IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 33, NO. 4, DECEMBER 2010 761 Development of High-Performance Optical Silicone for the Packaging of High-Power LEDs Yeong-Her Lin, Jiun Pyng You, Yuan-Chang Lin, Nguyen T. Tran, and Frank G. Shi Abstract —Silicone materials with a relatively high-refractive index have been introduced for the encapsulation of high- power light-emitting diodes (LEDs), and LEDs with relatively short wavelengths. However, most of those existing silicone encapsulants still suffer from thermal and radiation induced degradations and thus lead to reliability issues and a shorten lifetime. A new high-performance silicone has been developed and its performance is compared with other commercial silicone and optical grade epoxy in high-power white LEDs. The new materials had been found to suffer less loss in the lumen output during the aging test and high-temperature/high-humidity test, as well as the Joint Electron Devices Engineering Council (JEDEC) reliability test. It is concluded that this material is excellent for the packaging of high-power white LEDs and high-power colored LEDs, because of its ability in maintaining high-transparency and great radiation/thermal resistance. Index Terms—High-power LEDs, light-emitting diodes (LEDs), packaging, refractive index (RI), reliability, silicone, transmit- tance. I. Introduction L IGHT-EMITTING diodes (LEDs), and particularly white LEDs, are now being used in the general lighting as well as applications in automobile lighting, backlighting for liquid-crystal display laptops, mobile phones, and televisions. The most popular method for making white LEDs is to use YAG:Ce phosphors with GaN LEDs to produce white LEDs. The phosphor is embedded in an optical grade resin, such as epoxy or silicone, and excited by blue or ultraviolent (UV)–visible (VIS) light generated from the GaN chip. High- power LEDs can have high-cost effectiveness in large display, lighting, and automotive application. Surface mount device (SMD) LEDs have been a mainstream for a high-power LED packaging due to thermal advantage, low-profile design, and reduced assembly costs [1], [2]. The LED package includes LED chip mounted on the lead frame, gold wires for electrical connection, and encapsulant covering LED chip and gold wires. Encapsulant is an optical grade resin and has several functions in LED packages. First, Manuscript received June 21, 2009; revised December 9 2009 and March 11, 2010. Date of publication July 1, 2010; date of current version December 22, 2010. Recommended for publication by Associate Editor K. Zhang upon evaluation of reviewers’ comments. The authors are with Optoelectronics Packaging and Materials Labs, University of California, Irvine, CA 92697 USA (e-mail: yeonghel@uci.edu; jpyou20004@yahoo.com; grantycl@gmail.com; ntran3000@yahoo.com; fgshi2000@yahoo.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCAPT.2010.2046488 it protects the device from dust and moisture. Second, it behaves as a lens focusing the light in the desired way. Third, it improves the light output of LED device by increasing light-extraction from LED chip to air. A major challenge for the LED encapsulant is to achieve high-light-extraction efficiency, especially for high-power LED in general illumina- tion applications. Due to the relatively low-refractive index (RI) of the encapsulant that results in a low-critical angle at encapsulant-chip interface, there is a relatively high-light- extraction between the LED chip and the encapsulant with high-RI. The critical angle θ c can be calculated using the Snell’s equation θ c = sin -1  n 2 n 1  where n 1 is the RI of the LED chip and n 2 is the refractive index of the encapsulant. The critical angle θ c is decreased when a LED chip is encapsulated in a lower refractive index medium. The RI of a LED chip is between 2.5 and 3.5. However, the RI of an encapsulant, such as the epoxy or silicone, is only between 1.4 and 1.6. As light emitted from an active layer of an LED chip is incident on the interface between an LED chip and an encapsulant, light undergoes total internal reflection if the incident angle is larger than the critical angle or it is partially reflected back inside the medium in which it is propagating. In order to reduce the reflected light and increase light-extraction efficiency, materials with high-RI (n> 1.5) and TiO 2 nanoparticle-containing encapsulant are used in LED packaging [3]. The optical grade epoxy has been used as encapsulant materials for LED packages for many years due to its high- light transmission, high-RI, and high-glass transition temper- ature (T g ). However, several problems have been found in high-power LED packaged with epoxy encapsulants. Material yellowing and shorter lifetime are the most severe problems associated with epoxy encapsulant in high-power LED pack- ages. The yellowing issue of optical grade epoxy occurs during the exposure of the epoxy encapsulant to blue or UV light. Yellowing of encapsulant also occurs when the encapsulant material is subjected to large amounts of heat. In high-power LED packages, there are large amounts of heat generated and accumulated inside the package, especially around LED chips, resulting in yellowing of epoxy encapsulant surrounding LED chips. The yellowing of the epoxy results in a significant loss of light output over time. 1521-3331/$26.00 c  2010 IEEE