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The Effect of Strain Rate on the Ductility of Bismuth-Containing Solders
Materials Tech |
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Authored By:Keith Sweatman, Tetsuro Nishimura Nihon Superior Co., Ltd Osaka, Japan Stuart McDonald, Xiaozhou Ye, Qichao Hao, Beatrice Negura, Xin Fu Tan, , Kazuhiro Nogita The University of Queensland Queensland, Australia SummaryWith the identification of bismuth as an addition to lead-free solders that can reduce melting points and provide an additional strengthening mechanism, the electronics industry has moved to solder alloys that differ significantly from those on which it has previously relied for general assembly. Because of its low melting point, its unchallenged non-toxicity, and its unique ability to form intermetallic compounds with most of the substrates across which connections are needed, tin has provided the basis for most of the solders used in electronics manufacturing. The silver and copper additions used to reduce the melting point of tin in the first generation of lead-free solders have almost no solubility in the tin phase and are present in the microstructure as the intermetallic compounds Ag3Sn and Cu6Sn5, which provide, at least temporarily, the bonus of particle strengthening of the tin phase. The nickel addition, which enhances the fluidity of tin-copper solder and can stabilizes the hexagonal form of the Cu6Sn5 in all lead-free solders, is largely incorporated into the crystal structure of that intermetallic phase, partly replacing copper in the crystal lattice. Nickel in excess of that requirement forms other intermetallic phases that can provide additional particle strengthening of the tin phase. The matrix of these first-generation lead-free solders is essentially nearly pure tin. As an alloying addition in lead-free solders, bismuth differs from silver, copper and nickel in that it has substantial solid solubility in the tin phase. In solid solution the bismuth strengthens the tin phase, which has made it a popular ingredient in high reliability solder alloys. In that regard, the role of bismuth in lead-free solders is similar to that of lead in tin-lead solders. A critical difference, however, is that while lead in excess of the solid solubility in the tin phase at the prevailing temperature is present in the microstructure as a lead phase with a significant amount of tin in solid solution, bismuth at a level in excess of the solid solubility limit in the tin phase is present in the microstructure as a nearly pure bismuth phase. While the lead-tin solid solution, with its face-centred cubic structure is a tough ductile phase, the bismuth phase, with a rhombohedral crystal structure with limited slip planes, has a tendency to brittle failure. A further complication of solders containing bismuth is that over the range of temperatures to which electronic assembles are exposed in service they are closer to their solidus temperature than solders that do not contain bismuth. That means that the diffusion processes that can drive significant microstructural change proceed relatively rapidly. A consequence is that mechanical properties, which can be very dependent on microstructure, can change within relatively short time frames. In this paper the authors report some preliminary results from a wider study of the effects of bismuth on the properties and behaviour of solder alloys, in particular: 1. How the strain rate sensitivity of established leaded and lead-free solders differs from that of the tin-bismuth eutectic. 2. How a bismuth addition at less than the room temperature solid solubility in the tin phase on one of the established lead-free solders affects the strain rate sensitivity of strength and elongation 3. The relationship between the bismuth content of simple binary tin-bismuth alloys and their hardness and tensile strength. ConclusionsAlthough there remains much work to be done to fully characterize the behavior of solder alloys that contain significant amounts of bismuth the results reported here could provide the basis for improving the robustness of low melting point tin-bismuth alloys. Initially Published in the SMTA Proceedings |
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