Thermal Properties and Wetting Behavior of High Temperature Zn-Al-In Solders
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Solders for ultrahigh-temperature applications were defined by Vianco as those able to sustain working conditions with temperatures as high as 573 K, with momentary temperature rise up to 623 K. Zn-Al eutectic alloy (12 at.% Al) fits such defined criteria with respect to its melting temperature. It was found that small additions of indium to Zn-Al eutectic lower its melting temperature. The aim of this work is to assess if and to what extent thermal properties and wetting behavior are affected. It was found that addition of In increases electrical resistivity and coefficient of thermal expansion value. Wetting angles on Cu and Al substrates of liquid Zn-Al eutectic-based alloys containing up to 1.5 at.% of In were studied with the sessile drop method, after wetting at 773 K in the presence of flux. A decrease of apparent wetting angle was observed with increasing concentration of In. After wetting tests solidified alloy-substrate couples were cross-sectioned and examined with scanning electron microscopy coupled with electron dispersive X-ray analysis.
Keywordsmicrostructure thermal properties wetting Zn-Al-In alloys
Worldwide environment-protective legislation and following restrictions on the use of hazardous substances made replacement of Pb-containing solders necessary. The RoHS directive banned the use of Pb in solder joints, and every few years expands to another group of materials, including materials for the supervision and control, and medical devices. Numerous lead-free solders studies have been conducted in recent years (Ref 1, 2, 3, 4, 5), and focused on the solders with a melting point below 473 K, which can replace low-melting eutectic Sn-37Pb (wt.%). As a Pb-free replacement, Sn-based alloys such as: Sn-Ag-Cu and Sn-Zn, which have properties similar to the Pb-Sn eutectic alloy, have been proposed. However, in the case of devices utilizing Pb-based solders with melting point above 523 K, the solder is used in various types of applications not only as die-attach solders but also for assembling optical components, automobile circuit boards, circuit modules for step soldering, aircraft, space satellite, automotive, oil and gas well exploration (Ref 6). Vianco defined the ultrahigh-temperature solders as those working properly between 573 and 623 K (Ref 7). Therefore, alloys of similar properties to solders containing Pb and Cd are being developed. The literature survey indicates that promising candidate are alloys based on Zn-Al eutectic (654 K) (Ref 8, 9), with additives that lower the melting temperature. Another group of alloys is the Bi-Ag, but due to poor mechanical properties this group is neglected (Ref 10); as well as Au-X (Sn, Si, Ge), which because of the high price are only applicable in specific cases (Ref 11). Zn-Al alloys with additions of Mg and Ga (Ref 12) have been proposed as a replacement of Pb solders, due to their thermal conductivity twice as big, and the coefficient of linear expansion smaller than Pb-5Sn (wt%). These properties are correlated with Zn which is a major component of the alloys; also the very low price of Zn is a great advantage of these alloys. Shimizu et al. (Ref 12) also studied the hardness which was one order of magnitude greater than Pb-5Sn, as well as wettability at 563-593 K, which turned out to be worse than the Pb-based alloys. Zn-Al-Ga ternary and Zn-Al-Mg-Ga quaternary alloys were also tested by Rettenmayr et al. (Ref 13), in terms of melting temperature, mechanical properties, and impact of Ga and Mg addition on microstructure. To the best of the present authors’ knowledge, there are no papers published on thermal properties and wetting behavior of Zn-Al-In alloys. Therefore, the aim of this work is to study temperature properties of alloys based on Zn-Al eutectic with small additions of indium and their wetting behavior on Cu and Al substrates. In the present study, melting temperatures, electrical resistivity, thermal expansion and density in the solid state, wetting on Cu and Al pads and microstructure of solder/substrate interface were investigated.
The coefficient of thermal expansion (CTE), density in the solid and molar volume at 293 K, and melting temperatures of Zn-Al-In alloys
CTE, 10−6 K−1
Melting range, K
The melting behavior of the solder alloys was investigated with differential scanning calorimetric (DSC) method. The measurements were carried out on samples of about 25 mg, with the use of DSC microcalorimeter, with heating and cooling rate of 10 K/min, under protective atmosphere of high-purity Ar (99.9999%). The temperature was measured with an accuracy of 1 K.
Electrical Resistivity Measurements
The measurements were started at room temperature and subsequently the temperature was raised every 10 K. The sample was kept for 150 min. at each temperature. At constant temperature standard deviation of measured electric resistance value was less than 1%.
Density and Thermal Expansion in Solid Measurements
The measurements of density in the solid state were carried out with the setup utilizing Archimedes method. Distilled water was used as a reference liquid, temperature of measurement was 296 K, and the samples were cylindrical of 5 mm in diameter and 20 mm high. The experimental error of measured value of density in solid did not exceed 0.3%. For thermal expansion measurements samples of the same dimensions were used. Thermal expansion measurements were done by TMA setup, under the following conditions of experiment: range of temperature: −50 to 150 K, heating rate: 2 K/min, gas flow: 20 mL/min, force exerted on sample: 0.1 mN, protective gas: helium.
Measurements of wetting angle were carried out in air at temperature 773 K using the equipment described in (Ref 17). Before the tests, cylindrical samples of 5 mm in diameter and 4 mm high, as well as “as-delivered” Cu (99.9%) and Al (99.5%) substrates of 20 × 15 × 0.2 mm were degreased with acetone. For each type of substrate different flux was used, i.e., for Cu substrate flux Al700® and for Al substrate flux AFP200®. The samples were kept in the furnace for 1 and 3 min. Because layers of fluxes covering samples were relatively thick it was not possible to measure time dependence of wetting angle. For this reason, the apparent wetting angles reported in this work were determined on solidified samples after cleaning the flux residue with distilled water. The experimental error of wetting angle measurement did not exceed 2°. After measurements of wetting angle, solidified drops were cut on wire saw in the middle of a drop, perpendicular to the plane of the interface, mounted in conductive resin and polished for microstructure characterization. Thin layer of carbon was sputtered on samples’ surface to protect it against oxidation and improve SEM image quality. EDS analysis was performed at 20 kV and working distance of 10 mm.
Results and Discussion
The results of DSC measurements for Zn-Al-In alloys are presented in Table 1. The study of melting of high-temperature solders shows that the range of applicability of solders is high (up to 623 K) and through addition of alloying elements a reduction of melting temperature can be obtained. Too big addition of indium to alloys causes huge differences between melting and solidification temperatures (Ref 12). Also indium is a very expensive metal, given both the facts we decided to study the effect of small addition of indium to Zn-Al eutectic. As shown in Table 1, increasing concentration of indium causes slight decrease of melting temperatures. In the case of Zn-12Al-1.5In alloy, liquidus temperature is 10 K lower than eutectic temperature of Zn-12Al alloy. Present results when compared with literature data are higher than those for Pb-5.5Sn alloys (Ref 18) or other alloys based on Zn-Al eutectic (Ref 12), but solidus-liquidus gap is smaller.
Density in Solid and CTE Results
The results of EDS analysis performed on Zn-12Al-0.5In/Al couple at the positions indicated in Fig. 9
The results of EDS analysis performed on Zn-12Al-1.0In/Al couple at the positions indicated in Fig. 10
The results of EDS analysis performed on Zn-12Al-1.5In/Al couple at the positions indicated in Fig. 11
Thermal properties and wetting behavior of Zn-Al-In alloys were studied. It was found that the addition of indium reduces the melting temperature of the Zn-Al solder. The results of the electrical resistivity measurements show that the addition of indium to Zn-Al eutectic results in higher resistivity of ternary alloys compared to Zn-Al eutectic. The studied alloys expand linearly with increasing temperature over the whole temperature range. CTE for Zn-12Al-0.5In alloy is lower compared to Zn-Al eutectic alloy, while CTE for Zn-12Al-1.0In and Zn-12Al-1.5In is higher. The increasing concentration of indium in alloys, which is present in small precipitates, reduces the apparent wetting angle of the Zn-Al-In solders on the Cu and Al substrates. On Cu substrate interlayers are formed on solder/substrate interface, and their composition matches the intermetallics from the Cu-Zn system. On the other hand, in the case of Zn-Al-In/Al couples dissolution of aluminum substrate is observed. The results of EDS analysis indicate that Cu diffuses from the substrate and is present in the drop in the form of small precipitates.
This work was financed under the framework of the project POIG.01.01.02-00-015/09, co-funded by the European Regional Development Fund (ERDF) and the Government of Poland under the Innovative Economy Program. The authors are grateful to Dr A. Sypien for her help with EDS analysis.
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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