Journal of ELECTRONIC MATERIALS, Vol. 36, No. 1, 2007 Regular Issue Paper DOI: 10.1007/s11664-006-0002-7 Ó 2006 TMS Thermodynamic Properties of Liquid Ag-Bi-Sn Alloys ZUOAN LI,1 SABINE KNOTT,1 and ADOLF MIKULA1,2 1.—Institut für Anorganische Chemie/Materialchemie, Universität Wien, A-1090 Wien, Austria. 2.—E-mail: adolf.mikula@univie.ac.at As a promising lead-free solder, the thermodynamic properties of the liquid ternary Ag-Bi-Sn system were investigated. Using an appropriate galvanic cell, the partial free energies of Sn in liquid Ag-Bi-Sn alloys were determined as a function of concentration and temperature. Thermodynamic properties were obtained for 27 alloys. Their composition was situated on three cross sections with the constant ratios of Ag:Bi = 2:1, 1:1, and 1:2. The integral Gibbs free energy and the integral enthalpy for the ternary system at 900 K were calculated by Gibbs–Duhem integration. Key words: Silver-bismuth-tin, lead-free solders, liquid alloys, thermodynamic properties, electromotive force measurements INTRODUCTION Lead-tin base solders have long been the most popular materials for electronic packaging because of their low cost and superior properties required for interconnecting electronic components. However, the toxic nature of lead and the increasing awareness of its adverse effect on environment and health have led to the pressing need for development of lead-free solders in recent years. New lead-free solders, which will be used as the alternative of Sn-37Pb solder, must meet some required material properties, such as low melting temperature, good wettability, and excellent mechanical properties. Thermodynamic data are of great importance for the accurate calculation of phase diagrams, for the development of leadfree solder database, for the design of new lead-free solders, and for the prediction of physical and chemical properties of lead-free solders, such as surface tension and viscosity.1,2 In our group, the thermodynamic properties of a series of lead-free solders including Ag-Sn-Zn, Cu-Sn-Zn, In-Sn-Zn, and Al-Sn-Zn have been investigated completely.3–6 Previous investigations have proposed the Ag-BiSn alloy as a promising lead-free solder, because it is superior to other candidates with respect to melting properties, wettability, and mechanical properties.7,8 Unfortunately, the data of thermodynamic properties and the phase diagram are scarce for the (Received March 17, 2005; accepted June 27, 2005) 40 ternary Ag-Bi-Sn system. In the present investigation, the thermodynamic properties of tin were measured with an electromotive force (emf) method at three cross sections with a constant Ag:Bi molar ratio of 2:1, 1:1, and 1:2. A Gibbs–Duhem integration was carried out to determine the integral thermodynamic properties of the entire ternary system. EXPERIMENTAL PROCEDURE The ternary alloys were prepared from starting materials of high-purity 5 N metals (from Johnson Matthey GmbH, Karlsruhe, Germany). In order to remove the oxide layer from the surface, Sn was polished with a fine emery paper, Bi was cleaned prior to its use by melting it under vacuum and filtering it through quartz wool under a purified argon atmosphere, and Ag was heated in a carbon crucible for 10 min at 973 K to remove Ag2S. The metals were weighed and sealed in quartz tube and melted at 873 K for 5 days. Afterward, the samples were quenched in cold water. Approximately 2 g of each alloy was used for the emf measurements. The liquid electrolyte for the emf measurements was a eutectic mixture of KCl and LiCl. The preparation of electrolyte and the assembling of the cell is described in Ref. 9. The 0.5 mol.% of potentialforming Sn+2 ion salt—dehydrated SnCl2—could not be added directly to the KCl-LiCl because the chlorine gas would oxidize the Sn+2 to Sn+4, and it was added directly into the emf cell before the measurements. Tungsten wire was used for the Thermodynamic Properties of Liquid Ag-Bi-Sn Alloys current leads because of no reactions and solubility between the tungsten and the alloys.10 The emf measurements were carried out on heating and cooling. The temperature range of measurements was from the liquidus temperature of the alloys up to 1000 K. The heating and cooling rate was 10 KÆh–1. The emf and temperature were recorded automatically every 5 min. At the hightemperature and low-temperature limits, the temperature was kept constant for a longer period in order to check the stability of the emf. As far as the evaluation of the thermodynamic properties was concerned, only the cooling curve was used. For these emf measurements, the following cell arrangement was used: Snð‘Þ=Snþþ (KCl
LiCl)/Ag-Bi-Snð‘Þ Under reversible conditions, the Gibbs free energy change for the reaction at temperature T is given by DGSn ¼
zFE ¼ RTln aSn where z = 2, F is the Faraday constant (96,486 CÆmol–1), R the universal gas constant, E the measured emf of the cell, T the absolute temperature, and aSn the thermodynamic activity of tin in the ternary alloy, with the pure liquid tin constituent as reference state. At all three cross sections, the emf versus temperature curves were straight lines. Using the least-squares fit, the emf is expressed by the following equation: E(mV) ¼ a þ bT(K) Using the measured emf values, the activity of tin and the change of Gibbs free energy were calculated. From the temperature dependence of E, the partial molar entropy DSSn and enthalpy DH Sn were derived using the following equations: DSSn ¼ zF DH Sn  @E @T  ¼ 2bF x;P  @E ¼
zF E
T @T  x;P ! ¼ DGSn þ TDSSn ¼
2aF The Gibbs–Duhem equation given by Elliot and Chipman11 (Eq. (12) in their publication) was 41 applied to calculate the integral thermodynamic quantities for the entire ternary system from the emf data. RESULTS AND DISCUSSION The thermodynamic data of the three binary systems were taken from the literature. Silver-Bismuth System In this system, the thermodynamic properties were investigated by emf methods with liquid electrolyte12,13 and with solid electrolyte1,4–15 and by calorimetric measurements.16–19 The thermodynamic activities were also measured by a vapor pressure method20 and a torque effusion method.21 A review of this system was given by Kattner et al.22 Bismuth-Tin System In this system, the thermodynamic properties were investigated by calorimetric measurements23,24 and by the emf method.25,26 The results of the different measurements show large scatter and some contradictions. More recent experimental results by emf measurement given by Asryan and Mikula26 have shown that the enthalpy of mixing versus composition was asymmetric. The phase Table I. Emf Data of Liquid Ag-Bi-Sn Alloys xSn E (mV) Ag:Bi = 2:1 0.102 0.200 0.300 0.400 0.500 0.600 0.699 0.801 0.900 –40.006 –38.787 –33.530 –11.829 –9.543 –3.895 –3.891 –2.400 –0.787 + + + + + + + + + 0.1481 0.1098 0.0842 0.0508 0.0372 0.0224 0.0173 0.0106 0.0041 T/K T/K T/K T/K T/K T/K T/K T/K T/K Ag:Bi = 1:1 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 –74.370 –73.164 –52.553 –9.706 –12.574 –10.200 –2.033 –1.649 –1.294 + + + + + + + + + 0.1709 0.1427 0.1036 0.0481 0.0406 0.0302 0.0159 0.0101 0.0045 T/K T/K T/K T/K T/K T/K T/K T/K T/K Ag:Bi = 1:2 0.100 0.201 0.301 0.400 0.500 0.600 0.701 0.801 0.900 –143.118 –39.086 –67.203 –12.691 –10.313 –11.862 –4.351 –4.369 –5.047 + + + + + + + + + 0.2403 0.1098 0.1234 0.0471 0.0361 0.0304 0.0186 0.0130 0.0088 T/K T/K T/K T/K T/K T/K T/K T/K T/K 42 Li, Knoot, and Mikula Fig. 1. Activity of Sn at 900 K for the three cross sections: (a) Ag:Bi = 2:1, (b) Ag:Bi = 1:1, and (c) Ag:Bi = 1:2. diagram has been reviewed and thermodynamically calculated by Lee et al.27 Silver-Tin System The most recent work on this system has been done by Luef et al. in the temperature range of 773– 1523 K, which reports negative enthalpy values on the silver-rich side and positive ones on the tin-rich side.28 Several calorimetric works show the same tendency.29–32 The partial molar quantities were measured by an emf method.33–35 A critical review of this system has been done by Kubaschewski and Alcock.36 Silver-Bismuth-Tin System Only a few investigations exist in this ternary system. The enthalpy values were measured by the direct reaction method,16 and the experimental equilibrium phase diagram was investigated by Hassam.37 Kattner and Boettinger calculated phase diagrams according to the extrapolation of three binary systems.22 Limited information on the ternary alloy phase obtained from the CALPHAD estimation was also reported by Ohtani et al.38 We started our investigation from the binary Ag-Bi system by adding Sn. The activity of Sn was measured along three cross sections with a constant Ag:Bi ratio of 2:1, 1:1, and 1:2. At all cross sections, the temperature versus emf curves were straight lines. A least-squares fit was used and the emf is expressed by the following equation: E(mV) ¼ a þ bT(K) Thermodynamic Properties of Liquid Ag-Bi-Sn Alloys In Table I, the parameters are given for all alloys. The activity of Sn shows a positive deviation from RaoultÕs law, and it becomes a little more positive around Sn at 50 at.% for all three cross sections. The results of our investigation in the ternary system are shown in Fig. 1a–c and are given in Table II. The partial Gibbs energy, partial enthalpies, and partial entropies are also listed in Table II. The integral Gibbs free energy and the enthalpy of mixing for the ternary Ag-Bi-Sn system were calculated using an equation given by Elliott and Chipman.11 The integration was carried out along the line of the constant Ag to Bi ratios, and for the integration constant GXS binary (Ag
Bi), the values of Hultgren et al. were used.39 The results are given in Table III, and the isoGibbs free energy curves for the ternary system are plotted in Fig. 2. The data for the binary Bi-Sn system were taken from the latest emf measurements given by Asryan and Mikula26 and for Ag-Sn from the compiled data given by Hultgren et al.39 A similar procedure was used to calculate the integral enthalpy of mixing. In this case, the data given by Hassam at 878 K were used for the Table II. Activities and Partial Molar Quantities of the Ag-Bi-Sn System at 900 K aSn DGSn (J/g-atom) DH Sn (J/g-atom) DSSn (J/g-atomÆK) Ag:Bi 0.102 0.200 0.300 0.400 0.500 0.600 0.699 0.800 0.900 = 2:1 0.090 0.213 0.336 0.417 0.540 0.658 0.718 0.832 0.927 –18,006 –11,583 –8,151 –6,540 –4,617 –3,132 –2,480 –1,380 –567 7,720 7,485 6,470 2,283 1,841 752 833 463 152 28.585 21.186 16.246 9.803 7.177 4.315 3.682 2.047 0.799 Ag:Bi 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 = 1:1 0.129 0.240 0.350 0.421 0.539 0.645 0.729 0.826 0.932 –15,337 –10,665 –7,850 –6,481 –4,628 –3,282 –2,367 –1,427 –528 14,351 14,119 10,141 1,873 2,426 1,968 392 318 250 32.987 27.537 19.990 9.282 7.839 5.834 3.066 1.939 0.865 Ag:Bi 0.100 0.201 0.301 0.400 0.500 0.600 0.701 0.801 0.900 = 1:2 0.152 0.214 0.323 0.465 0.564 0.670 0.727 0.828 0.929 –14,111 –11,523 –8,460 –5,733 –4,286 –2,996 –2,391 –1,411 –551 27,618 7,543 12,968 2,449 1,990 2,289 840 843 974 46.365 21.184 23.809 9.091 6.974 5.872 3.589 2.505 1.694 xSn 43 Table III. Integral Thermodynamic Quantities of the Ag-Bi-Sn System at 900 K xSn DGM (J/g-atom) Ag:Bi = 2:1 0.000 –4,520 0.100 –6,498 0.150 –7,029 0.200 –7,411 0.250 –7,654 0.300 –7,771 0.350 –7,776 0.400 –7,681 0.450 –7,496 0.500 –7,232 0.550 –6,899 0.600 –6,505 0.650 –6,055 0.700 –5,552 0.750 –4,994 0.800 –4,372 0.850 –3,661 0.900 –2,811 0.950 –1,682 Ag:Bi = 1:1 0.000 –4,329 0.100 –6,164 0.150 –6,592 0.200 –6,910 0.250 –7,123 0.300 –7,239 0.350 –7,264 0.400 –7,205 0.450 –7,069 0.500 –6,860 0.550 –6,582 0.600 –6,240 0.650 –5,835 0.700 –5,366 0.750 –4,831 0.800 –4,217 0.850 –3,507 0.900 –2,657 0.950 –1,562 Ag:Bi = 1:2 0.000 –3,560 0.100 –5,400 0.150 –5,846 0.200 –6,195 0.250 –6,450 0.300 –6,615 0.350 –6,693 0.400 –6,687 0.450 –6,602 0.500 –6,439 0.550 –6,201 0.600 –5,891 0.650 –5,508 0.700 –5,053 0.750 –4,524 0.800 –3,914 0.850 –3,215 0.900 –2,403 0.950 –1,429 DH M (J/g-atom) DSM (J/g-atomÆK) 799 1,204 1,590 1,916 2,183 2,392 2,543 2,637 2,675 2,657 2,585 2,461 2,285 2,061 1,791 1,480 1,134 766 405 6.278 8.558 9.577 10.363 10.929 11.293 11.466 11.464 11.301 10.987 10.538 9.962 9.267 8.459 7.539 6.502 5.328 3.975 2.319 1,754 2,472 3,154 3,716 4,158 4,482 4,692 4,791 4,780 4,665 4,449 4,140 3,743 3,269 2,729 2,141 1,533 953 528 7.246 9.595 10.829 11.806 12.534 13.023 13.285 13.329 13.165 12.805 12.257 11.533 10.642 9.595 8.399 7.065 5.600 4.011 2.322 2,100 3,496 4,535 5,205 5,586 5,750 5,754 5,647 5,466 5,235 4,971 4,679 4,353 3,982 3,544 3,017 2,376 1,618 819 6.889 9.885 11.535 12.667 13.374 13.739 13.830 13.705 13.408 12.971 12.414 11.744 10.957 10.039 8.964 7.702 6.212 4.468 2.498 44 Li, Knoot, and Mikula Fig. 2. 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