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The effects of Ni element on the morphology of intermetallic compounds and some mechanical properties of SAC-Bi-XNi/Cu lead-free solder joints have been investigated. The corresponding mechanical and reliability behaviors were evaluated by performing shear test, fracture mode analysis, nanoindentation experiments and measurement of IMC before and after isothermal aging 200h and 400h at 160 . The results indicate d that with increasing the content of Ni (0~0.15 wt %), hardness of solder increased, shear strength of solder joints and modulus of elasticity of solder increased and then decreased. SAC-Bi-0.1Ni leadfree solder showed the largest shear strength and Young's modulus. The fracture dimples in SAC-Bi-0.1Ni solder joints were less than other three solders. The thickness of IMC increased originally and then decreased after reflowing, which was thinnest in SAC-Bi-0.05Ni/Cu solder joint. Ni apparently refined the grains of IMC.

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In this study, the nanoindentation properties of the assoldered low-Ag SAC-Bi-Ni solder alloys on Cu substrate were characterized. The overall evaluations on the elastic modulus, hardness, and creep properties of the new solder alloys were proposed based on the comparisons with low-Ag solder SAC0307, SAC0705, and high-Ag solder SAC305. Experimental results indicated that the hardness of the SAC solders without Bi and Ni addition varied from 127MPa to 196MPa. Meanwhile, the elastic modulus was between 29-36GPa, and the creep stress sensitivity was from 15 to 19. However, the hardness of low-Ag SAC-Bi-Ni solders was between 305-403MPa. Additionally, with Bi and Ni elements addition, the elastic modulus of SAC0705 increased to 35-63GPa, and the value of creep stress sensitivity reached to 25-45. It is clear that the addition of Bi and Ni in low-Ag SAC solder greatly improved its nanoindentation properties, which were critical for the solder joint reliability.