In-situ characterization of solidification and microstructural evolution during interrupted thermal fatigue in SAC305 and SAC105 solder joints using high energy X-ray diffraction and post-mortem EBSD analysis

Highlights: • Sudden emergence of new crystal orientations after solidification during cooling. • Systematic rotations of twin-related dominant orientations during first 100 cycles. • No significant further rotations observed during 650 additional thermal cycles. • Twin -related orientations emerged and disappeared during thermal cycling. • Dominant orientations agree between X-ray and EBSD cross section after 750 cycles. Sn based solders used in the electronic packaging industry exhibit unpredictable failures due to their highly anisotropic properties, where thermal stresses, position within a package and microstructure evolution affect the property evolution and hence, lifetime of solder joints. In the present work, non-destructive high energy-X-ray diffraction measurements were performed in-situ on Sn–1Ag-0.5Cu (SAC105) and Sn–3Ag-0.5Cu (SAC305) solder joints in the corner position in chip array ball grid array packages in order to follow the grain evolution in the same joints starting with melting, solidification/cooling rate, and thermal cycling up to 750 cycles. Most of the SAC305 joints had 'beachball' (solidification twinned) microstructures, whereas most of the SAC105 joints were single crystals. Following solidification, the initial solder microstructure was affected in some joints by the thermal strain history during cooling, as new orientations emerged during cooling. During the first 100 thermal cycles, a major shift of dominant orientations was observed in all of the samples, and these orientations were stable thereafter, but the relative volume fractions of three twin-related orientations changed with increasing number of thermal cycles. All of the joints were cross-sectioned following the last increment of thermal cycling (750 thermal cycles), indicating that all of the joints had fully cracked. Electron backscatter diffraction mapping is compared to x-ray measurements, indicating that most of the orientations in the x-ray measurements were present in the cross section, but there were also significant differences, consistent with the fact that the 2-D section represents a small fraction of the entire joint. There was no obvious effect on microstructure or fracture associated with the initial cooling rate.