Interface Reaction between Tin Solder and Nanocrystalline Ni and Ni-Mo Coatings Obtained by Electrodeposition

Nanocrystalline Ni-Mo coatings, obtained by electroplating, were investigated as an alternative barrier layer between the Cu substrate and Sn solder. The wetting behavior of molten Sn on Ni coatings of two variants (the equiaxed and columnar microstructure) was examined as the reference samples. Next, the wetting of liquid tin on Ni-Mo alloys coatings was analyzed by means of the sessile drop method, to define the contact angle values. The microstructure, chemical, and phase composition changes of the interface zone formed between coatings and the solder were explored using scanning and transmission electron microscopy techniques. It was found that the wettability of the Ni-Mo alloy coating by the liquid solder is affected by the surface morphology and roughness. Moreover, the Ni-Mo sublayer application inhibits the formation of the Ni₃Sn₄ intermetallic phase layer, observed for Ni coatings. As the result of the intensive Ni diffusion from the Ni-Mo solid solution, a thin, continuous Mo-rich layer is formed at the Ni-Mo/solder interface, which can hinder further diffusion of elements and growth of the Ni₃Sn₄ intermetallic phase.