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As a novel high-temperature solder with suitable eutectic temperature and excellent solderability, Au–30Ga eutectic alloy shows high application potential as a packaging material for high-power devices. In this work, the evolution of microstructure, shear strength and fracture mechanism of Au–30Ga/Ni solder joint during soldering and aging was studied. It was found that with the increase of soldering time, Ga was seriously consumed to form Ga–Ni interfacial intermetallic compounds (IMCs), resulting in the formation of Ga-poor region near the interface layer. Au7Ga3 phase would be first formed at the valley area of Ga3Ni2 IMC layer. The dominant IMCs growth mechanism eventually transformed from grain-boundary diffusion to volume diffusion after soldering for 0.49 min. The diffusion coefficients are 1.64 × 10−14 and 3.77 × 10−19 m2/s at 500 °C (soldering) and 300 °C (solid-state aging), respectively. With the coarsening of IMC layers, the fracture mechanism of joint changed from solder control mode to solder/IMC mixed control mode. When the thickness is 4.36 μm, the joint strength reaches the maximum value of 77.7 MPa. Furthermore, a reliability evaluation equation of Au–30Ga/Ni solder joint was established to optimize soldering process and solder composition.