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10 Reliability Analysis of Group III Nitride LEDs Devices
In a mounted LEDs device, the p-type and n-type electrodes are generally located
on the same side of the device. Due to the low thermal resistance and insulation
of the sapphire substrate, GaN/InGaN devices are very vulnerable to electrostatic
discharge damage. In general, in order to avoid permanent damage to the device by
ESD, a suitable reverse voltage regulator diode can be connected in parallel. This
structure allows voltage spikes in two directions to pass without damaging the LEDs
device, [14] or connecting a GaN Base Schottky diodes to improve the overall ESD
characteristics of nitride LEDs devices [15, 16], or anti-parallel shunt GaN-based
ESD protection diodes to improve the ESD reliability of GaN LEDs devices [17].
In addition, the use of SiC and GaN substrates with small lattice mismatch and Si
substrates with high thermal resistance can also improve the antistatic capability of
the device.
10.1.3 Packaging
(1) Lens and potting layer
The materials used for the lenses and potting layers in LED devices are mostly based
on polymeric materials such as epoxy, silicone polymers and polymethyl methacrylate (PMMA). When the junction temperature is too high or the injection current
density is high, the encapsulation layer and the phosphor layer prepared from these
materials will accelerate aging, resulting in color drift.
When a large amount of Joule heat is generated under a large current, or directly
in a high temperature environment, the surface encapsulation layer would be locally
carbonized to form a conductive path across the LED device. This effect can directly
destroy the device, which is called carbonization of the encapsulating material. After
the encapsulation layer is carbonized, the insulation resistance of the encapsulation
layer is lowered, and the electrical insulation between the bond alloy wires and
between the gold wires and the leads is severely damaged. When the temperature
of the working environment of the device is high enough, it will cause thermal
dissipation of the device. In this process, the bond alloy wire is partially melted due
to the high temperature and high current, where part of the current in the gold wire
passes through the encapsulation layer in the encapsulation layer. A large amount
of Joule heat is generated, which further reduces the insulation resistance of the
encapsulation layer and ultimately leads to carbonization of the encapsulation layer.
Under sustained, high current and high temperature environment, it is extremely easy
to form a conductive layer due to the carbonization at the contact area of package
layer and sealing layer region, resulting in shorting out of LED device. If continuous
high electrical stress is applied, the abrupt failure caused by the encapsulation layer
will occur. Figure 10.3 shows the cross-sectional view of the LED device undergoing
a high temperature stress aging test. The trace of carbonized reflective cup can be
clearly seen [18].
10 Reliability Analysis of Group III Nitride LEDs Devices
In a mounted LEDs device, the p-type and n-type electrodes are generally located
on the same side of the device. Due to the low thermal resistance and insulation
of the sapphire substrate, GaN/InGaN devices are very vulnerable to electrostatic
discharge damage. In general, in order to avoid permanent damage to the device by
ESD, a suitable reverse voltage regulator diode can be connected in parallel. This
structure allows voltage spikes in two directions to pass without damaging the LEDs
device, [14] or connecting a GaN Base Schottky diodes to improve the overall ESD
characteristics of nitride LEDs devices [15, 16], or anti-parallel shunt GaN-based
ESD protection diodes to improve the ESD reliability of GaN LEDs devices [17].
In addition, the use of SiC and GaN substrates with small lattice mismatch and Si
substrates with high thermal resistance can also improve the antistatic capability of
the device.
10.1.3 Packaging
(1) Lens and potting layer
The materials used for the lenses and potting layers in LED devices are mostly based
on polymeric materials such as epoxy, silicone polymers and polymethyl methacrylate (PMMA). When the junction temperature is too high or the injection current
density is high, the encapsulation layer and the phosphor layer prepared from these
materials will accelerate aging, resulting in color drift.
When a large amount of Joule heat is generated under a large current, or directly
in a high temperature environment, the surface encapsulation layer would be locally
carbonized to form a conductive path across the LED device. This effect can directly
destroy the device, which is called carbonization of the encapsulating material. After
the encapsulation layer is carbonized, the insulation resistance of the encapsulation
layer is lowered, and the electrical insulation between the bond alloy wires and
between the gold wires and the leads is severely damaged. When the temperature
of the working environment of the device is high enough, it will cause thermal
dissipation of the device. In this process, the bond alloy wire is partially melted due
to the high temperature and high current, where part of the current in the gold wire
passes through the encapsulation layer in the encapsulation layer. A large amount
of Joule heat is generated, which further reduces the insulation resistance of the
encapsulation layer and ultimately leads to carbonization of the encapsulation layer.
Under sustained, high current and high temperature environment, it is extremely easy
to form a conductive layer due to the carbonization at the contact area of package
layer and sealing layer region, resulting in shorting out of LED device. If continuous
high electrical stress is applied, the abrupt failure caused by the encapsulation layer
will occur. Figure 10.3 shows the cross-sectional view of the LED device undergoing
a high temperature stress aging test. The trace of carbonized reflective cup can be
clearly seen [18].