Low-Temperature Bonding of Bi_0.5Sb_1.5Te₃ Thermoelectric Material with Cu Electrodes Using a Thin-Film In Interlayer

A Bi_0.5Sb_1.5Te₃ thermoelectric material electroplated with a Ni barrier layer and a Ag reaction layer was bonded with a Ag-coated Cu electrode at low temperatures of 448 K (175 °C) to 523 K (250 °C) using a 4-μm-thick In interlayer under an external pressure of 3 MPa. During the bonding process, the In thin film reacted with the Ag layer to form a double layer of Ag₃In and Ag₂In intermetallic compounds. No reaction occurred at the Bi_0.5Sb_1.5Te₃/Ni interface, which resulted in low bonding strengths of about 3.2 MPa. The adhesion of the Bi_0.5Sb_1.5Te₃/Ni interface was improved by precoating a 1-μm Sn film on the surface of the thermoelectric element and preheating it at 523 K (250 °C) for 3 minutes. In this case, the bonding strengths increased to a range of 9.1 to 11.5 MPa after bonding at 473 K (200 °C) for 5 to 60 minutes, and the shear-tested specimens fractured with cleavage characteristics in the interior of the thermoelectric material. The bonding at 448 K (175 °C) led to shear strengths ranging from 7.1 to 8.5 MPa for various bonding times between 5 and 60 minutes, which were further increased to the values of 10.4 to 11.7 MPa by increasing the bonding pressure to 9.8 MPa. The shear strengths of Bi_0.5Sb_1.5Te₃/Cu joints bonded with the optimized conditions of the modified solid–liquid interdiffusion bonding process changed only slightly after long-term exposure at 473 K (200 °C) for 1000 hours.