New Method to Separate Failure Modes by Transient Thermal Analysis of High Power LEDs

Abstract

A high reliability of light emitting diode (LED) light sources is essential for general and automotive lighting applications, where exchange of LED components is expensive. Thermal management of modern high power LEDs is crucial for their lifetime. An important aspect is the thermal path for heat conduction. Many different defects can have an influence on this path of an electronic system: on the one hand process failures during production, e.g. voids inside the solder joint, on the other hand typical failures induced by thermo-mechanical stress during their lifetime, like cracks in the solder joint or delamination in the package. The transient thermal analysis (TTA) is a powerful tool to detect changes in the thermal path. Due to improvements in the TTA method during the last years, not only cracks can be detected but also failure modes can be separated, and the root cause can be analyzed by support of transient finite element analysis. In this paper, transient thermal testing is applied and further developed, to monitor the structural integrity of new wafer level LED packages during thermal stress testing. Failure modes are defined and separated. For failure analysis the different defects are simulated by transient finite element analysis and correlated to the TTA results. The simulation results, that solder cracks increase the peak height of the derivative of the transient thermal curves (b(z)). A delamination of an inner layer of the LED package creates additionally to the increase of the peak height also a separation of the b(z) curves between 1 µs and 5 µs. Therefore a transient thermal measurement equipment with a dead time