Synthesis and thermal properties of low melting temperature tin/indium (Sn/In) lead-free nanosolders and their melting behavior in a vapor flux Yang Shu, Karunaharan Rajathurai, Fan Gao, Qingzhou Cui, Zhiyong Gu ⇑ Department of Chemical Engineering, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, USA article info Article history: Received 2 October 2013 Received in revised form 27 November 2014 Accepted 28 November 2014 Available online 5 December 2014 Keywords: Lead-free solders Nanosolders, tin/indium Low melting temperature Reflow Flux abstract Low melting temperature tin/indium (Sn/In) nanosolders with a wide composition range were synthe- sized by a surfactant-assisted chemical reduction method in an aqueous solution at ambient conditions. The parameters that affect the synthesis process were studied to control the size, shape and composition of the Sn/In nanosolders. It was found that hydrolysis of the metal ions significantly affected the nano- solder formation and inhibition of hydrolysis resulted in better nanosolder quality. Under optimized con- ditions, the Sn/In nanosolders around 50/50 (wt%) composition had a size range of 30–90 nm. SEM, TEM, EDS and XRD were used to determine the morphology, composition, and crystal structure of the nanosol- ders. It was found that the Sn/In nanosolders were mainly composed of InSn 4 at low In content (30 wt% In). When the In content further decreased to 20%, a mixture of InSn 4 and b-Sn exist; however, when the In content increased to 40% or higher, pure In appeared in the nanosolders, and the nanosolders have an increasing amount of In nanocrystals with the increase of the In ratio. DSC measurements have been con- ducted to understand the melting behavior of the nanosolders, and low melting temperatures were achieved at a wide range of In compositions (from 30% to 70%). The lowest melting temperature was found to be 115.5 °C, indicating a new eutectic point around 30% In. Finally, the temperature and time effects on the melting behavior and solder reflow property of the nanosolders have been investigated in a vapor flux environment. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Soldering techniques have been widely used in microelectronics assembly, packaging, and MEMS integration [1,2]. The most widely used solder is eutectic tin–lead (Sn/Pb, 63/37) with a melting point of 183 °C. However, due to the toxicity of lead, Sn/Pb solders are being phased out and lead-free solders are more widely adopted for various applications [3–6]. Up to date, tin/silver/copper (Sn/ Ag/Cu) probably is the most widely used lead-free solder material. The Sn/Ag/Cu (SAC) system has a melting point around 217 °C or higher, depending on its respective compositions, and is mainly used in surface-mount technology (SMT) [7,8]. Due to the well- known melting temperature depression phenomenon of nanopar- ticles [9,10] and the potential to be used in the smaller feature sized assembly and packaging, nanosolders started to attract more and more attention. Up to now, several lead-free nanosolders have been proposed and utilized [11]. The most popular one is tin/silver/ copper (Sn/Ag/Cu) nanoparticles, which can be synthesized through a chemical reduction method [12–14] or electrodeposition method [15]. Some other lead-free nanosolder systems include Sn nanosolder particles [16], Sn/Ag alloy nanosolder particles [17,9,18,19] or nanowires [20,21], and Sn–Cu–Bi nanosolder parti- cles [22], etc. In addition to the requirement from the material point of view (elimination of the usage of Pb), in recent years, energy cost and demand have been increasingly high. In this sense, pursuit of less energy consuming or more energy efficient processes is desired. Furthermore, many electronics manufacturing processes require low temperature processing for electronics assembly and packag- ing, for example, flexible electronics where polymer based sub- strate exists, or electronic devices with thermal sensitive components (such as LEDs). However, up to now, commonly used lead-free alternatives have a relative high melting temperature (220 °C or above). They have to be processed (reflowed) at even higher temperatures (another 20–30 °C) [23,24]. Even with certain degree of melting point depression, the melting points of most lead- free nanosolder particles are still relatively high. Thus, to meet the requirement for energy saving as well as low temperature process- ing, low melting point lead-free solder materials are necessary for low temperature electronic assembly and joining of certain thermal-sensitive products. http://dx.doi.org/10.1016/j.jallcom.2014.11.173 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +1 978 934 3540; fax: +1 978 934 3047. E-mail address: Zhiyong_Gu@uml.edu (Z. Gu). Journal of Alloys and Compounds 626 (2015) 391–400 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom