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Ductile Low Temperature Solders for Mass Production of Assemblies
Materials Tech |
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Authored By:Keith Sweatman, Tetsuro Nishimura Nihon Superior Co., Ltd. Osaka, Japan Xin F. Tan, Qichao Hao, Stuart D. McDonald, Michael J. Bermingham, Kazuhiro Nogita The University of Queensland Australia Qinfen Gu ANSTO Australia SummaryWhile alloys based around the Sn-In system provide a great combination of low melting point, high thermal conductivity and ductility, their cost necessarily limits their use to special applications. If the wider electronics industry is to enjoy the benefits of low temperature solders (LTS), environmental, and economic, as well as technical, it will be necessary to develop a database that can provide the confidence the industry needs to use solders based on the Sn-Bi system. Because the fundamental metallurgy of Sn-Bi solders differs in several significant ways from that of other Pb-free solders, establishing that confidence will require systematic studies that will go beyond what was done to provide the electronics industry with the confidence to move from Sn-Pb to the first generation of Pb-free solders. When a solder based on the Sn-Bi system behaves in a way that might compromise its performance in soldering processes, or its reliability in service, the response of the material scientist is to look for additions to the base alloy, sometimes referred to as “dopants”, which could have the potential to improve its performance. Early reports indicated that an addition of Sb mitigated the embrittling tendency of the Bi phase in the microstructure of Sn-Bi solders by refining the eutectic phase. That in subsequent studies it has been found difficult to reproduce that beneficial effect consistently suggests that, in the experiments in which the beneficial effect was observed, there might have been factors additional to the presence of Sb that were contributing to the improved ductility. In this paper we report the results of a systematic study of the behaviour of Sb in a Sn-Bi alloy and consider the implications of the findings for the reliability of LTS solder joints. These results are considered in the context of a wider review of the characteristics that distinguish Sn-Bi alloys from the Pb-free solders on which the industry currently relies. ConclusionsThere are fundamental differences between the basic metallurgy of solder alloys based on Sn-Bi and that of the current generation of Pb-free solders. That means that the criteria for reliability developed for the current generation of Pb-free solders do not necessarily apply to Sn-Bi solders. Confidence in Sn-Bi as solders for the assembly of high reliability electronics will depend on the development of a deeper understanding of the behaviour of these alloys and, in particular the behaviour of the Bi phase that is the distinguishing feature of their microstructure. Evidence from historic studies suggests that it would be worth investigating the role that recrystallization might play in ductile deformation of Bi. Also, worth investigating would be whether a fine eutectic creates a microstructure more conducive to a ductile response to stress. In the study of the behaviour and effect of Sb additions to a hypoeutectic Sn-Bi alloy referenced in this paper [9]:
Initially Published in the SMTA Proceedings |
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