Detection of Solder Joint Cracking of High Power LEDs on Al-IMS During Temperature Shock Test by Transient Thermal Analysis

An innovative sensitive test method is developed to detect solder joint cracking for high power LED packages. The method is based on transient thermal analysis and can replace the dominating Light-On test. Test groups of LED packages were soldered with two different lead free solders (SnAgCu 305 and Innolot FL-640) on Aluminum Insulated Metal Substrate and exposed to temperature cycles. 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 and correlated with cracks in the solder joint by cross sections. The transient curves are reproduced by time resolved finite element analysis. Based on the simulation, a failure criterium is defined representing a crack length of 30% of the solder joint area. A higher creep resistance for the test group soldered with Innolot FL-640 compared to the test group soldered with SAC 305 is observed.