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9 Packaging of Group-III Nitride LED
technology, the process is anticipated to be further simplified. The flip-chip itself
may become a technical carrier of the wafer level package and directly used as
a package. The LED flip-chip process has the following advantages: the flip-chip
active layer is close to the substrate. The heat flow path from the heat source to the
substrate is shortened. The flip-chip has low thermal resistance and high thermal
response speed. The thermal stabilization process is fast and performance changes
are small. Light efficiency would be high under high injection level. Absence of
wire bonding reduces the probability of failure and the cost. Compact package size
enables easier secondary optical design. Therefore, it is a necessary path for major
development in LED encapsulation.
9.4 Package and System Cooling Technology
9.4.1 Packaging and System Cooling Technology
Chip cooling technology is a key issue that must be addressed in high power density
LED packages. When a high-power LED is in operation, a large amount of heat is
generated. Excessive heat accumulation will cause an increase in the temperature
of the LED chip, resulting in a series of problems that reduce luminous efficiency
and reliability of the device. High-power LED heat dissipation technology mainly
includes chip placement, packaging material selection (substrate material, thermal
interface material) and process, heat sink design and so on.
LED package thermal resistance mainly includes material internal thermal resistance and interface thermal resistance. The function of the heat dissipation substrate
is to absorb the heat generated by the chip and conduct it to the heat sink to achieve
heat exchange with the outside world. The commonly used heat-dissipating substrate
material is a copper-clad ceramic plate, which is sintered by a ceramic substrate
and a conductive layer (Cu) under high temperature and high pressure. It has good
thermal conductivity, high strength, strong insulation, and reduces thermal stress of
the packaging material.
At the same time, the package interface of the LED has a great influence on
the thermal resistance. If the interface cannot be processed correctly, it is difficult to
obtain a good heat dissipation effect. For instance, a well-contacted interface at room
temperature may have gaps at high temperatures. The warpage of the substrate may
also affect soldering robustness and local heat dissipation. The key step of improving
LED packaging is to reduce interface and interface contact thermal resistance and
enhance heat dissipation. Hence, the thermal interface material must be carefully
selected to meet the technical requirements. The use of low temperature or eutectic
solder, solder paste or conductive paste with nano-particles as the thermal interface
material can greatly reduce the interface thermal resistance.
9 Packaging of Group-III Nitride LED
technology, the process is anticipated to be further simplified. The flip-chip itself
may become a technical carrier of the wafer level package and directly used as
a package. The LED flip-chip process has the following advantages: the flip-chip
active layer is close to the substrate. The heat flow path from the heat source to the
substrate is shortened. The flip-chip has low thermal resistance and high thermal
response speed. The thermal stabilization process is fast and performance changes
are small. Light efficiency would be high under high injection level. Absence of
wire bonding reduces the probability of failure and the cost. Compact package size
enables easier secondary optical design. Therefore, it is a necessary path for major
development in LED encapsulation.
9.4 Package and System Cooling Technology
9.4.1 Packaging and System Cooling Technology
Chip cooling technology is a key issue that must be addressed in high power density
LED packages. When a high-power LED is in operation, a large amount of heat is
generated. Excessive heat accumulation will cause an increase in the temperature
of the LED chip, resulting in a series of problems that reduce luminous efficiency
and reliability of the device. High-power LED heat dissipation technology mainly
includes chip placement, packaging material selection (substrate material, thermal
interface material) and process, heat sink design and so on.
LED package thermal resistance mainly includes material internal thermal resistance and interface thermal resistance. The function of the heat dissipation substrate
is to absorb the heat generated by the chip and conduct it to the heat sink to achieve
heat exchange with the outside world. The commonly used heat-dissipating substrate
material is a copper-clad ceramic plate, which is sintered by a ceramic substrate
and a conductive layer (Cu) under high temperature and high pressure. It has good
thermal conductivity, high strength, strong insulation, and reduces thermal stress of
the packaging material.
At the same time, the package interface of the LED has a great influence on
the thermal resistance. If the interface cannot be processed correctly, it is difficult to
obtain a good heat dissipation effect. For instance, a well-contacted interface at room
temperature may have gaps at high temperatures. The warpage of the substrate may
also affect soldering robustness and local heat dissipation. The key step of improving
LED packaging is to reduce interface and interface contact thermal resistance and
enhance heat dissipation. Hence, the thermal interface material must be carefully
selected to meet the technical requirements. The use of low temperature or eutectic
solder, solder paste or conductive paste with nano-particles as the thermal interface
material can greatly reduce the interface thermal resistance.