Abstract
A full set of physical and thermophysical properties for lead-free solder (LFS) alloys have been calculated, which include liquidus/solidus temp-eratures, fraction solid, density, coefficient of thermal expansion, thermal conductivity, Young's modulus, viscosity and liquid surface tension, all as a function of composition and temperature (extending into the liquid state). The results have been extensively validated against data available in the literature. A detailed comparison of the properties of two LFS alloys Sn-20In-2.8Ag and Sn-5.5Zn-4.5In-3.5Bi with Sn-37Pb has been made to show the utility and need for calculations that cover a wide range of properties, including the need to consider the effect of non-equilibrium cooling. The modelling of many of these properties follows well established procedures previously used in JMatPro software for a range of structural alloys. This paper describes an additional procedure for the calculation of the liquid surface tension for multi-component systems, based on the Butler equation. Future software developments are reviewed including the addition of mechanical properties, but the present calculations can already make a useful contribution to the selection of appropriate new LFS alloys.
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. There is therefore still strong interest in developing new LFS alloys for improved performance, reliability and to reduce toxicity.
This has created a need for fundamental data that accurately describe the behaviour of these alloys in solder joints and which can also be used to develop appropriate reliability models. Although tremendous efforts have been made in this direction, there is still a definite lack of material data for LFS alloys. Most of the available references focus on data collected via experimental routes that are costly and time-consuming. Moreover, the fact that a large number of experiments are required to generate sufficient data to cover the multitude of proposed alloy types/compositions and conditions means experimentation is not always an option. Existing computer modelling work is normally based on finite-element analysis,dealing with real production and reliability issues. Although material properties are critical inputs for such typesof modelling, little work has been done on computer modelling the whole range of materials properties as a function of phase distributions calculated thermodynamically in real time.
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