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Effect of Oxygen on Surface Tension of Liquid Ag-Sn Alloys

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Effect of Oxygen on Surface Tension of Liquid Ag-Sn Alloys Materials Transactions, Vol. 45, No. 3 (2004) pp. 625 to 629 Special Issue on Lead-Free Soldering in Electronics #2004 The Japan Institute of Metals Effect of Oxygen on Surface Tension of Liquid Ag-Sn Alloys Joonho Lee1, Toshihiro Tanaka1, Masaya Yamamoto2 and Shigeta Hara1 1Department of Materials Science and Processing, Osaka University, Suita 565-0871, Japan 2Formerly Graduate Student, Department of Materials Science and Processing, Osaka University, Suita 565-0871, Japan The effect of oxygen on the surface tension of liquid Ag-Sn alloys used as one of the main components in lead-free soldering alloys was measured by the sessile drop method. The surface tension of liquid Ag-Sn alloys decreased with increasing tin content. The effect of oxygen was investigated for oxygen partial pressure ranging from about 10À17 to 10À12 atm. The effect of oxygen on the surface tension of liquid Ag-Sn alloys is closer to pure tin rather than pure silver. Thermodynamic simulation using Butler’s equation showed that the surface of liquid Ag-Sn alloys is enriched with tin. Accordingly, the effect of oxygen adsorption on the surface tension of liquid Ag-Sn alloys was considered to be closer to that of tin than silver. (Received September 22, 2003; Accepted December 2, 2003) Keywords: surface tension; liquid silver-tin alloys; sessile drop method; lead-free soldering 1. Introduction In the present work, the surface tension of liquid Ag-Sn alloys, one of the main components in lead-free solders, was Since the surface tension of liquid solder alloy determines measured by the sessile drop method with varying oxygen its wetting behavior, many researchers have investigated the partial pressure in atmosphere. In order to investigate the surface tension of the lead-free soldering alloys (e.g. Ag-Sn surface tension in the whole composition range, the exper- after Moser et al.1)). Although oxygen in atmosphere is imental temperature was determined as 1253 K. Then, the known to decrease the surface tension of liquid metals and oxygen adsorption behavior on the surface of liquid Ag-Sn alloys,2–10) studies on the effect of oxygen for lead-free alloys was interpreted by the surface concentration calculated solders have been only restricted to pure metals.8–10) using thermodynamic database. Generally, Gibbs adsorption isotherm (1) may express the 11) oxygen adsorption (ÀO) on liquid metals and alloys. 2. Experimental 1 d 2 d ÀO ¼ ¼ ð1Þ The experimental apparatus used for the measurements of RT d ln a RT d ln p O O2 the surface tension of liquid Ag-Sn alloys consisted of an where R: universal gas constant, T: temperature, : surface image capturing system with a He-Ne laser and a SiC heating tensions of liquid metal or alloy, aO: oxygen activity in the element furnace. Figure 1 shows a schematic diagram of the metal or alloy and pO2 : oxygen partial pressure, respectively. experimental setup. A quartz reaction tube (5.5 cm O.D., Hence, the information of the surface tension dependence on 5.0 cm I.D., 20.0 cm high) with investigation windows from a the oxygen partial pressure reflects the oxygen adsorption horizontal direction was used. The tube was sealed by means behavior in the surface of liquid metals and alloys. For of O-rings fitted onto copper water-cooled jackets at the ends example, if the surface of liquid metals and alloys is saturated of the tube. High purity samples (Ag: 99.99+% and Sn: with oxygen, d=d ln pO2 have a constant slope, yielding the 99.999+%) installed in an alumina crucible with a hole surface excess concentration of oxygen (oxygen adsorption) at saturation. The oxygen adsorption for pure metals has been Gas inlet investigated by many researchers. Table 1 summarizes some Alumina rod of the reported oxygen adsorption at saturation for pure 9) Cooling water inlet metals after Bernard and Lupis. However, there is no Oxygen sensor research on the oxygen adsorption for lead-free solder alloys siliconit as far as the authors concern. Alumina crucible Table 1 Saturation parameters of oxygen at the surface of pure liquid metals.9) Maximum surface excess Area per adsorbed atom, He-Ne laser CCD camera Temp. À System concentration, S/10 2 nm2 Sample alloy Alumina substrate T/K À6 À2 ÀO.sat/10 molÁm experiment calculation Copper jacket Thermocouple Ag 1253 5 33 23Ã/38þ Gas outlet Connected to Cu 1423 9.3 17.8 18Ã/30þ a vacuum pump þ Fe 1823 18:123:47:19:2 8 Fig. 1 Schematic diagram of experimental apparatus for the sessile drop Ã(100) face and þ(111) face of the solid compound measurements of surface tension. 626 J. Lee, T. Tanaka, M. Yamamoto and S. Hara fitted with an optical zoom lens and filters, where a pixel Alumina tube corresponds to 18.7166 mm with the CCD camera. The threshold shape of the drop was determined using image Alumina cement analysis software. Then, this drop profile was used to Glass determine the surface tension by comparing the observed Alumina particles shape with that of the solution to the Laplace equation using a computer program developed by Krylov et al.13) Reference electrode (Metal/Metal oxide, p ref . ) O2 3. Results and Discussion Solid electrolyte tube (ZrO –8mol%Y O ) 2 2 3 3.1 Surface tension of liquid Ag-Sn alloys Platinum wire Initially the method was applied to liquid Ag-Sn alloys in Gas ( p O ) 2 Ar-10%H2 atmosphere. In Fig. 3, the experimental results are plotted as a function of concentration of tin in the bulk phase. Fig. 2 Schematic diagram of zirconia oxygen sensor. For comparison, the surface tension of pure silver and tin measured by Bernard and Lupis9) (1253 K), and Taimatsu and Sangiorgi10) (1273 K) are also plotted. In addition, the (1.4 mm) at the bottom side, was set about 15 mm above an experimental results for alloys were compared with the alumina (sapphire) substrate. The alumina substrate was set calculated results based on Butler’s eq. (6) using thermody- in the uniform-temperature area of the furnace, and accu- namic database. rately leveled by adjusting three poles sustaining the furnace. ! S The temperature was measured with a Pt/Pt-13%Rh thermo- RT 1 À NAg ¼ Sn þ ln B couple, which was set directly under the substrate. After the SSn 1 À NAg sample was placed in the furnace, the quartz tube was 1 Ex,S S Ex,B B evacuated with a mechanical pump producing a vacuum of þ GSn ðT; NAgÞGSn ðT; NAgÞ SSn 1 Pa and filled with high-purity Ar-10%H2 gas mixture. This ! procedure was repeated for three times to eliminate residual RT NS ¼ þ ln Ag oxygen in the reaction tube. Then, the furnace was Ag B SAg NAg programmed for the heating cycle up to 1253 K in 3 h. When 1 temperature in the furnace was stabilized, the sample alloys Ex,S S Ex,B B þ GAg ðT; NAgÞGAg ðT; NAgÞ ð6Þ in the alumina crucible were dropped onto the alumina SAg substrate using an alumina rod. During experiments, the where R: universal gas constant, T: temperature, , : oxygen partial pressure was controlled with introducing a Sn Ag surface tensions of pure liquid tin and silver, respectively, suitable mixture of purified Ar-10%H and CO gases. In 2 2 S , S : molar surface areas of pure liquid tin and silver, order to confirm the oxygen potential in the gas, zirconia Sn Ag S S ¼ À S oxygen sensor equipped with reference electrodes (Fe/FeO, NAg: mole fraction of silver in the surface (NSn 1 NAg), B B B Ni/NiO and Cu/Cu2O couples) was placed in the furnace. A NAg: mole fraction of silver in the bulk (NSn ¼ 1 À NAg). schematic illustration of the oxygen sensor is shown in Fig. 2. Here, the ‘‘surface’’ is defined as outermost monolayer. Ex,S S When the effect of the electronic conduction is negligible, Gi ðT; Ni Þ: partial excess Gibbs energy of i in the surface S Ex,B B from the electromotive force (E) expressed by eq. (2) one can as a function of T and Ni , Gi ðT; Ni Þ: partial excess Gibbs obtain the oxygen potential in the atmosphere (p ). ! O2 RT pO2 E ¼ ln ð2Þ 900 4F pref. Calculation O2 Experiment where F: Faraday’s constant, pref.: the oxygen potential in the -1 Bernarad et al. O2 800 reference electrode. Here, the oxygen potential in the Taimatsu et al. reference electrodes was determined using reported values /mN m σ below.12) (pref. ¼ expðÁG=RT)) O2 700 Fe þ 1/2O2 ¼ FeO ÁG ¼263:4 þ 64:81T ðkJÞð3Þ Ni þ 1/2O2 ¼ NiO ÁG ¼233:6 þ 83:94 ðkJÞð4Þ 600 2Cu þ 1/2O2 ¼ Cu2O ÁG ¼166:9 þ 71:13 ðkJÞð5Þ Preliminary experiments showed that the equilibrium oxygen Surface Tension, 500 potential was obtained in 60 min in the present experimental condition. Surface tension measurements, therefore, were 0.0 0.2 0.4 0.6 0.8 1.0 conducted after 150 min to confirm the oxygen partial Concentration in the Bulk, N B pressure. In order to capture a clear drop shadow image, a Sn He-Ne laser ( ¼ 632:8 nm) was used. The drop shadow Fig. 3 Change in surface tension of liquid Ag-Sn alloys as a function of tin profile was taken using a CCD camera (512  512 pixels) content at 1253 K. Effect of Oxygen on Surface Tension of Liquid Ag-Sn Alloys 627 Table 2 Thermo-physical data and partial excess Gibbs energies for the calculation of the surface tension of Sn-Ag alloys.14,17,18) ¼ À À1 Sn 601 00886T i/mN m Ag ¼ 1207 À 0:2279T ¼ Á À6f þ Á À4ð À Þg 3 À1 VSn 17:0 10 1 0:87 10 T 504:99 Vi/m mol À6 À4 VAg ¼ 11:6 Á 10 f1 þ 0:98 Á 10 ðT À 1233:65Þg Ex G / NAgNSnfð3902:15 À 4:96927TÞþðÀ16974:05 þ 7:424515TÞðNAg À NSnÞ À1 2 3 Jmol þð14299:05 þ 10:67712TÞðNAg À NSnÞ þðÀ5979:25 þ 6:497125TÞðNAg À NSnÞ g B Table 3 The maximum limit of oxygen partial pressure forming SnO .
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