Effect of Ag addition in Sn on growth of SnTe compound during reaction between molten solder and tellurium
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SnTe is the most common compound formed at the bismuth telluride/metal soldered junction of thermoelectric modules. It affects the mechanical and electrical properties of the soldered junction. In the study we investigate the growth of SnTe compound during reaction between molten Sn–3.5Ag solder and tellurium at 250 C. We found that the growth of SnTe is suppressed by Ag–Te bilayer compounds that block further reaction between liquid Sn and Te. With increasing reaction time, the SnTe morphology becomes rough as a result of coarsening of SnTe grains. The growth of SnTe grains follows the conservative ripening kinetics with the mean particle size proportional to one-third power of reaction time. I. INTRODUCTION
Bismuth telluride based compounds have been widely used in thermoelectric cooling or power-generation modules operating at temperatures below 200 C. To assemble thermoelectric modules, a number of tiny bismuth telluride pallets are arranged in square array and soldered to metallic conductors that are prejointed on the ceramic plates of thermoelectric modules. Sn solder can react with bismuth telluride readily and form intermetallic compounds (IMCs) at the soldered junction, which may degrade the electrical and mechanical properties of the thermoelectric module.1 In general, a nickel layer has to be coated on the top and bottom sides of telluride elements as a barrier to prevent Sn from reacting with telluride directly. However, under high temperature and high electric current operation conditions, Sn-based solder will consume Ni to form Ni3Sn4 and penetrate through the barrier layer to react with telluride. Figure 1 shows the cross-sectional image of a commercial thermoelectric module after passing an electric current of 6 A at 140 C. It is clearly seen in the amplified image of the interface that Sn has penetrated through Ni at certain locations and reacted with bismuth telluride by forming SnTe compound. Thus, knowledge about the reaction kinetics between Sn-based solder and bismuth telluride becomes essential for developing robust and reliable thermoelectric modules. It is not unusual for the kinetics of IMC growth during soldering to be tailored by a trace element in molten solder. For example, a small amount of Cu added in the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0040 J. Mater. Res., Vol. 25, No. 2, Feb 2010
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Sn-based solder changes the kinetics of Cu–Sn IMC growth at the solder/Cu junctions.2,3 Some Sn–Bi alloys with various Pb, Cd, and Sb ingredients can react with bismuth telluride by forming PbTe, CdTe, and SnTe, respectively.4 Our previous investigation revealed that Pb1xSnxTe and SnTe are the major IMCs for Sn–37Pb and Sn–4Ag–0.5Cu solders reacting with bismuth telluride, respectively.1 Because SnTe is the most common IMC formed at the bismuth telluride/metal junction during a soldering reaction, it would be informative to investigate the reaction of molten Sn with tellurium
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