620 ℃ thermal aging was carried out on full Cu3Sn solder joints for various duration, the microstructure evolution of solder joints were investigated. The mechanical properties of solder joints after aging were characterized by nano indentation test and shear test. The results show that, during the thermal aging, the Cu20Sn6 is first precipitated in planar along Cu/Cu3Sn interface, it grows continuously until the complete consumption of Cu3Sn. Subsequently, Cu20Sn6 transforms to two-phase layer, which consists of Cu20Sn6 and Cu13.7Sn. As the aging time increases, Cu13.7Sn is precipitated in waves at the Cu/two-phase layer interface by consuming the two-phase layer and grows continuously until it occupies the entire interface area. Meanwhile, there is growth in the number and size of voids in the middle of the solder joint, which eventually coalesces into microcracks. The hardness of Cu20Sn6, Cu3Sn and Cu13.7Sn phases are 9.62, 7.15, 4.67 GPa, and the elasticity modulus are 146.5, 134.0, 133.2 GPa, respectively.With the increase of aging time, the shear strength of solder joints increases first and then decreases, and remains more than 20.1 MPa within 120 min. The fracture morphology and fracture path also change.
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