Next, to address the second issue, silicone manufacturers needed to assist the LED packager and develop a material with a higher RI to transmit this increased brightness. By increasing the refractive index, the material is able to reduce internal reflections improving light extraction. Previously, a dimethyl silicone was used and had heritage in several industries. As described above, altering the R groups attached to the siloxane backbone can alter the silicone polymer in many ways. For example, adding trifluoropropyl groups to the siloxane backbone lowers the refractive index to around 1.38. More important however, by altering the substituent R groups on the backbone of the silicone polymer chain with phenyl groups, a higher RI can be achieved compared to a traditional methyl system. Paramount to achieving a higher RI is the addition of phenyl. Depending on the amount of phenyl added, the RI can range from 1.43 to greater than 1.55 (Figure 4). In addition, adding phenyl provides a secondary benefit of lowering permeability of moisture and other various gases.3
However, LED applications that use silicones with increased RI should also take into account the increased thermal energy associated with increased brightness. While the physical properties of silicone can withstand a wide temperature range, typically -100°C to +200°C, studies have shown that phenyl containing silicones exposed to higher temperatures over time may begin to discolor, ultimately decreasing the transmittance over time.4
Here is where the manufacturer must make a decision. If a need for higher RI silicones is required, proper thermal management must be considered such as thermally conductive interfaces and heat sinks attached to the LED assembly. The more heat generated, the greater the need and size of the heat sink. This need for thermal management and higher phenyl content increases the cost of the LED. Applications that will exist in harsher environments and have a need for a longer operating life would benefit from using a dimethyl silicone such as the headlight of a car. On the other hand, lighting inside the car that is in a controlled environment could benefit from a higher IR, phenyl-containing silicone.
Obviously, the relationship between the LED packager and the material provider is crucial. Both entities are responsible in different ways for ensuring the end product is high quality. Because LEDs are a relatively new lighting source, industry standards are continuously refined as more is understood about LED performance in specific markets.
On the material level, Accelerated Life Testing (ALT) and other accelerated aging tests have set test conditions and industry standards for various applications. While it is important for the silicone material provider to supply initial ALT data, the LED packager has the most control over design, materials and processing. The packager must, ultimately, develop rigorous ALT testing regimes that ensure the chosen materials not only give expected light output over time, but also high yields once in production. A close relationship between the material supplier and LED packager can ensure the best candidate is chosen for the specific design and process, which will accelerate time to market and keep high yields.
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- 1 Yanagida, Naoto. To Stabilize LED Output, TOWA Pursues Compression Mold Technique. AEI, September, 2010.
- Riegler, Bill, Velderrain, Michelle. Solving the Process Problems of Delivering Phosphors with a Silicone. White LEDs Conference, Tokyo, Japan, November, 2007.
- Lipps, Nathan, Velderrain, Michelle. Moisture Permeability Case Study #2. NuSil Technology Literature. December, 2010.
- Randall, Elgin, Thomaier, Rob. How Temper ature Effects Transmission of Silicone Encap sulants. NuSil Technology Literature. September, 2007.