In-situ measurement and crystal plasticity analysis of internal stresses in advanced packaging under thermal cycling

Lead free solder joints in advanced packaging architectures often contain a limited number of grains, leading to a typical oligocrystalline microstructure in which the thermomechanical response is strongly governed by the crystallographic characteristics of individual grains. Under such conditions, conventional homogenized constitutive models cannot accurately capture the anisotropic deformation behavior and local stress evolution caused by grain orientation differences during thermal cycling. In this study, a combined experimental and numerical approach is employed. Piezoresistive sensors integrated within the package are used to monitor the internal stress state under thermomechanical loading, while the solder-joint microstructure is characterized using electron backscatter diffraction (EBSD) and incorporated into the crystal plasticity finite element model. The simulation results reproduce the overall stress evolution observed in the experiments and show good agreement with the measured data. The stress peaks at the central region increase from about 39.9 MPa to 46.9 MPa during thermal cycling, while the maximum lattice rotation angle in corner solder joints remains below 1.4°. Further microscopic analysis indicates that the initial crystal orientation significantly influences slip activity among different slip systems and the deformation pathway during thermal cycling, while the overall deformation accumulation during the first ten thermal cycles remains comparable among the orientations considered in the present study. These findings provide insight into thermomechanical deformation, slip-activity evolution, and energy accumulation behavior in oligocrystalline solder joints, contributing to a microstructure-sensitive understanding of reliability in advanced electronic packaging structures.