Secondary Effects Causing Early Solder Fatigue Failure of QFN Components Subjected to Test- and Field Cycling

Studies on solder fatigue failure of quad flat nonleaded (QFN) packages subjected to test cycling (e.g.,-40/125°C, 1 hour) and field cycling (18/83°C, 7.1 hour) indicated unexpectedly early failures. Daisy chained packages QFN36 and QFN40 dummy components were used on FR-4 test boards. Thermal cycling had been made up to 4,000 test cycles and 17,850 field cycles (15 years). The paper addresses questions as to why this early failure occurs. Microstructural analyses revealed that SAC357 solder degradation in test-And field cycling follow standard processes and cannot explain the deviations. "Secondary effects"are therefore studied. Results on multiple effects are reported based on metallographic analyses, microscopic/SEM analyses, fluorescence microscopy, optical out-of-plane and inplane thermal deformation analyses, and comparison to FEM. It is shown that considering cure induced package shrinkage somewhat affects solder joints loadings, while formation of electronic molding compounds (EMC) surface layers due to thermooxidative cross-linking cannot be proven, even after 15 years of field cycling. Two additional secondary effects were observed, which seriously affect solder fatigue: internal package delamination between leadframe and EMC, and standoff filling (peripheral solder gap) by solder mask and flux residues. A parametric FE study revealed that both delamination and gap filling can reduce solder fatigue life by 1.5X and more. Even more important, by including these effects in FE-based predictions, they come in reasonable agreement with testing results and previous predictions for other components.