New nanoalloys for high temperature soldering
On soldering components on a printed circuit board, it is crucial that the joints do not remelt in subsequent soldering operations. The electronics industry is therefore inclined to use solder material containing a high percentage of lead with a higher melting point (300 ˚C and above). One solution is to use so-called nanosolders based on tin-antimony (Sn-Sb) alloys. Nanoparticles have a lower melting point than the bulk substance. Funded by the EU, the project 'A chemical approach to lead-free nanosolders' (Nanosold) aimed to develop new lead-free high-temperature solders with Sn-Sb-M alloys. M is silver (Ag), copper (Cu) and nickel (Ni). Concentrating on the two ternary alloy systems Sn-Sb-Ag and Sn-Sb-Cu, the Nanosold project investigated the thermodynamic properties, the sum of the features of the individual phases. The team used the so-called 'Computer coupling of phase diagrams and thermochemistry' (Calphad) method enabling the scientists to reliably predict the thermodynamic properties without experimental information. To refine the phase relations in the Sn-Sb-Ni system, Nanosold used other complementary methods. These included powder X-ray diffraction, electron probe micro-analysis, scanning electron microscopy and differential thermal analysis. A reduction in melting point of up to 11 ˚C was achieved using nanoalloys rich in Sn prepared by a chemical reduction method. Particle size was modified to be in the range of 50 to 150 nm which would translate into a size-dependent lowering of soldering temperature in any practical application. Although there are further problems to be worked on, solder pastes based on nanoalloys would achieve a decrease in melting point. From an environmental point of view, the new nanoalloys remove a very toxic element from electronic appliances' manufacturing.
