Partial and integral enthalpies of mixing of Ag–Ga–Sn liquid alloys

Abstract

A Calvet type calorimeter was used for measurement of partial and integral enthalpies of mixing of Ag–Ga–Sn alloys. The Ag–Ga binary alloys have been studied with 0 < x_Ag < 0.79 composition and in the 803–1073 K temperature range. The mixing enthalpies at 803 K of Ga–Sn binary alloys have been determined for x_Sn < 0.35. The Ag–Ga–Sn ternary liquid alloys have been investigated at 803 K along the following sections: x_Ag/x_Ga = 1/3, x_Ag/x_Ga = 0.36/0.64, x_Ag/x_Ga = 1/1, x_Ga/x_Sn = 1/3, x_Ga/x_Sn = 1/1, x_Ga/x_Sn = 0.65/0.35 and x_Ga/x_Sn = 3/1.

Experimental data were used to obtain the binary interaction parameters by using the Redlich–Kister polynomial for the Ag–Ga binary system, while the Redlich–Kister–Muggianu method was used to determine the ternary interaction parameters for the Ag–Ga–Sn system. The experimental values and the fitted curves were compared with those predicted from the Muggianu and Toop methods.

Introduction

Pb–Sn eutectic solder has long been widely used. However, Pb has been listed as one of the most toxic elements, and the health hazards of Pb have increased with the development of the microelectronic industry. Because of environmental and legal issues, Pb must now be eliminated from printed circuit boards [1]. Enormous efforts have been made in the search for lead-free solders, and the alloys based on the Ag–Sn system are considered to be a promising replacement for Pb–Sn alloys. The eutectic point of the Ag–Sn binary system is 221 °C, still higher than 183 °C, the eutectic of Pb–Sn alloys. Moreover, the use of higher melting-temperature solders may cause damage to the other components. Ga is used in the formation of low-melting point alloys. In fact, only a small addition of Ga to alloys can significantly decrease the melting points. Meanwhile, the addition of Ga to lead free-solders may also improve the mechanical properties [2], [3] as well as the wetting ability on Cu substrate [4]. Knowledge of thermodynamic properties is thought to be essential for understanding the solidification behaviour as well as the final properties. New experimental data are needed in order to obtain the necessary knowledge of the phase diagram and the melting behaviour of the Ag–Ga–Sn alloys.

The research activity of this work, which regards high-temperature calorimetric measurements of liquid Ag–Ga–Sn alloys using a Calvet-type calorimeter, has been integrated in the European COST Action MP0602 “Advanced Solder Materials for High Temperature Application – HISOLD”.

The mixing enthalpies of Ag–Ga liquid alloys have been measured by Béja [5], [6], Itagaki and Yazawa [7] and Predel and Stein [8] at 773 K, 980 K, 1243 K and 1323 K, respectively, over almost the entire composition range. The data published up to 1973 have been assessed by Hultgren et al. [9]. All the measured mixing enthalpies of the liquid alloys show a “V-shape”, with the minimum value at about x_Ag ≈ 0.75. Near the Ga-rich side, the mixing enthalpies of the liquid phase show slightly positive values.

In the previous assessments of the Ag–Ga system [10], [11], the values by Itagaki and Yazawa [7] were considered to be not exothermic enough and therefore more weight was given to the results by Predel [8]. The results by Béja [5], [6] were the only data available around 800 K and consequently, all the assessments were based on these. Meanwhile, the association model has been used to describe the temperature dependence of the mixing enthalpies. In order to insert this system into the COST Action MP0602 database [12], which is now being developed, Watson has recently changed the description of the Gibbs energy function by using the Redlich–Kister polynomial [13].

Kleppa [14] measured the mixing enthalpy of Ag–Sn liquid alloys at 723 K for 0.64 < x_Sn < 0.99, using liquid tin calorimetry. Direct reaction calorimetry was employed by Wittig and Gehring [15] for 0.072 < x_Sn < 0.931 at 1248 K, by Castanet et al. [16] at 1280 K for 0.05 < x_Sn < 0.95, and by Rakotomavo et al. [17] at 1373 K for 0.02 < x_Sn < 0.934. Hultgren et al. [9] made a compilation of selected values based on work prior to 1973. Recently, Flandorfer et al. [18] has carried out calorimetric measurements at five temperatures in the range of 773–1523 K mainly in the Sn-rich region and at 1523 K in the Ag-rich region.

Cohen et al. [19] determined the enthalpy of solution of Ga in liquid Sn. Predel [20], Pool and Lundin [21] performed tin solution calorimetry measurements. Bros and Laffite [22], using microcalorimetry, determined the enthalpy of formation of the liquid alloys at 400–750 K in the whole concentration range. All data published up to 1973 have been compiled by Hultgren [9]. The mixing enthalpies of Ga–Sn alloys have been determined by Muggianu et al. [23] using a Calvet type microcalorimeter. Živković et al. [24] measured the mixing enthalpy of Ga–Sn liquid alloys in the 350–650 K temperature range by using an Oelsen calorimeter.

Weibke and Hesse [25] studied 22 ternary alloys and six Ag–Sn alloys by thermal, metallographic and X-ray analysis, and reported the isothermal section at 273 K of the Ag–Ga–Sn system and the vertical sections at 30 at% Ag and 30 at%. Sn. The results published by Weibke and Hesse [25] were included in the compendium of ternary alloys [26] with a change in the slope of the phase boundaries. Song et al. [27] studied the microstructure of Sn-3.5 mass% Ag–3 mass% Ga cast alloy and its thermal behaviour by DSC and cooling curve method.