Accelerated thermo-mechanical test method for LED modules

Abstract

High power light emitting diode (LED) has gained more and more importance over the last decade as a long-life general illumination source. To ensure a long-lasting lifetime of the LED module, knowledge about critical failures has to be generated. However, it is extremely time consuming and complicated to assess the failure mechanisms and the reliability of the whole LED system. In this work the focus lies mainly in the investigation of failure modes generated in wire-bonds of phosphor converted (pc) white LED modules. Therefore an accelerated test method was developed, where the time-consuming electrical switching test was transferred into a fast purely thermo-mechanical test. For this purpose, Finite Element simulations representing the thermal effect of an electrical off-switching cycle were performed to compare thermally induced strains and stresses with equivalent purely mechanical stresses causing similar strain/stress scenarios. The experimental setup consisted of a dynamic mechanical analyzer (DMA), where the electrical switching test was transferred into an equivalent accelerated mechanical compression test. Failure analysis methods such as X-Ray computed tomography (XR-CT) and cross section investigations by light microscopy or scanning electron microscopy (SEM) were used to analyze failure modes and to compare both testing setups (electrical vs. mechanical). Additionally, thermal impedance analysis was used to monitor changes in thermal device performance in a non-destructive way.