Abstract

    The demand for new and improved lead-free solder (LFS) alloys grows steadily as the need for reliable lead-free electronic products increases. Thermodynamic calculations have proved to be an important tool in providing information for the design and understanding of new LFS systems.

    However, such tools often fall short from directly providing the information that is actually required by the end users, such as physical and thermophysical properties. In the present work, models have been created such that a full set of such properties can be calculated for solder alloys for the multicomponent system Sn-Ag-Al-Au-Bi-Cu-In-Ni-Pb-Sb-Zn.The properties, given for both the overall alloy or for each phase if required, include coefficient of thermal expansion, densities, various modulii, thermal conductivity, liquid surface tension and viscosity, all as a function of composition and temperature (extending into the liquid state).

Introduction

    To meet the requirements arising from environmental and health issues concerning the toxicity of lead, lead-free solder (LFS) alloys have been developed during the past decade to replace conventional Pb-Sn alloys. Studies on LFS materials were particularly accelerated in the last years due to the introduction of RoHS (Restriction of Hazardous Substances)Directive on 1 July 2006, i.e. all electrical and electronic products in the EU market must now pass RoHS compliance.

    Although many industries serving the information communications technology and consumer electronics have claimed their production has been   completely redesigned to accommodate the newly developed LFS alloys, the long term effect of such a switch remains to be seen. It has become clear though that the cost, and increased risk, to industry is significantly greater that initially thought, and to close the remaining knowledge gaps could take several more years of investment and investigation. Therefore, interest in developing new LFS alloys will remain, if not increase, both for improved performance, reliability and to reduce toxicity.

    As part of the process of developing new LFS alloys, thermodynamic calculations have been extensively reported [1,2,3,4,5] and a number of the rmodynamic databases havebeen developed specifically for this purpose [1,2]. However, the limitations of a purely thermodynamic approach are well known, in that it does not provide direct information for general material properties, such as physical, thermophysical and mechanical properties, which are the key to the application of any new solders. Such material properties are also critical inputs for the manufacturing and reliability modelling of soldered components using finite element (FE) or finite difference (FD) tools.

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