Enhancement of bonding strength in BiTe-based thermoelectric modules by electroless nickel, electroless palladium, and immersion gold surface modification

In this study, an electroless nickel (EN), electroless palladium (EP), and immersion gold (IG) (ENEPIG) multilayer coat was developed for application on BiTe-based thermoelectric materials to increase the bonding strength between BiTe-based materials and Cu electrodes in thermoelectric modules. The ENEPIG-plated thermoelectric module exhibited a bonding strength of over 11 MPa, which was approximately 40% higher than those of conventional EN-plated modules. According to the interfacial analysis of the bonding areas via a field emission electron probe micro analyzer (FE-EPMA), the Ni layers in ENEPIG and EN acted as diffusion barriers to inhibit the formation of brittle Sn-Te intermetallic compounds (IMCs). Sn-Cu-Pd and Sn-Cu IMCs were formed below the Ni-plated area at the bonding interface of the ENEPIG and EN-plated modules, respectively. However, Sn-Cu-Pd IMCs in the ENEPIG interface exhibited thicker and more homogeneous IMC layers after heat treatment compared to the Sn-Cu IMCs in the conventional EN interface. The wettability of the ENEPIG-plated BiTe surface was significantly greater than that of the EN-plated surface. This directly induced fewer and smaller pores on the bonding interface of the BiTe-based materials with ENEPIG plating compared to those with EN plating, thereby resulting in the higher bonding strength of ENEPIG-plated modules.