Abstract:A novel Sn-2.5Ag-2.0Ni alloy was used for soldering SiC_p/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 SiC_p/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.

Key words: SnAgNi solder; electroless Ni(P); SiC_p/Al composites; intermetallic compound; interfacial reaction