Reactivity with tin and corrosion resistance of electroless Ni-P and Ni-P-Re coatings plated on copper

The electroless plated nickel (Ni-P) layers are widely applied in the electronics industry (ENIG coatings), where the quality of interconnections is directly related to the nickel solution and the resultant chemical composition (phosphorus content) of the coating. Depending on the concentration of phosphorus within the coating, the layer can be either fully amorphous or with nano Ni crystallites or crystalline. This translates into an increase of the temperature of its thermal stability, which directly impacts the processes occurring at elevated temperatures during the soldering process or later during the use of the final product (heating/cooling conditions). This work shows how the metallization/solder zone is transformed after their reaction, where, as the solder material pure tin was used, being the main component of currently used lead-free solder of SAC type. In the electronics industry, the typically used coatings have a phosphorous content in the range of 6-8 wt.%. Therefore, the coatings of also such compositions have been studied. The newly electroless plated coatings were first characterized in terms of surface morphology and chemical composition. Then, the interfacial reactivity between liquid Sn and coatings was studied using the sessile drop method, and the resulting substrate/coating/solder reaction zone was examined by scanning and transmission electron microscopy. The main goal of the research was to determine the quality of the interface, the width of the newly formed zones, and the presence of NixPy phases as well as the intermetallic phases formed as a result of the reaction. The addition of rhenium to Ni-P coatings inhibited the formation of the Ni2SnP phase in each of the tested phosphorus content ranges (low, medium, and high phosphorus), also after thermal cycles. Thus, no undesirable phenomenon of intermetallic phase detachment from the layer/solder interface was observed. Studies have shown that the corrosion resistance of samples increases as the pH of the solution from which the test coatings are deposited decreases, and the addition of rhenium to Ni-P coatings always improve their corrosion resistance.