10.2 The LED Aging Test and an Aging Mechanism
213
stress are taken as the accelerated stress; Peck model can be used when high temperature and high humidity are taken as the accelerated stress; Eyring accelerated life
test model can be used when compound stress is taken as the accelerated stress to
estimate the relationship between lifetime and accelerated environmental stress. In
the following, the mechanism of accelerated aging test under different temperature,
electrical stress, high temperature and humidity, and other composite stress as well
as aging mechanisms will be introduced.
10.2.2 Temperature Acceleration Test.
The failure of the LED device is related to the materials, interfaces, electrical connections, and various physical and chemical reactions that exist in the package structure.
As the temperature increases, the physicochemical reaction speed inside the LED
increases, and the failure process will be accelerated. The Arrhenius model can be
used to study the corresponding law between the reaction speed and the temperature
as shown by the formula:
d M
dt
= Ae
−
Ea
kT
(10.2)
where:
dM/dt is the reaction rate of the chemical reaction;
A is the pre-factor constant;
E a is the activation energy (eV) that causes the failure or degradation process;
k is the Boltzmann constant (0.000086174 eV/K);
T is the absolute temperature (K) under certain acceleration conditions;
Degradation rate of the device performance is exponentially proportional to the
activation energy inversely and the reciprocal of the temperature. At given constant
temperature, the failure state number is M 0 when the device is at the t 0 . At time t 1 ,
the device is in a failed state number M 1 . The obtained integral
M 1 − M 0 = Ae
−
Ea
kT (t 1 − t 0 )
(10.3)
Let M 0 = 0, t 0 = 0, t 1 = t, yield
t =
M 1
A
e
Ea
kT
(10.4)
Taking the natural logarithm of both sides of the equation
lnt = ln
M 1
A
+
E a
kT
(10.5)
213
stress are taken as the accelerated stress; Peck model can be used when high temperature and high humidity are taken as the accelerated stress; Eyring accelerated life
test model can be used when compound stress is taken as the accelerated stress to
estimate the relationship between lifetime and accelerated environmental stress. In
the following, the mechanism of accelerated aging test under different temperature,
electrical stress, high temperature and humidity, and other composite stress as well
as aging mechanisms will be introduced.
10.2.2 Temperature Acceleration Test.
The failure of the LED device is related to the materials, interfaces, electrical connections, and various physical and chemical reactions that exist in the package structure.
As the temperature increases, the physicochemical reaction speed inside the LED
increases, and the failure process will be accelerated. The Arrhenius model can be
used to study the corresponding law between the reaction speed and the temperature
as shown by the formula:
d M
dt
= Ae
−
Ea
kT
(10.2)
where:
dM/dt is the reaction rate of the chemical reaction;
A is the pre-factor constant;
E a is the activation energy (eV) that causes the failure or degradation process;
k is the Boltzmann constant (0.000086174 eV/K);
T is the absolute temperature (K) under certain acceleration conditions;
Degradation rate of the device performance is exponentially proportional to the
activation energy inversely and the reciprocal of the temperature. At given constant
temperature, the failure state number is M 0 when the device is at the t 0 . At time t 1 ,
the device is in a failed state number M 1 . The obtained integral
M 1 − M 0 = Ae
−
Ea
kT (t 1 − t 0 )
(10.3)
Let M 0 = 0, t 0 = 0, t 1 = t, yield
t =
M 1
A
e
Ea
kT
(10.4)
Taking the natural logarithm of both sides of the equation
lnt = ln
M 1
A
+
E a
kT
(10.5)