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A novel Sn-2.5Ag-2.0Ni alloy was used for soldering SiCp/Al composites substrate deposited with electroless Ni(5%P) (mass fraction) and Ni(10%P) (mass fraction) layers. It is observed that variation of P contents in the electroless Ni(P) layer results in different types of microstructures of SnAgNi/Ni(P) solder joint. The morphology of Ni3Sn4 intermetallic compounds (IMCs) formed between the solder and Ni(10%P) layer is observed to be needle-like and this shape provides high speed diffusion channels for Ni to diffuse into solder that culminates in high growth rate of Ni3Sn4. The diffusion of Ni into solder furthermore results in the formation of Kirkendall voids at the interface of Ni(P) layer and SiCp/Al composites substrate. It is observed that solder reliability is degraded by the formation of Ni2SnP, P rich Ni layer and Kirkendall voids. The compact Ni3Sn4 IMC layer in Ni(5%P) solder joint prevents Ni element from diffusing into solder, resulting in a low growth rate of Ni3Sn4 layer. Meanwhile, the formation of Ni2SnP that significantly affects the reliability of solder joints is suppressed by the low P content Ni(5%P) layer. Thus, shear strength of Ni(5%P) solder joint is concluded to be higher than that of Ni(10%P) solder joint. Growth of Ni3Sn4 IMC layer and formation of crack are accounted to be the major sources of the failure of Ni(5%P) solder joint.