**2. Syntheses and/or fabrications of low melting temperature solders**

In this section, we show how low melting temperature solders, consisting of various elements with higher melting temperatures, such as Sn (melting point of 231.9°C), In (melting point of 156.6°C), and Bi (melting point of 271.5°C), can be fabricated and/or synthesized using metallurgy principles or nanotechnology theory [8]. For example, the formation of an eutectic alloy with 42 wt.% Sn and 58 wt.% Bi can lead to a melting point decrease to 138°C due to the shift to the eutectic temperature [3, 4]. Due to the melting point drop according to the size decrease, the size reduction effect can also be used to synthesize a low melting point solder [4].

Low melting temperature Sn–In solder nanoparticles were successfully synthesized through a surfactant-assisted one-step chemical reduction method [9]. Different synthesis parameters, including pH, stirring speed, and surfactant concentration, were used to control the size, shape, and uniformity of the Sn–In solder nanoparticles [9]. At low In composition (20 wt.% In), the Sn–In solder nanoparticles were composed of InSn4 alloy and β-Sn phase [9]. When the In content increased to 30 wt.%, the Sn–In solder nanoparticles were composed mainly of InSn4 , with a melting temperature of 115.5°C [9]. This low melting temperature indicates a new eutectic point for the Sn–In solder nanoparticle system, which is lower than that of the bulk alloy system (around 50 wt.% In) [9]. At higher In compositions, the Sn–In solder nanoparticles are composed of both InSn4 and In phase [9].

**1. Introduction**

8 Recent Progress in Soldering Materials

More advanced solder bumps with low melting temperatures are crucial for use in flexible, bendable, and stretchable interconnection technology [1, 2]. In particular, the use of wearable devices requires the development of novel solders that can be reflowed at low temperature to avoid thermal damage to the usually temperature-sensitive components in these flexible devices, such as organic light-emitting diodes (OLEDs), polymer light-emitting diodes (PLEDs), and so on [3–6]. Thus, it is worthwhile to design a low melting temperature solder for more advanced interconnection technology and thus to impart more reliability to the solder bumps between future organic- or polymer-based microchips and flexible substrates. Furthermore, the continuous pursuit of multifunctionality in microelectronics has caused a significant increase in the number of solder bumps, to the level of 10,000 per chip [7]. It also has decreased the bump size to as small as 20 μm [7]. Thus, solder bumps with enhanced electrical and thermomechanical properties are needed to meet these demands. With these uses in mind, the properties of conventional solder materials with high melting temperatures (180–230°C) have been under scrutiny due to their reference characteristics, and whether implementing currently used soldering methods or inventing new ones, solution strategies to

overcome problems associated with novel solder materials have been implemented.

**2. Syntheses and/or fabrications of low melting temperature solders**

the size reduction effect can also be used to synthesize a low melting point solder [4].

nanoparticles were composed of InSn4

Low melting temperature Sn–In solder nanoparticles were successfully synthesized through a surfactant-assisted one-step chemical reduction method [9]. Different synthesis parameters, including pH, stirring speed, and surfactant concentration, were used to control the size, shape, and uniformity of the Sn–In solder nanoparticles [9]. At low In composition (20 wt.% In), the Sn–In solder

alloy and β-Sn phase [9]. When the In content increased to

In this section, we show how low melting temperature solders, consisting of various elements with higher melting temperatures, such as Sn (melting point of 231.9°C), In (melting point of 156.6°C), and Bi (melting point of 271.5°C), can be fabricated and/or synthesized using metallurgy principles or nanotechnology theory [8]. For example, the formation of an eutectic alloy with 42 wt.% Sn and 58 wt.% Bi can lead to a melting point decrease to 138°C due to the shift to the eutectic temperature [3, 4]. Due to the melting point drop according to the size decrease,

In this chapter, we focus on the electrical and thermo-mechanical properties of novel solder materials with a specific range of low melting temperatures (<150°C). Emerging carbon reinforcement materials, such as carbon nanotubes (CNTs), graphenes, and their nanocomposites, are also briefly discussed and linked to the increasing development of composite solder materials (in addition to their low melting temperatures). In particular, strategies for improving the performance of solder materials are proposed, along with the provision of insight into classic metallurgy principles. To engineer the properties of low melting temperature solder materials in intended directions, new approaches using nanostructures, nanocomposites, alloying, and doping are also suggested.

To increase the compatibility and usefulness of the low melting point solder, eutectic Bi–43Sn nanowires with diameters of 20, 70, and 220 nm were fabricated by a hydraulic pressure injection process using anodic aluminum oxide templates [10]. Their microstructure investigation showed that the fabricated nanowires are composed of alternating Bi and Sn segments along the wire axes [10].

Novel Sn–Bi nanocomposites reinforced with reduced graphene oxide nanosheets (RGONs) were successfully fabricated by a simple, scalable, and economical electrochemical deposition method [11]. The Sn–Bi nanocomposites, reinforced with reduced graphene oxide nanosheets, had fine grain size as well as reduced graphene oxide nanosheets dispersed throughout the Sn–Bi matrix [11].

For the microstructural transformation and thermoelectrical improvement of Sn–Bi solder, MWCNT was reinforced using the electrochemical codeposition method [11]. Electron microscopy analysis can confirm that pristine MWCNTs were trapped in the deposited composites [11].

Bi-based solder powders with three chemical compositions (binary Bi–Sn, ternary Bi–Sn–In, and quaternary Bi–Sn–In–Ga alloy systems) were fabricated using a gas atomization technique; subsequently, the powders were further ball-milled to fabricate smaller-sized particulates; in particular, the diameter of those in the Bi–Sn–In–Ga powders became less than 10 μm with an irregular shape due to the nature of intrinsic brittleness, the degree of surface oxidation, and the formation of Ga intermetallic compound (IMC), all of which produced fractures of the Ga-containing powders [3].

Ternary Bi–Sn–In micropowders and nanoparticles were prepared as a composite solder material via gas atomization process and a chemical reduction method, respectively [3, 4]. To be specific, two types of Bi-based micropowders, composed of binary Bi–Sn and ternary Bi–Sn–In, were fabricated using a gas atomizer [4]. Then, the gas-atomized powders were classified using a series of standard sieves to obtain powders of a specific size range [4]. Bi (III) nitrate pentahydrate, Sn (II) 2-ethylhexanoate, In (III) nitrate hydrate at a Bi/Sn/In weight ratio of 43.5/31.5/25.0, and 1,10-phenanthroline were added to methanol, and the solution was stirred for 2 h [4]. Then, sodium borohydride was added, and the reaction continued at 50°C for 1 h [4]. The as-synthesized nanoparticles were centrifuged at 4000 rpm for 15 min, washed with methanol, and then dried in a vacuum oven for 24 h [4].
