Effect of Electromigration on the Mechanical Performance of Sn-3.5Ag Solder Joints with Ni and Ni-P Metallizations

The effect of moderate electric current density (1 × 10³ to 3 × 10³ A/cm²) on the mechanical properties of Ni-P/Sn-3.5Ag/Ni-P and Ni/Sn-3.5Ag/Ni solder joints was investigated using a microtensile test. Thermal aging was carried out at 160°C for 100 h while the current was passed. The interfacial microstructure and intermetallic compound (IMC) growth were analyzed. It was found that, at these levels of current density, there were no observable voids or hillocks. Samples aged at 160°C without current stressing failed mostly inside the bulk solder with significant prior plastic deformation. The passage of current was found to cause brittle failure of the solder joints and this tendency for brittle failure increased with increasing current density. Fractographic analysis showed that, in most of the electrically stressed samples, fracture occurred at the interface region between the solder and the joining metals. The critical current density that caused brittle fracture was about 2 × 10³ A/cm². Once brittle fracture occurred, the tensile toughness, defined as the energy per unit fractured area, was usually lower than ~5 kJ/m², compared with the case of ductile fracture where this value was typically greater than ~9 kJ/m². When comparing the two types of joint, the brittle failure was found to be more severe with the Ni than with the Ni-P joint. This work also found that the passage of electric current affects the IMC growth rate more significantly in the Ni than in the Ni-P joint. In the case of the Ni joint, the Ni₃Sn₄ IMC at the anode side was appreciably thicker than that formed at the cathode side. However, in the case of electroless Ni-P metallization, this difference was much smaller.