206
10 Reliability Analysis of Group III Nitride LEDs Devices
recombination centers include an increase in series resistance and forward operating
voltage, current crowding effect, reverse leakage current caused by the tunnel effect,
and decrease in luminous efficiency. Kohler et al. found that high concentration of Mg
near the active region at low growth temperature can enhance luminous efficiency
due to the increase in hole concentration. However, it increases the non-radiative
recombination rate in the opposite condition. In addition, the use of gradual doping
can also effectively reduce the diffusion of Mg to the active layer and improve device
performance [10].
10.1.2 Sudden Failure
The sudden failure of LEDs devices mainly manifests as an open circuit or a short
circuit. Most flip-chip LEDs packages fail suddenly due to short circuits. Lift-off of
leads and ball solder is open circuits. Sudden failures can be divided into intrinsic
failures and wear-out failures. If we describe the distribution law of the failure rate
of a product throughout its life cycle, we get the bathtub curve. The intrinsic failure
corresponds to the flat part of the bathtub curve. For wear failures, the failure rate
increases over time.
(1) Die cracking
Due to the different characteristics of the materials such as the difference in thermal
expansion coefficient, the LEDs packaging layer will suffer from a greater impact of
thermal and electrical stress when the device is suddenly placed in a high-temperature
environment or a large current injection. If such an effect is large enough, it will
directly damage the chip of the GaN-based LEDs device. Whether the thermal shock
or the electric heating effect caused by high current density leads to the rapid heating
or cooling of the material, the huge temperature difference causes a large amount of
heat exchange in a short period of time. This can produce great mechanical stress on
material. Thus, in order to prevent the die from cracking under high stress, the thermal
expansion coefficients of the substrate and the epitaxial layer need to be adjusted
to make them match as much as possible as shown in Fig. 10.2. An appropriate
intermediate layer can be grown between the substrate and the epitaxial layer or using
flexible substrates, which can both effectively reduce the chance of die cracking.
(2) Fatigue or fracture of wire ball bonding caused by electric heating overstress
Wire bonding is the most commonly used method to connect the LEDs die and
external electrodes, i.e. to complete the current path between the chip and the external
pins of the package. Welding bonds are formed between these metal interfaces. The
bonding wire is the key to connecting the LEDs chip and the substrate. The failures in
wire bonding mainly include wire breakage and fracture, bond point disintegration,
and wire ball fatigue. Most of the failures are abrupt. The annealing process is used
to solder the gold wire. The area above the solder ball point processed by annealing
is called the heat affected zone (HAZ) which is the most vulnerable part in the entire
10 Reliability Analysis of Group III Nitride LEDs Devices
recombination centers include an increase in series resistance and forward operating
voltage, current crowding effect, reverse leakage current caused by the tunnel effect,
and decrease in luminous efficiency. Kohler et al. found that high concentration of Mg
near the active region at low growth temperature can enhance luminous efficiency
due to the increase in hole concentration. However, it increases the non-radiative
recombination rate in the opposite condition. In addition, the use of gradual doping
can also effectively reduce the diffusion of Mg to the active layer and improve device
performance [10].
10.1.2 Sudden Failure
The sudden failure of LEDs devices mainly manifests as an open circuit or a short
circuit. Most flip-chip LEDs packages fail suddenly due to short circuits. Lift-off of
leads and ball solder is open circuits. Sudden failures can be divided into intrinsic
failures and wear-out failures. If we describe the distribution law of the failure rate
of a product throughout its life cycle, we get the bathtub curve. The intrinsic failure
corresponds to the flat part of the bathtub curve. For wear failures, the failure rate
increases over time.
(1) Die cracking
Due to the different characteristics of the materials such as the difference in thermal
expansion coefficient, the LEDs packaging layer will suffer from a greater impact of
thermal and electrical stress when the device is suddenly placed in a high-temperature
environment or a large current injection. If such an effect is large enough, it will
directly damage the chip of the GaN-based LEDs device. Whether the thermal shock
or the electric heating effect caused by high current density leads to the rapid heating
or cooling of the material, the huge temperature difference causes a large amount of
heat exchange in a short period of time. This can produce great mechanical stress on
material. Thus, in order to prevent the die from cracking under high stress, the thermal
expansion coefficients of the substrate and the epitaxial layer need to be adjusted
to make them match as much as possible as shown in Fig. 10.2. An appropriate
intermediate layer can be grown between the substrate and the epitaxial layer or using
flexible substrates, which can both effectively reduce the chance of die cracking.
(2) Fatigue or fracture of wire ball bonding caused by electric heating overstress
Wire bonding is the most commonly used method to connect the LEDs die and
external electrodes, i.e. to complete the current path between the chip and the external
pins of the package. Welding bonds are formed between these metal interfaces. The
bonding wire is the key to connecting the LEDs chip and the substrate. The failures in
wire bonding mainly include wire breakage and fracture, bond point disintegration,
and wire ball fatigue. Most of the failures are abrupt. The annealing process is used
to solder the gold wire. The area above the solder ball point processed by annealing
is called the heat affected zone (HAZ) which is the most vulnerable part in the entire