Thermophysical and Mechanical Properties of Nano-composite Sn-Ag-Cu Alloys

The development of the new lead-free solders has a long and rich history, where tin-silvercopper (SAC) alloys play one of the central roles. However, there are some physical-chemical parameters, which require improvement. The high melting temperature and an excessive growth of intermetallic compounds (IMCs) belong to the most undesirable characteristics and should be improved. At the same time, metal-based nano-composite alloys are the subject of great interest in basic science as well as in current technology due to their unique physical and chemical properties compared to coarse-grained bulk ones. It is well known that in contrast to bulk materials the properties of the nano- scale particles have size dependencies. The increase of the ratio of surface area to volume makes it possible to obtain new quantum mechanical effects. The good chemical reactivity of nanoparticles and their low melting temperature compared to the bulk open up a new way to improve existing materials used in industry. For instance, the addition of nanoparticles significantly enhances the creep resistance, hardness and other mechanical properties, which are very important for industrial applicability in general, and for solders in particular. Therefore, materials that are refined by nanoparticles have a broad range of potential applications. According to the interaction mechanism between nanoparticles and coarse-grained bulk constituent, the big variety of nano-inclusions can be divided into reactive metal nanoparticles and nonreactive ceramic nanoparticles. In our opinion, metal nanoparticles have a few advantages compared to the latter ones. For example, the commonly used ceramic nano-inclusions such as Al2O3 and Si2O have a lower density than most metals. Therefore, we expect that such types of reinforcement will lead to particle segregation after joint formation with quite different properties of the connection after soldering in comparison with the solder itself. In contrast to them, metal nanoparticles and nano IMCs should distribute regularly in the bulk. In addition, these nano-inclusions can react with the matrix and decrease the melting temperature of the nano-composite alloy. In this case, it is very important to evaluate the amount of dissolved nano-additions and study the impact of the remaining nano- inclusions after the solidification. In conclusion, the proposed work is focused on studying the influence of reactive nano-inclusions on thermophysical properties (viscosity, electrical and thermal conductivity and contact angle), thermodynamic properties (surface energy, surface tension and enthalpy of mixing) and the structure of the Sn95.5Ag3.8Ag0.7 (SAC 387) alloy. The mechanical properties play a certain role in analyzing the effects of active nano-inclusions on the characteristics of the SAC 387 alloy. Therefore, measurements of physical-mechanical properties will be carried out during the project. Another goal of the work is the manufacturing of metal nanoparticles and nano IMCs using a chemical precipitation method, and the production of bulk nanostructured IMCs by the high pressure torsion technique.

Nanocomposite metal alloys are of high interest for many different applications in various industrial sectors. However, there are no general guidelines for the production and disposal of these materials, while the development of materials employing metal nanoparticles is of special challenge due to their high chemical reactivity. At the same time, nanocomposite lead-free solders have been under discussion as possible new generation of solders for the electronic industry over the past ten years. For instance, it is expected that minor additions of metal nanoparticles up to 2 wt.% will reinforce the microstructure and enhance the mechanical reliability of the produced solder joints. The present project was focused on two main topics: - behavior of metal nanoparticles in the lead-free solder depending on temperature; - effect of additions of metal nanoparticles on the microstructure and mechanical properties of the solder joints. In order to avoid immediate oxidation of metal nanoparticles in air, metal nanoparticles covered by an oxide shell were employed for the first time in such research. For this reason, various core/shell metal/oxide nanoparticles were synthesized by a chemical reduction method. It is expected that during reflow soldering minor amounts of metal nanoparticles should be dissolved in the liquid tin-silver-copper (Sn-Ag-Cu; SAC) solder. The performed calorimetric measurements showed that the oxide shell of these metal nanoparticles plays a crucial role in their dissolution. Furthermore, metal nanoparticles with an oxide shell could behave like non-reactive ceramic nanoparticles if an exchange reaction between oxide shell and atoms of the liquid matrix does not occur. The investigation of the structure and of various thermo- physical properties of liquid nanocomposite SAC alloys showed that they are in an inhomogeneous state over a certain temperature region after melting. In another series of experiments with solder joints it could be shown that the most beneficial influence of nanosized inclusions was found for solder joints employing a nanocomposite SAC solder paste with 0.5 wt.% of nanoparticles and for joints employing a nanocomposite SAC solder ribbon with 0.3 wt.% of nanoparticles. There was an extensive co-operation with other researchers in this field. With the outcome of this study, it will be possible to model the dissolution process of metal nanoparticles, both purely metallic as well as and covered by an oxide shell, in the liquid metal matrix as required for the development of nanocomposites for a possible industrial application.