**5. Concluding remarks**

**4. Strategies for enhancing the electrical and thermo-mechanical** 

The electrical and thermo-mechanical properties of many different low melting point solders are affected by a variety of processing factors, such as the reflow temperature, time, flux used and its effectiveness, and temperature of measurement [15, 36, 37, 40, 41, 45]. Furthermore, the change of chemical composition of solders according to the addition of supplementary additives is also considered as a main factor that modifies or even improves a solder's electrical and thermomechanical properties [15, 36, 37, 40, 41, 45]. Three main methods are used to improve the electrical and thermomechanical performance of low melting point solders: (i) doping with a small amount of certain elements via diffusion reactions, (ii) alloying with a large amount of certain elements, and (iii) reinforcing with metal or ceramic elements. Sometimes, (iv) all of these methods are combined for the enhancement of the intended properties of low melting temperature

Transient liquid phase bonding was conducted using the Sn–Bi solder with 30 wt.% Cu particles added [37]. However, this process caused the melting point of the solder joints to increase from 139 to 201°C; In addition, the solder joints contained large voids, resulting in a consider-

On a Cu substrate, the conventional Sn–58Bi solder was alloyed with 0.7 wt.% Zn to improve the interfacial reaction, tensile strength, and creep resistance during liquid-state aging [37]. However, the overgrown IMC layers between the Sn–58Bi solders and Cu substrates signifi-

Lin et al. added minor amounts of Ga, ranging from 0.25 to 3.0 wt.% to Sn–58Bi solder [14]. As

Four different concentrations of Ni (0.05, 0.1, 0.5, and 1.0 wt.%) were individually added to the Sn–58Bi solder [29]. The optimal concentration of Ni necessary to enhance the tensile strength of the alloy was 0.1 wt.%, but the elongation of the alloy was inversely correlated to

Wojewoda-Budka and a coworker demonstrated excellent diffusion soldering process results for Bi–22 at.% In on Cu interconnections; this was proved by the presence

Graphene nanosheets were successfully incorporated at various percentages (0, 0.01, 0.03, 0.05, and 0.10 wt.%) into Sn–58Bi solder; the microstructure, tensile properties, wettability,

Sun et al. introduced a low melting temperature Sn–57.6Bi–0.4Ag solder reinforced with different concentrations of MWCNTs or Ni-coated MWCNTs [35]. With the addition of

corrosion resistance, hardness, and creep behavior were significantly improved [19].

phase present in the Cu/In–22Bi/Cu interfaces in the temperature range of

O3

nanoparticles to slow

**performance of low melting temperature solder materials**

solders.

32 Recent Progress in Soldering Materials

the Ni content [29].

of Cu11In9

85–200°C [47].

able degradation in shear strength [37].

cantly degraded the reliability of the electronic products [37].

a result, the growth of IMC layers was effectively suppressed [14].

Hu et al. fabricated an Sn–58Bi composite solder reinforced with Al2

down electromigration and to improve the shear strength and microhardness [39].

A recent trend in solder research mentioned that low melting temperature solder materials and their nanocomposite materials will be suitable for flexible interconnection applications in the near future. Thus, fabrications and/or syntheses, as well as elaboration of the electrical and thermomechanical properties, of various low melting temperature solder materials are discussed in detail. The various determination factors regarding the electrical and thermomechanical properties of solder materials are also elucidated with theoretical and experimental support. Subsequently, a promising approach to enhancing the performance of solder materials using supplementary additives, such as nanostructures, nanocomposites, alloying, and doping, is described with examples. It is possible to conclude that low melting temperature solders may enable significant advancement in interconnecting components in various applications and soldering technologies for the flexible microelectronic packaging industry.
