Thermal fatigue life optimization of QFN package based on Taguchi Method

Quad Flat non-lead (QFN) packages known with many outstanding properties. However the thermal stresses and plastic strain accumulation in solder joints during temperature and power cycling can be induced due to the significant mismatch in the coefficients of thermal expansion (CTE) of dissimilar materials. The damage caused by these stresses and plastic strain accumulation, which are induced during accelerated temperature cycling due to the CTE mismatch between the various materials that encompass the QFN package, ultimately cause the failures of solder joints. In this paper the ANSYS software is adopted to build a quarter of model about QFN, which is subjected to the thermal recycling of -40~125°C. The thermal fatigue life of QFN is calculated based on the maximum strain range and the Coffin-Manson equation. Anand´s viscoplastic material model is used to describe the constitutive equation of the solder joints in this study. A L₂₇(3⁸) orthogonal array is built based on Taguchi Method to figure out the optimized conditions. Besides, in order to avoid accidental error, ANOVA is carried out to estimate the error and find the most important factors. The results indicate that only five material and structural factors are the most important ones, and the other factors are selected by price or convenience. Finally, the fatigue life after optimal design by Taguchi method has 3.43 folds on the fatigue life over the original design.