The materials used in the present research are pure Sn metal and Sn-0.5% Cu, Sn-3.5%Ag, Sn-0.3%Sb, and Sn-3.5%Ag-0.5%Cu alloys. Effects of Cu, Ag and Sb on the creep-rupture strength of lead-free solder alloys have been investigated. Creep tests are performed at the stress and temperature range of 3 to 12 MPa and 378 to 403 K, respectively. A 3.5% addition of Ag had the largest contribution to the creep-rupture strength of Sn metal among the single addition of 0.5%Cu, 3.5%Ag, and 0.3%Sb. The combined addition of 3.5%Ag and 0.5%Cu makes the largest creep-rupture strength. The effects of these elements on the microstructure of the lead-free alloys are also investigated with optical microscope (OM) and transmission electron microscope (TEM) observations.

INTRODUCTIONAs discussed by Morris and Reynold, 1 there is a large possibility that important mechanical properties, such as the creep strength of eutectic solder alloys, can change by orders of magnitude if the microstructure is altered. Morris et al. have performed vigorous research work 2-5 on the effect of microstructure, particularly the grain size effect, on the mechanical properties of Sn-Pb eutectic solder alloys and solder joints. Their studies show that, as the cooling rate during solidification increases, the microstructure of the solder alloys changes from the lamellar type typical of slow cooling to a more equiaxed and refined structure. 4 Such microstructure induces the superplastic creep, 2,5 reduces the tensile strength at relatively slow strain rate and high temperature, 3 and increases the fatigue cycle at 50% load drop by enhancing the grain boundary deformation mechanism. 4 The decreasing of yield strength and Vickers hardness or the increasing of fatigue life with the reducing of grain size caused by the increased cooling rate during solidification are also reported by Rack and Maurin, 6 Sutliff and Notis, 7 and Wild, 8 respectively. These studies, however, have been made on the very-fine-grained alloys, where superplasticity occurs.On the other hand, there are few studies that investigated the microstructural effect on the mechanical properties of Pb-free solder alloys. Recently, Kim et al. 9 investigated the effect of microstructure on the ultimate tensile strength (UTS), 0.2% proof strength, and elongation of Sn-3.0 to 3.9%Ag-0.5 to 0.7%Cu alloys at room temperature. The microstructure was altered by changing the cooling rate during solidification of the alloys. Kim et al. concluded that slow cooling decreased the UTS and 0.2% proof strength because of the coarsened eutectic structure and the formation of large primary Ag 3 Sn precipitates.However, to the authors' knowledge there are no studies on the effect of microstructure on the creep strength of Sn-3.5%Ag alloy. In the present research, the effect of microstructure on the creep-rupture strength of Sn-3.5%Ag alloy has been investigated after altering the microstructure by adopting different solidification and cooling rates of the alloy cast into the mold.The effect of microstructure obtained by rapid or slow solidification and cooling of a Sn-3.5%Ag lead-free solder alloy on the creep strength has been investigated. The rapidly cooled alloy showed that the microstructure consisted of the primarily crystallized Sn phase and the quasi-eutectic phase, where fine Ag 3 Sn particles dispersed in the Sn matrix. In the slowly cooled alloy, large platelets of Ag 3 Sn were formed sparsely in the Sn matrix. A difference of about 2.5 orders of magnitude in the cooling rate translates to about 1.5 orders of magnitude in the creep-rupture time. Accordingly, fine particle dispersion of Ag 3 Sn is considered to be very beneficial for the restraining of creep deformation, that is, for the decreasing of creep rate of the Sn-3.5%Ag alloy, compared with...

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Creep and rupture behavior of Cu wire/lead-free solder-alloy joint specimens have been investigated using Sn-3.5% Ag and Sn-0.5% Cu alloys. A Sn-37% Pb solder alloy is also used as a reference material. The present authors have fabricated a creep-rupture testing machine for Cu wire/solder-alloy joint specimens, performed creep and rupture tests at 303 K and 403 K, analyzed the characteristics of the creep and rupture behavior, and compared these to test specimens cut from the same alloy ingots. It is also found that the rupture strength of the joint specimens is related to the rupture strength of the alloys.

The influence of spray casting technique on martensitic transformation temperature, magnetic properties and compressive properties of Ni50Mn25+xGa25-x (x = 0, 1.25, 2.5, 3.75 and 5) alloys was investigated. With the increase in Mn content, martensitic transformation temperature of the spray casting samples is increased, but slightly lower than that of the corresponding as-cast ones. The saturation magnetization test shows the opposite tendency and proves the spray casting technique can effectively enhance the magnetic properties. Compression experiments showed that the spray casting process did not change the fracture strain, but lead to the greatly increase in the compressive strength and improve the brittleness of the Ni-Mn-Ga alloys. This study may offer a new processing technique for developing high performance ferromagnetic shape memory alloys.