With the advent of high-flux density permanent magnets based on rare earth elements such as neodymium (Nd) in the 1980s, permanent magnet-based electric machines had a clear performance and cost advantage over induction machines when weight and size were factors such as in hybrid electric vehicles and wind turbines. However, the advantages of the permanent magnet-based electric machines may be overshadowed by supply constraints and high prices of their key constituents, rare earth elements, which have seen nearly a 10-fold increase in price in the last 5 years and the imposition of export limits by the major producing country, China, since 2010. We outline the challenges, prospects, and pitfalls for several potential alloys that could replace Nd-based permanent magnets with more abundant and less strategically important elements.
Alloys of tin-silver-copper have been identified as promising replacements for conventional Sn-40Pb (wt.%) and Sn-37Pb (wt.%) solders; 1 in particular, compositions near the Sn-Ag-Cu ternary eutectic have attracted considerable attention (for example, Ref. 2). In recent work, Sn-Ag-Cu alloys have been modified with minor transition metal additions (primarily Co and Fe) to refine the as-solidified morphology and to suppress the growth rate of intermetallic compounds upon annealing of the solder/substrate (Cu) interfacial and joint interior regions. 3,4 Considerable effort has been applied in recent years to developing a quantitative understanding of the mechanical behavior of these new compositions, which is understandable given the paramount importance of shear strength and fatigue response in device applications. One parameter not yet fully characterized is the in-situ temperature dependence of electrical resistivity of Pb-free solder alloys in a joint configuration. Electrical characterization of drawn wire has limited usefulness in the design of electronic packaging components because formation of intermetallic phases, such as Cu 6 Sn 5 and Ag 3 Sn, in addition to diffusion of metal atoms from the substrate or interconnect wires can change the microstructure and composition of a solder relative to that of the nominal as-drawn wire condition. Moreover, certain fourth element additions to Sn-Ag-Cu solders, such as Bi, serve as solid solution strengthening agents by partial substitution for Sn in the solder matrix. One consequence of solid solution substitution in the -Sn phase is the potential increase in the electrical resistivity of the solder, relative to the unmodified ternary alloy. The increase in resistivity with increasing solute concentration is predicted by the Nordheim relationship and is a consequence of electron scattering from the resulting aperiodic lattice potential. This paper discusses the electrical resistivity and microstructure at 293 K and 423 K of solder joints prepared from four Pb-free solder alloys with compositions near the Sn-Ag-Cu eutectic: Sn-3.0Ag-0.5Cu, Sn-3.6Ag-1.0Cu, Sn-3.7Ag-0.9Cu, and Sn-3.7Ag-0.7Cu-0.2Fe (wt.%). For comparison, a broader series of solder alloys was characterized for ambient temperature resistivity both in a solder joint and as a solid wire. EXPERIMENTAL PROCEDUREThe solder alloys were prepared from 99.99%Sn, Ag, Cu, Co, Fe, and Bi by the Materials Preparation Center of the Ames Laboratory. Ingots were synthesized by melting the constituents in fused-quartz am-The electrical resistivity of solder joints prepared from Sn-Ag, Sn-Ag-Cu, and Sn-Ag-Cu-X alloys (where X ϭ Co, Fe, or Bi) was characterized by a four-point probe technique and interpreted in terms of microstructure and composition. The resistivity is also reported of drawn solid wires of these alloys. The solderjoint samples were prepared by hand soldering to copper substrates and were electrically characterized over a temperature range from 293-423 K, covering the anticipated range of elevated-tempera...
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