Reliability Physics Behind the QFN State of Stress

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Journal of Materials Science: Materials in Electronics

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A physically meaningful analytical stress model is developed for the explanation of the reliability physics underlying the relief in the thermally induced interfacial shearing stresses in a typical Quad Flat No-lead (QFN) assembly. The stress relief in a QFN design is, in effect, an implementation of the general concept developed and quantified by the first author about a decade ago of using inhomogeneous bonds for lower thermal stresses in adhesively bonded or soldered assemblies. It is shown also that the maximum stress in the QFN design can be minimized, if the lengths of the peripheral zones are established based on the requirement that the stresses at the ends of the assembly are made equal to the stresses at the ends of its mid-portion. In the carried out numerical example the gain in the relief in the maximum interfacial shearing stress owing to the application of a bond with an elevated compliance is 9.94 %. When the optimized inhomogeneous bond is applied, the gain is 16.87 %. The gain in the stress relief owing to the optimized inhomogeneous bond (with respect to the homogeneous compliant bond) is 10.66 %. The computed interfacial stresses are below the yield stress of the solder material. © 2016, Springer Science+Business Media New York.



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