9.3 LED Packaging Technology
195
also introduced SMD and COB packaged UV-LEDs products. Despite the diversity
of packaging methods, it still has a long way to meet customer needs. It is urgent to
find a stable and reliable packaging method.
Packaging raw materials also have a critical impact on the performance and reliability of UV LEDs. The choice of encapsulant and lens material directly affects
the luminous efficiency and lifetime of UV-LEDs. Generally, the encapsulant should
have high transmittance, high refractive index, good thermal stability, good fluidity
and easy for spraying. Meanwhile, in order to improve the reliability of the UV-LED
package, it is ideal for the material to have low hygroscopicity, low stress, weather
resistance and environmental protection. For UV-LED packaging, quartz glass has
the highest transmittance (up to 95%), followed by silicone. The UV transmittance
of epoxy resin decreases sharply with wavelength. However, despite the high ultraviolet light transmittance of quartz glass, its high thermal processing temperature is
not suitable for sealing. Therefore, in UV-LED packaging process, quartz glass is
generally used as the outermost lens material. Epoxy resin has a poor resistance to
UV and heat resistance, while silicone resin has an obvious advantage in UV region.
Therefore, epoxy resin is generally not used as the encapsulating lens material of
UV-LED, but silicon resin and glass are used as the encapsulating material. In order
to improve performance and reliability of UV-LEDs, glass encapsulated UV-LED is
currently one of the trends to avoid undesired materials aging issues induced by UV
exposure.
9.3.3 High Power Density Packaging Technology
A large amount of heat is generated during the operation of high-power LED. The
generated heat should be transferred to the heatsink as soon as possible to avoid
heat accumulation. Otherwise, excessive heat will reduce luminous efficiency and
reliability of the LED. In 1998, Lumileds in the US launched high-power LED (1 W
Luxeon device), which enables the possibility of using LEDs for general lighting,
rather than just working as indicating light source [28].
In order to meet the requirements of surface mount technology, Cree uses ceramic
substrate as the carrier of LED chip instead of the original metal plastic lead frame
and heat sink, which has good heat dissipation effect, small size and is suitable for
SMD process. From 2007 to 2009, Cree company launched xp-c, xp-e and xp-g
series of optical lenses with the size of 3.5 mm × 3.5 mm and the direct modeling
of optical lenses on ceramic substrates, eliminating metal reflecting cups and optical
glass lenses.
Early power LED suffers from low degree of integration and productivity due to the
use of conventional lead frame structure rather than the planar structure. The recently
introduced high-power LED packaging process uses a planar, array- arrangement,
together with encapsulation of the silicone optical lens structure in a planar area,
which can be called a planar array packaging process. During the processing, the
195
also introduced SMD and COB packaged UV-LEDs products. Despite the diversity
of packaging methods, it still has a long way to meet customer needs. It is urgent to
find a stable and reliable packaging method.
Packaging raw materials also have a critical impact on the performance and reliability of UV LEDs. The choice of encapsulant and lens material directly affects
the luminous efficiency and lifetime of UV-LEDs. Generally, the encapsulant should
have high transmittance, high refractive index, good thermal stability, good fluidity
and easy for spraying. Meanwhile, in order to improve the reliability of the UV-LED
package, it is ideal for the material to have low hygroscopicity, low stress, weather
resistance and environmental protection. For UV-LED packaging, quartz glass has
the highest transmittance (up to 95%), followed by silicone. The UV transmittance
of epoxy resin decreases sharply with wavelength. However, despite the high ultraviolet light transmittance of quartz glass, its high thermal processing temperature is
not suitable for sealing. Therefore, in UV-LED packaging process, quartz glass is
generally used as the outermost lens material. Epoxy resin has a poor resistance to
UV and heat resistance, while silicone resin has an obvious advantage in UV region.
Therefore, epoxy resin is generally not used as the encapsulating lens material of
UV-LED, but silicon resin and glass are used as the encapsulating material. In order
to improve performance and reliability of UV-LEDs, glass encapsulated UV-LED is
currently one of the trends to avoid undesired materials aging issues induced by UV
exposure.
9.3.3 High Power Density Packaging Technology
A large amount of heat is generated during the operation of high-power LED. The
generated heat should be transferred to the heatsink as soon as possible to avoid
heat accumulation. Otherwise, excessive heat will reduce luminous efficiency and
reliability of the LED. In 1998, Lumileds in the US launched high-power LED (1 W
Luxeon device), which enables the possibility of using LEDs for general lighting,
rather than just working as indicating light source [28].
In order to meet the requirements of surface mount technology, Cree uses ceramic
substrate as the carrier of LED chip instead of the original metal plastic lead frame
and heat sink, which has good heat dissipation effect, small size and is suitable for
SMD process. From 2007 to 2009, Cree company launched xp-c, xp-e and xp-g
series of optical lenses with the size of 3.5 mm × 3.5 mm and the direct modeling
of optical lenses on ceramic substrates, eliminating metal reflecting cups and optical
glass lenses.
Early power LED suffers from low degree of integration and productivity due to the
use of conventional lead frame structure rather than the planar structure. The recently
introduced high-power LED packaging process uses a planar, array- arrangement,
together with encapsulation of the silicone optical lens structure in a planar area,
which can be called a planar array packaging process. During the processing, the