In this study, electroless-plating of a nickel-phosphor (Ni–P) thin film on surface-controlled thermoelectric elements was developed to significantly increase the bonding strength between Bi–Te materials and copper (Cu) electrodes in thermoelectric modules. Without electroless Ni–P plating, the effect of surface roughness on the bonding strength was negligible. Brittle SnTe intermetallic compounds were formed at the bonding interface of the thermoelectric elements and defects such as pores were generated at the bonding interface owing to poor wettability with the solder. However, defects were not present at the bonding interface of the specimen subjected to electroless Ni–P plating, and the electroless Ni–P plating layer acted as a diffusion barrier toward Sn and Te. The bonding strength was higher when the specimen was subjected to Ni–P plating compared with that without Ni–P plating, and it improved with increasing surface roughness. As electroless Ni–P plating improved the wettability with molten solder, the increase in bonding strength was attributed to the formation of a thicker solder reaction layer below the bonding interface owing to an increase in the bonding interface with the solder at higher surface roughness.

In this study, we examined the thermal stability, bonding strength, and interfacial characteristic of a Ni–W–P coating applied for solder joint of Bi₂Te₃ thermoelectric modules. This layer had uniform thickness and was confirmed to be amorphous even after annealing at 200°C for 150 h, demonstrating its thermal stability. Moreover, the barrier layer hindered the interdiffusion between Sn and Te, eliminating the formation of brittle intermetallic at the solder joint. Thus, the bonding strengths of p- and n-type thermoelectric units were improved approximately 1.3- and 2.8-fold, respectively, compared to the bonding strengths obtained without a Ni–W–P layer. In addition, the bonding strength was not affected by heating at 200°C for 150 h. The reliability of bonding strength was caused by the excellent thermal stability, and solder wettability of the barrier layer, which resulted in a bonding interface without brittle intermetallic and defects.

In this study, a method for electroless Ni–P plating with excellent adhesion via chemical wet etching to fabricate Bi–Te thermoelectric modules is proposed. The electroless Ni–P plating formed through the proposed method showed excellent adherence without peeling, even under heat treatment of 200 °C for 24 h. Wet etching and electroless Ni–P plating was performed on a Bi–Te thermoelectric module, which showed the excellent bond strength of approximately 10 MPa. The surface roughness of the Bi–Te thermoelectric element was increased significantly by the wet etching process, which secured the adherence of the Ni–P plating by anchoring to this induced surface roughness.

In this study, electrolytic etching, anodic oxidation, and copper electroplating were applied to aluminum to produce a plate on which a copper circuit for a thermoelectric module was formed. An oxide film insulating layer was formed on the aluminum through anodic oxidation, and platinum was coated by sputtering to produce conductivity. Finally, copper electroplating was performed directly on the substrate. In this structure, the copper plating layer on the insulating layer served as a conductive layer in the circuit. The adhesion of the copper plating layer was improved by electrolytic etching. As a result, the thermoelectric module fabricated in this study showed excellent adhesion and good insulation characteristics. It is expected that our findings can contribute to the manufacture of plates applicable to thermoelectric modules with high dissipation performance.

This study investigates a brazing method for manufacturing PbTe thermoelectric modules using a Ag-based filler metal with a melting point of about 650 °C. To improve the bonding strength between the Ag-based brazing layer and the PbTe thermoelectric module, an electroless Ni-P plating layer is formed on the surface of the thermoelectric module as a diffusion barrier layer. The bonding strength of the PbTe thermoelectric module manufactured by the electroless Ni-P plating and Ag-based brazing has a high value of approximately 8.3 MPa. No defects such as pores or cracks were observed at the bonding interface between the thermoelectric element and the brazing layer. Furthermore, because of the high bonding strength of the manufactured thermoelectric module, fractures occur inside the thermoelectric element rather than at the bonding interface. Accordingly, the electroless Ni-P plating and Ag-based brazing method proposed in this study is found to be effective in manufacturing PbTe-based thermoelectric modules with high bonding strength.

A method for directly bonding thermoelectric elements onto copper electrodes without applying a solder paste was developed in this study. A tin coating of thickness approximately 50 μm was deposited via electroplating onto the surface of a Bi₂Te₃-based thermoelectric element, which had a nickel diffusion barrier layer. The resulting structure was subsequently subjected to direct thermocompression bonding at 250 °C on a hotplate for 3 min at a pressure of 1.1 kPa. Scanning electron microscopy imaging confirmed that a strong and uniform bond was formed at the copper electrode-thermoelectric element interface, and the melted or solidified tin layer remained defect-free. The thermoelectric module fabricated using tin plating had an average bonding strength similar to that fabricated using soldering.