Silicone materials development for LED packaging
The properties of silicone materials make them ideal for new,
demanding applications such as protective encapsulants and lenses for
high-brightness LEDs.
Authors: Ann Norris, Maneesh Bahadur and Makoto Yoshitake of
Dow Corning
Silicone-based materials are attracting considerable attention for
use as encapsulants and lenses in a variety of LED device designs. The
trends driving silicone materials for LED packaging are higher
brightness, longer device lifetime, market growth of blue and white
light LEDs, and lead-free solder reflowable processing.
The key attributes of silicones that make them attractive materials
for high-brightness (HB) LEDs include their high transparency in the
UV-visible region, controlled refractive index (RI), and stable thermo-opto-mechanical
properties.
Dow Corning is a leader in new material developments related to
applications being pursued by the major LED manufacturers. This includes
injection-mold processing of novel resin-based silicones, which is a new
challenge that must be demonstrated for high-throughput LED device
manufacturing.
Properties of silicones
Silicone-based materials have a long history of excellent performance
in very demanding applications, such as protective materials for
semiconductor devices. Their high optical clarity and stable properties
upon exposure to heat and humidity make silicones an excellent choice
for use in new, demanding applications such as protective encapsulants
and lenses for HBLEDs.
Silicone elastomers and gels are well known for their
stress-relieving characteristics and are often used for encapsulating
and protecting stress-sensitive electronic devices. Key attributes for
silicones in optical applications include:- High optical transmittance in the UV-visible region
- Can be synthesized to cover a wide range of refractive indices from
1.38-1.58
- Excellent photo-thermal stability
- A variety of cure chemistries for easy processing
- High purity
- A wide range of cured moduli from gels to hard resins
Silicones for HB-LEDs
Silicones are finding wide applicability as packaging materials for
HBLEDs. The silicone polymers can be synthesized as a linear polymer
with varying organic groups attached to the silicon atom; this group
tailors the refractive index. Silicones are also highly transparent in
the UV-visible wavelength region. With minimal to no absorption or
scattering losses, light that is produced by the LED is transmitted
efficiently through the silicone material.
Silicones can also be formulated to achieve a wide range of cured
modulus values. The hardness can range from soft compliant gels, to
harder yet flexible elastomers, up to very hard resinous materials. The
cured modulus is dictated by two factors: the crosslink density and the
ratio of linear to branched silicon species in the polymer.
When compliant gels and soft elastomers are used to encapsulate
devices, they provide a soft, stress-relieving character that can
cushion the devices from internal and external stresses. A critical
characteristic of a good encapsulant is adhesion, and silicones can be
designed to have good adhesion to the various substrates and components
used to build LEDs.
Another key attribute of silicones is their unique cure chemistry.
Silicones developed for the LED market are thermoset materials and as
such are cured with a thermal process. This cure system has several
advantages; it can be offered in either one-part or two-part
compositions, it can be accelerated with heat, it shows little to no
cure shrinkage, and there are no cure by-products.
Molding and lens fabrication
Silicone resin-based compositions have been designed for precision
molding applications and can be used to fabricate small, complex lenses.
Nanometer- and micrometer-sized features can now be replicated on a
silicone surface. These new resinous silicones have been successfully
fabricated into optical parts using a variety of techniques such as
casting, compression molding, and injection molding. Selected examples
of molded parts from recent products under development at Dow Corning
are shown in Figure 1.
Molding processes using multi-cavity molds are ideal for high-volume,
low-cost production of optical parts such as LED lenses. Molding with
silicones also allows for designing unique optical elements and features
for improved extraction of light from LED devices. While Dow Corning is
not a supplier of molded parts (the companys focus is on materials),
limited molding studies have been carried out to support molders on the
use of these new silicone products.
Figure 1 shows three examples of molded parts of different geometries
made with silicone hard resin (SR 7010) having a refractive index of
1.53. In the upper left is a picture of a molded button-shaped part,
upper right is a molded disc, and in the middle is a hollow lens shape.
In all three cases, the molded parts can be released from the mold,
using a cycle time as short as one minute, with high optical clarity.
The optimal mold process conditions are often not too different from
those used to mold traditional thermoplastics. Commercial fabrication of
LED lenses has begun by various suppliers using the two-part addition
curing silicone compositions.
Photo-thermal stability of silicones
Silicones are highly transparent in the UV-visible wavelength region.
It is critical to the LED performance that transparency be retained
throughout the lifetime of the device. Because HBLEDs are running at
higher junction temperatures, retention of properties upon exposure to
high temperatures is critical.
The high-temperature stability that is demonstrated by siloxanes is
partially due to the inherent nature of the stable silicon-oxygen bond.
Silicones have been shown to withstand the high temperatures of
lead-free solder reflow with no change in their properties. Retention of
optical transparency as a function of high-temperature exposure for Dow
Cornings SR 7010 material is shown in Figure 2. SR 7010 shows
stable transmission in the visible region (400-800 nm) when exposed to a
temperature of 200C over seven days aging. Silicone resins exhibit
excellent resistance to yellowing and provide better reliability to
HBLED devices. If formulated correctly, silicones will retain their
optical transparency and other physical properties when exposed to
extended high temperatures of operation and high optical flux in the
visible region.
Silicones have been shown to be highly stable when exposed to 370 nm
light at 150C, as opposed to epoxies which easily discolor under
equivalent conditions. The UV-stability test results on a silicone
elastomer product, EG 6301 (refractive index of 1.4), are shown in
Figure 3, benchmarked with an optical-grade epoxy encapsulant.
In this test, 1mm-thick samples of both materials were exposed to a
UV light, 140 mW/cm2 at 365 nm at room temperature. Changes
in percent transmission after 70 hours of exposure are compared to the
initial values. The EG 6301 material shows stable performance with no
changes in transparency as compared to the epoxy. Additional testing is
ongoing to better understand the stability of silicones under various
conditions of temperature, wavelength and flux.
Summary
Silicones can be designed to accommodate a wide range of refractive
indices and cured moduli. They can be formulated to various viscosities,
which allows for a wide range of processing options. For LED
applications, materials can be either dispensed when used as
encapsulants, or molded into lenses. Silicone materials have high
transparency and can be formulated for excellent photo and thermal
stability as well as excellent retention of their initial transparency
throughout the lifetime of the LED devices. Finally, silicones are
excellent materials for encapsulating LEDs and for manufacturing
discrete optical lenses.
About the Authors
Ann W. Norris and Maneesh Bahadur are with Dow Corning Corporation,
PO Box 994, Midland, Michigan 48642, USA and Makoto Yoshitake is with
Dow Corning Toray, Chiba, Japan.
Dow Corning Corporation (www.dowcorning.com)
is a globally integrated provider of materials, application technology
and services, and is focused on providing innovative technology for all
segments of the electronics industry. For more information, contact
electronics@dowcorning.com.
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