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9 Packaging of Group-III Nitride LED
9.4.2 LED Thermal Testing Technology
The generation and extraction of heat is one of the essential factors that affecting
the performance of LED devices. Thermal management study in LED encapsulation
includes material selection, process design and interface like plurality of aspects. Poor
thermal management of LED devices will lead to a list of undesired results such as
rising junction temperature, decreased luminous efficiency, wavelength drift, color
temperature variation, electrical performance degradation, thermal stress increase,
component lifetime loss, and phosphor quality degradation. One of the main indicators for evaluating the quality of thermal management is the thermal resistance of the
device. It is an indicator mainly reflects the heat dissipation capability caused by the
medium used by the device and the related device structure. Variety of factors would
affect the thermal resistance, which comprises a LED chip structure and materials,
thermal interface materials, heat sink material, sectional area of heat path, thermal
structure design, temperature change of the thermal conductivity, ambient temperature, etc. In order to maximize LED output power per unit area, higher power density
LED package shape is the future development trend, and LED’s thermal management
becomes increasingly important.
At present, LED thermal testing mainly includes infrared thermal imaging, electrical parameters, transient thermal testing, spectral and optical power methods, and
pin temperature methods. These methods measure or derive the junction temperature
and thermal resistance of the LED based on different principles.
Infrared thermal imaging refers to the equipment or method that detects the
infrared energy emitted by an object, converting the infrared energy into a temperature
value, and thereby the real-time temperature distribution of the measured objected
can be obtained in the form of image. The measurement can be made in a distance,
without making direct contact with the object.
The principle of electrical parameter method is based on a good linear relationship
between the forward voltage drop VF of LED and the temperature of pn junction,
commonly known as K factor. After obtaining the K factor, the junction temperature
of LED can be obtained by measuring the voltage change of LED. During the whole
measurement process, LED is not only the measured object, but also the sensor.
Transient thermal test method: the thermal characteristics of LED can be described
by equivalent RC circuit. The relationship between the transient response of LED to
thermal step input and the physical characteristics of each part of the package is used
to obtain the thermal resistance-heat capacity curve of each part of the LED package
[32].
9.5 Development Trend of LED Encapsulation Form
According to the application requirements, LED packaging technology has gone
through four stages: lead-type LED package, surface mount electronic SMT package,
COB chip-on-board LED package and SiP system electronic package-like LED
9 Packaging of Group-III Nitride LED
9.4.2 LED Thermal Testing Technology
The generation and extraction of heat is one of the essential factors that affecting
the performance of LED devices. Thermal management study in LED encapsulation
includes material selection, process design and interface like plurality of aspects. Poor
thermal management of LED devices will lead to a list of undesired results such as
rising junction temperature, decreased luminous efficiency, wavelength drift, color
temperature variation, electrical performance degradation, thermal stress increase,
component lifetime loss, and phosphor quality degradation. One of the main indicators for evaluating the quality of thermal management is the thermal resistance of the
device. It is an indicator mainly reflects the heat dissipation capability caused by the
medium used by the device and the related device structure. Variety of factors would
affect the thermal resistance, which comprises a LED chip structure and materials,
thermal interface materials, heat sink material, sectional area of heat path, thermal
structure design, temperature change of the thermal conductivity, ambient temperature, etc. In order to maximize LED output power per unit area, higher power density
LED package shape is the future development trend, and LED’s thermal management
becomes increasingly important.
At present, LED thermal testing mainly includes infrared thermal imaging, electrical parameters, transient thermal testing, spectral and optical power methods, and
pin temperature methods. These methods measure or derive the junction temperature
and thermal resistance of the LED based on different principles.
Infrared thermal imaging refers to the equipment or method that detects the
infrared energy emitted by an object, converting the infrared energy into a temperature
value, and thereby the real-time temperature distribution of the measured objected
can be obtained in the form of image. The measurement can be made in a distance,
without making direct contact with the object.
The principle of electrical parameter method is based on a good linear relationship
between the forward voltage drop VF of LED and the temperature of pn junction,
commonly known as K factor. After obtaining the K factor, the junction temperature
of LED can be obtained by measuring the voltage change of LED. During the whole
measurement process, LED is not only the measured object, but also the sensor.
Transient thermal test method: the thermal characteristics of LED can be described
by equivalent RC circuit. The relationship between the transient response of LED to
thermal step input and the physical characteristics of each part of the package is used
to obtain the thermal resistance-heat capacity curve of each part of the LED package
[32].
9.5 Development Trend of LED Encapsulation Form
According to the application requirements, LED packaging technology has gone
through four stages: lead-type LED package, surface mount electronic SMT package,
COB chip-on-board LED package and SiP system electronic package-like LED