Microstructural approach to evaluate the reliability of lead-free electronic assemblies in thermomechanical fatigue

With the prohibition on the use of certain hazardous substances by the European RoHS directive, such as lead in a series of electrical products, the SnPb eutectic, which was the most used solder alloy in electronic assemblies, is replaced by SnAgCu (SAC) alloys. Despite the early appearance of these materials, their insertion into the industry is hindered by the observation of early failures of solder joints under thermomechanical fatigue. These alloys have a complex and variable microstructure that significantly influences their behavior.The main objective of this work is to improve the prediction of the lifetime of SAC interconnections by developing a generic damage criterion, based on the microstructural evolution of the alloy and which can be implemented in a fatigue model. This work also involves studying the various factors that can affect the aging of SAC solders in thermomechanical fatigue.The initial microstructure of the SAC alloy and its evolution due to thermomechanical fatigue were examined based on accelerated tests and microstructural investigation by EBSD analyses. Recrystallization of β-SN grains and coarsening of Ag3Sn intermetallics appear to be precursors to intergranular cracking of the different solders studied (BGA, R1206 and QFN). The QFN case reveals another mixed dynamic where we observe a competition between the intergranular cracking and the interfacial failure that occurs along the intermetallics layer, without microstructural evolution of the SAC alloy.The analysis of microstructural damage indicators is approached with the quantitative characterization of recrystallization and Ag3Sn particles coarsening. The size of the recrystallized grains varies from 1 to 30 µm independently of the solder geometry. This is a characteristic of the SAC alloy. A low degree of recrystallization seems sufficient to trigger crack propagation in BGA and R1206 joints (8% and 10% respectively). A higher degree of recrystallization is required for crack propagation of QFN solder joints (27%). This may be due to the mixed cracking mode observed on this geometry. Regarding the coalescence of intermetallics, particles with a diameter greater than 2 µm seem to promote recrystallization. It would be interesting to be able to implement these damage indicators in the prediction models.The study of the different parameters influencing the thermomechanical performance of SAC solders shows that isothermal aging, the location of the solder joint and the initial microstructure have an effect on the thermomechanical behavior of SAC interconnections. They are essential factors to take into account when developing the thermomechanical fatigue model.