a given component, and in turn
limits the
current that may be passed through the component. Since the emitted flux of
semiconductor optical emitters are typically proportional to the electrical
current passed
through them, limitations upon maxixnum electrical current also create
limitations on
flux generated.
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[0031] Thus, it would be advantageous to reduce internal operating temperature
without
having to reduce device power, or alternately to maintain internal operating
temperature
while increasing device power by means of reducing the device thermal
resistance
without increasing device vulnerability to transient thermal processing damage
from
soldering.
[0032] Other prior art devices have avoided these constraints, but have
achieved high
performance only by ignoring the needs of standardized, automated electronic
assembly
operations and adopting configurations incompatible with these processes.
Still other
prior art devices have achieved high performance by employing unusually
expensive
materials, sub-components, or processes in their own construction.
[0033] For example, one prior art approach that has been used to overcome
these
limitations uses hermetic semiconductor packaging, hybrid chip-on-board
techniques,
exotic materials such as ceramics, KOVAR and glass, or complex assemblies
instead of
or in addition to polymer encapsulation. While relevant for certain high-cost
aerospace
and telecommunications applications (where component cost is not a significant
concern), such devices require expensive materials and unusual assembly
processes.
This results in high cost and restricted manufacturing capacity - both of
which effectively
preclude the use of such components in mass-market applications. The devices
disclosed
in U.S. Patent No. 4,267,559 issued to Johnson et al. and U.S. Patent No.
4,125,777
issued to O'Brien et al. illustrate good examples of this.
[0034] The Johnson et al. patent discloses a device which includes both a TO-
18 header
component and a heat coupling means for mounting an LED chip thereto and
transferring internally generated heat to external heat dissipating means. The
header
consists of several components, including a KOVAR member, insulator sleeves
and
electrical posts, and is manufactured in a specialized process to ensure that
the posts are
electrically insulated as they pass through the header. The heat coupling
means is a
separate coinponent from the header and is composed of copper, copper alloys,
aluminum or other high thermal conductivity materials. According to the
teachings of
Johnson et al., the KOVAR header subassembly and copper heat coupling means
must
be bonded together with solder or electrically conductive adhesive for
electrical
continuity, allowing flow of electrical current into the heat coupling means
and
subsequently into the LED chip. Furthermore, the header and heat coupling
means of
the Johnson et al. patent are made of completely dissimilar materials and must
be so
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because of their unique roles in the described assembly. The header must be
made of
KOVAR in order that it may have a similar coefficient of thermal expansion to
the
insulator sleeves that run through it. At least one such sleeve is necessary
to electrically
isolate electrical pins from the header itself. However, KOVAR has relatively
low
thermal conductivity, necessitating the inclusion of a separate heat coupling
means made
of a material such as copper with a higher thermal conductivity. Since the
header is a
complex subassembly itself and is made of different materials than the heat
coupling
means, it must be made separately from the heat coupling means and then later
attached
to the heat coupling means with solder or an electrically conductive adhesive.
[0035] LED devices made similarly to the teachings of the Johnson et al.
patent are
currently being marketed in specialized forms similar to a TO-66 package.
These
devices are complex and typically involve insulated pin and header
construction and/or
include specialty sub-components such as ceramic isolation sheets within them.
[0036] Another approach which has been used to avoid damage to opto-electronic
emitters
from soldering has been to prohibit soldering of the component altogether or
to otherwise
require use of laser spot soldering or other unusual electrical attachment
method. This can
allow construction of a device with low thermal resistance from the
semiconductor emitter
within to the electrical pins without danger of device damage from soldering
operations. The
SNAPLED'''' 70 and SNAPLEDTM 150 devices made by Hewlett Packard illustrate
this
approach. In these devices, electrical connections are made to circuitry by
mechanically
stamping the leads to a simple metal circuit rather than soldering. The
resultant devices are
capable of continuous power dissipation as high as 475 mW at room temperature.
This
configuration, however, may complicate integration of such components with
electronic
circuits having higher complexity - such circuits are conventionally made
using printed
circuit boards, automated insertion equipment, and wave or reflow solder
operations.
[0037] A fmal approach is illustrated by an LED package called the SUPERFLUX'
package
(also known as the "PIRANHA"'), available from Hewlett Packard. The SUPERFLUXI
device combines moderate thermal resistance between the encapsulated chip and
the solder
standoff on the pins with a high-grade optical encapsulant and specialized
chip materials and
optical design. It achieves a moderate power dissipation capability without
resorting to a non-
solderable configuration such as the SNAPLEDTM. However, there are several
significant
problems with this configuration that inhibit its broader use.
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[0038] The package geometry of the SUPERFLUXTM package renders it incompatible
with conventional high-speed THD radial or axial insertion machinery or by SMT
chip
shooters known to the present inventors. Instead, it must be either hand-
placed or
placed by expensive, slow, robotic odd-form insertion equipment. The
SUPERFLUX'
package geometry is configured for use as an "end-on" source only - no readily
apparent convenient lead-bend technique can convert this device into a 90-
degree "side-
looker" source. The moderate thermal resistance of the solderable pins of this
device
and relatively low heat capacity may leave it vulnerable to damage from poorly
controlled solder processes. It may be inconvenient or costly for some
electronic
circuit manufacturers to control their soldering operations to the degree
needed for this
configuration. Finally, there is no convenient mechanism known to the
inventors to
outfit a SUPERFLUX' package with a conventional active or passive heat sink.
[0039] A principle factor impeding further application of these and other LED
devices in
signaling, illumination and display applications is that there is not
currently available a
device that has a high power capability with high emitted flux where the
device is easily
adaptable to automated insertion and/or mass-soldering processes. These
limitations
have either impeded the practical use of LEDs in many applications requiring
high flux
emission, or they have mandated the use of arrays of many LED components to
achieve
desired flux emission.
[0040] Conventional "5 mm" or "T 1-3/4" devices have a high thermal
resistance,
typically in excess of 240 degrees C per watt and usually are limited by clear
encapsulation materials that lead to unreliability if the emitter in the
device is operated
continuously, routinely or cyclically above 130 degrees C (less for any but
the best
materials clear available). With typical ambient temperatures commonly
exceeding 85
degrees C in the automotive environment, the temperature rise in these devices
must be
limited to 45 degrees C in order to properly avoid these material limits. This
means that
the device power must be limited to approximately 0.18 W. With a reasonable
design
tolerance of 33 percent to accommodate manufacturing variances, the practical
reliable
power limit of this device must be approximately 0.12 W. This is not a lot of
power,
and the emitted flux of these devices is thus limited. To overcome this, many
of these
devices are often used in combination to produce the luminous or radiant flux
needed for
an application (e.g., up to 50 for an automotive CHSML, up to 400 for a
traffic signal
lamp).
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[0041] Hewlett Packard's SUPERFLUX' or Piranha devices have a lower thermal
resistance than "5 mm" or "T 1-3/4" devices, typically around 145 degrees C
per watt. As
with "5 mm" or "T 1-3/4" devices, SUPERFLUXT" or Piranha devices usually are
limited
by clear encapsulation materials that lead to unreliability if the emitter in
the device is
operated continuously, routinely, or cyclically above 130 degrees C (less for
any but the
best materials clear available). With typical ambient temperatures commonly
exceeding 85
degrees C in the automotive environment, the temperature rise in these devices
must be
limited to 45 degrees C in order to properly avoid these material limits. This
means that
the device power must be limited to approximately 0.3 W. Because these devices
are
attached subsequently with thermally stressful wave or other solder
operations, and because
their thermal resistance from lead to junction is reduced, they are more
susceptible to
damage during processing into circuits. Thus, a higher design tolerance of 40
percent
should be used to accommodate manufacturing variances and increased
susceptibility, and
the practical reliable power limit of this device must be approximately 0.18
W. This is a
substantial increase (33 percent) compared to "5 mm" or "T 1-3/4" devices, it
still is not a
lot of power and the emitted flux of these devices is thus also limited. To
overcome this,
many of these devices are often used in combination to produce the luminous or
radiant
flux needed for an application (e.g., up to 30 for an automotive CHSML).
[0042] Hewlett Packard's SNAPLEDTM devices have a lower thermal resistance
than "5
mm" or "T 1-3/4" or SUPERFLUXI or PIRANHA' devices, as low as 100 degrees C
per watt. As with "5 mm" or "T 1-3/4" or SUPERFLUXI, PIRANHA', or
SNAPLED' devices usually are limited by clear encapsulation materials that
lead to
unreliability if the emitter in the device is operated continuously,
routinely, or cyclically
above 130 degrees C (less for any but the best materials clear available).
With typical
ambient temperatures commonly exceeding 85 degrees C in the automotive
environment,
the temperature rise in these devices must be limited to 45 degrees C in order
to properly
avoid these material limits. This means that the device power must be limited
to
approximately 0.45 W. As noted above, because the thermal resistance of these
devices
from lead to junction is so low, they cannot be soldered by conventional means
without
being damaged. This severely limits their utility, but they still are suitable
for some
applications. Because these devices are attached subsequently with
mechanically stressful
clinching operations, they remain susceptible to damage during processing
operations.
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Thus, a higher design tolerance of 40 percent should be used to accommodate
manufacturing variances and potentially increased processing damage
susceptibility,
and the practical reliable power limit of this device must be approximately
0.27 W.
This is a significant increase compared to "5 mm" or "T 1-3/4" or SUPERFLUXTM
or
PIRANHA' devices, but it still is not a lot of power (and is achieved at a
sacrifice in
conventional solderability). To overcome the resulting limited flux from these
devices, many are often used in combination to produce the luminous or radiant
flux
needed for an application (e.g., up to 12 for an automotive CHSML and up to 70
for
an automotive rear combination stop/turn/tail lamp).
[0043] Surface mount devices such as the TOPLED, PLCC and Hewlett Packard's
"HIGH-FLUX"" or "BARRACUDA"' devices use dissimilar polymer materials in
their construction, the first in order of assembly being a plastic material
that forms the
basic structure of the device body and holds the device leads together.
However, this
approach requires that the lead frames be processed initially via insert
molding (to
emplace the first supporting material around the lead frame), then die
mounting, then
wire bonding and then a second stage of molding. The second stage of molding
must
be the optical molding (to first provide an opportunity for die bonding and
wire
bonding). Such a design and process are difficult and expensive to execute
with high
yield and high quality. Accumulated variances would be excessive from the
multistage
molding scheme, interrupted by die and wire bonding.
[0044] An additional problem faced by designers of conventional LED devices is
that the
wire bond used to join one of the LED leads to the LED chip can break or lose
contact
with the lead or the chip. Such failure can occur, for example, due to shear
forces that
are transferred to the wire bond through the encapsulant or thermal
expansion/contraction of the encapsulant around the wire bond.
[0045] The other forms of radiation emitters mentioned above also experience
performance degradation, damage, increased failure probability, or accelerated
decay if
exposed to excessive operating temperatures.
[0046] Consequently, it is desirable to provide a radiation emitter device
that has the
capacity for higher emission output than conventional LED devices while being
less
susceptible to failure due to a break in the wire bond contact or other defect
that may be
caused by excessive operating temperatures.
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[0047] Additionally, it is desirable to provide a radiation emitter device
having improved
emission output over that of conventional LED devices while retaining the same
size and
shape of the conventional LED devices so as to facilitate the immediate use of
the
inventive LED devices in place of the conventional LED devices while also
requiring
minimal modification to the apparatuses that are used to manufacture the LED
devices.
SUMMARY OF THE INVENTION
[0048] Accordingly, it is an aspect of the present invention to provide a
radiation emitter
device that overcomes the above-noted problems and that provides improved
performance and less vulnerability to fatal damage. According to one
embodiment of
the present invention, a radiation emitting device comprises at least one
radiation
emitter, first and second electrical leads electrically coupled to the
radiation emitter, and
an integral encapsulant configured to encapsulate the radiation emitter and a
portion of
the first and second electrical leads. The encapsulant has at least a first
zone and a
second zone. The second zone exhibits at least one different characteristic
from the first
zone. The different characteristic may be a physical, structural, and/or
compositional
characteristic. For example, the at least one different characteristic may
include at least
one or more of the following: mechanical strength, thermal conductivity,
thermal
capacity, specific heat, coefficient of thermal expansion, adhesion, oxygen
impermeability, moisture impermeability, and transmittance for radiation
emitted from
the radiation emitter.
[0049] A method of making a radiation emitting device according to the present
invention comprises the steps of (1) attaching and electrically coupling at
least one
radiation emitter to a lead frame to form a subassembly; (2) inserting the
subassernbly
into a mold cavity; (3) partially filling the mold cavity with a first
encapsulant material;
(4) filling the remainder of the mold cavity with a second encapsulant
material; and (5)
removing the encapsulated subassembly from the mold cavity.
[0050] For a wide range of otherwise-conventional discrete opto-electronic
emitters, the
present invention accomplishes a significant increase in the reliable package
power
capacity and accomplishes significant reductions in the package thermal
resistance and
package damage susceptibility in novel ways.
[0051] These and other features, advantages, and objects of the present
invention will be
further understood and appreciated by those skilled in the art by reference to
the
following specification, claims, and appended drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0052] In the drawings:
[0053] Fig. 1 is a perspective view of a radiation emitter device constructed
in
accordance with a first embodiment of the present invention;
[0054] Fig. 2 is a top.view of the radiation emitter device shown in Fig. 1;
[0055] Fig. 3 is a cross-sectional view of the radiation emitter device shown
in Figs. 1
and 2 taken along line 3-3' as represented in Fig. 2;
[0056] Fig. 4 is a side view of a lead frame subassembly from which radiation
emitter
device shown in Figs. 1-3 may be constructed;
[0057] Fig. 5 is a perspective view of the lead frame subassembly shown in
Fig. 4
inverted and inserted into a mold in accordance with the inventive method for
making
radiation emitter devices;
[0058] Fig. 6 is a partial cross-sectional view taken along line 6-6' in Fig.
5, which
shows a portion of the lead frame subassembly inverted and inserted into the
mold
shown in Fig. 5 prior to a first molding step;
[0059] Fig. 7 is a partial cross-sectional view taken along line 6-6' in Fig.
5, which
shows a portion of the lead frame subassembly inverted and inserted into the
mold
shown in Fig. 5 after a first molding step;
[0060] Fig. 8 is a partial cross-sectional view taken along line 6-6' in Fig.
5, which
shows a portion of the lead frame subassembly inverted and inserted into the
mold
shown in Fig. 5 after a final molding step;
[0061] Fig. 9 is a side view of the final lead frame assembly following
removal from the
mold;
[0062] Fig. 10 is a flow chart showing the steps of the inventive method for
producing
radiation emitter devices in accordance with the present invention;
[0063] Fig. 11 is a cross-sectional view of a radiation emitter device
constructed in
accordance with a second embodiment of the present invention;
[0064] Fig. 12 is a perspective view of a radiation emitter device constructed
in
accordance with a third embodiment of the present invention;
[0065] Fig. 13 is a perspective view of a radiation emitter device constructed
in
accordance with a fourth embodiment of the present invention;
[0066] Fig. 14 is an exploded perspective view of the radiation emitter device
shown in
Fig. 13;
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[0067] Fig. 15 is a perspective view of a radiation emitter device constructed
in
accordance with a fifth embodiment of the present invention; and
[0068] Fig. 16 is a graph showing the average normalized luminous flux as a
function of
applied power for a conventional T-1 V4 LED device and for the inventive T-1
~/4 LED
device shown in Figs. 1-3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0069] Reference will now be made in detail to the present preferred
embodiments of the
invention, examples of which are illustrated in the accompanying drawings.
Wherever
possible, the same reference numerals will be used throughout the drawings to
refer to
the same or like parts.
[0070] For purposes of description herein, the ternis "upper," "lower,"
"right," "left,"
"rear," "front," "vertical," "horizontal," "top," "bottom," and derivatives
thereof shall
relate to the invention as viewed by a person looking directly at the
radiation emitting
device along the principal optical axis of the source. However, it is to be
understood
that the invention may assume various alternative orientations, except where
expressly
specified to the contrary. It is also to be understood that the specific
device illustrated in
the attached drawings and described in the following specification is simply
an
exemplary embodiment of the inventive concepts defined in the appended claims.
Hence, specific dimensions, proportions, and other physical characteristics
relating to
the embodiment disclosed herein are not to be considered as limiting, unless
the claims
expressly state otherwise.
[0071] Several embodiments of the present invention generally relate to an
improved optical
radiation emitting device useful in both high and low power applications. Such
embodiments
of the present invention are particularly well suited for use in limited power
applications such
as vehicles, portable lamps, and specialty lighting. By vehicles, we mean over-
land vehicles,
watercraft, aircraft and manned spacecraft, including but not limited to
automobiles, trucks,
vans, buses, recreational vehicles (RVs), bicycles, motorcycles and mopeds,
motorized carts,
electric cars, electric carts, electric bicycles, ships, boats, hovercraft,
submarines, airplanes,
helicopters, space stations, shuttlecraft, and the like. By portable lamps, we
mean camping
lanterns, head or helmet-mounted lamps such as for mining, mountaineering, and
spelunking,
hand-held flashlights, and the like. By specialty lighting we mean emergency
lighting
activated during power failures, fires or smoke accumulations in buildings,
microscope stage
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illuminators, billboard front-lighting, backlighting for signs, etc. The light
emitting
assembly of the present invention may be used as either an illuminator or an
indicator.
Examples of some of the applications in which the present invention may be
utilized are
disclosed in commonly assigned U.S. Patent No. 6,441,943 entitled "INDICATORS
AND
ILLUMINATORS USING A SEMICONDUCTOR RADIATION EMITTER PACKAGE,"
filed on October 29, 2000, by J. Roberts et al.
[0072] Some of the embodiments of the present invention provide a highly
reliable, low-
voltage, long-lived, light source for vehicles, portable lighting, and
specialty lighting
capable of producing white light with sufficient luminous intensity to
illuminate subjects
of interest well enough to be seen and to have sufficient apparent color and
contrast so as
to be readily identifiable. 'Several of the radiation emitter devices of the
present
invention may be well suited for use with AC or DC power sources, pulse-width
modulated DC power sources, and electronic control systems. The radiation
emitting
devices of the present invention may further be used to emit light of various
colors
and/or to emit non-visible radiation such as IR and UV radiation.
[0073] As used herein, the term "radiation emitter" and "radiation emitting
device"
shall include any structure that generates and emits optical or non-optical
radiation,
while the term "optical radiation emitter" or "optical radiation emitting
device" includes
those radiation emitters that emit optical radiation, which includes visible
light, near
infrared (IR) radiation, and/or ultraviolet (UV) radiation. As noted above,
optical
radiation emitters may include electroluminescent sources or other solid-state
sources
and/or photoluminescent or other sources. One form of electroluminescent
source
includes semiconductor optical radiation emitters. For purposes of the present
invention,
"semiconductor optical radiation emitters" comprise any semiconductor
component or
material that emits electromagnetic radiation having a wavelength between 100
nm and
2000 nm by the physical mechanism of electroluminescence, upon passage of
electrical
current through the component or material. The principle function of a
semiconductor
optical radiation emitter within the present invention is the conversion of
conducted
electrical power to radiated optical power. A semiconductor optical radiation
emitter
may include a typical IR, visible or UV LED chip or die well known in the art
and used
in a wide variety of prior art devices, or it may include any alternate form
of
semiconductor optical radiation emitter as described below.
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[0074] Alternate forms of semiconductor optical radiation emitters which may
be used in
the present invention are light emitting polymers (LEPs), polymer light
emitting diodes
(PLEDs), organic light emitting diodes (OLEDs), and the like. Such materials
and opto-
electronic structures made from them are electrically similar to traditional
inorganic
LEDs, but rely on organic compositions such as derivatives of the conductive
polymer
polyaniline for electroluminescence. Such semiconductor optical radiation
emitters are
relatively new, but may be obtained from sources such as Cambridge Display
Technology, Ltd. of Cambridge, and from Uniax of Santa Barbara, California.
[0075] For brevity, the term semiconductor optical radiation emitter may be
substituted
with the term LED or the alternate forms of emitters described above or known
in the
art. Examples of emitters suitable for the present invention include varieties
of LED
chips with associated conductive vias and pads for electrical attachment and
that are
emissive principally at P-N or N-P junctions within doped inorganic compounds
of
AlGaAs, AlInGaP, GaAs, GaP, InGaN, AlInGaN, GaN, SiC, ZnSe and the like.
[0076] LEDs are a preferred electroluminescent light source for use in the
radiation
emitting devices of the present invention because LEDs do not suffer
appreciable
reliability or field-service life degradation when mechanically or
electronically switched
on and off for millions of cycles. The luminous intensity and illuminance from
LEDs
closely approximates a linear response function with respect to applied
electrical current
over a broad range of conditions, making control of their intensity a
relatively simple
matter. Finally, recent generations of AIInGaP, AIGaAs, InGaN, AlInGaN, and
GaN
LEDs draw less electrical power per lumen or candela of visible light produced
than
incandescent lamps, resulting in more cost-effective, compact, and lightweight
illuminator wiring harnesses, fuses, connectors, batteries, generators,
alternators,
switches, electronic controls, and optics. A number of examples have
previously been
mentioned and are incorporated within the scope of the present invention,
although it
should be recognized that the present invention has other obvious applications
beyond
the specific ones nlentioned which do not deviate appreciably from the
teachings herein
and therefore are included in the scope of this invention.
[0077] Another preferred radiation source that may be used in the inventive
light
emitting assembly is a photoluminescent source. Photoluminescent sources
produce
visible light by partially absorbing visible or invisible radiation and re-
emitting visible
radiation. Photoluminescent sources are phosphorescent and fluorescent
materials,
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which include fluorescent dyes, pigments, crystals, substrates, coatings, as
well as
phosphors. Such a fluorescent or phosphorescent material may be excited by an
LED or
other radiation emitter and may be disposed within or on an LED device, or
within or on
a separate optical element, such as a lens or diffuser that is not integral
with an LED
device. Exemplary structures using a fluorescent or phosphorescent source are
described further below.
[0078] Figs. 1-3 show a radiation emitter device 10 constructed in accordance
with a
first embodiment of the present invention. As shown, radiation emitter device
10
includes an encapsulant 12, first and second electrical leads 14 and 16, and a
radiation
emitter 35. Encapsulant 12 encapsulates emitter 35 as well as a portion of
each of
electrical leads 14 and 16. Electrical leads 14 and 16 may have optional
respective
standoffs 18 and 20, which are provided to aid in the auto insertion of the
device when
constructed for THD applications.
[0079] As best shown in Fig. 3, an upper end of first electrical lead 14
extends
horizontally outward and defines a reflective cup 36 on which radiation
emitter 35 is
preferably attached. Electrical connection of a first contact terminal of
radiation emitter
35 to first electrical lead 14 may be made through a die-attach (not shown) or
may be
made by way of a wire bond or other connector. Device 10 further includes a
wire bond
38 or other means for electrically coupling a second contact terminal of
radiation emitter
35 to second electrical lead 16. The upper ends of first lead 14 and second
lead 16 are
electrically insulated from another by their spaced relation and by the fact
that the
encapsulant 12 is preferably made of a material having a relatively high
electrical
resistance.
[0080] The radiation emitter device 10 shown in Figs. 1-3 is intended to have
the same
relative size and shape as a conventional 5 mm/T-13/4 LED device or 3 mm T-1
device,
and accordingly, encapsulant 12 includes a lower rim 22 and a flat side 24,
which
facilitate registration of the radiation emitter device by an auto inserter.
[0081] As best shown in Fig. 3, an optional glob-top 40 or other optical or
physical
moderator is deposited over radiation emitter 35 and at least a portion of
wire bond 38.
Glob-top 40 may be made of silicone or silastic and may include an optional
phosphor or
other photoluminescent material. Use of a glob-top 40 is beneficial to help
protect
radiation emitter 35 and its connection to wire bond 38 during the molding
processes.
Other advantages of providing a glob-top 40 are discussed further below.
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[0082] The encapsulant 12 of the inventive radiation emitter device 10
includes at least
two functional zones 30 and 32 with a transition region 31 between zones 30
and 32.
Two separate functional zones 30 and 32 are provided based upon the inventors'
recognition that different portions of an encapsulant may serve different
functions from
other portions of the encapsulant such that the first zone 30 may have at
least one
different characteristic than the second zone 32 so as to optimize performance
of the
function(s) to be performed by that particular zone. For example, first zone
30 should
be at least partially transmissive to the wavelengths of radiation emitted
from radiation
emitter 35, while second zone 32 need not be transparent to such wavelengths.
This
allows the radiation emitter device of the present invention to make use of
the
extraordinary benefits of high performance power semiconductor encapsulation
and
transfer-molding compounds in the second zone. These characteristics can
include
relatively low coefficient of thermal expansion; relatively high thermal
conductivity;
relatively high Tg; relatively high specific heat; relatively low permeability
to oxygen,
gas, or water vapor; and relatively high physical strength properties. The
compounds
used for packaging or potting many high-power non-optical electronic devices
are
superior by a large margin in many of these categories to those traditionally
used for
conventional opto-electronic emitters. One of the main reasons for the
disparity is that
the high performance materials under discussion are usually opaque mixtures -
not
transparent to the band of radiation emitted in discrete opto-electronic
emitter devices.
The opacity of these functionally attractive materials is intrinsically linked
to their
beneficial properties (by virtue of the performance-enhancing mineral, metal,
and metal-
oxide fillers, for example), and thus, these materials had not been previously
considered
for use in opto-electronic components due to their opacity. However, by
limiting the use
of such materials to a zone of encapsulant 12 that does not require
transparency, the
present invention enjoys all the benefits of these material characteristics.
[0083] First zone 30 of encapsulant 12 is preferably a substantially
transparent material
to preserve optical performance. First zone 30 may optionally be partially
diffuse. First
zone 30 may be made of any conventional transparent encapsulant commonly used
for
opto-electronic emitter devices. First zone 30 of lens 12 preferably covers,
envelopes,
protects, and supports radiation emitter 35, the die-attach (if present) and a
portion of
any wire bonds 38 connected to radiation emitter 35.
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[0084] First zone 30 of encapsulant 12 may be comprised of two or more
portions, with
the innermost being a silicone or silastic glob-top 40 preapplied to radiation
emitter 35
prior to the first stage of molding of the encapsulant of the present
invention. This
innermost portion of first zone 30 may alternately be a high performance
epoxy,
silicone, urethane, or other polymer material possibly including optically
translucent or
transparent fillers or diffusants.
[0085] First zone 30 of encapsulant 12 is preferably made of a composition
comprising
an optical epoxy mixture that is substantially transparent to the radiation
emitted by
radiation emitter 35. However, other clear materials may also be used, and the
materials need not be transparent in bands outside the primary emission band
of the
radiation emitter.
[0086] Second zone 32 of encapsulant 12 is preferably made of a material that
optimizes
the function of that region of encapsulant 12. As noted above, second zone 12
need not
be transparent. However, a specialized function of zone 32 is generally to
minimize
catastrophic failure, stress, and accumulated fatigue from mechanical stresses
propagated
up electrically conductive leads 14 and 16. Not only may a material that is
better suited
for this purpose be selected given that it need not be transparent, but also
the material
may have higher strength properties, including higher tensile and
compressional
strength, adhesion, and/or cohesion.
[0087] Another function served by second zone 32 of encapsulant 12 is to serve
as a
barrier to oxygen, molecular water vapor, or other reagents that may otherwise
propagate upward into the device through second zone 32 or through the
interface
between leads 14 and 16 and encapsulant 12. Thus, second zone 32 should
effectively
protect radiation emitter 35, the die-attach (if present) wire bond 38, the
encapsulated
portions of the lead frame plating, and other internal device constituents
including any
photoluminescent material that may be present, from oxygen, molecular water
vapor,
and other reagents. Because second zone 32 of encapsulant 12 need not be
transparent,
second zone 32 may be constructed with improved barrier properties compared to
those
present in conventional transparent encapsulants.
[0088] Perhaps one of the more significant different characteristics that
second zone 32
may have from first zone 30 is that it may have improved thermal
characteristics. To
achieve lower device thermal resistance, second zone 32 preferably has a high
thermal
conductivity, at least in the critical region of the device surrounding
electrical leads 14
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and 16 and in thermal coupling to the portion of the leads that supports
radiation emitter
35 (i. e. , reflective cup 36, if present). To preserve relatively high
thermal resistance
protection from soldering operations, the bottom portion of second zone 32 of
encapsulant 12 extends no closer to the solderable portion or ends of
electrically
conductive leads 14 and 16 than the standoffs 18 and 20 (if present) or an
equivalent
point on the leads destined to remain substantially out of contact with molten
solder
during processing, if standoffs are not present.
[0089] By forming second zone 32 of encapsulant 12 to have a high heat
capacity,
second zone 32 will help suppress transient temperature spikes during
processing or
operation. Also, by configuring second zone 32 to have a low coefficient of
thermal
expansion, catastrophic failure, stress, and accumulated fatigue from thermal
expansion
and contraction within the device is minimized.
[0090] To achieve different functional cllaracteristics for the first and
second zones 30
and 32 of encapsulant 12, the two zones may have different physical
properties. Such
physical properties may be structural or compositional. Such different
structural
characteristics may be obtained using the same general composition for both
first and
second zones 30 and 32 but by causing a change in grain size or micro-
structural
orientation within the two zones. Such structural characteristics may be
modified during
the molding process by treating the zones differently by annealing, radiation
curing, or
other radiation treatment. Further, the micro-structural orientation may be
changed by
applying a magnetic field to one or more of the zones forming encapsulant 12.
[0091] In the event two different compositions are utilized to form first and
second zones
30 and 32, it is preferable that the material compositions are compatible for
molding in
the same mold, as is discussed further below with reference to the inventive
process for
making a preferred embodiment of the present invention. By integrally molding
first
and second zones 30 and 32, a cohesive bond may be formed at transition 31
between
zones 30 and 32. Such a cohesive bond is desirable to improve the strength of
the
encapsulant as a whole and to prevent oxygen, water vapor, or other reagents
from
reaching radiation emitter 35 via any interface between zones 30 and 32 that
otherwise
may be present. Further, such a cohesive bond provides continuity of the outer
surface.
As discussed further below, it is desirable that the compositions used for
first and second
zones 30 and 32 partially intermix at transition 31. Transition 31 may be a
fairly narrow
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cross section of encapsulant 12 or may be broader and larger if a composition
gradient is
formed using the compositions of first and second zones 30 and 32.
[0092] An additional advantage of making second zone 32 of encapsulant 12
opaque is
that any back-scattering from device 10 may be substantially reduced. Such
back-
scattering may be a problem when a light sensor is mounted in the same housing
as
radiation emitting device 10, as is often the case when such radiation emitter
devices are
mounted in an electrochromic rearview mirror assembly for an automobile.
[0093] Having described the general physical structure of the radiation
emitting device
of the present invention, an inventive method for making such a radiation
emitter device
is described below. It will be appreciated, however, that radiation emitter
device 10
may be made using other methods.
[0094] Fig. 10 shows a flowchart showing the steps and optional steps for the
inventive
method. Reference will be made to Fig. 10 simultaneously with references to
Figs. 4-9,
which show various stages of the device assembly. The first step 100 in the
inventive
method is to prepare a lead frame. An example of a lead frame is shown in Fig.
4 and is
designated with reference numeral 52. The lead frame may be made in any
conventional
configuration using any conventional techniques. Lead frame 52 is preferably
made of
metal and may be stamped and optionally post-plated. Lead frame 52 may also
undergo
optional ultrasonic or other cleaning. As shown in Fig. 4, lead frame 52
includes the
first and second electrically conductive leads 14 and 16 for a plurality of
radiation
emitter devices. Leads 14 and 16 are held together by a first tie bar 54 and
by a second
tie bar 56 that extend substantially perpendicularly to leads 14 and 16. Lead
frame 52
may further include vertical frame members 58 that extend between first and
second tie
bars 54 and 56 at both ends of lead frame 52 and between pairs of leads 14 and
16.
[0095] Lead frame 52 is also preferably shaped to include a support
(preferably a
reflective cup 36) at one end of first electrical lead 14. Reflective cup 36
may be
polished or plated to increase its reflectivity.
[0096] The next step in the process (step 102) is to attach one or more
radiation emitter
35 to each reflective cup 36 on lead frame 52. In the most preferred
embodiment,
radiation emitters 35 are LED chips and are die-bonded with conductive epoxy
attach or
eutectic bond into/onto each cup 36 or other support structure in the lead
frames. The
LED chips, if used, may be any conventional LED chip or any LED chip or other
radiation emitter subsequently developed. As part of this step, the attach
epoxy may
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optionally be degassed by vacuum and then cured/cooled. This structure may
then
optionally be subjected to ultrasonic or otlier cleaning following the above
steps.
[0097] For the most preferred embodiment, radiation emitters 35 are then wire-
bonded
with bond wire to establish the desired conductive path for radiation emitter
35 (step
104). Then, in step 106, an optional phosphor, glob-top, or other optical or
physical
moderator is then deposited on radiation emitter 35. Note that more than one
such
optical or physical moderator may be used (e. g. , a phosphor can first be
applied and
cured/dried followed by a silicone glob-top application). Whatever is applied
at this
stage is normally then dried and cured (step 108). Optionally, any such
optional optical
or physical moderator may be degassed by vacuum prior to proceeding to the
next step.
[0100] The next step (step 110) involves the optional application of a clear
epoxy within
the reflective cup 36 followed by an optional degassing step, which may be
performed
by vacuum. This optional application of clear epoxy may be performed to
prevent
bubbles from forming in and around the reflective cup during the subsequent
molding
operation. The clear epoxy applied may be the same material that is applied
during the
first molding stage described further below. Following step 110, the
construction of
lead frame subassembly 50 (Fig. 4) is complete and such subassembly is ready
for
molding.
[0101] The next step (step 112) is thus to invert lead frame subassembly 50
and insert
and register portions of the lead frame subassembly into encapsulation mold
cavities 62
formed in a mold 60. As shown in Fig. 5, the mold preferably includes a
plurality of
lead frame supports 64 for receiving and registering lead frame subassembly 50
in a
proper position relative to mold cavities 62. Fig. 6 shows a cross-sectional
view of one
such mold cavity 62 with a corresponding portion of lead frame subassembly 50
inverted
and inserted into cavity 62.
[0102] The next step (114) is to perform the first stage of encapsulation
whereby a clear
epoxy lens material is dispensed (preferably by injection) into encapsulant
mold cavity
62. Precise metering or feedback is preferably used to fill the clear epoxy
just up to or
over the inverted lip of the reflective cup 36 or surfaces of the radiation
emitter 35, if
for some reason there is no reflective cup in the device. See, for example,
Fig. 7.
Next, an optional degas step (step 116) is performed to remove bubbles by
vacuum from
the clear epoxy. A step (118) of precuring the clear epoxy may then optionally
be
performed. This optional precure may be just enough of a cure to minimize free
mixing
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of the two primary encapsulation materials, but also be not so much as to
prevent some
mixing. Some minor mixing in the transition boundary 31 is believed to be good
for
homogenous strength, cohesive bonding, etc.
[0103] The next step (step 120) is to perform the second stage of
encapsulation molding
in which a base epoxy is dispensed within mold cavity 62 (preferably by
injection) so as
to fill the remainder of mold cavity 62. Precise metering or feedback is
preferably used
to fill just up to the designed bottom of the device body or the top of
standoffs 18 and
20, if present. Fig. 8 shows an appropriate filled mold cavity 62 following
the second
stage.
[0104] After step 120, step 122 is optionally performed whereby the base
encapsulation
material is degassed by vacuum to remove any bubbles.
[0105] Then, in step 124, the base encapsulation material is cured along with
any
residual curing of any other previously in-place materials that are not yet
fully cured.
Next, in step 126, the nearly finished lead frame structure is ejected from
mold 60. An
optional post-cure step 128 may then be performed followed by an optional
cleaning/deflash step 130. The resultant structure is shown in Fig. 9.
[0106] The next step is a singulation step 132 whereby second tie bar 56 and
vertical
lead frame members 58 are cut away from the finished lead frame assembly and
first tie
bar 54 is cut between first and second electrical leads 14 and 16 for each
device as well
as between each device. If standoffs are not desired, first tie bar 54 may be
removed in
its entirety, otherwise the portions of tie bar 54 that remain may serve as
standoffs 18
and 20.
[0107] After the singulation step 132, an optional testing step 134 may be
performed and
the device may then be packed and shipped. Variations of this method are
discussed
below with reference to the alternative embodiments of the invention.
[0108] Fig. 11 shows a radiation emitter device 150 constructed in accordance
with a
second embodiment of the present invention. Device 150 differs from device 10
discussed above in that it includes a plurality of radiation emitters 35a and
35b. Both
emitters 35a and 35b may be mounted in a common reflective cup 36 or may be
mounted
in separate cups provided on the same or separate leads. Depending upon the
electrical
connection and control desired, an additional lead 16b may be provided.
Radiation
emitters 35a and 35b may be connected in series or in parallel and may be
identical or
have different constructions so as to emit light of different wavelengths. In
one
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preferred embodiment, emitters 35a and 35b emit light of a binary
complementary
nature to produce effective white light. LED chips and devices suitable for
such
application are disclosed in commonly assigned U.S. Patent No. 5,803,579
entitled
"ILLUMINATOR ASSEMBLY INCORPORATING LIGHT EMITTING DIODES,"
filed on June 16, 1996, by Robert R. Turnbull et al.
[0109] The base epoxy used to form second zone of encapsulant 12 may be
distinct from
the clear lens epoxy used to form first zone 30 not only in composition, but
additionally
or alternatively distinct in one or more physical properties (spectral
transmittance at a
wavelength of interest, diffuse scattering properties at one or more
wavelengths of
interest, microcrystalline structure, strength, thermal conductivity, CTE, Tg,
etc.). The
transition zone 31 between first zone 30 and second zone 32 may occur at a
transition
boundary zone 31, which may be narrow (effecting a more abrupt transition in
properties) or broad (effecting a more gradual transition or gradient in
properties). As
discussed above, the distinction between lens epoxy and base epoxy may be
compositional and achieved by using two different material mixtures in the
manufacturing process. A narrow transition boundary zone 31 between zones 30
and 32
might then be achieved by ensuring two formulations that are substantially
immiscible or
by slightly or completely precuring one inaterial before the other is added. A
broad
boundary zone 31 might be achieved by not precuring the first material
completely prior
to adding the second material and by ensuring the formulae of the two
materials allow
some mixing at their boundary.
[0110] In the event that a distinction desired between lens epoxy and base
epoxy is not
primarily a compositional distinction, but rather a physical distinction, then
alternate
means may be used to accomplish this, if the above-noted means is
insufficient. For
example, material property enhancement to a compositionally identical base
epoxy
portion may be achieved by post-treating the base epoxy portion after
dispensing into the
mold. Such post-treatment may be differential heating (such as by having
established a
temperature gradient in the mold or by using a stratified oven or stratified
heated
airflow). Such pretreatment may additionally or alternatively be differential
irradiation
with zonal IR, UV, visible, microwave, X-ray, or other electromagnetic
radiation source
or by E-beam or other particle beam. Also, certain microstructural effects
(grain
migration, lamination, orientation, size, agglomeration, etc.) may be effected
by
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exposing all or part of the device materials to electric fields, magnetic
fields,
centrifugal/centripetal forces or gravity before, during, or after dispensing.
[0111] Fig. 12 shows a radiation emitter device constructed in accordance with
a third
embodiment of the present invention. The device 200 shown in Fig. 12 is
configured to
have the same size and shape as Hewlett Packard's SuperFlux or Piranha
devices. As
shown, device 200 differs, however, in that it incorporates an encapsulant 212
having a
first zone 230 and a second zone 232 applied in a similar manner as applied in
the first
two embodiments.
[0112] Figs. 13 and 14 show a radiation emitter device 250 constructed in
accordance
with a fourth embodiment of the present invention. As apparent from Figs. 13
and 14,
this fourth embodiment is intended to resemble, in both size and shape, the
configuration
of Hewlett Packard's SnapLED device. By configuring various embodiments of the
invention to resemble conventional products in size and shape, the devices of
the present
invention may be readily substituted for the conventional devices without
requiring any
modification to equipment used to populate a circuit board. Additionally, by
configuring
these embodiments to resemble conventional structures, the same encapsulant
mold
cavities that are used to make the conventional devices may be used to make
the
inventive embodiments thereby eliminating the need to significantly modify the
apparatus
used to manufacture these radiation emitter devices.
[0113] Fig. 15 shows a radiation emitter device 300 constructed in accordance
with a
fifth embodiment of the present invention. Device 300 includes an additional
heat
extraction member 310 that extends outward from the encapsulant separate and
apart
from the electrical leads. Other suitable constructions utilizing a heat
extraction member
are disclosed in commonly assigned U.S. Patent No. 6,335,548.
[0114] With the construction shown in Fig. 15, it may be desirable to form a
micro-groove
lens, such as a Fresnel lens directly in the encapsulant. This is particularly
advantageous
when LED chips of different colors are provided in a single discrete LED
device where the
light from the chips is to mix to form another color such as white light. A
reflective
element may also be secured to the light output surface of the device to
further modify
the light emitted from the device. An example of an LED device having a micro-
groove
lens and such a reflective element is disclosed in commonly assigned U.S.
Patent No.
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CA 02430747 2007-02-28
6,670,207 entitled RADIATION EMITTER DEVICE HAVING A MICRO-GROOVE
LENS, by John K. Roberts.
[0115] The invention will be further clarified by the following example, which
is
intended to be exemplary of the invention and is not intended in any manner to
limit the
invention.
EXAMPLE
[0116] To demonstrate the effectiveness of the present invention, two LED
devices were
constructed and tested. The first LED device was a conventional T-1 * LED
device,
while the second LED device had an identical construction with the exception
that
encapsulant 12 included a second zone 32 as disclosed above with respect to
the first
embodiment of the present invention. The conventional T-1 U LED device was
constructed using HYSOL OS4000 transparent epoxy available from Dexter
Electronic
Materials Division. The inventive T-13/a LED device was constructed using the
same
transparent epoxy for first zone 30. The second zone 32, however, was formed
using
HYSOL E00123 casting compound, which is also available from Dexter. The two
LED devices were then operated by DC operation at room temperature, and their
average normalized luminous flux was measured and plotted in the graph shown
in Fig.
16. As apparent from this graph, the inventive LED device had much greater
luminous
flux, particularly at higher powers.
[0117] It should be understood that, for this sample, increased illuminance at
each
indicated power level for the inventive LED device relative to the
conventional LED
device is an indication of reduced junction operating temperature and reduced
assembly
thermal resistance.
[0118] While the present invention is generally described as employing two or
three
zones in the encapsulants that are arranged substantially vertically when the
primary
optical axis of the device is vertical, it will be appreciated that the zones
may be oriented
with respect to each other to the left or right of the central optical axis or
one inside or
outside the other. Such an inside/outside arrangement of the encapsulant zones
may be
affected by achieving a "cure gradient" from the outside to the inside whereby
the inside
is not fully cured and is left soft for a while into the life of the radiation
emitter device.
Such a configuration may also be used by curing the inside of the LED using
the heat
generated by the radiation emitter 35 itself. This may be advantageous when
low
residual mechanical stress is desired.
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[0119] In certain embodiments of the present invention, thermal resistance
from the
radiation emitter junction to the ambient environment is reduced without
reducing
thermal resistance (junction to lead), and therefore, better operating
temperature (i. e. ,
lower operating temperature) at a given power may be achieved without
increased
susceptibility to lead-soldering thermal damage.
[0120] The above description is considered that of the preferred embodiments
only.
Modifications of the invention will occur to those skilled in the art and to
those who
make or use the invention. Therefore, it is understood that the embodiments
shown in
the drawings and described above are merely for illustrative purposes and not
intended
to limit the scope of the invention, which is defined by the following claims
as
interpreted according to the principles of patent law, including the doctrine
of
equivalents.
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