Nano-structured Nano-composite Solders and Solder Joints

Reliability has been a concern in electronics with the introduction of new lead-free solders, based mainly on Sn- Ag, Sn-Cu, or Sn-Ag-Cu (SAC) alloys. To improve the reliability of solder interconnects a considerable amount of research has been recently devoted to so-called composite solders which are reinforced by micro- or even nano- sized particles. Some of these composite solders were in fact shown to have improved mechanical and thermo- mechanical properties. On the other hand, the higher melting temperatures of the currently used lead-free solders have also been of concern because of increased thermal stress both on the printed circuit board (PCB) and all electronic components soldered to it. Therefore, the use of nano-solders has been considered for which the nano- effect would cause a considerable reduction of the melting temperature whereas the solidified solder joint would remain solid up to the bulk melting temperature. In order to combine these two beneficial effects it is proposed to prepare nano-composite nano-solders in this project and to test some of their properties that influence reliability. In a first step, nano SAC solders will be prepared by a chemical reduction method from aqueous solutions, using sodium borohydride (NaBH 4 ) as reducing agent. The nano-solders will be characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential thermal analysis (DTA). In a separate step, cobalt nano-particles will be produced by a similar chemical reduction procedure and characterized in an analogous way. The nano-particles will be mechanically mixed with the nano-solders to produce the desired nano-composite nano- solders with varying amounts and sizes of nano-particles. In a later stage it will be searched for possible chemical procedures to synthesize the nano-solders with the reinforcing nano-particles in situ. Alternatively, some other types of nano-particles (Ni, Ta, Mo) will also be prepared by chemical reduction and tested with the nanoscopic solder materials. The solders will be used to prepare model solder joints with copper as base materials. These solder joints will be aged at various temperatures, and they will be characterized for their microstructure depending on aging time and temperature. They will then be thoroughly tested for their mechanical and thermo-mechanical properties which will be studied as a function of solder type, of type, size and concentration of reinforcing nano-particles, and of aging temperature and aging time. It is hoped that in this way a solder material with clearly improved properties could be produced.

The so-called RoHS1 Directive of the European Union came into force on July 1, 2006, and since then a number of hazardous substances in electronic and electrical equipment has been restricted. Thus, the use of lead as a component of solder materials has also been prohibited, although a number of exemptions still exist. Unfortunately, the corresponding lead-free solders (mostly tin alloys with minor contents of silver and/or copper) have melting temperatures nearly40 degrees higher than the old lead-tin solders which causes a considerably higher thermal stress on the electronic components during soldering. So-called nanosolders have been discussed as one possible solution, i.e. solder alloys consisting of nanoscopic particles (smaller than about 0.00002 mm) which are known to have considerably lower melting temperatures, depending on the particle size.In this project, the synthesis of tin-silver-copper nanosolders (SAC 387, i.e. tin with 3.8 % silver and 0.7 % copper) by chemical reduction from aqueous solutions was optimized, and the particle size could be varied by changing the conditions of synthesis. It was observed that the majority of the nanoparticles consisted of a core of metallic tin, surrounded by a shell of amorphous tin hydroxide which converted into tin and tin dioxide on heating up to 300 to 400C. Using commercial fluxes, these nanosolders were turned into corresponding solder pastes which were then employed to prepare model solder joints with copper contacts. It was found that the described shell of tin oxide/hydroxide prevented the formation of stable contacts. Only a purification and reduction process of the nanoparticles in a mildly acidic solution, prior to mixing them into the flux, resulted in solder pastes that gave reliable solder joints, comparable with joints prepared from commercial solder pastes.Using semi-empirical model calculations, the melting behavior of the tin-silver-copper alloys could be calculated as a function of the particle size, not only in the concentration range of the synthesized nanosolders but over the entire composition range of the ternary alloy.In another series of experiments, ternary tin-zinc-copper nanoalloys which are of potential industrial interest were synthesized and characterized.Within an international cooperation, the properties of solder joints were characterized that had been prepared from solder pastes reinforced with oxide nanoparticles. The influence of the concentration of the nanoparticles in the solder paste on the properties of the joints could be determined. In addition, calorimetric experiments allowed to measure the additional energy of nanoparticles that is due to their increased surface area. The experimental values agreed quite well with those calculated from theoretical considerations.