Silicone delivers thermal stability to LED packaging
LED packaging materials must enhance the light transmission, reliability and longevity of a device. However, with a recent move to using lead-free solders requiring reflow temperatures of up to 260 °C, HB-LED packagers have had difficulty in maintaining the quality of light transmission. Traditional epoxy-based encapsulants crack or yellow when exposed - even for a short time - to these higher soldering temperatures.
Highlighting the importance of encapsulants, a patent application from Cree submitted earlier this year (US patent 6,740,906) notes that certain die-attach methods involving high thermal cycles can discolor clear epoxy encapsulants, resulting in unwanted light attenuation.
Encapsulant of choiceIn a recent paper regarding the performance of high-power LED illuminators, Lumileds Lighting states: "Key elements for high power, [long] lifetime, and high-efficiency operation are the LED chip itself, the soft silicone gel [which is used as an encapsulant], the metal slug directly below the actual LED chip [providing a very low thermal resistance path for heat conduction] and a high-temperature resistant plastic lens." Lumileds has confirmed that testing the physical and optical characteristics of silicone have proved it to be the encapsulant of choice for its HB-LEDs.
In fact, Lumileds now uses silicone to encapsulate its Luxeon products, and rival manufacturers Osram Opto Semiconductors and Nichia also use the material in their Golden Dragon, Jupiter and Rigel HB-LEDs.
While the soldering process represents a temporary temperature spike in the life of the LED, engineers must also consider the operating conditions of the diode. Both temperature and UV exposure can affect the optical properties of the encapsulant material.
Researchers at Lumileds recognized that materials such as glass, acrylic, polycarbonate, optical nylon, and cast-molded epoxy all suffered from a degradation in optical transparency over time under normal LED operation. However, they found silicone to be stable following extensive temperature and humidity tests and exposure to ambient UV radiation, and this is one of the reasons that the Californian company s devices have such a long lifetime.
Silicone materials work well in encapsulation applications because, typically, they have higher coefficient of thermal expansion (CTE) values than epoxy, acrylic and other adhesive systems (see table). This means that silicones are more flexible and are thus ideal bridge materials between two substrates with dissimilar CTE values. Another factor to consider is the stress that expanding or contracting materials exert on wires in packaged assemblies. Stresses associated with high modulus values can break these wires or compromise the wire bond. However, silicone has a relatively low modulus value compared with other encapsulant materials, eliminating this problem.
A material with an industrial history spanning 60 years, silicone maintains its elastomeric properties in continuous temperatures of 260 °C. The polymer structure contributes significantly to the material s stability and versatility. Maintaining elastomeric properties after extreme temperature cycling or radiation exposure can be attributed to a relatively small rotational energy in a silicone polymer. This low energy is due in part to the negligible interactions between siloxane chains. Additionally, siloxane elastomers have physical properties that allow engineers to easily incorporate additives such as phosphors with little effect on the function of the elastomer system.
The high refractive index outputs of the dies used in HB-LEDs, which can be as great as 2.5, ideally require an encapsulant with a similarly high refractive index. Modifying the silicone backbone by incorporating relatively large molecular groups such as phenyls can boost the refractive index of these silicone encapsulants from 1.38 to 1.57.
According to a recent patent application submitted by Emcore (US patent 6,746,889), this index-matching material should be lens-shaped to improve light extraction from the device. The Emcore team further states that this material should have a refractive index "substantially equal" to the geometric mean of the epitaxial region and surrounding light-guiding-medium refractive indexes. Light-guiding media can include glass fibers and light pipes. The index-matching material may be a polymer, and silicone is suggested as a material option for this application.
Further reading
Performance of high-power LED illuminations in projection displays, G Harbers et al. Lumileds Lighting (white paper).
US patent application 6,590,235.
