The Nickel-Phosphorous-Tin Phase Diagramm

As a common practice in electronics, Cu or Al contacts for solder joints are protected by a thin layer of an Ni-P alloy. This layer is commonly used due to its superior characteristics (excellent solderability, corrosion resistance, uniform thickness, ). The P content in the Ni-P layer is due to the deposition procedure: "Electroless" Ni is deposited by chemical reduction of an aqueous Ni solution by means of phosphonic acid or phosphonates leading to the inclusion of about 5 to 15 wt% P. As a consequence, the intermetallic Ni-Sn compounds formed in the soldering process (with solders based on Sn alloys) contain likewise variable amounts of P, and it is clear that the composition of these intermetallics has a great influence on the mechanical properties of the solder joints, and thus on their reliability. Although the microstructure and the growth kinetics of such solder joints have been studied in various laboratories, no systematic investigation is available on the ternary Ni-P-Sn phase equilibria which would help to understand the corresponding reactions. Such a knowledge is of even higher importance considering the fact that any solders of the future will certainly continue to contain Sn as the main element, independently of other components. Therefore it is proposed to investigate the phase relationships and some thermochemical properties in the ternary Ni-P-Sn system experimentally, and to attempt a thermodynamic optimization of the phase diagram by means of the well-established CALPHAD method. X-ray powder diffraction (XRD) methods and electron microprobe analyses (EPMA) are to be used, together with the study of diffusion couples, to establish a series of isothermal sections (200, 500, 750C); thermal analyses methods will be used to determine transformation and melting temperatures in a number of isopleths, with a description of the liquidus surface and a Scheil diagram as the final result. Enthalpies of formations of solid compounds and enthalpies of mixing of liquid alloys will be determined calorimetrically, and it will be attempted to measure phosphorous vapor pressures in the binary Ni-P system and in parts of the ternary system by an isopiestic vapor pressure method. All experimental information on phase equilibria and on thermodynamic properties will be combined for a CALPHAD-type optimization of the system that will result in a phase diagram consistent with the available thermodynamic data. This research will be a contribution to the European COST Action 531 ("Lead-free Solder Materials").

As a common practice in electronics, Cu or Al contacts for solder joints are protected by a thin layer of an Ni-P alloy. This layer is commonly used due to its superior characteristics (excellent solderability, corrosion resistance, uniform thickness, ...). The P content in the Ni-P layer is due to the deposition procedure: "Electroless" Ni is deposited by chemical reduction of an aqueous Ni solution by means of phosphonic acid or phosphonates leading to the inclusion of about 5 to 15 wt% P. As a consequence, the intermetallic Ni-Sn compounds formed in the soldering process (with solders based on Sn alloys) contain likewise variable amounts of P, and it is clear that the composition of these intermetallics has a great influence on the mechanical properties of the solder joints, and thus on their reliability. Although the microstructure and the growth kinetics of such solder joints have been studied in various laboratories, no systematic investigation is available on the ternary Ni-P-Sn phase equilibria which would help to understand the corresponding reactions. Such a knowledge is of even higher importance considering the fact that any solders of the future will certainly continue to contain Sn as the main element, independently of other components. Therefore it is proposed to investigate the phase relationships and some thermochemical properties in the ternary Ni-P-Sn system experimentally, and to attempt a thermodynamic optimization of the phase diagram by means of the well- established CALPHAD method. X-ray powder diffraction (XRD) methods and electron microprobe analyses (EPMA) are to be used, together with the study of diffusion couples, to establish a series of isothermal sections (200, 500, 750C); thermal analyses methods will be used to determine transformation and melting temperatures in a number of isopleths, with a description of the liquidus surface and a Scheil diagram as the final result. Enthalpies of formations of solid compounds and enthalpies of mixing of liquid alloys will be determined calorimetrically, and it will be attempted to measure phosphorous vapor pressures in the binary Ni-P system and in parts of the ternary system by an isopiestic vapor pressure method. All experimental information on phase equilibria and on thermodynamic properties will be combined for a CALPHAD-type optimization of the system that will result in a phase diagram consistent with the available thermodynamic data. This research will be a contribution to the European COST Action 531 ("Lead-free Solder Materials").