| Environmental concerned regulations in developed countries require the use of lead-free solders in place of conventional Sn-Pb solders in electronic industry in every country including China. Eutectic and near-eutectic Sn-Ag-Cu alloys are the lead-free solder in the main stream and have acquired some applications. Sn-Zn alloy has attracted much attention due to its marked advantages such as low cost, low melting points and good mechanical properties, and is expected to be one of the main-streamed alternative alloys for the next generation lead-free solders. However, Sn-9Zn has poor wettability to materials to be connected such as Cu, which is the obstacle to its application and is the focus of research efforts.The existing research in Sn-Zn alloys as lead-free solders is based on eutectic Sn-9Zn, following normal thinking of using eutectic alloys which has the lowest melting point the binary system. Based on the detrimental effects of Zn and the feature of Sn-Zn, we suggest that eutectic Sn-9Zn is not necessarily the best alloy as lead-free solder, whereas Sn-Zn alloys with slightly lower Zn concentration may be better options. We hence carried out investigations at this point, including the effect of Zn concentration on the wettability of Sn-Zn to Cu, the interfacial microstructure and adhesive strength of Sn-Zn/Cu, on the microstructure, tensile properties and creep resistance of bulk Sn-Zn. We also studied the effect alloying on the selected hypoeutectic Sn-Zn alloy.1. The spreading tests (in parallel to reflow soldering process) and wetting force test ( in parallel to wave soldering process ) show that, the wettability of Sn-Zn alloys reaches its peak when the concentration of Zn is around 6.5% by weight. The reason is that on one hand, Zn is detrimental to wettabil'ity; on the other hand, Zn lowers the melting point of Sn-Zn alloys which favors wetting. The detrimental effect of Zn in Sn-Zn solders can not be accounted for by the higher surface tension of Zn than Sn One of the reasons for the harmful effect of Zn on wettability is that it causes the formation ZnO on the surface of the system, while ZnO reacts with rosin which is the basic composition of flux, which results in the thickening and loss of fluidity of the flux to some extent, which in turn prevents spreading and wetting of the alloy system Lower Zn concentration reduces amount of ZnO and retards or even suppresses thickening of rosin, and thus improve wettability of the alloy. Systematic studies show that there existing a critical ZnO concentration, below which no thickening is seen in the rosin system.2. During non-equilibria heating (≥5℃/min), Sn-6.5Zn exhibits almost the same melting behavior as Sn-9Zn. As far as practical application is concerned, there is not much difference between the melting temperature of Sn-6.5Zn and that of eutectic Sn-gZn alloy.3. When Zn concentration is higher that 0.7%wt., the IMC (Intermetallic Compound) formed and the microstructure at the hypoeutectic Sn-Zn and Cu interface are the same. However, the thickness of the IMC decreases as the concentration of Zn decreases. Sn-6.5Zn/Cu withstands highest shear force in all Sn-xZn/Cu lap joints investigated, in accordance with the best wettability of Sn-6.5Zn to Cu.4. Under air cooling condition, coarse rod-like Zn rich phase is seen in the solidification microstructure of eutectic Sn-9Zn, in addition to the eutectic structure, whereas for Sn-6.5Zn, only complete eutectic structure appears and no rod-like Zn rich phase. Only under furnace cooling condition, does microstructure appear as complete eutectic structure in Sn-9Zn. This result suggests that Sn-6.5Zn has advantage over Sn-9Zn in microstructure.5. The creep resistance of Sn-Zn alloys (Wzn≥2.5%) is far better than that of conventional Sn-37Pb, and slightly worse than that of Sn-3.SAg-0.7Cu. The creep property of Sn-Zn alloys is not sensitive to Zn concentration. When Zn concentration is in the range 2.5～9% by weight, the stress exponents n are very close in the range of 3.15～3.3, corresponding to grainboundary (interface) creep mechanism. Tensile tests at high strain rate (10-1/s) and low strain rate (10-3/s) show that tensile strength of Sn-Zn alloys is not sensitive to Zn concentration, maintained at the strength level of Zn-9Zn. The ductility however, increases considerably with decreasing Zn concentration. As to mechanical property, lowering Zn concentration with respect to eutectic point results in no loss in strength, while ductility is increased.6. Similar to the conclusions in alloying Sn-9Zn, adding Cu (2%wt),Bi (3%wt) or P (0.5%wt) improves the wettability of Sn-6.5Zn to Cu, and has no significant harmful influence on its microstructure, mechanical properties and related physical properties.7. Sn-6.5Zn has better corrosion resistance than Sn-9Zn in solder flux. Addition of Cu and P causes decrease in corrosion resistance, while Bi affects corrosion resistance of Sn-6.5Zn the otherwise.Summarizing the research results, we propose for the first time that development of Sn-Zn lead-free solders should be based on Sn-6.5Zn, not the most attended eutectic Sn-9Zn so far instead.
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