9.3 LED Packaging Technology
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9.3.1 White LED Package Technology
White LED packaging technology is the basis for the development of solid-state
lighting technology. Different approaches are employed to realize white LED, such
as phosphor conversion and color-mixed approach. Taking phosphor conversion as
an example, the method was first invented by Nichia, the blue light emitted from
the LED chip excites phosphor to obtain yellow-green light, yellow-green light and
blue light synthesize white light. Color rendering index of the design can be further
improved by adding red and green phosphors and the colorimetric parameters needs to
be optimized. Combination of ultraviolet LED and trichromatic phosphor is another
promising method, which should provide better light color parameters. However, the
efficiency is still not high enough for practical application.
The structure and technology of phosphor coating can be classified as coating and
prefabrication. The coating method includes mixing phosphors in the encapsulating
gel, which covers the LED chip. When the light from the LED chip passes through
the encapsulating mixture, the phosphors can be excited and producing white light.
This is the traditional packaging structure because the packaging method is simple
and widely used in industry.
With the development of technology, a new type of phosphor coating structure is
proposed. In such design, fluorescent material is directly coated on LED chip. The
advantage of this structure is fluorescence material directly coated onto the chip,
which gives the possibility of controlling the thickness and uniformity of fluorescent
materials and improving the luminescence efficiency and white light quality. It needs
to be pointed out that due to density differences between phosphor powder and silica
gel, phosphor powder would gradually precipitate in the mixture and may lead to the
phenomenon of precipitation or concentration stratification in the phosphor layer. In
addition, the viscosity of silica gel will change obviously at different temperatures.
In the curing process of phosphor glue, the viscosity of silica gel will first drop and
then rise as the temperature rises, which will lead to the precipitation of phosphor
powder in silica gel to different degrees. Therefore, pre-prepared fluorescent film has
been proposed and used during the process to improve the issue.
As far as color-mixed approach is concerned, the method is the trend of future
development of solid-state lighting. Such design would enable a wide range of
dynamic color adjustment. However, it would require complex control and feedback
system.
9.3.2 UV Packaging Technology
With the maturity of the visible light field, short-wavelength ultraviolet (UV) light
has gradually become the focus of research. According to the wavelength, ultraviolet
light can be divided into near-ultraviolet UVA (320–400 nm), UVB (275–320 nm)
and far-ultraviolet UVC (100–275 nm). UV-LED has the advantages of compact
193
9.3.1 White LED Package Technology
White LED packaging technology is the basis for the development of solid-state
lighting technology. Different approaches are employed to realize white LED, such
as phosphor conversion and color-mixed approach. Taking phosphor conversion as
an example, the method was first invented by Nichia, the blue light emitted from
the LED chip excites phosphor to obtain yellow-green light, yellow-green light and
blue light synthesize white light. Color rendering index of the design can be further
improved by adding red and green phosphors and the colorimetric parameters needs to
be optimized. Combination of ultraviolet LED and trichromatic phosphor is another
promising method, which should provide better light color parameters. However, the
efficiency is still not high enough for practical application.
The structure and technology of phosphor coating can be classified as coating and
prefabrication. The coating method includes mixing phosphors in the encapsulating
gel, which covers the LED chip. When the light from the LED chip passes through
the encapsulating mixture, the phosphors can be excited and producing white light.
This is the traditional packaging structure because the packaging method is simple
and widely used in industry.
With the development of technology, a new type of phosphor coating structure is
proposed. In such design, fluorescent material is directly coated on LED chip. The
advantage of this structure is fluorescence material directly coated onto the chip,
which gives the possibility of controlling the thickness and uniformity of fluorescent
materials and improving the luminescence efficiency and white light quality. It needs
to be pointed out that due to density differences between phosphor powder and silica
gel, phosphor powder would gradually precipitate in the mixture and may lead to the
phenomenon of precipitation or concentration stratification in the phosphor layer. In
addition, the viscosity of silica gel will change obviously at different temperatures.
In the curing process of phosphor glue, the viscosity of silica gel will first drop and
then rise as the temperature rises, which will lead to the precipitation of phosphor
powder in silica gel to different degrees. Therefore, pre-prepared fluorescent film has
been proposed and used during the process to improve the issue.
As far as color-mixed approach is concerned, the method is the trend of future
development of solid-state lighting. Such design would enable a wide range of
dynamic color adjustment. However, it would require complex control and feedback
system.
9.3.2 UV Packaging Technology
With the maturity of the visible light field, short-wavelength ultraviolet (UV) light
has gradually become the focus of research. According to the wavelength, ultraviolet
light can be divided into near-ultraviolet UVA (320–400 nm), UVB (275–320 nm)
and far-ultraviolet UVC (100–275 nm). UV-LED has the advantages of compact