Electrodeposition and Characterisation of Novel Ni-NbOx Composite Coatings as a Diffusion Barrier for Liquid Solder Interconnects - Part II: Diffusion Barrier Performance

Electronics products are increasingly required to operate at high temperatures in harsh environments, for instance in oil well, geophysical drilling and aerospace applications. Conventional solder interconnects, that have been extensively utilised in consumer electronics, are no longer adequate. This is primarily due to excessive intermetallic compounds (IMCs) that can form and continuously grow during high temperature operation. With the intention of reducing such excessive IMC growth, electrically conducting niobium sub-oxide containing Ni coating has been produced which acts as a barrier layer between the solder and substrate. This then promises the possibility of developing a new solder barrier material with electrical conductivity and chemical resistance as required.

The present study considers a novel electrochemical route to produce Ni-NbO_x composite coatings of good uniformity, smoothness, compactness and purity. The electrolyte is derived from a Type IV eutectic solvent consisting of NbCl₅ and propylene glycol (PG). The addition of a small amount of Ni ions (molar ratio: Ni : Nb=1 : 30) was found to enable the co-deposition of Nb species. The introduction of NaBH₄ into the electrolyte has elevated the maximum deposit thickness (above 15 µm), although this led to a reduction in the co-deposited Nb content. The effects of cathodic current density, metal precursor and NaBH₄ concentrations on the surface morphology, composition and thickness of the coatings were examined. Scanning transmission electron microscopy (STEM) revealed a nanocrystalline structure resembling that of a simple FCC structure of Ni, with a slightly fluctuating composition across the coating thickness and with Nb uniformly distributed (See Fig. 1). From X-ray photoelectron spectroscopy (XPS), a 0.7 eV shift to lower binding energy for the Nb (V) species signifies the partial chemical reduction of Nb₂O₅ due to possible Ni-Nb bonding. It is thus proposed that a Ni-NbO_x deposit was formed by a two-step mechanism: the simultaneous electrochemical reduction of adherent Ni metal and inadherent niobium oxides on Cu, followed by nickelation of the NbO_x. The composite coating’s electrical conductivity, solderability and diffusion barrier characteristics (the latter preventing excessive IMC growth between molten Sn-In solder and Cu substrate) were assessed and showed promising results. From diffusion studies, a Nb-enriched layer was observed at the solder/substrate interface after ageing at 200°C suggesting evident effectiveness of the diffusion barrier characteristics.

Figure 1 - STEM cross sectional image a) of a 95Ni-5Nb coating with corresponding Energy-dispersive X-ray spectroscopy (EDX) maps of b) Ni Kα & c) Nb Lα.