Effect of electric current on the mechanical properties and interfacial microstructure of Ni-P/Sn-3.5Ag and Ni/Sn-3.5Ag solder joints

The effect of electric current on the mechanical properties and interfacial microstructure of electroless Ni-P/Sn-3.5Ag and pure Ni/Sn-3.5Ag solder joints were investigated for different DC current densities ranging from 1 times 10³ to 3 times 10³ A/cm². It was observed that electric current causes the brittle failure of solder joints and the tendency of brittle failure increases with the increase in current density. In most of the electrically-stressed samples, fracture occurred between brittle interfacial compounds (IFCs) and/or at the brittle solder/IFC interface rather than inside the ductile solder. The electric current induced brittle failure of solder joint was more prominent in the case of pure Ni as compared to the electroless Ni-P metallization. Microstructure analysis of fracture surfaces showed that in the case of pure Ni, the cohesive strength of Ni-Sn IFCs was very low as most of the brittle fractures occurred between the Ni-Sn IFCs, whereas in the case of electroless Ni-P, brittle fractures occurred mainly at the Ni₃Sn₄/solder interface. Interfacial microstructure of solder joints revealed that electric current also influences the IFC growth depending on the metallization. In the case of pure Ni metallization, the thickness of Ni₃Sn₄ IFC formed at anode side Ni/Sn-3.5Ag interface (where electrons flow from solder to Ni) was thicker than that formed at cathode side Ni/Sn-3.5Ag interface (where electrons flow from Ni to solder). However in the case of electroless Ni-P metallization, no significant difference was observed in the thickness of Ni₃Sn₄ formed at the cathode side and anode side Ni-P/Sn-3.5Ag interfaces.