Stress Analysis of Hygrothermal Delamination of Quad Flat No-Lead (QFN) Packages

Interfacial delamination is a major reliability issue of Quad Flat No-lead (QFN) packages under the JEDEC-MSL preconditioning and reflow process. Failures will occur when the hygrothermal stress exceeds the interfacial strength. Simulation based on finite element model is a popular method for studying the failure mechanism. However, the non-accurate material properties and the lack of experiment validations always constrain the Finite Element Analysis (FEA) at the artificial parametric study stage. To further investigate the interfacial delamination, a complex system including both simulation and experiment validation is established in this study. A dummy QFN is fabricated first as the test vehicle for the subsequent study. Then the related finite element model is built to reveal the interfacial stress distribution when the packages are subjected to the pure thermal loading and hygrothermal loading, respectively. Once the interfacial stress is derived, the strength approach is applied here to indicate the high risk area where delamination will occur. Finally, the analyses from simulation are verified by Moisture Sensitivity Level (MSL) tests using dummy samples. In this paper, a superposition method is used to integrate the thermo-mechanical and hygro-mechanical stress, with considering the non-uniform moisture distribution during reflow. Such a method is different from the previously method in literature. Results show that the shear stress is dominant along all the interfaces. From the comparison between simulation and experiments, the strength approach is applied to evaluate the package reliability successfully. Both simulation and experiment results show that the molding compound/lead-frame interface around the junction of die attach fillet would be the initiation of delamination.