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9 Packaging of Group-III Nitride LED
epoxy groups in its molecule. It has the following advantages: (1) no by-product and
the volume shrinkage during solidification process; (2) less restriction on the amount
of filling agent, (3) good adhesion for most of the materials; and (4) excellent electrical and mechanical characteristics. However, epoxy resin has disadvantages such
as weak ozone resistance and yellowing effect on the light transmission [7]. The
basic structural unit of silica gel is composed of silicon-oxygen chain links. The side
chains are connected with other organic groups. The special composition and molecular structure make it good performance compared with other polymer materials [8].
For instance, it has good thermal stability, stable chemical bonds of the molecules
at high temperatures (or radiation), and operational over a wide temperature range.
In addition, electrical properties of the materials are not significantly affected by
temperature and frequency. In comparison with the same molecular weight hydrocarbon, it is a stable electrical insulating material with lower viscosity, weak surface
tension, small surface energy and strong film forming ability. Currently, methyl
silicone-based material is widely used in LED packaging.
9.1.6 Thermal Interface Material
LED heat dissipation could be affected by various factors including chip structure,
thermal interface material, heat sink substrate material, and package structure. In
order to improve the LED heat dissipation, besides the effort to reduce the thermal
resistance of the chip and packaging structure, the thermal interface material between
LED chip and heat sink is needed to fill the voids between the contact surfaces of
the two materials to reduce the thermal resistance. Since air has a thermal conductivity of only 0.024 W / (m • K), existence of air voids between LED chip and heat
sink would lead to relatively high thermal resistance if they are directly mounted
together. Therefore, it is necessary to fill in these voids with a thermal interface
material with high thermal conductivity to eliminates air and to establish an effective heat conduction path between the electronic component and the heat sink. Such
a design would greatly reduce the contact thermal resistance, and fully utilizing
the function of the heat sink. Good thermal interface materials should have high
thermal conductivity, low thermal resistance, compressibility and softness, surface
wettability, proper viscosity, good stability during thermal cycling, etc. Meanwhile,
development of low cost and preparation technology should be explored. Currently,
LED chip packaging generally includes: conductive silver paste, thermal paste, solder
paste and tin-gold alloy eutectic solder.
Conductive silver paste is made by adding silver powder into epoxy resin. Its
hardening temperature is low, and its heat conduction coefficient is about 20 W/(m·K).
It has good thermal conductivity and good adhesion strength, but its light absorption
is relatively large, resulting in a decrease in light efficiency.
The thermal conductive adhesive combines the matrix with some high thermal
conductivity fillers such as SiC, AlN, Al 2 O 3 , SiO 2 , carbon tubes, graphene to improve
9 Packaging of Group-III Nitride LED
epoxy groups in its molecule. It has the following advantages: (1) no by-product and
the volume shrinkage during solidification process; (2) less restriction on the amount
of filling agent, (3) good adhesion for most of the materials; and (4) excellent electrical and mechanical characteristics. However, epoxy resin has disadvantages such
as weak ozone resistance and yellowing effect on the light transmission [7]. The
basic structural unit of silica gel is composed of silicon-oxygen chain links. The side
chains are connected with other organic groups. The special composition and molecular structure make it good performance compared with other polymer materials [8].
For instance, it has good thermal stability, stable chemical bonds of the molecules
at high temperatures (or radiation), and operational over a wide temperature range.
In addition, electrical properties of the materials are not significantly affected by
temperature and frequency. In comparison with the same molecular weight hydrocarbon, it is a stable electrical insulating material with lower viscosity, weak surface
tension, small surface energy and strong film forming ability. Currently, methyl
silicone-based material is widely used in LED packaging.
9.1.6 Thermal Interface Material
LED heat dissipation could be affected by various factors including chip structure,
thermal interface material, heat sink substrate material, and package structure. In
order to improve the LED heat dissipation, besides the effort to reduce the thermal
resistance of the chip and packaging structure, the thermal interface material between
LED chip and heat sink is needed to fill the voids between the contact surfaces of
the two materials to reduce the thermal resistance. Since air has a thermal conductivity of only 0.024 W / (m • K), existence of air voids between LED chip and heat
sink would lead to relatively high thermal resistance if they are directly mounted
together. Therefore, it is necessary to fill in these voids with a thermal interface
material with high thermal conductivity to eliminates air and to establish an effective heat conduction path between the electronic component and the heat sink. Such
a design would greatly reduce the contact thermal resistance, and fully utilizing
the function of the heat sink. Good thermal interface materials should have high
thermal conductivity, low thermal resistance, compressibility and softness, surface
wettability, proper viscosity, good stability during thermal cycling, etc. Meanwhile,
development of low cost and preparation technology should be explored. Currently,
LED chip packaging generally includes: conductive silver paste, thermal paste, solder
paste and tin-gold alloy eutectic solder.
Conductive silver paste is made by adding silver powder into epoxy resin. Its
hardening temperature is low, and its heat conduction coefficient is about 20 W/(m·K).
It has good thermal conductivity and good adhesion strength, but its light absorption
is relatively large, resulting in a decrease in light efficiency.
The thermal conductive adhesive combines the matrix with some high thermal
conductivity fillers such as SiC, AlN, Al 2 O 3 , SiO 2 , carbon tubes, graphene to improve