Sn-based lead-free solders such as Sn-Ag-Cu, Sn-Cu, and Sn-Bi have been used extensively for a long time in the electronic packaging field. Recently, low-temperature Sn-Bi solder alloys attract much attention from industries for flexible printed circuit board (FPCB) applications. Low melting temperatures of Sn-Bi solders avoid warpage wherein printed circuit board and electronic parts deform or deviate from the initial state due to their thermal mismatch during soldering. However, the addition of alloying elements and nanoparticles Sn-Bi solders improves the melting temperature, wettability, microstructure, and mechanical properties. Improving the brittleness of the eutectic Sn-58wt%Bi solder alloy by grain refinement of the Bi-phase becomes a hot topic. In this paper, literature studies about melting temperature, microstructure, inter-metallic thickness, and mechanical properties of Sn-Bi solder alloys upon alloying and nanoparticle addition are reviewed.

Transient liquid phase (TLP) bonding is a novel bonding process for the joining of metallic and ceramic materials using an interlayer. TLP bonding is particularly crucial for the joining of the semiconductor chips with expensive die-attached materials during low-temperature sintering. Moreover, the transient TLP bonding occurs at a lower temperature, is cost-effective, and causes less joint porosity. Wire bonding is also a common process to interconnect between the power module package to direct bonded copper (DBC). In this context, we propose to review the challenges and advances in TLP and ultrasonic wire bonding technology using Sn-based solders for power electronics packaging.

Nanocomposite Sn-Bi solders received noticeable attention for flexible electronics due to their improved mechanical properties. The main limitation is the dispersion of nanoparticles in the solder alloy. Accordingly, in this work, varying additions of ZnO nanoparticles were successfully dispersed into Sn57Bi solder via the liquid-state ultrasonic treatment. Nanocomposite solders were prepared using the melting and casting route. The solder alloys were then characterized for microstructure, spreading and mechanical properties. With increasing ZnO addition, the microstructure revealed significant refinement of Bi-and Snrich phases. Consequently, the eutectic lamellar spacing also decreases. The spreading improved up to 0.1 wt.% ZnO addition. For higher additions, nanocomposite solders experienced deterioration in spreading characteristics. The tensile strength of the solder increases with an increase in the amount of ZnO nanoparticles. High ductility is achieved for nanocomposite solder containing 0.05 wt.% ZnO. An attempt was made, to explain the effect of increasing ZnO nanoparticle addition on microstructural, spreading, and mechanical properties of Sn57Bi solder.

Joining technology of silicon semiconductors devices to direct bond copper (DBC) substrates in high-temperature power electronics packages is of utmost importance today. In this study, Sn–Cu solder was prepared by electroplating on a direct bonded copper (DBC) substrate. The electroplated DBC system thus prepared was TLP bonded with Si chip at 250 °C for 10 min under a vacuum atmosphere. The effect of electrical charge used for plating Sn–Cu solder, void fraction in the joint, Sn–Cu solder composition on the joining characteristics, and shear strength of the Si-DBC system were analyzed. The experimental results showed that the plating thickness increased almost linearly with plating time and electrical charge. A sound Sn–Cu solder plating thickness was obtained at 40 mA cm−2, 11 C cm−2, 20 min with 20 at% Cu in the deposit. Furthermore, the plated Sn–Cu solder layer transformed to Cu6Sn5 and Cu3Sn after joining at 250 °C for 10 min. The shear bonding strength of the Si/DBC joint increased with Cu content in the Sn–Cu solder until 20 at% in the Sn–Cu interlayer.

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