**1. Introduction**

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.

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.
