Application of Bi-Sn nanosized alloys

The alloy design of lead-free solders by using nanoparticles will be realized on the basis of chemistry, thermodynamics and metallurgy. A different approach based on the use of nano-size particles can be helpful for decreasing the melting temperature of new lead-free solders. Nowadays there are already some literature data and patents on the development of nano-solder alloys and their manufacturing in the form of lead-free pastes. The last mentioned point is associated to the idea that the addition of nanoparticles to lead-free solders can influence their properties, such as melting temperature, wetting behaviour and joint reliability. Sn, Bi and Sn-Bi alloy nanoparticles were synthesized by a low temperature chemical reduction method in aqueous solution adopting potassium borohydride as reducing agent. The experimental investigation of the nanoparticles stability and their thermodynamic properties with special reference to the measurement of the depression melting temperature has been performed by using a DSC (Differential Scanning Calorimetry) method. These results will be used to validate available theoretical models concerning the dependence of the melting temperature depression on the nanoparticle size of the alloys. The morphology of the solidified samples as well as the microstructure evolution have been analysed by using SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) techniques. X-Ray powder diffraction (XRD) have been used to determine the crystalline nature of the samples, the grain size and size distribution and the results have been compared with those obtained by Transmission Electron Microscopy (TEM). Key experiments and thermodynamic modelling will be combined in a recursive procedure using the CALPHAD (CALculation of PHase Diagrams) method by which the corresponding Bi-Sn phase diagram of nanosized alloys will be assessed.