Non-destructive orientation tracking of individual β-Sn grains in die-attach solder joints.

Understanding the failure mechanisms of Pb-free solder joints during thermal cycling is a critical issue in electronic packaging, where solder joints strongly influence device reliability and lifetime. While electron backscatter diffraction (EBSD) has been widely used for this purpose, its destructive nature prevents tracking the microstructural evolution of solder joints from their pristine state. Non-destructive X-ray-based orientation microscopy techniques can address this limitation but typically require cylindrical, pillar-shaped specimens, which are incompatible with the planar geometry of practical solder joints. To overcome this constraint, we apply inclined scanning three-dimensional X-ray diffraction microscopy (i-S3DXRD), which is specifically designed for plate-like specimens. Using i-S3DXRD, we found that thermal cycling induces preferential reorientation of β-Sn grains toward specific crystallographic directions relative to the Cu substrate. This reorientation is associated with recrystallization-assisted grain boundary migration and grain coalescence, and is closely related to the thermal fatigue behavior of Pb-free solder joints.