Microstructural studies of ultrarapidly quenched foils of zinc-doped indium–tin eutectic alloys

J Mater Sci METALS Metals Microstructural studies of ultrarapidly quenched foils of zinc-doped indium–tin eutectic alloys Vasiliy G. Shepelevich1, Olga V. Gusakova2, Elena L. Koukharenko3,* , and Sofia V. Husakova1 1 Department of Solid State Physics, Belarusian State University, Nezavisimosti Ave, 4, 220030 Minsk, Belarus Department of Nuclear and Radiation Safety, International Sakharov Environmental Institute of Belarusian State University, Dolgobrodskaya St., 23/1, 220070 Minsk, Belarus 3 Electronics and Computer Science, University of Southampton, Highfield, Southampton SO17 1BJ, UK 2 Received: 16 June 2018 ABSTRACT Accepted: 24 September 2018 Alloys with the composition of Sn (46.5 at.%)–In (50.7 at.%)–Zn (2.8 at.%) were fabricated by using the ultrarapid quenching process, with quenching rate up to 105 K/s. These materials were obtained in the shape of foils with a thickness varying from 30 to 70 lm. Their phase composition, microstructure, grain structure, and texture have been analyzed and revealed that these alloys consist of solid solutions based on the b phase (In3Sn) and c phase (InSn4) with inclusions of zinc. Aging processes in the foils revealed that the volume fraction of zinc (VZn) increases with the increase in the samples exposure time to the air at room temperature. The electron backscatter diffraction analysis has shown that these foils have a microcrystalline structure. The mechanism of the texture formation in these materials has been explained.  The Author(s) 2018 Introduction The most popular soldering materials used in electronic packaging are the eutectic Sn–37Pb and near eutectic Sn–Pb. However, recent environmental concerns and the increasing awareness of health risk associated with lead-containing solder alloys have pushed the electronics industry toward lead-free compositions, leading to environmental concerns over the amount of lead ending up in landfill [1, 2]. In Europe, the waste electrical and electronic equipment (WEEE) directive by EU has banned the use of Pb in consumer goods, while the restriction of hazardous substances (ROHS) compliance has Address correspondence to E-mail: https://doi.org/10.1007/s10853-018-2964-2 claimed that Pb is the most common material that must be eliminated [3]. Hence, in recent years, there has been a significant amount of efforts dedicated by the research community and its related industrial users to investigate Pb-free eco-friendly solders alternatives suitable for a wide range of applications [4, 5]. To this end, new types of solders are being currently developed by using complex multicomponent alloys, time-consuming, costly or sophisticated fabrication techniques requiring a careful approach to melting, using noble metals or rare earths (Au, Ag, RE). Several groups focused their research on Sn–Ag-based alloys. Chen et al. [6] investigated the SnAgCu–RE alloy J Mater Sci manufactured by multiple steps of melting and using one of the rarest elements. Li et al. developed Sn–Bi– In alloys by multiple re-melting steps [7] as it has been difficult to obtain homogeneous mixture required for good quality solders. Hindler et al. [8] investigated the thermodynamics of Au-based alloys, including Au–Sb–Sn and Au–Sb, by using conventional quenching in iced water, a time-consuming process. In general, while there are many research publications about high-temperature solders, like Sn–Ag– Cu, there is very limited amount of research reported in literature that is dedicated to solders with melting temperature around 110 C [6]. Very recently, Maruya et al. [9] investigated samples combining Sn–Bi–Ag alloys and gold fabricated by electroplating that are expected to be a good candidate for low-temperature lead-free soldering. However, this approach has shown several drawbacks, such as being time-consuming and requiring sophisticated processing, as it involves multiple fabrication steps at different temperatures ramping. Besides, current low melting temperature solders face several key challenges, which include more intermetallic compound formation (thus becoming more brittle), high cost associated with the addition of supplementary elements to decrease the melting point temperature, and an increase in the possibility of thermal or popcorn cracking (reliability problems) [9]. Thus, the pursuit of more advanced low melting temperature solders for interconnections is timely. The motivation of this research is to develop highperformance and environmentally friendly solders, fabricated by easy and low-cost fabrication processes. The focus of this research is Zn-doped indium–tin eutectic alloys. The fabrication of solders with low melting point of about 120 C is possible by using the eutectic system of indium–tin [10]. In such low melting point solders, plastic deformation is possible at operating temperatures that occurs according to the mechanism of the grain-boundary slip, which is manifested in creep processes [11]. Therefore, in order to reduce the effect of grain-boundary slippage, various components are added to the eutectic, such as zinc, bismuth and antimony [12]. Since zinc has a limited solubility in tin and indium, its inclusions can be formed in the alloy at the boundaries of the phases forming the eutectic, which effectively prevent grain-boundary slippage. At low and medium cooling rates of the melt, dendritic, cellular and coarse-grained structures are formed, which adversely affects the alloys mechanical and physical properties, as well as their processing parameters. For example, the inhomogeneity in the phase distribution in solders and the formation of large inclusions of the third component influence the melting temperature of the alloy [13]. At the ultrarapid liquid quenching process, the cooling rate of the liquid phase can be higher than 105 K/s. The melt reaches a deep supercooling state before the beginning of solidification, and the solidification occurs with a high speed of movement at the interface between the melt and solid phases. This leads to the formation of a structure which can not be obtained by using traditional fabrication technologies or by employing heat treatment. The use of ultrarapid cooling rate of the liquid phase (more than 105 K/s) leads to the formation of a fine-grained structure, the formation of supersaturated solid solutions and also amorphization [14–16]. In addition, high-speed solidification is one of the energy-saving technologies that allows to lower the cost of solders [14]. The ultrarapid quenching enables obtaining solder in the shape of foils, which is convenient to use for certain types of soldering, for example, in automated processes. The foils obtained by the ultrarapid quenching are expected to have a more homogeneous structure with dispersed precipitates of phases and grains, which increases the quality of the material [17]. There is a very limited amount of studies available on solders obtained by the ultrarapid liquid quenching, and (...truncated)