Potential interconnect technologies for high power LEDs assemblies

Abstract

In this study, the reliability of several potential interconnect methods for high temperature applications are evaluated by using accelerated thermal cyclic tests on typical LED test carriers. The applied interconnect materials are SAC (Sn-Ag-Cu), SAC+(SAC with doping elements), and an eutectic AuSn solder alloy. The degradation behavior and typical failure modes in the different interconnect materials are analyzed at different test intervals. SAC+ shows better fatigue resistance than SAC based solder interconnects at the early phase of tests. However, the leading position decreases at relatively long test cycles and high stress situations. This could be explained by the different behavior of crack propagation in SAC and SAC+ interconnects caused by the changing microstructures under cyclic temperature loads. Although well processed AuSn shows relatively higher fatigue resistance to cyclic thermal loads than SAC/SAC+ based interconnects, some issues like cracks in the component or substrate, or the crack in the plating layers could be an attention point.In addition, the process need be taken carefully to avoid some quality issues like (micro-) cracks in the component.Three dimensional numerical models of typical LED modules are developed. The time and temperature dependent creep behavior of the interconnect material is considered. The developed models enable the analysis of the thermalmechanical performance of solder interconnects under cyclic temperature loads. Furthermore, the sensitivity of fatigue resistance with respect to reliability parameters is predicted for several designs. It is concluded that the fatigue resistance is most sensitive for changes in thickness and meniscus of solder interconnects. Also it is shown that AuSn is less sensitive to meniscus changes than SAC/SAC+. IntroductionReplacing traditional lighting with LED lighting is becoming one of the great changes in human lifestyle in this decade. As a promising light source, LED packaging should not only meet optical and thermal specifications, but should also be reliable.For automotive and high-brightness applications, the environment is extremely harsh. The cyclic thermal load of LED devices can be up to 150ºC for several thousands of cycles. For such a high temperature application, the thermal stability and the thermal cyclic fatigue resistance of interconnect of the LED to the next level substrate/heat spreader is a critical factor for the reliability of the LED module.Above this, the exemption from European RoHS regulations on using Pb containing solders in specific applications is only within a limited time period. It is needed to find a feasible alternative solution to replace traditional