Grain deformation and strain in board level SnAgCu solder interconnects under deep thermal cycling

Digital image correlation and cross polarizer, optical microscopy were used to quantify the deformation behavior under deep thermal cycling of near eutectic SnAgCu (SAC) solder in board level interconnects. Maps with sub micron spatial resolution of the strain levels and von Mises strain were produced for selected cross sections. Large spatial variations in the thermo mechanical response of the solder joints were observed and were correlated with Sn grain boundaries or intermetallic precipitates. Such observations are consistent with the anisotropic nature of the mechanical properties of Sn, and the differences in the mechanical responses of Sn and the intermetallic precipitates in SAC solder. The demonstrated anisotropic thermomechanical response of many SAC solder joints sheds doubt on any model which considers these joints to be composed of isotropic material.