NANOSOLD · A Chemical Approach to Lead-free Nanosolders

A Chemical Approach to Lead-free Nanosolders

Nanosold - A Chemical Approach to Nano Solder R. Mishra, H. Ipser Chemistry Division, BARC, Trombay, Mumbai-400085, India Dept. of Inorg. Chem. / Mater. Chem., Univ. Vienna, A-1090 Wien, Austria (herbert.ipser@univie.ac.at) Soldering in electronics is done in consecutive stages. For example, if electronic components are soldered to the printed circuit board (at temperatures around 230°C) it is important that solder joints within the component do not re-melt again. Therefore, the electronics industry uses high-temperature solders which have higher melting temperatures (up to 300°C) but are still based on alloys with high lead contents. On the other hand, higher temperatures create additional thermal strain on all electronic parts besides causing additional energy costs. One of the proposed solutions is the use of so-called nano solders which would have reduced melting temperatures on first heating but, after solidification, would re-melt only at the equilibrium melting temperature of the bulk material. Since solders based on Sn-Sb alloys and modified by additional elements had been discussed in the past as possible lead-free high-temperature solders, it was proposed to investigate ternary Sn-Sb-M alloys (M = Ag, Cu, Ni) and to select compositions that would fulfill the necessary temperature requirements. The work was planned in cooperation within COST Action MP 0602. In this Action it was agreed that colleagues in Genoa (Italy) and Cracow (Poland) would concentrate on the two alloy systems Sn-Sb-Ag and Sn-Sb-Cu. Therefore, it was decided to investigate first phase equilibria and thermodynamic properties of bulk Sn-Sb-Ni alloys and to optimize the ternary phase diagram using the so-called CALPHAD method. In a second step it would be attempted to prepare nano alloys via a chemical route and to study their special properties. Standard methods like powder X-ray diffraction, electron probe micro-analysis, scanning electron microscopy, and differential thermal analysis were used to clarify the phase relations in the ternary Sn-Sb-Ni system. Special emphasis was placed on Sn-rich compositions which were assumed to be of particular importance for soldering purposes. The results are presented as isothermal sections (200°C, 400°C, 900°C), as isopleths (constant Sn contents and constant Ni/Sn ratios of 3/1, 3/2, and 3/4) and as a liquidus projection which shows all solidification reactions in this composition range in diagram form. A vapor pressure method was used to determine partial thermodynamic properties of Sb in liquid ternary Sn-Sb-Ni alloys along two sections with constant Ni/Sb ratios of 3/1 and 3/2. At the same time, calorimetric measurements were performed by colleagues in our laboratory to determine enthalpies of mixing for liquid ternary alloys. All were used as input in a CALPHAD-type optimization of the entire ternary system in close cooperation with col-leagues from Brno (Czech Republic). Tin-rich nano alloys were prepared by a chemical reduction method. Their particle size was deliberately modified to be between about 50 and 150 nm. Melting temperatures were found to be depressed by up to 11°C (2 to 11°C depending on particle size) which would correspond to a reasonable lowering of the soldering temperature in any practical application. The additional surface energy due to the nano sized particles was determined in a series of calorimetric experiments. As a consequence, the results of the project showed that a decrease of the soldering temperature could be achieved if solder pastes based on nano alloys were employed although, of course, many practical problems would still have to be solved. At the same time it is hoped that the experience of the researcher (RM) here in Europe will have consequences for his future scientific development, but also that the idea of removing toxic elements from electronics will catch on in his country which has developed into a huge market for electronic appliances.

Data: CORDIS, © European Union