Oxygen tensioactive effect on molten Bi-Sn system

D. Giuranno1, E. Ricci1, R. Novakovic1 and E. Arato2

1National Research Council of Italy-Institute for Energetics and Interphases, Italy
2University of Genoa-Department of Civil, Environmental and Architectural Engineering, Italy

Keywords: lead free solder; oxidation; thermodynamic modeling
property: surface tension
material: Bi-Sn

Bi-Sn is a basic system for many multicomponent alloys basically used as soldering materials. Similar to others complex metal systems, the development of new and improvement of existing, require a precise knowledge of the thermophysical properties of the liquid phase as a function of temperature and composition as well as other parameters, particularly those related to solder process which is determined by the physico-chemical properties of the liquid solder/solid substrate system and by the environmental conditions. The interfacial forces between molten solder, working atmosphere and substrate influence the degree of wetting and thus, the performances of the joints. The presence of any other elements which play an “active” role at interfaces, lowering the interfacial specific free energy and changes drastically the wetting behavior. Among all active additions, oxygen could be considered the most important one which often play the role of “contaminant”, and its control is very important in determining the surface and interfacial properties. In this work, the surface tension behavior of a set of liquid Bi-Sn alloys has been investigated both as a function of composition and under variable oxygen content in the surrounding atmosphere. The measurements were carried out at equilibrium both under “oxygen-free” conditions and oxygen containing atmospheres. The experimental data have been analyzed together with the available literature data as well as with the corresponding theoretical values calculated by applying different thermodynamic models. In addition, dynamic surface tension measurements have been performed to study the evolution of the molten alloy surface in the presence of trace amounts of oxygen in the surrounding atmosphere under two different diffusional regimes: under a vacuum (Knudsen regime) and under a fluxing atmosphere of a inert gas (Fick regime) . The general physico-mathematical formalism developed by our group to study liquid binary alloys and their oxides has been applied to describe the interactions in Bi-Sn melts under an oxidizing atmosphere.

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