**3. Conclusion**

*Lead Free Solders*

**Figure 4.**

presence of the voids.

**2. Electrical conductivity**

transport across the joints.

In order to control the IMC thickness across the solder joint interface, many researchers have also tried to add the fourth element, which would get mixed up with the solder alloy and would restrict the diffusion of Cu from substrate toward solder and thereby restricting tin to get diffused from solder toward substrate. But, an optimized growth rate of IMC formation has not been achieved yet. Ni-doped SAC solder alloy has been studied by Benabou et al. Ni is considered to be one of the best elements in restricting the diffusion of copper across the interface [26]. Study shows that though Ni-doped solder alloy produces joints with thinner IMC layer, it could not be able to restrict the formation of voids in Cu3Sn layer with aging time and it produces cavities in (Cu, Ni)6Sn5 phase, which would be the point of initiation of failure of solder joint. Also, it increases the electrical resistivity of the solder joint system due to the discontinuity in the electrical pathways because of the

*(a) Plot of total IMC thickness of Cu-SAC-Cu solder joint with variation of aging time in hours. (b) Derivative plot of total IMC thickness of Cu-SAC-Cu solder joint with variation of aging time in hours.*

Kang et al. have reported SAC solder alloy with surface finish of Ni(P)/Au, which produces Ni3Sn4 IMC layer, which grows faster than Cu-Sn IMC (Cu6Sn5 and Cu3Sn phases) on bare Cu and remains detached from the substrate. This detachment of the Ni3Sn4 phase from the substrate decreases the shear strength as well as increases the electrical resistivity of the solder joint due to the discontinuity in the electrical pathways [27]. However, our proposed multilayer structure in the Cu-Sn/ SAC/Sn-Cu solder joint shows neither void at the Cu3Sn phase nor any cavities at Cu6Sn5 phase; moreover, a controlled rate of IMC formation has been observed. This may be attributed due to the formation of solid solution of β-Sn phase in the interface, which restricts the Cu diffusion from substrate to solder. On the other hand, a significant improvement in the electrical properties has been observed for the solder joints produced using this TLP-like soldering and multilayered structure.

The electrical resistivity across the solder joints has been investigated by four probe methods. For the Cu-Sn/SAC/Sn-Cu solder joint it was found 1.4\*10<sup>−</sup><sup>5</sup> Ω-mm for as soldered samples and 2.66\*10<sup>−</sup><sup>5</sup> Ω-mm after 1200 h of thermal aging, whereas the electrical resistivity of the conventional Cu-SAC-Cu solder joint has been found to be 4.9\*10<sup>−</sup><sup>5</sup> Ω-mm at 1200 h of thermal aging, which is almost twice that of Cu-Sn/SAC/Sn-Cu solder joint aged up to 1200 h. Thinner interfacial IMC layers cause lower associated localized Joule heating, resulting in enhanced electronic

**4**

This introductory chapter discusses the overview of the lead-free solders, its importance, and necessity in the cutting-edge research carried out across the globe for making environment friendly electronic devices. For any solder joints, the interfacial characteristics like mechanical and functional properties along with the microstructural morphology play the mostly vital role. Therefore, a systematic and detailed knowledge of the interfacial products and their structures are of fundamental interest for understanding the behavior of the solder joints. The concept of transient liquid phase-like soldering technique has been discussed, and the effect of thin multilayered film of Sn has been reported toward obtaining a more reliable lead-free solder joint.

We have demonstrated that incorporation of thin multilayer of Sn film along with SAC305 paste could produce good solder joints at 230°C. Cu pads have been joined by TLP-like soldering. For Cu-Sn/SAC/Sn-Cu solder joints, homogeneous interfacial microstructures have been formed across the solder joint interface as compared to scallop like IMC in the conventional Cu-SAC-Cu solder joint. Incorporation of thin multilayers of Sn film is capable of reducing the IMC growth rate up to 10% compared to the conventional Cu-SAC-Cu solder joint. Thinner interfacial IMC layers cause lower associated localized Joule heating, resulting in enhanced electronic transport across the joint, thereby reducing the electrical resistivity and increasing the conductivity across the solder joint interface.
