Effect of P and aging on microstructure and shear strength of Sn-2.5Ag-2.0Ni/Ni(P) solder joints

A novel Sn-2.5Ag-2.0Ni alloy is developed for soldering of SiCp/Al composite substrate with various types of Ni coatings. Electroplated Ni layer, electroless Ni(4 wt.% P), and Ni(10 wt.% P) layers have been evaluated. It has been observed that the microstructure of as-deposited Ni coatings transforms form nanocrystalline to amorphous with an increase of P contents and this transformation leads to an increase of the diffusion rate of Ni into solder and furthermore results in a high growth rate of Ni3Sn4 intermetallic compounds (IMCs). The formation of Ni2SnP, which significantly affects the reliability of solder joints, has been suppressed by lowering the P contents in as-deposited Ni coatings. It has also been observed that the P contents in as-deposited Ni coatings play a crucial role to evaluate the reliability of solder joint. With high P content, the formation of Ni2SnP phase, P-rich Ni layer and voids between Ni layer and substrate have a detrimental effect on the mechanical reliability. While, with the low P content, thermal stress and formation of cracks in IMCs are considered to be the major sources of failure.