Novel WO3 nanoparticles modified electroless metallization to retard interfacial reaction and reinforce the reliability of solder interconnection

The role of under bump metallization becomes more important to control the sharp interfacial reaction. In the study, a Ni-P-WO₃ composite layer was developed as a novel UBM for solder interconnection. It´s the first time tungsten oxide nanoparticles been used in solder metallization to reinforce the joint. After surface treatment, magnetic stirring and ultrasonically processing, the uniform distribution of WO₃ nanoparticles was proved by SEM and XRD analysis. The conventional Ni-P layer was used for comparison. Microstructure evolution shows solder/Ni-P-WO₃ joint has successfully slowed down the IMC growth rate at interface under various aging conditions. WO₃ nanoparticles act as barrier to limit the atom inter-diffusion, result in controlled IMC spalling and less voids formation. Top-view IMC grains shows a finer structure on Ni-P-WO₃ UBM. The activation energy of IMC growth in solder/Ni-P-WO₃ calculated to be 47.2 KJ/mol, which is higher than that of plain solder joint. The alleviation of interfacial reaction is explained by the schematic diffusional mechanism in detail. With the incorporation of WO₃ nanoparticles, mechanical properties like shear strength and tensile strength of the solder joints have been improved. Ni-P-WO₃ UBM in solder interconnection with reinforced reliability is proved to be a potential replacement for conventional UBM for advanced electronic packaging, even can be extend to micro-bump in 3D packaging bonding.