Intermetallic Reactions during the Solid-Liquid Interdiffusion Bonding of Bi2Te2.55Se0.45 Thermoelectric Material with Cu Electrodes Using a Sn Interlayer

The intermetallic compounds formed during the diffusion soldering of a Bi2Te2.55Se0.45 thermoelectric material with a Cu electrode are investigated. For this bonding process, Bi2Te2.55Se0.45 was pre-coated with a 1 μm Sn thin film on the thermoelectric element and pre-heated at 250 ̋C for 3 min before being electroplated with a Ni barrier layer and a Ag reaction layer. The pre-treated thermoelectric element was bonded with a Ag-coated Cu electrode using a 4 μm Sn interlayer at temperatures between 250 and 325 ̋C. The results indicated that a multi-layer of Bi–Te–Se/Sn–Te–Se–Bi/Ni3Sn4 phases formed at the Bi2Te2.55Se0.45/Ni interface, ensuring sound cohesion between the Bi2Te2.55Se0.45 thermoelectric material and Ni barrier. The molten Sn interlayer reacted rapidly with both Ag reaction layers to form an Ag3Sn intermetallic layer until it was completely exhausted and the Ag/Sn/Ag sandwich transformed into a Ag/Ag3Sn/Ag joint. Satisfactory shear strengths ranging from 19.3 and 21.8 MPa were achieved in Bi2Te2.55Se0.45/Cu joints bonded at 250 to 300 ̋C for 5 to 30 min, dropping to values of about 11 MPa for 60 min, bonding at 275 and 300 ̋C. In addition, poor strengths of about 7 MPa resulted from bonding at a higher temperature of 325 ̋C for 5 to 60 min.