Reliability Study of High-Efficiency LEDs for Road Lighting

However, in the actual packaging parameter test, it is found that the packaging thermal resistance during the test is much larger than the theoretical calculated value. One reason for this is that the actual value of the thermal conductivity of the die-bonding material deviates from the rated value given by the manufacturer. For example, when using silver glue to bond the die, this deviation is related to the storage, coating, and curing process of the silver glue material. Another reason is that the control of the die-bonding thickness is far from ideal. Generally speaking, the die-bonding thermal resistance is proportional to the thickness of the die-bonding layer. Therefore, the surface flatness of the base material, the microbubbles and impurities inside the colloid after curing, and the deviation of the thickness will cause the die-bonding thermal resistance to increase.

In order to examine the quality of the actual die-bonding layer, a batch of LED devices with the same process, the same material packaging, and different thermal resistance parameters were dissected and analyzed. Figures 5 and 6 are cross-sectional views. The thermal conductivity of the silver glue is about 12. It can be seen from the figure that the actual thickness of the die-bonding layer will have different values ​​in the same batch of products, and the difference is not small.

Figure (4) Cross-section of the solid crystal layer (solid crystal layer thickness 15 microns)

Figure (5) Cross-section of the solid crystal layer (solid crystal layer thickness 60 microns)

The thermal resistance of the solid crystal layer shown in Figure (4) is less than 1.25K/W, while the thermal resistance of the solid crystal layer shown in Figure (5) is more than 5K/W. This is only a theoretical calculation. The actual measured thermal resistance value will be even greater. The use of this type of LED device in street lamps will undoubtedly have an adverse effect on the life of the entire lamp system.
It can be seen that when selecting solid crystal materials, it is necessary to comprehensively consider the thermal conductivity and the minimum thickness that the solid crystal layer can achieve, and the packaging process needs to consider how to ensure that the solid crystal layer is as thin as possible and can remain consistent.
Therefore, some current solid crystal materials (such as Sn80Au20) not only have high thermal conductivity, but also because of the use of eutectic welding technology, the thickness of the solid crystal layer is also thinner than traditional silver glue, and the heat dissipation effect is better than traditional silver glue solid crystal.

3. Reliability analysis of LEDs
The failure of light-emitting diodes is manifested as sudden failure and slow failure. The sudden failure is mainly caused by electrostatic breakdown, gold wire breakage, and aging of solid crystal materials. The causes of slow failure are more complicated, including physical failure of phosphors and chips.
In the application of power light-emitting diodes, the two main stresses that affect the life are temperature and current. Therefore, in this experiment, the temperature and current stresses are used to accelerate the aging of the device, and the relationship between the life and temperature and current is analyzed.
The Arrhenius model can be used when only temperature is considered.

Accelerated aging test can choose temperature or current accelerated aging. When temperature accelerated aging, select two or more different temperatures T1, T2 for aging test, get the life at two different temperatures, according to the relationship between life and temperature in Arrhenius model, we can fit the activation energy Ea and coefficient A of the reaction, from which we can infer the life at other temperatures; when current accelerated aging, select two or more different currents I1, I2 for test at the same temperature, fit the coefficient B and exponent , the life at other currents can also be deduced by the expression.
We conducted accelerated aging test on 2 batches of different high-power LEDs. Under different stress conditions, the attenuation rate of luminous flux is different. According to the analysis of reaction rate theory model, luminous flux will decay in exponential form, the decay rate is the reaction rate R (T, S1, S2, ...), the life defined by σ , the time when the luminous flux decays to 70% is defined as the life of the light-emitting diode, and the life of the light-emitting diode at different temperatures can be inferred according to the fitted attenuation curve .
Table 1 Life of LED under different stress conditions


By exponentially fitting the life data under different junction temperatures, we can get the curve of life changing with temperature: , we can get the activation energy Ea of the performance degradation of this type of light-emitting diode is 1239K, and we can predict the life of the light-emitting diode at other temperatures, such as T=30℃ Life hours.
4. Conclusion
Chip bonding technology is an important aspect of LED packaging. The quality of bonding has a great influence on the heat dissipation performance. Different bonding materials have different thermal conductivity, so it is particularly important to use a suitable process to ensure the thickness of the bonding layer. At the same time, the correct testing technology of the thermal resistance of the package is also an indispensable means to improve the quality of high-power LED packaging.
The life of the power light-emitting diode under different stresses was obtained by aging under certain external stress conditions. The reaction rate model was used to infer the change of the life of the light-emitting diode with temperature or current, which is a more reliable reference for predicting the life of LEDs of different types and qualities.