Analysis of crack length and crack position in the Solder Joints of High Power LEDs by Transient Thermal Measurements and Finite Element Simulations

Abstract

In SMT devices, the CTE mismatch between the substrate and the component makes the solder joint region the most susceptible to crack. Due to temperature cycles experienced by the device during field life, the solder region often cracks leading to system failure. For high power devices that have to dissipate large thermal load through the solder joint, it was shown in earlier publication that transient thermal analysis is very sensitive to determine the solder joint reliability. For Transient Thermal Analysis the forward voltage (Vf) of the device is measured after switching the thermal load. By normalizing the transient Vf plots, an algorithm was introduced which eliminate the influence of k factor (a linear factor between Vf and T) and thermal load (Pth) for calculating the relative thermal resistance [1]. In this paper experimental studies are presented were the reliability of the solder joint of high power ceramic LED packages is investigated. The LEDs are soldered with two different lead free solders (SnAgCu 305 and Innolot FL-640) on Aluminum Insulated Metal Substrate (Al-IMS). Test batches were prepared and exposed to temperature cycles at −40°C / +125°C. Transient thermal measurements were performed directly after assembly and after specific cycle numbers. It is seen that as the temperature cycle number increases, the thermal signal corresponding to the solder region changes. After appropriate data processing the increase of the relative thermal resistance between the initial signal at ‘0’ cycles and ‘n’ cycles is calculated. Based on the increase of the relative thermal resistance a failure criterion is introduced and the cumulative failure probability is calculated. We observed a higher creep resistance for the test group soldered with Innolot FL-640 compared to the test group soldered with SAC 305. Transient finite element analysis (FEA) is applied to simulate the experimental curves. The influence of crack length and crack position in the solder joint region of a high power ceramic LED used for automotive applications is analyzed and it is shown that the crack length and position has a significant impact on the relative increase in thermal resistance. Within the simulations the crack length and crack position is varied to match the peak-height difference and peak shift of the experimental curves by simulation. It is observed that the increase of relative thermal resistance strongly depends on the position of the crack. Therefore it is shown that the relative thermal resistance increase takes place not only within the solder joint but also within the dielectric layer of the Al-IMS. It is show that as the crack propagates from the exterior to the interior region of the solder joint, the effective area available for heat flow through the Al-IMS is significantly reduced and adds onto the increase of the thermal resistance within the solder joint.