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

Interfacial delamination is the 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 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 both including simulation and experiment validation is established in this study. A dummy QFN is fabricated first as the test vehicle for subsequent study. Then the related finite element model is also built up to reveal the interfacial stress distribution when the packages are subjected to the pure thermal loading and hygrothermal loading. 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 together, considering the non-uniform moisture distribution during reflow. The shear stress is found to be dominated 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/leadframe interface around the junction of die attach fillet would be the initiation of delamination. INTRODUCTION Quad Flat No-lead (QFN) package known as one of the Surface Mount Technology Device (SMTD) is widely used in industry. The major features are low cost, low pin-count requirement and high thermal dissipation. In particular, there is one critical challenge in SMT applications which is interfacial delamination during reflow. This failure could be observed in Quad Flat Packages (QFP), Thin Array Plastic Package (TAPP), Flat Bump Package (FBP) and other lead-frame based packages. The reasons for interfacial delamination could be traced to thermal mismatch effect, moisture absorption and degradation of interfacial adhesion. Simulation based on finite element model is a popular method to study the failure mechanism. Tee and Zhong [1] established an integrated stress model in order to consider all the effects together. The thermo-mechanical, hygro-mechanical and vapor stress were calculated separately with lots of modeling techniques then integrated into one model. However, the failure mechanism is still not clear since the critical interfacial stress was not discussed and the simulation result lacks experiment validation. Driel et al [2] also discussed the initiation of delamination in QFN using interfacial fracture mechanics. But the simulation result was really dependent on the location and the length of the precrack. Therefore, the contribution to the failure of each effect needs to be identified with more experiment validations. To break the gap between FEA and experiment validations, a complex system shown in Figure 1 is established to study the failure mechanism of hygrothermal delamination. Two kinds of dummy QFN packages with different lead-frame will be manufactured as the test vehicles for this study. The research methodology generally divides into two routes. One follows the simulation route, starting from material characterization to finite element modeling. Strength approach is then applied to evaluate the package reliability. In the other route, dummy QFN are fabricated and tested to provide experiment result for later comparison. The stress analysis with red background will be discussed in this paper, more details about the sample preparation and experiment tests with yellow background is introduced in reference [3]. The major objectives of this paper are given as follows:  A finite element model will be established to figure out the interfacial stress distribution along different interfaces when dummy QFN is subjected to the pure thermal loading and hygrothermal loading.