Application of thermal analysis for the development of reliable high power LED modules

For the design of reliable light sources based on light emitting diodes (LED) efficient accurate testing is required. An innovative sensitive test method is developed to detect and distinguish failures in LED modules. The method, i.e. measurement of the relative thermal resistance, is based on transient thermal analysis and measures the structural integrity of the package by changes of the relative thermal resistance of the LED. Failure modes can be separated in the time domain. For detailed analysis and identification of the failure mode the transient signals are simulated by time-resolved finite element (FE) simulations. The sensitivity of the method is demonstrated in the paper. Test groups of ceramic LED packages were soldered on different Aluminum Insulated Metal Substrate (Al-IMS) and exposed to temperature shock tests. Transient thermal measurements were performed directly after assembly and after specific cycle numbers. After data processing the increase of the relative thermal resistance between the initial signal at `0´ cycles and `n´ cycles is obtained. Different failures can be observed, i.e. solder joint cracking and failures on die level. A large increase of relative thermal resistance of the LED package on the Al-IMS is observed for an Al-IMS with a thin dielectric layer. Due to the thin and hard dielectric layer the thermo-mechanical stress on the solder contact is higher and faster cracks growth takes place. The cracks are detected by their influence on the heat flow path within a certain time domain, i.e. 20-100ms. The new test method simplifies the thermal transient analysis by eliminating k-factor and thermal load measurement. It enables automatic data analysis of the transient thermal data required for processing large amount of data in production and reliability testing. Based on the method initial testing of the integrity of the LED packages and more accurate lifetime prediction is obtained.