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In a wide variety of electronic devices, silicones have a long demonstrated
history of providing useful properties such as clarity, stability, stress
relief, processability and heat dissipation.
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Optical clarity and lumen maintenance
Silicones have some of the highest clarity (transmission) values of any
polymeric based materials. Light that is produced by the LED is
transmitted efficiently with little absorbance within the silicone
material. Silicones have the capacity to provide a wide range of
refractive index (RI) values, from 1.40 to 1.58. By closer matching of the
refractive index of the silicone to that of the substrate, interfacial losses
can be minimized. The combination of clarity and refractive index of
silicones can lead to increased light output and improved lumen
maintenance. Silicone elastomers and resins can also be precisely molded
into lenses to help focus and direct light output, again contributing to
brightness within the targeted area of light distribution.
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Optical Stability and Stress Relief
Another strong advantage of silicones is their thermal and photo stability
in a variety of harsh environments. These features minimize the risk of
yellowing or changes in physical properties during device fabrication and
use. Their use also products provides thermal cycling reliability for
protecting LEDs.
And because of the special materials used to fabricate these devices, many
components in the package assemblies including the LED chips and their
wirebonds can be sensitive to stress damage. Silicones have a soft, stress
relieving nature that can cushion these devices from outside stress. Stability
and the stress-relieving nature of silicones can significantly contribute to
thermal cycling reliability and long lifetimes for the packaged
LEDs.
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Processability
Many of today’s packaging and surface mount assembly processes for
electronic and optical assemblies use lead-free solders, requiring high
temperatures for reflow. The thermal stability of silicones helps them to
survive these processes. And for the finished package, silicones have very
low moisture absorption values. This means they can meet various levels of
reliability requirements for subsequent processing operations with minimal
handling concerns.
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Heat Dissipation
And finally, an undesirable by-product of today’s HBLEDs is excessive heat
generation. In order to increase lumen output, devices are being
designed for continuous operation at 165C and higher junction temperatures.
Silicone thermal management materials are well known today in high
performance IC applications such as CPUs in flip-chip packages with heater
spreaders and heat sinks. A variety of product forms such as adhesives,
gels, phase-change pads and simple greases offer an array of options to address
thermal management issues.
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So, there are a wide variety of reasons to use silicones in the packaging of
high brightness LEDs and, in many situations, they are becoming the material of
choice for evolving LED applications.
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