Chemical Short Range Order CSRO in solder materials

The recent EU legislation, Directive WEEE (Waste from Electrical and Electronoc Equipment) and the Directive RoHS (Restriction of Hazardous Substances) prohibit the use of low melting (<220C) lead containing solders in many applications starting July1st, 2006. However research is lacking on high temperature (<350C) solders with a high lead content. These materials are exempt from the new legislation. To fill this gap of understanding a new COST Action COST MP0602 will start to address these problems. Our research group will also be involved in this Action. The ongoing miniaturization in the electronic industry, currently solder balls of the size of 50m are already in use and 25m are in the experimental state, poses serious questions concerning reliability. At this size of the solder balls the surface to volume ratio becomes more important and also the segregation of some elements to the surface. Therefore we will investigate what influences this segregation at the surface and does the chemical short range order (CSRO) in the liquid play any part in it. For application as a solder material the reaction of the liquid surface of the solder with the substrate to form intermetallic compounds (IMCs) must be known. To investigate the surface of the liquid solders we will use small angle X-ray scattering and especially for the surface the Grazing Incidence Small Angle X-ray Scattering (GISAXS) at HASYLAB in Hamburg and Neutron Diffraction measurements. The thermodynamic properties, which are necessary for certain models will be investigated with EMF and Calorimetric measurements. Shear tests of the solder material on different substrates like gold, copper and nickel will be carried out. To get a better understanding of the experimental results we will apply different models and if necessary develop some models for the surface tension and surface properties. With this research we hope to get a better understanding of the reactions on the surface of liquid alloys and with the substrate. We hope to contribute to the development of new high temperature lead-free solders.