Thermomechanical stability of Bi₂Te₃-based thermoelectric modules employing variant diffusion barriers

An electroless nickel (EN) layer and EN, along with electroless palladium (EP), and immersion gold (IG) (ENEPIG) multilayers were two variant diffusion barriers employed for Bi₂Te₃-based thermoelectric materials. After the electroless plating process, coated Bi₂Te₃-based species were bonded with Cu electrodes using a soldering method with Sn-based alloy as a solder. Heat treatment was conducted at 200 °C for 200, 500, and 1000 h to investigate the thermomechanical stability of the Bi₂Te₃/solder interface adopting different coatings. First, the Bi₂Te₃/solder interface characteristics were analyzed by compositional identification and morphology observation. Intermetallic compounds (IMCs) were generated below the plating area in both EN and ENEPIG. The phase composition of these IMCs was identified by qualitative analysis. Additionally, the thickness of IMCs with ENEPIG was larger than those with EN regardless of annealing duration. The IMC morphology without and with 200 h of heating showed columnar grains, while those with heating over 500 h exhibited equiaxed grains. The ENEPIG coating generally yielded a higher bonding strength for the Bi₂Te₃/solder interface than the EN layer. Meanwhile, the bonding strength for EN and ENEPIG was constant after 200 h of heating, followed by a significant drop at 500 h, and it finally leveled off after heating for 1000 h. Furthermore, the fracture caused by the shear test was characterized by the brittle fracture mode. Ultimately, the IMC morphology was found to influence the bonding strength along with the increase in annealing duration rather than IMC thickness and composition.