Deformation behavior of dilute SnBi(0.5 to 6 at. pct) solid solutions
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I.
INTRODUCTION
IN the development of new lead-free solder alloys for the electronics industry, research has mainly been concentrated on eutectic or near-eutectic two- or three-component alloys. Such alloys have been considered to be the only serious alternatives to the eutectic SnPb solder. However, a new solder alloy, which is intended for use in electronics mass production, faces several metallurgical and economic requirements. None of the alloys known at present can be regarded as a real ‘‘drop-in’’ alternative to the eutectic SnPb. Therefore, other approaches to this problem must be sought. One such approach, which also allows fully fluxless, Pb-free soldering of electronic components on costeffective flexible substrates, is based on the use of metallurgically compatible solder coatings. In this ‘‘transfusion bonding’’ technique, which is described elsewhere,[1] the surfaces to be joined are chemically coated with thin layers of Sn and Bi. The surfaces, which are well protected with the Bi top-coat layer, are then pressed lightly together and heated to within the temperature range of 160 7C to 180 7C for a short period of time, such as from 15 to 30 seconds, to cause rapid intermixing of thin liquid surface layers of the mating materials. In the beginning, the Sn-rich liquid interlayer melts some more of the tin, while Bi atoms principally diffuse along the grain boundaries of the newly formed Sn-rich solid solution. Since, in most cases, the local nominal composition of the reaction zone does not essentially change from the liquidus composition during the
transfusion bonding, the liquid reaction zone solidifies during cooling.[2] Thus, the resulting joints are dilute Sn-based solid solutions containing small, randomly distributed Bi particles. Since bismuth is one of the greatest strengtheners of tin,[3] even low concentrations of dissolved Bi are expected to affect the deformation behavior of thermodynamically stable or metastable SnBi solid-solution joints in the stress and temperature ranges involved in electronics assemblies. Most of the available experimental data related to the mechanical properties of the SnBi alloys are based on the eutectic system.[4,5] The mechanical behavior of dilute SnBi alloys has been investigated only by Alden.[6,7] He found that, with small grain sizes (,10 mm), the flow stress exhibits a strong strain-rate sensitivity and the alloys deform superplastically, with a major part of the strain being accommodated by grain-boundary sliding. Superplastic deformation can only be obtained with a very small grain size, and, for the samples used by Alden, a special mechanicalthermal treatment was used to obtain this desired structure. However, in practical applications, the grain size of Sn is considerably larger, generally on the order of 100 mm, which most likely gives rise to different rate-controlling deformation mechanisms from those in Alden’s samples. It was, therefore, considered necessary to study the stressstrain and creep behavior of the dilute and relatively largegrained
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