Silicone Materials for LEDs

Silicones for LED applications are driving the evolution of LEDs.

LEDs (light-emitting diodes) consume less energy, are long-lasting, and are eco-friendly. As they have become higher in brightness, they have come to be used in a wide variety of applications. LEDs used for illumination and TV backlights in particular must provide long-lasting reliability, which makes silicone a vital material for the manufacture of such LEDs.

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Silicone is used as an encapsulant and die-bonding adhesive, and manufacturers are working toward using it as a reflector material. Silicone is driving the evolution of LEDs.

Structure of a LED and the areas where the silicone material is used.

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.

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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.

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