High phosphorus electroless NiP coatings – Effect of rhenium addition on coating/solder interface

The current study was dedicated to the modification of the high-phosphorus NiP coatings by addition of up to 1 wt. % of rhenium to improve their thermal stability and make them suitable to replace the Electroless Nickel Immersion Gold (ENIG) plating nowadays used in electronics. The NiPRe coatings were produced with the same amount of phosphorus as the corresponding NiP ones. The coatings’ surface morphology, chemistry, thermal stability, corrosion resistance were studied along with their wetting behavior and reactivity with liquid tin by means of the sessile drop method. Reducing phosphorus content in the coatings increased their crystallization temperature, while in the coatings with 1 wt. % of Re the crystallization temperature was higher by 10 ◦C compared to the reference NiP coatings. Corrosion tests showed that coatings produced at lower pH (3.0) had the best corrosion resistance due to higher amorphous phase content, while increasing of pH improved their wettability. After the reaction with tin, the coating close to the copper substrate remained amorphous, while on the solder-side it was a mixture of the Ni3P, Ni12P5, Ni8P3, Ni2P and Ni5P4 phases and nanocrystalline nickel. Additionally, the formation of Ni3Sn4 and (Cu,Ni)3Sn phases was observed. Coatings with the highest phosphorus content, despite better corrosion resistance, are not recommended for electronics, because of defectiveness of such interconnections. The NiPRe coatings produced at 4.0 pH with economically favorable low level of rhenium showed decrease of the Ni2PSn phase formation kinetics, being responsible for spalling phenomenon of Ni3Sn4 phase from the coating/tin interface.