**4. Strategies for enhancing the electrical and thermo-mechanical performance of low melting temperature solder materials**

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 solders.

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 considerable degradation in shear strength [37].

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

Lin et al. added minor amounts of Ga, ranging from 0.25 to 3.0 wt.% to Sn–58Bi solder [14]. As a result, the growth of IMC layers was effectively suppressed [14].

Hu et al. fabricated an Sn–58Bi composite solder reinforced with Al2 O3 nanoparticles to slow down electromigration and to improve the shear strength and microhardness [39].

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 the Ni content [29].

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 of Cu11In9 phase present in the Cu/In–22Bi/Cu interfaces in the temperature range of 85–200°C [47].

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

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 MWCNTs and Ni–MWCNTs, the fractured surface of the solder joints became rougher, leading to a better bonding structure [35]. Though both MWCNTs and Ni–MWCNTs have the capability to improve the mechanical performance of solder joints, Ni–MWCNTs worked much better [35]. The Ni coating was proved to significantly inhibit the aggregation of MWCNTs, which can solve cracks and wetting problems and even improve the strength of solder joints [35].
