Evaluation of SAC solder joint thermomechanical fatigue in different types of components

As lead-free solder joints mechanical behavior is dependent on their microstructural characteristics, locations and geometries, developing accurate solder joint fatigue models implies correctly identifying the microstructural changes that undergo the solder alloy during thermal cycling. The present work aims at evaluating thermomechanical SAC (Sn-Ag-Cu) solder joints reliability by studying different types of components. Lifetimes data coupled to the microstructural investigation allowed us to study the microstructural process leading to SAC solder joints failure and to identify the different factors that must be considered in Finite Element Models (FEM) of electronic lead-free assemblies under thermomechanical loading. Tin recrystallization and strain-enhanced precipitate coarsening led to intergranular cracking in the solder joints, which was the first identified failure mode. A brittle interfacial crack that does not need tin grain recrystallization to propagate was the second detected cracking mode. The different behavior of one solder joint compared to another can be explained either by the presence of an unexpected microstructure highlighted during the characterization of the as-reflowed solder or by the design and the materials properties that unify each package.