Abstract (eng)
In the electronic industry nickel is often used for protective coatings of metal surfaces, e.g. Cu, to protect them against corrosion, to increase the hardness or the solderability. These protective layers are usually applied by means of electroless plating using phosphorus-containing solutions. The Ni-P layers that are formed in this process can have a P content of up to 15 at%. During the soldering process there is an interaction of these Ni(P)-layers with the tin-rich solder alloy. This causes the formation of so called intermetallic compounds (IMC) in the solder joint. In order to explain the occurring reactions and the reaction products that are formed during the soldering process, it is necessary to investigate the corresponding phase diagrams.
In the present thesis, the systems Sn-P and Ni-Sn-P have been investigated. The binary system shows a strong dependence on the vapor pressure of phosphorus. Hence, isopiestic experiments with different vapor pressures of P were performed. Successful measurements were carried out at nine different pressures in a range from 0.006 to 0.69 bar and for these pressures phase diagrams could be created up to a P content of 70 at%. With the obtained data the change of the equilibrium temperatures of the formation reactions for the three binary compounds (Sn4P3, Sn3P4, SnP3) with increasing pressure could be analyzed and extrapolated to 1 bar. Further, the pressure-dependent maximum solubility of phosphorus in the tin-rich liquidus was investigated according to the obtained results. Finally, partial thermodynamic properties of phosphorus, such as partial enthalpies of mixing and activities, as well as integral Gibbs energies of formation were derived and compared with literature data.
In the ternary system samples in the Sn-rich part of the phase diagram were prepared and then investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). With these data an isothermal section at 300 ° C should have been created. However, phosphorus and its vapor pressure caused significant problems during sample preparation and interpretation of the results. Samples with a phosphorus content of more than 60 at% could not be used for evaluation. Some quartz tubes burst during the sample preparation, in other samples P condensed during quenching on the quartz wall. Further, most samples with a P-content of more than 15 at% were not in thermodynamic equilibrium. These non-equilibrium samples were most likely obtained due to the influence of the gas phase and the low annealing temperature. As a result of these experimental difficulties, no reliable and consistent isothermal section of the Ni-Sn-P phase diagram could be created.