The multi-scaled failure mechanism of SSL system

Because the diversity and complexity of the LED lighting product design, it results in system failure modes which can occur in various value chains, and interact with each other. The same failure modes in different value chain often exhibits different, complex failure mechanisms influences.

Because of the characteristics of the GaN-based LED devices, its reliability issues including the hetero-epitaxial substrate material and growth of a compound semiconductor, denature of the doped GaN p-type material, electrode metallization and ohmic contacts, with respect to the external loadings, such as the current, temperature and electrostatic forces and other factors. The main failure mode is the decay of the output light, color drift and catastrophic failure. Among them, the main reason causing the light fades and color drift failure is the material defects propagation as the loading of the current and thermal stress. The catastrophic failure occurs mainly due to the material defects, residual stress and thermal stress and mechanical damages.

Numerous studies indicate that the failures caused by the LED packages are more often than the chip itself. Package failure mainly from polymer optical materials and interface inside the package. In addition, some other failure modes, such as failure caused by humidity, and gradually get attention, but related failure mechanism remains unclear. Package failure is a relatively slow failure process. Hence, most of the package failure mechanisms cannot be completely avoided, but can be slow down by improving packaging design and process way delay, to ensure the proper performance during the guaranteed lifetime.

There are two major interfaces within a LED package. They are the chip/phosphors layer and chip/substrate. The failure mechanism of the interfaces is mostly dominated by the CTE (Coefficient of thermal expansion) mismatches induced thermal stress. The propagation of the defects (cracks) along the interfaces will cause the increasing of the thermal resistance and then induce the efficacy drop, against both LED chip and phosphors layer.

There is another reliability factor is the corrosion that caused by the humanity intake to the packaging material. These corrosions will cause the breaking of the wires (wire bonding), PCB traces, etc. The moveable ionic might stay at top of the LED chip and cause the leakage. The bubbles, caused by the unstable manufacturing packaging process, will cause the corrosion of the LED packages as well. A improper treatment of the silver (Ag) will cause the darken of the reflection surface.

Another important part of the SSL product is the electronic drivers. The qualities of the individual components, such as semiconductors, resistors, capacitors, inductors, decide the driver quality. These small components are always under electric, thermal, magnetic, mechanical stress during working time. Moreover, there is often the malfunction of single component will cause catastrophic failure of the entire system.